From our small world we have gazed upon the cosmic ocean for untold thousands of years. Ancient astronomers observed points of light that appeared to move among the stars. They called these objects planets, meaning wanderers, and named them after Roman deities Jupiter, king of the gods; Mars, the god of war; Mercury, messenger of the gods; Venus, the god of love and beauty, and Saturn, father of Jupiter and god of agriculture. The stargazers also observed comets with sparkling tails, and meteors or shooting stars apparently falling from the sky. Science flourished during the European Renaissance. Fundamental physical laws governing planetary motion were discovered, and the orbits of the planets round the Sun were calculated. In the 17th century, astronomers pointed a new device called the telescope at the heavens and made startling discoveries. But the years since 1959 have amounted to a golden age of solar system exploration. Advancements in rocketry after World War II enabled our machines to break the grip of Earth’s gravity and travel to the Moon and to other planets.
The United States has sent automated spacecraft, then human-crewed expeditions, to explore the Moon. Our automated machines have orbited and landed on Venus and Mars; explored the Sun’s environment; observed comets, and made close-range surveys while flying past Mercury, Jupiter, Saturn, Uranus and Neptune. These travellers brought a quantum leap in our knowledge and understanding of the solar system. Through the electronic sight and other senses of our automated spacecraft, colour and complexion have been given to worlds that for centuries appeared to Earth-bound eyes as fuzzy disks or indistinct points of light. And dozens of previously unknown objects have been discovered. Future historians will likely view these pioneering flights through the solar system as some of the most remarkable achievements of the 20th century.
The National Aeronautics and Space Administration’s (NASA’s) automated spacecraft for solar system exploration come in many shapes and sizes. While they are designed to fulfil separate and specific mission objectives, the craft share much in common.
Each spacecraft consists of various scientific instruments selected for a particular mission, supported by basic subsystems for electrical power, trajectory and orientation control, as well as for processing data and communicating with Earth. Electrical power is required to operate the spacecraft instruments and systems. NASA uses both solar energy from arrays of photo-voltaic cells and small nuclear generators to power its solar system missions. Rechargeable batteries are employed for backup and supplemental power. Imagine that a spacecraft has successfully journeyed millions of miles through space to fly but one time near a planet, only to have its cameras and other sensing instruments pointed the wrong way as it speeds past the target
To help prevent such a mishap, a subsystem of small thrusters is used to control spacecraft. The thrusters are linked with devices that maintain a constant gaze at selected stars. Just as Earth’s early seafarers used the stars to navigate the oceans, spacecraft use stars to maintain their bearings in space. With the sub-system locked onto fixed points of reference, flight controllers can keep a spacecraft’s scientific instruments pointed at the target body and the craft’s communications antennas pointed toward Earth. The thrusters can also be used to fine-tune the flight path and speed of the spacecraft to ensure that a target body is encountered at the planned distance and on the proper trajectory.
Between 1959 and 1971, NASA spacecraft were dispatched to study the Moon and the solar environment; they also scanned the inner planets other than Earth — Mercury, Venus and Mars. These three worlds, and our own, are known as the terrestrial planets because they share a solid-rock composition. For the early planetary reconnaissance missions, NASA employed a highly successful series of spacecraft called the Mariners. Their flights helped shape the planning of later missions. Between 1962 and 1975, seven Mariner missions conducted the first surveys of our planetary neighbours in space. All of the Mariners used solar panels as their primary power source. The first and the final versions of the spacecraft had two wings covered with photo-voltaic cells.
Other Mariners were equipped with four solar panels extending from their octagonal bodies. Although the Mariners ranged from the Mariner 2 Venus spacecraft, weighing in at 203 kilograms (447 pounds), to the Mariner 9 Mars Orbiter, weighing in at 974 kilograms (2,147 pounds), their basic design remained quite similar through-out the program. The Mariner 5 Venus spacecraft, for example, had originally been a backup for the Mariner 4 Mars flyby. The Mariner 10 spacecraft sent to Venus and Mercury used components left over from the Mariner 9 Mars Orbiter program.
In 1972, NASA launched Pioneer 10, a Jupiter spacecraft. Interest was shifting to four of the outer planets — Jupiter, Saturn, Uranus and Neptune — giant balls of dense gas quite different from the terrestrial worlds we had already surveyed. Four NASA spacecraft in all — two Pioneers and two Voyagers — were sent in the 1970s to tour the outer regions of our solar system. Because of the distances involved, these travellers took anywhere from 20 months to 12 years to reach their destinations. Barring faster spacecraft, they will eventually become the first human artefacts to journey to distant stars. Because the Sun’s light becomes so faint in the outer solar system, these travellers do not use solar power but instead operate on electricity generated by heat from the decay of radioisotopes.
NASA also developed highly specialised spacecraft to revisit our neighbours Mars and Venus in the middle and late 1970s. Twin Viking Landers were equipped to serve as seismic and weather stations and as biology laboratories. Two advanced orbiters — descendants of the Mariner craft — carried the Viking Landers from Earth and then studied Martian features from above. Two drum-shaped Pioneer spacecraft visited Venus in 1978. The Pioneer Venus Orbiter was equipped with a radar instrument that allowed it to “see” through the planet’s dense cloud cover to study surface features. The Pioneer Venus Multiprobe carried four probes that were dropped through the clouds. The probes and the main body — all of which contained scientific instruments — radioed information about the planet’s atmosphere during their descent toward the surface. A new generation of automated spacecraft — including Magellan, Galileo, Ulysses, and Cassini — is being developed and sent out into the solar system to make detailed examinations that will increase our understanding of our neighbour-hood and our own planet.
The solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes the satellites of the planets, numerous Comets, Asteroids, Meteoroids, and the Interplanetary Medium. The sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun's nearest known stellar neighbour is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away (a light year is the distance light travels in a year, at the rate of 300,000 km per second). The whole solar system, together with the local stars visible on a clear night, orbits the centre of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large and Small Magellanic Clouds. Our galaxy, one of billions of galaxies known, is travelling through Intergalactic space. The planets, most of the satellites of the planets, and the asteroids revolve around the sun in the same direction, in nearly circular orbits. The sun and planets rotate on their axes. All the planets orbit the sun in or near the same plane, called the Ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the sun than is Neptune.
| MERCURY | VENUS | EARTH | MARS | JUPITER | |
|---|---|---|---|---|---|
| Equatorial Diameter (km) | 4,878 | 12,104 | 12,756 | 6,794 | 142,800 |
| Sidereal period | 58.65d | 243.16d | 23h 56m | 24h 37m | 9h 55m |
| Inclination of axis to orbit | 0 | 178 | 23 27' | 24 46' | 3 04' |
| Density (Earth=1) | 0.997 | 0.95 | 1.0 | 0.71 | 0.13 |
| Mass (Earth=1) | 0.06 | 0.81 | 1.0 | 0.11 | 317.89 |
| Surface Gravity (Earth=1) | 0.38 | 0.90 | 1.0 | 0.38 | 2.64 |
| Escape Velocity (km per sec) | 4.3 | 10.36 | 11.2 | 5.03 | 60.22 |
| Albedo (100% refl=1) | 0.06 | 0.76 | 0.36 | 0.16 | 0.73 |
| Mean distance from the Sun in millions of km | 57.9 | 08.2 | 149.6 | 227.9 | 778.3 |
| SATURN | URANUS | NEPTUNE | PLUTO | SUN | |
|---|---|---|---|---|---|
| Equatorial Diameter (km) | 120000 | 52800 | 48400 | 2400 | 1392530 |
| Sidereal period | 10h 4m | 17h* | 16h* | ** | 25d 9h 7m |
| Inclination of axis to orbit | 26 44' | 97 53' | 28 48' | 50* | 7 15' |
| Density (Earth=1) | 0.04 | 0.23 | 0.31 | 0.36* | 0.25 |
| Mass (Earth=1) | 95.14 | 14.52 | 17.21 | 0.10* | 332.945 |
| Surface Gravity (Earth=1) | 1.16 | 1.11 | 1.21 | ** | 28.0 |
| Escape Velocity (km per sec) | 32.26 | 22.5 | 23.9 | 1.16* | 617.3 |
| Albedo (100% refl=1) | 0.76 | 0.93 | 0.62 | 0.54* | ----- |
| Mean distance from the Sun in millions of km | 1,427 | 2,869.6 | 4,496.7 | 5,899 | ----- |
A discussion of the objects in the solar system must start with the Sun. The Sun dwarfs the other bodies, representing approximately 99.86 percent of all the mass in the solar system; all of the planets, moons, asteroids, comets, dust and gas add up to only about 0.14 percent. This 0.14 percent represents the material left over from the Sun’s formation. One hundred and nine Earth’s would be required to fit across the Sun’s disk, and its interior could hold over 1.3 million Earth’s. As a star, the Sun generates energy through the process of fusion. The temperature at the Sun’s core is 15 million degrees Celsius (27 million degrees Fahrenheit), and the pressure there is 340 billion times Earth’s air pressure at sea level. The Sun’s surface temperature of 5,500 degrees Celsius (10,000 degrees Fahrenheit) seems almost chilly compared to its core-temperature. At the solar core, hydrogen can fuse into helium, producing energy. The Sun also produces a strong magnetic field and streams of charged particles, both extending far beyond the planets. The Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so.
At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow Earth. After a billion years as a “red giant,” it will suddenly collapse into a “white dwarf”— the final end product of a star like ours. It may take a trillion years to cool off completely.
Many spacecraft have explored the Sun’s environment, but none have got any closer to its surface than approximately two thirds of the distance from Earth to the Sun. Pioneers 5-11, the Pioneer Venus Orbiter, Voyagers 1 and 2 and other spacecraft have all sampled the solar environment. The Ulysses spacecraft, launched on October 6, 1990, is a joint solar mission of NASA and the European Space Agency. On February 8, 1992, Ulysses flew close to Jupiter and used Jupiter’s gravity to hurl it down below the plane of the planets. Although it will still be at great distance from the Sun, Ulysses will fly over the Sun’s polar regions during 1994 and 1995 and will perform a wide range of studies using nine onboard scientific instruments. We are fortunate that the Sun is exactly the way it is. If it were different in almost any way, life would almost certainly never have developed on Earth.
To the Egyptians it was Ra, Amen, Aten, or Osiris, each with a different religious significance. The winged globe in Egyptian art is a familiar representation of the solar orb. Atenism, the first impersonal concept of the Deity, worshipped only “the power which came from the Sun,” and forbade any emblem or idol that would tend to substitute a symbol for the thing itself. To The Persian it was Mithras; to the Hindu, Brahma; to the Chaldean, Bel; and to the Greek, Adonis and Apollo. In Free-masonry Sol-om-on, the name of the Sun in three languages, is an expression of light.
Actually the Sun has no visible motion, although we know it moves because nothing in the universe can hold its place by standing still. However, ancient astrology dealt with things as they appear rather than as they are; just as the wind which blows South was to the ancients the North wind because it came out of the North. Therefore, when astrology speaks of the Sun’s motion we must not overlook the fact that what we actually mean is the Earth’s motion which we measure by or describe in the terms of the apparent motion of the Sun. That the ancient masters knew this, can be seen in the order of the planetary hours: Saturn, Jupiter, Mars, Sun, Venus — the placing of the Sun between Mars and Venus clearly showing that it represents the Earth in this sequence.
The Nodes at which the Earth intercepts the plane of the Sun’s equator, lie at heliocentric longitudes 75° and 255°, which the Earth crosses in Junc and December. The Sun’s North Pole is inclined toward the Earth by 7° in July, and away from the Earth by 7° in January. The plane of the Sun’s orbit is not known, but since the Milky Way galaxy is a flat disc of stars it is probable that the Sun’s orbit does not deviate to any great extent from the average of the stars within the galaxy — similar to the orbits of the planets which lie within a narrow band that extends some 7° on either side of the Ecliptic.
We do know that the plane of our ecliptic is inclined to the plane of the Milky Way galaxy at a steep angle of approximately 50° hence the three-dimensional motion of the Earth with reference to the orbit of the Sun must involve a considerable degree of elevation and depression above and below the plane of the Sun’s orbit; also that there must be a considerable declination of the Sun’s pole with reference to its orbit, not unlike that of the Earth’s pole to which we ascribe our seasonal variations. Because of this, the Nodes where the Earth intersects the Sun’s equator are not the same as those at which the Earth intersects the plane of the Sun’s orbit. It is not improbable that the latter nodes may pursue a precessional cycle not unlike that of the Moon’s Nodes.
The Sun is a variable star, unlike any other star yet discovered. It revolves from East to West; i.e., looking down on its North pole, it moves counter-clockwise. Its period of rotation at the Equator is 24.65 d.; at the pole, 34 d. Its mean period as seen by the Earth is 25.38 d.; but its synodical period of rotation is 27.25 d.
The diameter of the Sun is 864,392 miles. Driving in an automobile at the rate of 500 miles a day, it would require 14 y, 10 m, 2 d, to circle the Sun.
Its weight in tons is 2,200 plus 24 ciphers, or 2.2 octillion tons. In bulk it could contain 1,300,000 Earths.
The Sun-Earth distance — 92,897,416 miles — is taken as a unit of measurement of inter-solar system space, and is known as one Astronomical Unit. Its light requires 498.59 seconds, or about 8 1/3 minutes, to reach the Earth. To travel the distance by an airplane at 300 miles per hour, would consume 35 years; to walk at 4 m.p.h., 6300 y.
Hugh Rice, astronomer of the Hayden Planetarium of New York, says, “The Sun is the source of almost all the power, heat and life on the Earth.” Heat reaching the Earth amounts to 1.94 calories per minute, per square mile of the Earth’s surface. One caloric is the amount of heat required to raise one gram of water by one degree of temperature.
In terms of power the Sun’s radiation amounts to 1.51 h.p. per sq. yard of the Earth’s surface, or 643,000 h.p. per sq. mile. Were it not for loss by curvature and reflection it would amount to 4,690,000 h.p. per sq. mile, or for the entire surface of the Earth, 127 Plus twelve ciphers, or 127 trillions of horsepower — more than we could possibly use. Actually our absorption amounts to from 0.34 to 0.38 h.p. per sq. yard, or the equivalent of a 60-watt lamp in continuous operation. When it is recalled that the Earth as seen from the Sun is a point in the sky apparently less than half as large as Venus when it is our brilliant evening star, and that this is the tiny object which intercepts a total of 230 million-million horsepower of solar radiation, it becomes evident that the Sun radiates an incomprehensible amount of energy. Indeed, we find that it radiates nearly 2,200,000,000 times as much energy as that which lights and warms and gives life to our planet, and hundreds of millions of times as much energy as is intercepted by all the planets, satellites, and planetoids combined.
Most of the Sun has a temperature of a million degrees. Its energy travels at the rate of 186,271 miles per second. The Sun’s heat would melt a block of ice the size of the Earth in 16.6 minutes; a block of iron of the same size, in Less than 3 hours. Its heat for a year is equal to the burning of tons of coal amounting to 400 Plus 21 ciphers.
The Sun’s Spectrum of visible light extends from 7700 Angstrom units on the red end, to 3600 Angstrom units on the violet end. An Angstrom unit is one ten-millionth of a millimeter. A millimeter is 1/25th of an inch. A wave of red light measures one 32-thousandths of an inch; of violet, one 64-thousandths. Hence the visible Spectrum consists of one octave, although 40 octaves are known to Scicncc.
The ultra-violet band extends from 3600 to 1000 Angstrom units. However, the ozone in the Earth’s atmosphere cuts out all rays shorter than 2900 A.U. Tanning is nature’s way of protecting the body against an excess of ultra-violet radiation.
The light of the Sun is 465,000 times brighter than the Full Moon; 900,000,000 times brighter than Venus at its brightest. In the Zenith this has been computed at 103,000 meter-candles. A meter-candle is the light received from a candle at a distance of a meter.
According to the latest astronomical computations the Sun’s proper motion in orbit is approx. 200 miles per second; its apparent motion towards a point in the constellation Hercules is 12 miles per second.
Obtaining the first close-up views of Mercury was the primary objective of the Mariner 10 spacecraft, launched on November 3, 1973, from Kennedy Space Center in Florida. After a journey of nearly five months, which included a flyby of Venus, the spacecraft passed within 703 kilometres (437 miles) of the solar system’s innermost planet on March 29, 1974. Until Mariner 10, little was known about Mercury. Even the best telescopic views from Earth showed Mercury as an indistinct object lacking any surface detail. The planet is so close to the Sun that it is usually lost in solar glare. When the planet is visible on Earth’s horizon just after sunset or before dawn, it is obscured by the haze and dust in our atmosphere. Only radar telescopes gave any hint of Mercury’s surface conditions prior to the voyage of Mariner 10. The photographs Mariner 10 radioed back to Earth revealed an ancient, heavily cratered surface, closely resembling our own Moon. The pictures also showed huge cliffs criss-crossing the planet. These apparently were created when Mercury’s interior cooled and shrank, buckling the planet’s crust. The cliffs are as high as 3 kilometres (2 miles) and as long as 500 kilometres (310 miles). Instruments on Mariner 10 discovered that Mercury has a weak magnetic field and a trace of atmosphere — a trillionth the density of Earth’s atmosphere and composed chiefly of argon, neon and helium. When the planet’s orbit takes it closest to the Sun, surface temperatures range from 467 degrees Celsius (872 degrees Fahrenheit) on Mercury’s sunlit side to -183 degrees Celsius (-298 degrees Fahrenheit) on the dark side. This range in surface temperature — 650 degrees Celsius (1,170 degrees Fahrenheit) — is the largest for a single body in the solar system. Mercury literally bakes and freezes at the same time.
Days and nights are long on Mercury. The combination of a slow rotation relative to the stars (59 Earth days) and a rapid revolution around the Sun (88 Earth days) means that one Mercury solar day takes 176 Earth days or two Mercury years — the time it takes the inner-most planet to complete two orbits around the Sun Mercury appears to have a crust of light silicate rock like that of Earth. Scientists believe Mercury has a heavy iron-rich core making up slightly less than half of its volume. That would make Mercury’s core larger, proportionally, than the Moon’s core or those of any of the planets. After the initial Mercury encounter, Mariner 10 made two additional flybys — on September 21, 1974, and March 16, 1975 — before control gas used to orient the spacecraft was exhausted and the mission was concluded. Each flyby took place at the same local Mercury time when the identical half of the planet was illuminated; as a result, we still have not seen one-half of the planet’s surface.
Veiled by dense cloud cover, Venus — our nearest planetary neighbour — was the first planet to be explored. The Mariner 2 spacecraft, launched on August 27, 1962, was the first of more than a dozen successful American and Soviet missions to study the mysterious planet. As spacecraft flew by or orbited Venus, plunged into the atmosphere or gently landed on Venus’ surface, romantic myths and speculations about our neighbour were laid to rest. On December 14, 1962,
A small planet, with pale bluish light; the planet closest to the Sun. Never more than 28 degrees from the Sun, it is rarely visible to the naked eye. The Roman god Mercury and the Greek god Hercules, the winged messenger of the Gods, were endowed with the qualities that are associated with the influence of the planet Mercury. To the Chaldeans it was Nebo, the planet of warning; also associated with Buddha, the wise.
Ancient astrologers considered the existence of a planet nearer to the Sun than Mercury, to which they gave the name Vulcan. It has not as yet been discovered by astronomers.
From a stationary point about 28ş in advance of the Sun, it retrogrades to an inferior conjunction with the Sun — after which it becomes a “morning star,” visible on the Eastern horizon shortly before Sunrise. From a stationary point about 20ş behind the Sun, it advances by direct motion to a superior conjunction with the Sun — after which it becomes an “evening star,” visible on the Western horizon shortly after Sunset.
As with the Moon, and all satellites with reference to the planet around which they revolve, Mercury always turns the same face toward the Sun, except for a libration of 23ş 7’ in both directions: making a 47ş zone of temperate conditions, and 132ş zones of perpetual heat and cold.
As seen from the Earth, Mercury presents phases, similar to those of the Moon, because of which its visible size varies from 36’ to 104’ — its crescent or new moon phase occurs at its inferior conjunction; its full moon phase at its superior conjunction. Its minor elongation, about 18ş, occurs 22 days before and after its inferior conjunction; its major elongation, about 28ş, 36 days before and after its superior conjunction. At its maximum its visible size is 3¬ times its diameter. Two of Jupiter’s moons are larger than the planet Mercury.
To locate Mercury in the evening sky, find in the ephemeris the dates of its major elongation before or after a superior conjunction, and for 10 and 5 days before and after. Transfer into hours its R.A. and declination on these five dates, and plot its course on a star map, making note its nearness to known bright stars. Tilt this map toward the celestial North pole, and assume a horizon about 23ş below the Mercury position. If weather conditions permit it can be seen with the aid of a field glass — sometimes even with the naked eye. Mercury made a transit across the face of the Sun on May 11, 1937.
Mariner 2 passed within 34,839 kilometres (21,648 miles) of Venus and became the first spacecraft to scan another planet; onboard instruments measured Venus for 42 minutes. Mariner 5, launched in June 1967, flew much closer to the planet. Passing within 4,094 kilo-metres (2,544 miles) of Venus on the second American flyby, Mariner 5’s instruments measured the planet’s magnetic field, ionosphere, radiation belts and temperatures. On its way to Mercury, Mariner 10 flew by Venus and transmitted ultraviolet pictures to Earth showing cloud circulation patterns in the Venusian atmosphere.
In the spring and summer of 1978, two spacecraft were launched to further unravel the mysteries of Venus. On December 4 of the same year, the Pioneer Venus Orbiter became the first spacecraft placed in orbit around the planet. Five days later, the five separate components making up the second spacecraft — the Pioneer Venus Multiprobe entered the Venusian atmosphere at different locations above the planet. The four small, independent probes and the main body radioed atmospheric data back to Earth during their descent toward the surface. Although designed to examine the atmosphere, one of the probes survived its impact with the surface and continued to transmit data for another hour. Venus resembles Earth in size, physical composition and density more closely than any other known planet. However, spacecraft have discovered significant differences as well. For example, Venus’ rotation (west to east) is retrograde (backward) compared to the east-to-west spin of Earth and most of the other planets.
Approximately 96.5 percent of Venus’ atmosphere (95 times as dense as Earth’s) is carbon dioxide. The principal constituent of Earth’s atmosphere is nitrogen. Venus’ atmosphere acts like a greenhouse, permitting solar radiation to reach the surface but trapping the heat that would ordinarily be radiated back into space. As a result, the planet’s average surface temperature is 482 degrees Celsius (900 degrees Fahrenheit), hot enough to melt lead. A radio altimeter on the Pioneer Venus Orbiter provided the first means of seeing through the planet’s dense cloud cover and deter-mining surface features over almost the entire planet. NASA’s Magellan spacecraft, launched on May 5, 1989, has been in orbit around Venus since August 10, 1990. The spacecraft used radar-mapping techniques to provide high-resolution images of 98 percent of the surface.
Magellan’s radar revealed a landscape dominated by volcanic features, faults and impact craters. Huge areas of the surface show evidence of multiple periods of lava flooding with flows lying on top of previous ones. An elevated region named Ishtar Terra is a lava-filled basin as large as the United States. At one end of this plateau sits Maxwell Montes, a mountain the size of Mount Everest. Scarring the mountain’s flank is a 100-kilometre (62-miles) wide, 2.5-kilometre (1.5-miles) deep impact crater named Cleopatra. (Almost all features on Venus are named for women; Maxwell Montes, Alpha Regio and Beta Regioare the exceptions.) Craters survive on Venus for perhaps 400 million years because there is no water and very little wind erosion.
Extensive fault-line networks cover the planet, probably the result of the same crustal flexing that produces plate tectonics on Earth. But on Venus the surface temperature is sufficient to weaken the rock, which cracks just about everywhere, preventing the formation of major plates and large earth quake faults like the San Andreas Fault in California. Venus’ predominant weather pattern is a high-altitude, high-speed circulation of clouds that contain sulphuric acid. At speeds reaching as high as 360 kilometres (225 miles) per hour, the clouds circle the planet in only four Earth days. The circulation is in the same direction — west to east — as Venus’ slow rotation of 243 Earth days, whereas Earth’s winds blow in both directions — west to east and east to west — in six alternating bands. Venus’ atmosphere serves as a simplified laboratory for the study of our weather.
A brilliant planet reflecting a silvery-white light, it is the most brilliant object that illuminates the evening sky. The Greeks associated it with Aphrodite. To the Romans, it was known as Lucifer, when the Morning Star: and Vesper, when the Evening Star. To the Chaldeans it was Ishtar, and compared to the Sumerian virgin mother, the “Lady of Heaven,” and the goddess of fertility.
Like Mercury, Venus exhibits phases, from a large twin crescent at the Inferior Conjunction, when it is closest to the Earth, and some- times visible in daylight if you know where to look for it, to a small round orb at the Superior Conjunction, when it is on the opposite side of the Sun from the Earth. After the Superior Conjunction it is an Evening Star, and thus is visible in the evening, sky after sun- down, setting later each evening until it reaches its maximum elongation of about 47ş — at which time it sets about 3 hours after the Sun.
Shortly thereafter it attains to its greatest brilliancy, then grows rapidly smaller as it again comes closer behind the Sun, until at its Inferior Conjunction it becomes invisible. Thereafter it reappears on the other side of the Sun and becomes again visible as the Morning Star. Its motion as a Morning Star, as measured from the Earth, is slower because of its greater distance from the Earth: 26 million miles at the Inferior Conjunction, as compared to 16o million miles at the Superior Conjunction.
Its rotation period has never been established because of the layer of clouds in which it is perpetually enveloped. Its period has been variously estimated at from 68 hours to 225 days. Its axis is inclined to its orbit plane at an angle of 5 degrees. Its low albedo, or reflecting power (.59), is due to this constant cloud covering. The periods when it is a Morning and Evening Star are of about 10 months’ duration each.
Transits over the Sun are rare and occur only when the Sun is within 1ş 45’ of the node, with the Earth also at the node. Though infrequent, they come in pairs. The last such transits occurred in 1874 and 1882. It will not recur until June 8, 2004 and June 6, 2012. The duration of such a transit is about 8 hours.
As viewed from space, our world’s distinguishing characteristics are its blue waters, brown and green land masses and white clouds. We are enveloped by an ocean of air consisting of 78 percent nitrogen, 21 percent oxygen and 1 percent other constituents. The only planet in the solar system known to harbour life, Earth orbits the Sun at an average distance of 150 million kilometres (93 million miles). Earth is the third planet from the Sun and the fifth largest in the solar system, with a diameter just a few hundred kilometres larger than that of Venus. Our planet’s rapid spin and molten nickel-iron core give rise to an extensive magnetic field, which, along with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Earth’s atmosphere protects us from meteors as well, most of which burn up before they can strike the surface. Active geological processes have left no evidence of the pelting Earth almost certainly received soon after it formed - about 4.6 billion years ago. Along with the other newly formed planets, it was showered by space debris in the early days of the solar system.
From our journeys into space, we have learned much about our home planet. The first American satellite — Explorer 1 — was launched from Cape Canaveral in Florida on January 31, 1958, and discovered an intense radiation zone, now called the Van Allen radiation belts, surrounding Earth. Since then, other research satellites have revealed that our planet’s magnetic field is distorted into a teardrop shape by the solar wind — the stream if charged particles continuously ejected from the Sun. We’ve learned that the magnetic field does not fade off into space but has definite boundaries. And we now know that our wispy upper atmosphere, once believed calm and uneventful, seethes with activity — swelling by day and contracting by night. Affected by changes in solar activity, the upper atmosphere contributes to weather and climate on Earth.
Besides affecting Earth’s weather, solar activity gives rise to a dramatic visual phenomenon in our atmosphere. When charged particles from the solar wind become trapped in Earth’s magnetic field, they collide with air molecules above our planet’s magnetic poles. These air molecules then begin to glow and are known as the auroras or the northern and southern lights. Satellites about 35,789 kilometres (22,238 miles) out in space play a major role in daily local weather forecasting. These watchful electronic eyes warn us of dangerous storms. Continuous global monitoring provides a vast amount of useful data and contributes to a better understanding of Earth’s complex weather systems. From their unique vantage points, satellites can survey Earth’s oceans, land use and resources, and monitor the planet’s health. These eyes in space have saved countless lives, provided tremendous conveniences and shown us that we may be altering our planet in dangerous ways.
The planet we inhabit. Astrologically, the Earth is the center of its universe, since one is concerned not with the position of the planets in reference to the Sun, but with the angle from which their reflected frequencies enter into the experience of those who dwell upon the Earth. When one speaks of the Sun’s position he is but expressing the position of the Earth in its orbit in terms of the apparent position of the Sun. The Earth’s orbit is an ellipse, of an eccentricity of about 1.60 — but which is slowly diminishing. Its longest diameter is its major axis. Its half length, or semi-axis, taken as the Mean distance from the Earth to Sun, amounts to about 92,900,000 miles. At perihelion the Earth is more than three million miles closer to the Sun than at aphelion; or about 3% of the maxi- mum distance. The velocity of the Earth in its orbit is approximately 18.5 miles per second.
The Precession of the Equinoctial Point amounts to 360 degrees in about 24,800 years. The Earth’s rotation appears to be slowing down at a rate which if continued will amount to 1 second in about 120,000 years.
The common center around which the Earth and the Moon revolve has been computed to be about 3000 miles from the Earth’s center — or 1000 miles below the crust of the Earth. That this point is a variable one has been used by some as a basis for a computation based on the assumption that as this point approaches the surface of the Earth there result phenomena known as Earthquakes.
The Earth curves from a straight line at the rate of about 1/9th of a degree per second. Its Diameter at the poles is 7900 m.; at the Equator, 7926 m. The inclination of its axis to the Ecliptic, 66ş 33’.
The Moon is Earth’s single natural satellite. The first human foot-steps on an alien world were made by American astronauts on the dusty surface of our airless, lifeless companion. In preparation for the human-crewed Apollo expeditions, NASA dispatched the automated Ranger, Surveyor and Lunar Orbiter spacecraft to study the Moon between 1964 and 1968. NASA’s Apollo program left a large legacy of lunar materials and data. Six two-astronaut crews landed on and explored the lunar surface between 1969 and 1972, carrying back a collection of rocks and soil weighing a total of 382 kilograms (842 pounds) and consisting of more than 2,000 separate samples. From this material and other studies, scientists have constructed a history of the Moon that includes its infancy. Rocks collected from the lunar highlands date to about 4.0-4.3 billion years old. The first few million years of the Moon’s existence were so violent that few traces of this period remain. As a molten outer layer gradually cooled and solidified into different kinds of rock, the Moon was bombarded by huge asteroids and smaller objects, and their collisions with the Moon created basins hundreds of kilometres across.
This catastrophic bombardment tapered off approximately four billion years ago, leaving the lunar highlands covered with huge, overlapping craters and a deep layer of shattered and broken rock. Heat produced by the decay of radioactive elements began to melt the interior of the Moon at depths of about 200 kilometres (125 miles) below the surface. Then, for the next 700 million years — from about 3.8 to 3.1 billion years ago — lava rose from inside the Moon. The lava gradually spread out over the surface, flooding the large impact basins to form the dark areas that Galileo Galilei, an astronomer of the Italian Renaissance, called maria, meaning seas. As far as we can tell, there has been no significant volcanic activity on the Moon for more than three billion years. Since then, the lunar surface has been altered only by micro-meteorites, by the atomic particles from the Sun and stars, by the rare impacts of large meteorites and by spacecraft and astronauts. If our astronauts had landed on the Moon a billion years ago, they would have seen a landscape very similar to the one today. Thousands of years from now, the footsteps left by the Apollo crews will remain sharp and clear.
The origin of the Moon is still a mystery. Four theories attempt an explanation: the Moon formed near Earth as a separate body; it was torn from Earth; it formed somewhere else and was captured by our planet’s gravity, or it was the result of a collision between Earth and an asteroid about the size of Mars. The last theory has some good support but is far from certain.
A satellite of the Earth, which to different civilizations has also been known as Luna, Soma, Isis; the “mother of the Earth.” It has given us the name for the first day of the week-Monday; also lunacy, lunatic, moonstruck.
The Moon, reflecting the light of the Sun, emits a degree of heat which can be registered by concentrating the rays on the bulb of a thermometer. It may have some slight vegetation, but because of the apparent absence of atmosphere or clouds it lacks sufficient water to support vegetation such as is on the Earth.
The period of the Moon’s axial rotation is the same as its period of revolution, hence the same side of the Moon is always turned toward the Earth. That its orbit was formerly smaller and its velocity correspondingly greater is proved by comparing records of ancient eclipses to tables based on observation of its present motion.
The Moon’s mean distance from the Earth is 238,840 miles, or 60 times the Earth’s radius. It travels a trifle faster than its diameter per hour. Nor is it entirely the nearest body to the Earth, for in part of its orbit the minor planet Hermes (disc. in 1937) approaches to a distance of only 200,000 Miles. Traveling by airplane at 200 m.p.h. one would traverse the Earth-Moon distance in 5o days; but it would take a rocket ship speed of 7 m.p.s. to get beyond the Earth’s gravitational field-at which rate we could arrive in 2 days.
Lifetimes have been devoted to the study of its incredibly complex motions. Among its various perturbations are the Equation of the Center, the retrogression of the Nodes, Evection, the anomalistic period, Lunar Variation, Annual Equation, and Secular Acceleration.
Galilee, in 1610, was the first selenographer to study the Moon through a telescope. In 1647 Hevelius published a chart of the Moon’s surface that was not improved upon for a century. Its phases are familiar: The crescent of the new moon, and the reverse crescent of the fourth quarter of its circuit; the gibbous phase of the second and third quarters, when more than half of the moon is light; and the Earth-shine, when the Earth reflects a dim light upon the surface of the Moon during a few days before and after the Lunation.
Because of its faster motion near perigee we are able to see 7°45’ around the Eastern and Western edges. This is termed its Libration in Longitude. Because of the inclination of the plane of the Moon’s orbit to that of the Earth, we are able at times to see 6°41’ beyond each of the poles. This is termed Libration in Latitude. There is also a Diurnal Libration of 1° on the Eastern limb of the Moon when rising, and on the Western when setting. The net combined result is that 41% of the Moon’s surface is visible all the time, with another 18% that is visible part of the time, leaving 41%. that has never been seen from the Earth.
Meton discovered the recession of the Moon’s node in 432 B.C. and reformed the calendar in accordance therewith. He determined that there were 235 synodic periods in 19 years, varying by i day according to the number of leap years contained in the period.
The node recesses 360° in 6793.5 days or 18 2/3 years, or roughly 1˝ years to a sign. The Draconitic period of the Moon’s motion, that from node to node, is 27.2122 days. The moon rises 50 minutes later each night.
Harvest MoonAt this season of the year the Moon’s path more nearly parallels that of the Earth, hence it remains near to the horizon for several days, at the same hour. Similarly with the Hunter’s Moon, which is the nearest Full Moon to September 23rd. This effect is further intensified when the descending node is at 0ş Aries. For example, with the Ascending node at 0ş Aries : 23ş 27’, Plus 5ş 9’, equals 28ş 36’. With the Descending node at 0ş Aries : 23ş 27, minus 5ş 9’, equals 18ş 18ş’. The Full Moon rides low in Summer but high in Winter, thus making Winter the season of least sunlight but of most moonlight.
Moonlight contains streaks of bright rays, apparently from some special mineral that fails to absorb light, or which may have some such property as radioactivity — to conjecture on a point regarding which scientists fail to agree. The rays consist largely of shades of yellow and gray, and from certain areas a shade of green. The Earth’s surface has a reflective power six times greater than that of the Moon.
The Lunar spectrum is much the same as that of the Sun, except that the light is yellower, and more diffused because of the rough- ness of the Moon’s surface. At the quarter, the Moon’s light has a brilliance of one-millionth that of the Sun; at the Full, 1/465 thousandths. However, the Moon absorbs 93% of the light it could reflect.
The Moon’s aspects by Right Ascension differ some minutes from those by Geocentric Longitude. Tropical period minus Precession from 0ş Aries : 6.9 seconds per period. The color white is often associated with the Moon to symbolize purity. That it is chemically white is due to the absence of all color. Prismatically it is the presence of all colors of the spectrum, or the three primary colors in the proportions of three parts of yellow, five of red, and eight of blue.
Of all the planets, Mars has long been considered the solar system’s prime candidate for harbouring extraterrestrial life. Astronomers studying the red planet through telescopes saw what appeared to be straight lines criss-crossing its surface. These observations — later determined to be optical illusions — led to the popular notion that intelligent beings had constructed a system of irrigation canals on the planet. In 1938, when Orson Welles broadcast a radio drama based on the science fiction classic War of the Worlds by H.G. Wells, enough people believed in the tale of invading Martians to cause a near panic. Another reason for scientists to expect life on Mars had to do with the apparent seasonal colour changes on the planet’s surface. This phenomenon led to speculation that conditions might support a bloom of Martian vegetation during the warmer months and cause plant life to become dormant during colder periods. So far, six American missions to Mars have been carried out. Four Mariner spacecraft — three flying by the planet and one placed into Martian orbit — surveyed the planet extensively before the Viking Orbiters and Landers arrived.
Mariner 4, launched in late 1964, flew past Mars on July 14, 1965, coming within 9,846 kilometres (6,118 miles) of the surface. Transmitting to Earth 22 close-up pictures of the planet, the spacecraft found many craters and naturally occurring channels but no evidence of artificial canals or flowing water. Mariners 6 and 7 followed with their flybys during the summer of 1969 and returned 201 pictures. Mariners 4, 6 and 7 showed a diversity of surface conditions as well as a thin, cold, dry atmosphere of carbon dioxide. On May 30, 1971, the Mariner 9 Orbiter was launched on a mission to make a year-long study of the Martian surface. The spacecraft arrived five and a half months after lift-off, only to find Mars in the midst of a planet-wide dust storm that made surface photography impossible for several weeks. But after the storm cleared, Mariner 9 began returning the first of 7,329 pictures; these revealed previously unknown Martian features, including evidence that large amounts of water once flowed across the surface, etching river valleys and flood plains.
In August and September 1975, the Viking 1 and 2 spacecraft — each consisting of an orbiter and a lander — lifted off from Kennedy Space Center. The mission was designed to answer several questions about the red planet, including, Is there life there? Nobody expected the spacecraft to spot Martian cities, but it was hoped that the biology experiments on the Viking Landers would at least find evidence of primitive life — past or present. Viking Lander 1 became the first spacecraft to successfully touch down on another planet when it landed on July 20, 1976, while the United States was celebrating its Bicentennial. Photos sent back from the Chryse Planitia (“Plains of Gold”) showed a bleak, rusty-red landscape. Panoramic images returned by the lander revealed a rolling plain, littered with rocks and marked by rippled sand dunes. Fine red dust from the Martian soil gives the sky a salmon hue. When Viking Lander 2 touched down on Utopia Planitia on September 3, 1976, it viewed a more rolling landscape than the one seen by its predecessor — one without visible dunes.
The results sent back by the laboratory on each Viking Lander were inconclusive. Small samples of the red Martian soil were tested in three different experiments designed to detect biological processes. While some of the test results seemed to indicate biological activity, later analysis confirmed that this activity was inorganic in nature and related to the planet’s soil chemistry. Is there life on Mars? No one knows for sure, but the Viking mission found no evidence that organic molecules exist there. The Viking Landers became weather stations, recording wind velocity and direction as well as atmospheric temperature and pressure. Few weather changes were observed. The highest temperature recorded by either craft was - 14 degrees Celsius (7 degrees Fahrenheit) at the Viking Lander 1 site in midsummer. The lowest temperature, -120 degrees Celsius (-184 degrees Fahrenheit), was recorded at the more northerly Viking Lander 2 site during winter. Near-hurricane wind speeds were measured at the two Martian weather stations during global dust storms, but because the atmosphere is so thin, wind force is minimal. Viking Lander 2 photographed light patches of frost — probably water-ice.
The Martian atmosphere, like that of Venus, is primarily carbon dioxide. Nitrogen and oxygen are present only in small percentages. Martian air contains only about 1/1,000 as much water as our air, but even this small amount can condense out, forming clouds that ride high in the atmosphere or swirl around the slopes of towering volcanoes. Local patches of early morning fog can form in valleys. There is evidence that in the past a denser Martian atmosphere may have allowed water to flow on the planet. Physical features closely resembling shorelines, gorges, river-beds and islands suggest that great rivers once marked the planet. Mars has two moons, Phobos and Deimos. They are small and irregularly shaped and possess ancient, cratered surfaces. It is possible the moons were originally asteroids that ventured too close to Mars and were captured by its gravity.
The Viking Orbiters and Landers exceeded by large margins their design lifetimes of 120 and 90 days, respectively. The first to fail was Viking Orbiter 2, which stopped operating on July 24, 1978, when a leak depleted its altitude-control gas. Viking Lander 2 operated until April 12, 1980, when it was shut down because of battery degeneration. Viking orbiter 1 quit on August 7, 1980, when the last of its attitude-control gas was used up. Viking Lander1 ceased functioning on November 13, 1983. Despite the inconclusive results of the Viking biology experiments, we know more about Mars than any other planet except Earth.
The nearest planet to the Earth, and frequently visible, it may be recognized through the distinct reddish hue of its ray. Mars was known as Ares, the god of war; and as Nimrod, the god of the chase, whose mission it was apparently to dispel terror and fear. To the Greeks, it was Pyrois, the fire. The Romans celebrated the festival of Mars in March, before an altar in the Campus Martius. From it comes our word martial, war like — as martial music. To the Chaldeans it was Nergal, called the “raging king” and the “furious one”; to the Babylonians, the god of war and pestilence, said to preside over the nether-world. For the Alchemists, it represented Iron.
Mars has two satelites: Deimos, 6 miles in diameter, distant from Mars by 6.9 radii; and Phoetus, with a revolutionary period of 7h 39M. Deimos has a sidereal period of 30h 18m. Phoetus makes 1330 eclipses a year.
The solar system has a large number of rocky and metallic objects that are in orbit around the Sun but are too small to be considered full-fledged planets. These objects are known as asteroids or minor planets. Most, but not all, are found in a band or belt between the orbits of Mars and Jupiter. Some have orbits that cross Earth’s path, and there is evidence that Earth has been hit by asteroids in the past. One of the least eroded, best preserved examples is the Barringer Meteor Crater near Winslow, Arizona. Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometres (932 miles) across — less than half the diameter of our Moon. Thousands of asteroids have been identified from Earth. It is estimated that 100,000 are bright enough to eventually be photographed through Earth-based telescopes.
Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what’s left strikes Earth’s surface and is called a meteorite. One of the best places to look for meteorites is the ice cap of Antarctica. Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials. Stony meteorites are the hardest to identify since they look very much like terrestrial rocks. Since asteroids are material from the very early solar system, scientists are interested in their composition. Spacecraft that have flown through the asteroid belt have found that the belt is really quite empty and that asteroids are separated by very large distances. Current and future missions will fly by selected asteroids for closer examination. The Galileo spacecraft, launched by NASA in October 1989, investigated the main-belt asteroid Gaspra on October 29, 1991 and will encounter Ida on August 28, 1993 on its way to Jupiter. One day, space factories will mine the asteroids for raw materials.
Asteroids are also called Minor Planets or Planetoids.Thousands of small bodies invisible to the naked eye, nearly all of whose orbits lie between Mars and Jupiter (but see CHIRON).
The four largest - CERES, PALLAS, JUNO, and VESTA - were discovered in 1801, 1802, 1804, and 1807, respectively.
Here are some key words for the four major asteroids:
Ceres Loss and return, grief, nourishing love, adoption, concern about children, motherhood, famine, agriculture, grain, bakeries, food industry.Vesta Devotion, dedication to duty, renunciation, preservation of the sacred, the center, wholeness, centeredness, inner fire and spirit, hearth and home.
Pallas Politics and government, strategies to win approval, fight for causes, justice, defending the underdog, career orientation, efficiency, assertion, creative intellect, struggle to balance masculine and feminine issues.
Juno Legal partnership, power balance in relationships, the need for intimacy, commitment, loyalty, jealously, and fidelity.
Chiron Chiron is an asteroid whose orbit lies between Saturn and Uranus. It was recently discovered (1977). Astrologically it is the archetype of the teacher and healer.
An orbit, approximately midway between those of Mars and Jupiter, occupied by a large number of planetoids or minor planets: variously explained as fragments of a major planet broken up in some prehistoric catastrophe; or particles drawn out of the Sun which failed to coalesce into a single planet. In all there are estimated to be some 50,000 of these Asteroids, of which 1380 had been identified in 1937. As many as 5000 are estimated to have been seen, and again lost. Many of them are more readily visible than Pluto, and may have some astrological significance not as yet identified. Their average diameter is less than 100 miles.
The Astronomischer Rechen-Institut at Dahlem, near Berlin, was world headquarters for Asteroid research, and up to World War 11 published a yearly ephemeris of the larger Asteroids for the periods when they are best observed.
Statistics of the five principal Asteroids are as follows:
| Name | Diameter (miles) | Magnitude | Albedo(rel. to Sun) | Discovered |
|---|---|---|---|---|
| Ceres | 480 | 7.4 | 0.06 | 1801 |
| Pallas | 304 | 8.0 | 0.07 | 1802 |
| Juno | 120 | 8.7 | 0.12 | 1804 |
| Vesta | 240 | 6.5 | 0.26 | 1807 |
| Astraca | - | 9.9 | - | 1845 |
The next five, in the order of their discovery, are Hebe (1847), Iris (1847), Flora (1847), Metis (1848), Hygeia (1849).
The orbit of 944 Hidalgo has an eccentricity of 0.65 — more elongated than some comets. At its aphelion distance (9.6 units) it extends into Saturn’s orbit.
That of 1177 Gounessia, has an eccentricity of 0.006399, more circular than that of Venus, the most circular among the major planets.
That of 846 Lipperta, is almost parallel with that of the Earth, with an inclination of 0ş.244 — more nearly parallel than that of Uranus 0ş.77.
That of 2 Pallas has an inclination of 34ş.726 — double that of Pluto’s 17ş.1.
Three Asteroids come closer to the Earth than do any of the major planets. They are Amor, Apollo, and Adonis. 1936 CA or Adonis was discovered in 1936 by Delporte in Belgium. Its orbit has an eccentricity of 0.78, an inclination to the Ecliptic of 1ş.48, and a major axis of 1.969 units. On February 7, 1936, it approached to within 1,200,000 miles of the Earth, in the sign Leo. It had reached perihelion in December 1935, at a point slightly outside Mercury’s orbit, at a distance of less than half an astrom. unit. Its diameter is less than ˝ mile. At aphelion it will go almost to the Jupiter orbit. Its period is about 2 years.
Another asteroid was discovered in 1940 that had approached to within 110,000 miles beyond the Moon’s orbit.
Beyond Mars and the asteroid belt, in the outer regions of our solar system, lie the giant planets of Jupiter, Saturn, Uranus and Neptune. In 1972, NASA dispatched the first of four spacecraft slated to conduct the initial surveys of these colossal worlds of gas and their moons of ice and rock. Jupiter was the first port of call. Pioneer 10, which lifted off from Kennedy Space Center in March 1972, was the first spacecraft to penetrate the asteroid belt and travel to the outer regions of the solar system. In December 1973, it returned the first close-up images of Jupiter, flying within 132,252 kilometres (82,178 miles) of the planet’s banded cloud tops. Pioneer 11 followed a year later. Voyagers 1 and 2 were launched in the summer of 1977 and returned spectacular photographs of Jupiter and its family of satellites during flybys in 1979. These travellers found Jupiter to be a whirling ball of liquid hydrogen and helium, topped with a colourful atmosphere composed mostly of gaseous hydrogen and helium. Ammonia ice crystals form white Jovian clouds. Sulphur compounds (and perhaps phosphorus) may produce the brown and orange hues that characterise Jupiter’s atmosphere. It is likely that methane, ammonia, water and other gases react to form organic molecules in the regions between the planet’s frigid cloud tops and the warmer hydrogen ocean lying below. because of Jupiter’s atmospheric dynamics, however, these organic compounds — if they exist — are probably short-lived.
The Great Red Spot has been observed for centuries through telescopes on Earth. This hurricane-like storm in Jupiter’s atmosphere is more than twice the size of our planet. As a high- pressure region, the Great Red Spot spins in a direction opposite to that of low-pressure storms on Jupiter; it is surrounded by swirling currents that rotate around the spot and are sometimes consumed by it. The Great Red Spot might be a million years old. Our spacecraft detected lightning in Jupiter’s upper atmosphere and observed auroral emissions similar to Earth’s northern lights at the Jovian polar regions. Voyager 1 returned the first images of a faint, narrow ring encircling Jupiter. Largest of the solar system’s planets, Jupiter rotates at a dizzying pace — once every 9 hours 55 minutes 30 seconds. The massive planet takes almost 12 Earth years to complete a journey around the Sun. With 16 known moons, Jupiter is something of a miniature solar system. A new mission to Jupiter — the Galileo Project — is under way. On December 7, 1995, after a six year cruise that takes the Galileo Orbiter once past Venus, twice past Earth and the Moon and once past two asteroids, the spacecraft will drop an atmospheric probe into Jupiter’s cloud layers and relay data back to Earth. The Galileo Orbiter will spend two years circling the planet and flying close to Jupiter’s large moons, exploring in detail what the two Pioneers and two Voyagers revealed.
The largest planet in the solar family: larger in fact than all other planets combined. Yet it is exceeded in brightness by Venus, because of her greater proximity to the Earth. To the Greeks, known as Zeus; also associated with Marduk, one of the gods of the Pantheon; known to the Hindus as Brahmanaspati.
Jupiter has 11 satellites. The first four were among the earliest discoveries of Galileo, and can be seen with the aid of a field glass. Statistics concerning the first five are as follows:
| Period | Distance | Diameter | |
|---|---|---|---|
| Io. | 1d.8 | 262,000 | 2109 |
| Europa | 3d.6 | - | 1865 |
| Ganymede | 7d.2 | - | 3273 |
| Callesta | 16d.7 | 1,000,000 | 3142 |
| V | 11h.57m. | 112,600 | 100 est. |
| VI | - | - | 100 |
| VII | - | - | 40 |
The dates of discovery are V, 1892; VI, 1904; VII, 1905; VIII, 1908; IX, 1914; X, 1938; XI, 1938. The orbits of the outer four are so far distant from the planet that their motion is affected by perturbations due to the Sun’s attraction, to such an extent that they can hardly be said to have an orbit.
No. IX has an orbital inclination in excess of 90ş, to that of Jupiter’s orbit. No. VIII has an orbital eccentricity of 0.38, whereby its distance varies from 9 to 20 million miles.
In 1610, Galileo Galilei aimed his telescope at Jupiter and spotted four points of light orbiting the planet. For the first time, humans had seen the moons of another world. In honour of their discoverer, these four bodies would become known as the Galilean satellites or moons. But Galileo might have happily traded this honour for one look at the dazzling photographs returned by the Voyager spacecraft as they flew past these planet sized satellites.
One of the most remarkable findings of the Voyager mission was the presence of active volcanoes on the Galilean moon Io. Volcanic eruptions had never before been observed on a world other than Earth. The Voyager cameras identified at least nine active volcanoes on Io, with plumes of ejected material extending as far as 280 kilometres (175 miles) above the moon’s surface. Io’s pizza-coloured terrain, marked by orange and yellow hues, is probably the result of sulphur-rich materials brought to the surface by volcanic activity. Volcanic activity on this satellite is the result of tidal flexing caused by the gravitational tug-of-war between Io, Jupiter and the other three Galilean moons.
Europa, approximately the same size as our Moon, is the brightest Galilean satellite. The moon’s surface displays a complex array of streaks, indicating the crust has been fractured. Caught in a gravitational tug-of-war like Io, Europa has been heated enough to cause its interior ice to melt — apparently producing a liquid-water ocean. This ocean is covered by an ice crust that has formed where water is exposed to the cold of space. Europa’s core is made of rock that sank to its centre. Like Europa, the other two Galilean moons — Ganymede and Callisto —are worlds of ice and rock.
Ganymede is the largest satellite in the solar system — larger than the planets Mercury and Pluto. The satellite is composed of about 50 percent ice or slush and the rest rock. Ganymede’s surface has areas of different brightness, indicating that, in the past, material oozed out of the moon’s interior and was deposited at various locations on the surface
Callisto, only slightly smaller than Ganymede, has the lowest density of any Galilean satellite, suggesting that large amounts of water are part of its composition. Callisto is the most heavily cratered object in the solar system; no activity during its history has erased old craters except more impacts. Detailed studies of all the Galilean satellites will be performed by the Galileo Orbiter.
No planet in the solar system is adorned like Saturn. Its exquisite ring system is unrivalled. Like Jupiter, Saturn is composed mostly of hydrogen. But in contrast to the vivid colours and wild turbulence found in Jovian clouds, Saturn’s atmosphere has a more subtle, butter-scotch hue, and its markings are muted by high-altitude haze. Given Saturn’s somewhat placid-looking appearance, scientists were surprised at the high-velocity equatorial jet stream that blows some 1,770 kilometres (1,100 miles) per hour. Three American space-craft have visited Saturn. Pioneer 11 sped by the planet and its moon Titan in September 1979, returning the first close-up images. Voyager 1 followed in November 1980, sending back breathtaking photographs that revealed for the first time the complexities of Saturn’s ring system and moons. Voyager 2 flew by the planet and its moons in August 1981. The rings are composed of countless low-density particles orbiting individually around Saturn’s equator at progressive distances from the cloud tops. Analysis of spacecraft radio waves passing through the rings showed that the particles vary widely in size, ranging from dust to house-sized boulders. The rings are bright because they are mostly ice and frosted rock. The rings might have resulted when a moon or a passing body ventured too close to Saturn. The unlucky object would have been torn apart by great tidal forces on its surface and in its interior. Or the object may not have been fully formed to begin with and disintegrated under the influence of Saturn’s gravity. A third possibility is that the object was shattered by collisions with larger objects orbiting the planet.
Unable either to form into a moon or to drift away from each other, individual ring particles appear to be held in place by the gravitational pull of Saturn and its satellites. These complex gravitational interactions form the thousands of ringlets that make up the major rings. Radio emissions quite similar to the static heard on an AM car radio during an electrical storm were detected by the Voyager spacecraft. These emissions are typical of lightning but are believed to be coming from Saturn’s ring system rather than its atmosphere, where no lightning was observed. As they had at Jupiter, the Voyagers saw a version of Earth’s auroras near Saturn’s poles. The Voyagers discovered new moons and found several satellites that share the same orbit. We learned that some moons shepherd ring particles, maintaining Saturn’s rings and the gaps in the rings. Saturn’s 18th moon was discovered in 1990 from images taken by Voyager 2 in 1981.
Voyager 1 determined that Titan has a nitrogen-based atmosphere with methane and argon — one more like Earth’s in composition than the carbon dioxide atmospheres of Mars and Venus. Titan’s surface temperature of -179 degrees Celsius (-290 degrees Fahrenheit) implies that there might be water-ice islands rising above oceans of ethane-methane liquid or sludge. Unfortunately, Voyager’s cameras could not penetrate the moon’s dense clouds. Continuing photochemistry from solar radiation may be converting Titan’s methane to ethane, acetylene and — in combination with nitrogen — hydrogen cyanide. The latter compound is a building block of amino acids. These conditions may be similar to the atmospheric conditions of primeval Earth between three and four billion years ago. However, Titan’s atmospheric temperature is believed to be too low to permit progress beyond this stage of organic chemistry.
The exploration of Saturn will continue with the Cassini mission. Scheduled for launch in the latter part of the 1990s, the Cassini mission is a collaborative project of NASA, the European Space Agency and the federal space agencies of Italy and Germany, as well as the United States Air Force and the Department of Energy. Cassini will orbit the planet and will also deploy a probe called Huygens, which will be dropped into Titan’s atmosphere and fall to the surface. Cassini will use radar to peer through Titan’s clouds and will spend years examining the Saturnian system.
The planet next smaller in magnitude to Jupiter, and next more remote from the Sun, is remarkable for its engirdling system of rings. It was the most remote planet known to the ancients. The surface of Saturn shows markings somewhat similar to those of Jupiter, but fainter. Spectroscopic observations have confirmed the theory that the rings are composed of a dense swarm of small solid bodies. of ten identified satellites of Saturn, the brightest is Titan. The ninth, Phobe, is fainter and more distant than any of the others. The tenth, Themis, lies between Titan and Hyperion. When the Alchemists and early Chemists used the name Saturn they referred to its association with the metal lead. Lead poisoning was once called the Saturnine colic.
Saturn was the ancient god of the seed sowing. His temple in Rome, founded in 497 B.C., was used as a state treasury. In 2I7 B.C. the worship of Saturn was conformed to that of its Greek counterpart, Cronus, son of Uranus, and god of Boundless Time and the Cycles. There was a myth that Saturn in Italy, as Cronus in Greece, had been king during an ancient golden age — hence was the founder of Italian civilization. Also associated with the Greek god Phoenon, “the cruel one,” and the Assyrian god Ninib, patron of Agriculture, and one of the gods of the Pantheon. From it we have the English word Saturnian or Saturnine. The Saturnine colic was lead poisoning. Its atmosphere contains a high percentage of methane and ammonium gases, with no oxygen. For some unexplained reason it changes color from year to year. There are 25,824 Saturn days in one Saturn year.
The Saturn rings consist of an outermost ring, about 11,000 miles in width; a middle ring, about 18,000 miles in width; and an inside ring, the gauze or crepe ring, about 11,000 miles in width. Between it and the surface of the planet is a gap of about 5,000 miles. Separating outer and middle rings is the Cassini division, a dark strip some 2,300 miles in width.
Because the planet’s equator is inclined about 28ş to the plane of the ecliptic, the Saturn ring as seen from the Earth passes through phases: from Saturn’s equinoctial point, where the rings are visible only as a thin — line, to Saturn’s solstices, where they lie transverse to us in a wide expanse. The edgewise view occurs in longitudes 172ş and 352ş; the maximum elongation, in longitudes 82ş and 262ş. The edgewise view was had in 1921 and 1936; the full-faced view in 1929 and 1944. As this constitutes a 15-year cycle, it is possible that there are related variations and fluctuations in the resultant astrological influences, which further research will be able to reduce to usable distinctions.
| Saturn's Moons | Discovered | Distance Thousands | Period Days | Eccent. | Diam. Miles |
|---|---|---|---|---|---|
| 1. Mimas | (1789) | 115 | 0.9 | 0.0190 | 370 |
| 2. Enceladus | (1789) | 148 | 1.4 | 0.0046 | 460 |
| 3. Tethys | (1684) | 183 | 1.9 | 0.0000 | 750 |
| 4. Dione | (1684) | 234 | 2.7 | 0.0020 | 900 |
| 5. Rhea | (1672) | 327 | 4.5 | 0.0009 | 1150 |
| 6. Titan | (1655) | 759 | 15.9 | 0.0289 | 3550 |
| 7. Hyperion | (1848) | 919 | 21.3 | 0.119 | 310 |
| 8. Iapetus | (1671) | 2,210 | 79.3 | 0.029 | 1100 |
| 9. Phoebe | (1898) | 8,044 | 550.4 | 0.1659 | 160 |
| 10. Themis | (1905) | c.800 | - | - | - |
In January 1986, four and a half years after visiting Saturn, Voyager 2 completed the first close-up survey of the Uranian system. The brief flyby revealed more information about Uranus and its retinue of icy moons than had been gleaned from ground observations since the planet’s discovery over two centuries ago by the English astronomer William Herschel. Uranus, third largest of the planets, is an oddball of the solar system. Unlike the other planets (with the exception of Pluto), this giant lies tipped on its side with its north and south poles alternately facing the sun during an 84-year swing around the solar system. During Voyager 2’s flyby, the south pole faced the Sun. Uranus might have been knocked over when an Earth sized object collided with it early in the life of the solar system. Voyager 2 found that Uranus’ magnetic field does not follow the usual north-south axis found on the other planets. Instead, the field is tilted 60 degrees and offset from the planet’s centre, a phenomenon that on Earth would be like having one magnetic pole in New York City and the other in the city of Djakarta, on the island of Java in Indonesia.
Uranus’ atmosphere consists mainly of hydrogen, with some 12 percent helium and small amounts of ammonia, methane and water vapour. The planet’s blue colour occurs because methane in its atmosphere absorbs all other colours. Wind speeds range up to 580 kilometres (360 miles) per hour, and temperatures near the cloud tops average -221 degrees Celsius (-366 degrees Fahrenheit). Uranus’ sunlit south pole is shrouded in a kind of photochemical “smog” believed to be a combination of acetylene, ethane and other sunlight-generated chemicals. Surrounding the planet’s atmosphere and extending thousands of kilometres into space is a mysterious ultraviolet sheen known as electroglow." Approximately 8,000 kilometres (5,000 miles) below Uranus’ cloud tops, there is thought to be a scalding ocean of water and dissolved ammonia some 10,000 kilometres (6,200 miles) deep. Beneath this ocean is an Earth-sized core of heavier materials.
Voyager 2 discovered 10 new moons, 16-169 kilometres (10- 105 miles) in diameter, orbiting Uranus. The five previously known — Miranda, Ariel, Umbriel, Titania and Oberon — range in size from 520 to 1,610 kilometres (323 to 1,000 miles) across. Representing a geological showcase, these five moons are half-ice, half-rock spheres that are cold and dark and show evidence of past activity, including faulting and ice flows.
The most remarkable of Uranus’ moons is Miranda. Its surface features high cliffs as well as canyons, crater-pocked plains and winding valleys. The sharp variations in terrain suggest that, after the moon formed, it was smashed apart by a collision with another body — an event not unusual in our solar system, which contains many objects that have impact craters or are fragments from large impacts. What is extraordinary is that Miranda apparently reformed with some of the material that had been in its interior exposed on its surface. Uranus was thought to have nine dark rings; Voyager 2 imaged 11. In contrast to Saturn’s rings, which are composed of bright particles, Uranus’ rings are primarily made up of dark, boulder sized chunks.
Its discovery by Sir William Herschel on March 13, 1781, added a new factor to the problems of Astrology, and incidentally widened the horizon of observation of planetary influence upon human life. Inserting the planet into the existing horoscopes, revealed that Uranus had been the previously inexplicable cause of violent dislocations, fractures, separations, mental disturbances and deaths. With its discovery there came a new interpretation to the old phrase “by visitation of God.” Herschel called it Georgium Sidus, but England continues to use the name Herschel — from which derives the symbol ( W ) although the rest of the world adopted the name Uranus by which Bode referred to it in 1783. Astrologers had long speculated upon its existence, referring to it as Ouranos. It is sometimes called “The cataclysmic planet.”
The astronomers’ symbol is one of the few cases in which astronomers and astrologers fall to employ the same symbols. As its Equator is inclined by 82ş to the plane of its orbit, the regions of perpetual day and night reach to within 8ş of the Equator.
Its satellites are:
| Name | Disc. | Sidereal Period | Magnitude | Diam. |
|---|---|---|---|---|
| Ariel | 1851 | 2d 12.489h | 16 | 560 |
| Umbriel | 1851 | 4d 3.460h | 16-17 | 430 |
| Titania | 1787 | 8d 16.941h | 14 | 1000 |
| Oberon | 1787 | 13d 11.118h | 14 | 900 |
Voyager 2 completed its 12-year tour of the solar system with an investigation of Neptune and the planet’s moons. On August 25, 1989, the spacecraft swept to within 4,850 kilometres (3,010 miles) of Neptune and then flew on to the moon Triton. During the Neptune encounter it became clear that the planet’s atmosphere was more active than Uranus’. Voyager 2 observed the Great Dark Spot, a circular storm the size of Earth, in Neptune’s atmosphere. Resembling Jupiter’s Great Red Spot, the storm spins counter clockwise and moves westward at almost 1,200 kilometres (745 miles) per hour. Voyager 2 also noted a smaller dark spot and a fast moving cloud dubbed the “Scooter,” as well as high-altitude clouds over the main hydrogen and helium cloud deck. The highest wind speeds of any planet were observed, up to 2,400 kilometres (1,500 miles) per hour. Like the other giant planets, Neptune has a gaseous hydrogen and helium upper layer over a liquid interior. The planet’s core contains a higher percentage of rock and metal than those of the other gas giants. Neptune’s distinctive blue appearance, like Uranus’ blue colour, is due to atmospheric methane. Neptune’s magnetic field is tilted relative to the planet’s spin axis and is not centred at the core. This phenomenon is similar to Uranus’ magnetic field and suggests that the fields of the two giants are being generated in an area above the cores, where the pressure is so great that liquid hydrogen assumes the electrical properties of a metal. Earth’s magnetic field, on the other hand, is produced by its spinning metallic core and is only slightly tilted and offset relative to its centre.
Voyager 2 also shed light on the mystery of Neptune’s rings. Observations from Earth indicated that there were arcs of material in orbit around the giant planet. It was not clear how Neptune could have arcs and ho these could be kept from spreading out into even, unclumped rings. Voyager 2 detected these arcs, but they were, in fact, part of thin, complete rings. A number of small moons could explain the arcs, but such bodies were not spotted. Astronomers had identified the Neptunian moons Triton in 1846 and Nereid in 1949. Voyager 2 found six more. One of the new moons — Proteus is actually larger than Nereid, but since Proteus orbits close to Neptune, it was lost in the planet’s glare for observers on Earth.
Triton circles Neptune in a retrograde orbit in under six days. Tidal forces on Triton are causing it to spiral slowly towards the planet. In 10 to 100 million years (a short time in astronomical terms), the moon will be so close that Neptunian gravity will tear it apart, forming a spectacular ring to accompany the planet’s modest current rings. Triton’s landscape is as strange and unexpected as those of Io and Miranda. The moon has more rock than its counterparts at Saturn and Uranus. Triton’s mantle is probably composed of water ice, but the moon’s crust is a thin veneer of nitrogen and methane. The moon shows two dramatically different types of terrain: the so-called “cantaloupe” terrain and a receding ice cap.
Dark streaks appear on the ice cap. These streaks are the fallout from geyser-like volcanic vents that shoot nitrogen gas and dark, fine-grained particles to heights of 2 to 8 kilometres (1 to 5 miles). Triton’s thin atmosphere, only 1/70,000th as thick as Earth’s, has winds that carry the dark particles and deposit them as streaks on the ice cap — the coldest surface yet found in the solar system (-235 degrees Celsius, -391 degrees Fahrenheit). Triton might be more like Pluto than any other object spacecraft have so far visited.
Until the discovery of Pluto in 1930, Neptune was supposed to be the outermost member of the solar system. It was discovered September 23, 1846 by Galle in Berlin, in the region suggested by Leverrier of Paris; but later was identified as the “star” observed in 1795 by Lalande of Paris. Agrippa dedicated a temple to Neptune in honor of the naval victory of Actium. To the Greeks, known as Poseidon. It is a greenish disc of the magnitude of 7.7, and is distant from Earth by 30o astrom. units. Its revolutionary period is 164y.
It has one known satellite, Triton, about the size of our Moon, and 220,000 miles distant from the planet. It has a magnitude of 13. Its period is 5d, 21h, its orbit inclined to the Neptune orbit by an angle of 40ş; its motion retrograde, with a recession period of 58oy, or 140ş direct. Inclination of Triton’s orbit to Neptune’s equator is 20ş.
Neptune was in Virgo from 1435 to 1449; from 1600 to 1614; from 1761 to 1778; and most recently from 1921 to 1942. It was in Libra from 1450 to 1465; from 1615 to 1635; from 1779 to 1793; and 1943 to 1957.
Pluto is the most distant of the planets, yet the eccentricity of its orbit periodically carries it inside Neptune’s orbit, where it has been since 1979 and where it will remain until March 1999. Pluto’s orbit is also highly inclined — tilted 17 degrees to the orbital plane of the other planets. Discovered in 1930, Pluto appears to be little more than a celestial snowball. The planet’s diameter is calculated to be approximately 2,300 kilometres (1,430 miles), only two-thirds the size of our Moon. Ground-based observations indicate that Pluto’s surface is covered with methane ice and that there is a thin atmosphere that may freeze and fall to the surface as the planet moves away from the Sun. Observations also show that Pluto’s spin axis is tipped by 122 degrees. The planet has one known satellite, Charon, discovered in 1978. Charon’s surface composition is different from Pluto’s: the moon appears to be covered with water-ice rather than methane ice. Its orbit is gravitationally locked with Pluto, so both bodies always keep the same hemisphere facing each other. Pluto’s and Charon’s rotational period and Charon’s period of revolution are all 6.4 Earth days. Although no spacecraft have ever visited Pluto, NASA is currently exploring the possibility of such a mission.
The outermost planet of the solar system so far identified, was discovered in 1930. It lies 800 million miles beyond Neptune. The nearest conjunction of Neptune and Pluto occurred in 1892. A previous exact conjunction occurred in prehistoric times, and will not recur for several thousand years, when they will remain close together for 100 years. As 3 Neptune revolutions take 494Y. and 2 of Pluto 496y, an approximate conjunction occurs every 492.328 years.
Pluto was discovered by Percival Lowell, who delayed publication of the news until his birthday, March 13, 1930 — the day on which Uranus had been discovered 140 years before.
The name Pluto, beginning with P.L., the initials of the discoverer was suggested by an eleven-year-old English girl.
The size or volume of Pluto has not been ascertained, but its mass is less than that of the Earth. The extreme eccentricity of its orbit brings it at times nearer to the Sun than Neptune. There is no certainty that the orbits do not cross, in which event a collision is not impossible. Experience seems to increase the probability of the eventual discovery of other Trans-Neptune planets.
The years in which Pluto entered the various signs is shown in the following tabulation, from which its position in ay year can readily be approximated:
| Aries | 1575 | 1824 | Libra | 1724 |
| Taurus | 1606 | 1851 | Scorpio | 1737 |
| Gemini | 1638 | 1882 | Sagittarius | 1749 |
| Leo | 1692 | 1937 | Aquarius | 1777 |
| Virgo | 1710 | 1958 | Pisces | 1799 |
The outermost members of the solar system occasionally pay a visit to the inner planets. As asteroids are the rocky and metallic remnants of the formation of the solar system, comets are the icy debris from that dim beginning and can survive only far from the Sun. Most comet nuclei reside in the Oort Cloud, a loose swarm of objects in a halo beyond the planets and reaching perhaps halfway to the nearest star. Comet nuclei orbit in this frozen abyss until they are gravitationally perturbed into new orbits that carry them close to the Sun. As a nucleus falls inside the orbits of the outer planets, the volatile elements of which it is made gradually warm; by the time the nucleus enters the region of the inner planets, these volatile elements are boiling. The nucleus itself is irregular and only a few miles across, and is made principally of water-ice with carbon monoxide, carbon dioxide, methane and ammonia — materials very similar to those composing the moons of the giant planets As these materials boil off of the nucleus, they form a coma or cloud-like “head” that can measure tens of thousands of kilometres across. The coma grows as the comet gets closer to the Sun. Solar charged particles push on gas molecules and the pressure of sunlight pushes on the cloud of dust particles, blowing them back like flags in the wind and giving rise to the comet’s “tails.” Gases and ions are blown directly back from the nucleus, but dust particles are pushed more slowly. As the nucleus continues in its orbit, the dust particles are left behind in a curved arc.
Both the gas and dust tails are blown away from the Sun; in effect, the comet chases its tails as it recedes from the Sun. The tails can reach 150 million kilometres (93 million miles) in length, but the total amount of material contained in this dramatic display would fit in an ordinary suitcase. Comets — from the Latin cometa, meaning “long-haired” — are essentially dramatic light shows. Some comets pass through the solar system only once, but others have their orbits gravitationally modified by a close encounter with one of the giant outer planets. These latter visitors can enter closed elliptical orbits and repeatedly return to the inner solar system.
Halley’s Comet is the most famous example of a relatively short period comet, returning on an average of once every 76 years and orbiting from beyond Neptune to within Venus’ orbit. Confirmed sightings of the comet go back to 240 B.C. This regular visitor to our solar system is named for Sir Edmond Halley, because he plotted the comet’s orbit and predicted its return, based on earlier sightings and Newtonian laws of motion. His name became part of astronomical lore when, in 1759, the comet returned on schedule. Unfortunately, Sir Edmond did not live to see it. A comet can be very prominent in the sky if it passes comparatively close to Earth. Unfortunately, on its most recent appearance, Halley’s Comet passed no closer than 62.4 million kilometres (38.8 million miles) from our world. The comet was visible to the naked eye, especially for viewers in the southern hemisphere, but it was not spectacular. Comets have been so bright, on rare occasions, that they were visible during day-time. Historically, comet sightings have been interpreted as bad omens and have been artistically rendered as daggers in the sky. Several spacecraft have flown by comets at high speed; the first was NASA’s International Cometary Explorer in 1985. An armada of five spacecraft (two Japanese, two Soviet and the Giotto spacecraft from the European Space Agency) flew by Halley’s Comet in 1986. Additional comet missions are being examined in the United States and abroad.
Erratic members of the Solar system, usually of small mass. Luminous bodies, wandering through space, or circulating around the Sun, and visible only when they approach the Sun. They usually consist of three elements: nucleus, envelope, and tail. The superstitious once considered them to be evil omens. Those pursuing an elongated orbit are periodic and return at fixed intervals. Those with a parabolic or hyperbolic orbit are expected never to return.
The astrological significance of comets has been the subject of much study, but so far no definite conclusions have been reached. Suggestion has been advanced that Donate’s comet, which made its first appearance of record in June 1858 and attained its maximum brilliancy on October 9th, was a factor in the nativity of Theodore Roosevelt, born October 27, 1858. It is presumed that comets presage history-making events; but operating through individuals whose birth coincides with their appearance, their effects are so delayed as often to be overlooked. Donati’s comet was one of the most beautiful of comets. Its tall was curved. The nucleus had a diameter of 5,600 miles.
“When beggars die there are no comets seen;
The heavens themselves blaze forth the death of princes." Shakespeare.
The year of F. D. Roosevelt’s birth was also marked by the appearance of one of the brightest comets of record, which was visible in broad daylight - even at noon.
LIST OF PERIODIC COMETS The following lists of comets afford a basis for their further study:
| Periodic Comets | Period | Distance from Sun | Incl. to Ecliptic | Perihelion Passage |
|---|---|---|---|---|
| Barnard’s (1884) | 5.40 | 1.28 - 4.89 | 5°28’ | 1906.2 |
| Barnard’s (1892) | 6.31 | 1.43 - 5.38 | 31°40’ | 1905.6 |
| Biela’s | 6.69 | 0.88 - 6.22 | 12°22’ | 1866.1 |
| Brooks’s | 7.10 | 1.96 - 5.43 | 6°04’ | 1903.9 |
| Brorsen’s | 5.46 | 0.59 - 5.61 | 29°24’ | 1890.2 |
| Cunningham’s | - | - | - | 1940.9 |
| D’Arrest’s | 6.69 | 1.33 - 5.77 | 15°43’ | 1897.4 |
| DeVico-E. Swift’s | 6.40 | 1.67 - 5.22 | 3°35’ | 1901.1 |
| Donati’s | 5000. | - | - | 1858.8 |
| Encke’s | 3.30 | 0.34 - 4.09 | 12°36’ | 1905.1 |
| Faye’s | 7.39 | 1.65 - 05.94 | 10°38’ | 1903.4 |
| Finlay’s | 6.56 | 0.97 - 06.04 | 3°03’ | 1900.2 |
| Halley’s | 76.08 | 0.69 - 35.22 | 162°13’ | 1910.3 |
| Holmes’s | 6.87 | 2.13 - 05.1 | 20°48’ | 1899.3 |
| Olters’s | 72.65 | 1.02 - 33.62 | 44°34’ | 1887.8 |
| Pons-Brooks’s | 71.56 | 0.78 - 33.7 | 74°3’ | 1884.1 |
| Temple’s | 6.54 | 2.09 - 4.90 | 10°47’ | 1898.8 |
| Temple’s | 5.28 | 1.39 - 4.68 | 12° 39’ | 1904.8 |
| Temple-L. Swift’s | 5.68 | 1.15 - 5.21 | 5°26’ | 1903.1 |
| Tuttle’s | 13.67 | 1.02 - 10.41 | 54°29’ | 1899.3 |
| Winnecke’s | 5.83 | 0.92 - 5.55 | 17° | 1004.1 |
| Wolf’s | 6.82 | 1.59 - 5.60 | 25°15’ | 1905.3 |
*In terms of Earth’s Mean Distance.
Cunningham’s Comet, first observed in 1940, had a tail of an estimated length of 60 million miles, pointing directly upward. It was of a magnitude of 1.7.
Halley’s Comet, 1835 and 1910, is the most historic comet. Every appearance has been traced back to 240 B.C.
The head of Holmes’s Comet had a diameter in excess of a million miles. It is one of the largest of record.
The great comet of 1843, which seems not to have been given a name, was apparently a Periodic Comet, with an orbit of 400 years. A tail 200 million miles in length, the longest tail of any comet of record, made it a sight of grandeur. Its perihelion distance, 300,000 miles, was extremely short, and carried it through the Sun’s corona.
Non-Periodic Comets. Among the records of non-periodic comets are: Great comet of 1729. The greatest of record, yet details are lacking. Its perihelion distance, approximately 384 million miles, over four times distance of sun to earth, brought it no closer to Sun than Jupiter’s orbit, although it did go around the Sun. Had it come as close as the average comet, its splendor would have transcended that of any other comet.
De Cheseaux’s Comet, 1744 — an unusual comet, six tails — Great Comet of 1811. The largest comet in actual size ever observed, except the 1729 comet of which little is known. The head was 1,125,000 miles in diameter — larger than the Sun. The tail was 100,000,000 miles in length. It was a magnificent sight. Its aphelion-distance was 14 times the distance of Neptune from the Sun. The wine in France was particularly good that season, and for years was famed as “Comet Wine.”
Great Comet of 1861. Earth passed through the tail which subtended over 100° of arc. At one time the comet was brighter than any star or planet except Venus at its brightest, and a peculiar glow suffused the entire sky. One of the finest, probably the brightest comet. Could be seen in broad daylight, even at noon.
Morehouse’s Comet, 1908, showed the most rapid variations in appearance — the tail changing so much from day to day that sometimes it could not be recognized as the same comet.
Comet 1925a. In perihelion distance it was one of the largest — nearly as far away as Jupiter.
Collision with Earth. On June 30, 1908, occurred in Siberia the greatest meteorite fall in historic times. It was probably the head of a small comet. It had no connection with Morehouse’s Comet. Another and larger collision caused Meteor Crater in Arizona, but it was pre-historic-probably 40,000 years ago.
Despite their efforts to peer across the vast distances of space through an obscuring atmosphere, scientists of the past had only one body they could study closely — Earth. But since 1959, space flight through the solar system has lifted the veil on our neighbours in space. We have learned more about our solar system and its members than anyone had in the previous thousands of years. Our automated spacecraft have travelled to the Moon and to all the planets beyond our world except Pluto; they have observed moons as large as small planets, flown by comets and sampled the solar environment. Astronomy books now include detailed pictures of bodies that were only smudges in the largest telescopes for generations. We are lucky to be alive now to see these strange and beautiful places and objects.
The knowledge gained from our journeys through the solar system has redefined traditional Earth sciences like geology and meteorology and spawned an entirely new discipline called comparative planetology. By studying the geology of planets, moons, asteroids and comets, and comparing differences and similarities, we are learning more about the origin and history of these bodies and the solar system as a whole. We are also gaining insight into Earth’s complex weather systems. By seeing how weather is shaped on other worlds and by investigating the Sun’s activity and its influence throughout the solar system, we can better understand climatic conditions and processes on Earth.
We will continue to learn and benefit as our automated spacecraft explore our neighbourhood in space. Missions to each type of body in the solar system are in flight or under development or study. We can also look forward to the time when humans will once again set foot on an alien world. Although astronauts have not been back to the Moon since December 1972, plans are being formulated for our return to the lunar landscape and for the human exploration of Mars and even the establishment of Martian outposts. One day, taking a holiday may mean spending a week at a lunar base or a Martian colony
