Appendix A-2

Planetary Exploration


Ground-based radar measurements determined (1965) that the rotation period is 59 days, not 88 days as had long been believed.

Mariner 10 made the first spacecraft flyby of Mercury in 1974 (in fact, it flew past Mercury three times), and obtained several thousand photographs.

Among the results of the Mariner 10 investigations were the following:

photo of Mercury's cratered surface
The surface of Mercury:
ancient craters and a strange trench.


Ground-based radar measurements in the early and mid-1960s showed that the rotation period of Venus is about 240 days and that the rotation is retrograde, so that it spins in the opposite sense with respect to the Earth.

The 1962 flyby of Venus by Mariner 2 led to an accurate determination of the planet's mass. This spacecraft, and ground-based observations, also measured high temperatures, around 300 C, in the atmosphere of Venus.

Ground-based observations detected various minor components of the Venus atmosphere, including the gases hydrochloric and hydrofluoric acid (1967), carbon monoxide (1968), water (1972), and sulfuric acid (1978). In 1973, it was found that the upper cloud layer is composed of sulfuric acid droplets.

two photos of  cloud covered Venus's changing surface
The changing swirls of Venus' thick atmosphere, recorded day by day by the Pioneer Venus spacecraft.

From the USSR's Venera 7 in 1970, it was found that the atmosphere is largely carbon dioxide, with a pressure at the planetary surface about 100 times that of the Earth's atmosphere. Venera 7 also measured a sur face temperature of about 500 C.

Mariner 10 flew past Venus in 1974. Among the results were:

In 19 75, the Venera 9 and 10 lander spacecraft found that the surface of the planet is firm and rocky. Their other results included:

The 1978 Pioneer Venus mission in cluded both atmospheric probes and an orbiting spacecraft. Among the results were:

Lightning and thunder are present on Venus, according to results from the Venera 11 and 12 landers in 1978.


Ground-based observations detected water vapor on Mars in 1964.

Mariner 4 flew past Mars in 1965 and photographed a heavily cratered, moonlike surface.

Mariner 4 found that the Martian atmosphere is thin, with less than 1 percent the pressure of the Earth's atmosphere, and is composed largely of carbon dioxide.

The Mariner 6 and 7 flybys took place in 1969. Instruments on these spacecraft found that nitrogen is virtually absent from the atmosphere and that solid carbon dioxide ("dry ice") occurs in the clouds and near the polar caps.

Mariner 6 and 7 found that the dust particles in the Martian atmosphere probably consist of silicate materials derived from the planetary surface.

The first soft landing on Mars was executed by the USSR Mars 3 spacecraft in 197 1; the spacecraft ceased operating 20 seconds after the landing.

a photo of the shining surface of Mars in the blackness of space
Landfall on Mars:
the Viking 1 spacecraft approaches the Red Planet.

Mariner 9 became the first Mars orbiting spacecraft in 1971. It obtained over 7300 photographs. Among the results of this mission were:

dark photo of the sun slipping below the Martian horizon
Martian sunset: the Viking I lander records the last fading light on the red surface of Mars.

The Viking 1 and 2 landers and orbiters reached Mars in 1976. Among their many findings were:

Viking 2 photo of the rock strewn martian surface
The Viking 2 lander,
surrounded by a croud
of Martian rocks.


The Planet

According to a theory formulated in 1958, the interior of Jupiter in cludes a large core of metallic hydrogen.

Pioneer 10, in 1973, and Pioneer I 1~ in 1974, made the first flybys of Jupiter; both spacecraft survived pas sage through the asteroid belt and through the Jovian trapped radiation. Among the findings from these space craft were:

The Atmosphere

From ground-based studies, it was deduced that the atmosphere of Jupiter must consist mostly of hydrogen and helium, in an approximately 2 to 1 ratio. The bright bands in the atmosphere ("zones") are cooler than the dark ones ("belts").

A theory proposed in 1958 interprets the great Red Spot as the top of a rotating column in the atmosphere.

Among the results of the Pioneer 10 and 11 missions were:

photo of the swirling atmosphere of Jupiter
Swirls and storms in Jupiter's atmosphere, recorded by Voyager 1.

Voyager 1 and Voyager 2 flew past Jupiter and its moons in 1979; among the findings from this pair of space craft were:

close up photo of Jupiters prominent red swirling covering
Stormy weather around Jupiter's
great Red Spot.

Minor components of the Jovian atmosphere that have been detected at various times include deuterium (heavy hydrogen) compounds; organic molecules such as ethane and acetylene; water vapor (found using the Kuiper Airborne Observatory in 1975); carbon monoxide; phosphine (PH3); and germane (GeH4).

The Magnetic Field and Trapped Radiation of Jupiter

The discovery of naturally generated radio bursts from Jupiter was accomplished with a ground-based radio telescope in 1955. Theory indicated that a strong magnetic field must be present in order to account for the bursts, and additional radio observations showed that the occurence of many bursts is related to the position of the Jovian moon Io.

photo of the orange colored moon Io
Jupiter's moon lo, with a large active volcano.

Among the findings of Pioneer 10 and 11 were:

Among the findings of Voyager I and 2 were:

The Moons and Ring of Jupiter

Groundbased studies of the jovian moons in the 1970's revealed the presence of two additional small moons and the existence of water ice on the surfaces of the three outermost Galilean satellites, Europa, Ganymede, and Callisto.

Findings about the jovian satellites from Pioneer 10 and 11 included:

photo of Io's orange hot volcanic surface
Io: a volcanic landscape sculptured by flows and eraptions of molten sulfur lava.

Results of the Voyager I and 2 missions included:


The Planet

photo of Saturn's prominent rings and two of her moons
Ringworld: giant Saturn and two of its moons.

Among important recent findings from Earth-based observations of the Saturn system have been the detection of a deuterium compound in the atmosphere of Saturn, indications that the small ring particles are composed of or covered by ice, the detection of ice on the moons Rhea, Iapetus, and Dione, and the discovery of methane in the atmosphere of Titan.

The first spacecraft flyby was accomplished by Pioneer Saturn in 1979. Among the findings were:

The Voyager 1 encounter (November, 1980) provided a closer look at Saturn and its surroundings. Some of the new discoveries were:

The Rings

photo of light browm belts encircling the planet Saturn
Saturn's atmosphere: belts of quiet colors.

The Pioneer Saturn flyby in 1979 made several new discoveries about the rings:

Voyager 1 provided much more detail on the beauty, complexity, and some times baffling nature of the rings. Among the discoveries were:

photo of Saturn illustrating the different layers of atmosphere
Saturn's atmosphere, color-enhanced to bring out details.

The Moons

Nine (possibly ten) moons had been detected from Earth. The encounter of Pioneer Saturn (September, 1979) provided several new discoveries:

Saturn's small white moon, Diane against the orange giant
Saturn's moon Dione against the red back ground of Saturn itself.

A much closer look at several of Saturn's moons was provided by Voyager 1. The new results included:


A wholly unanticipated system of thin and narrow rings was discovered around the planet Uranus in 1977 by telescopic observations from the Kuiper Airborne Observatory. There may be as many as nine of the very dark rings.


Cloud patterns were detected in the atmosphere of Neptune by ground based observations in 1978.


All studies of Pluto thus far have been done from the ground. In recent years, a surface layer of frozen methane has been detected, and a moon (Charon) was discovered. It appears that Pluto is much smaller, less dense, and less massive than previously believed.

Asteroids and Meteorites

Although nearly all asteroid observa tions thus far have been made from the ground, the asteroid belt was safely traversed by Pioneer 10 in 1973 and subsequently by other spaceprobes.

Based on the study of reflected sun light, asteroids have been classified into six mai or CORIDositional families since 1970.

Since 1970, diameters have been estimated for an increasing number of asteroids by means of infrared observations.

In 1968, a radar echo was successfully obtained from the small asteroid Icarus during its close approach to the Earth. Another radar contact was made in 1980.

Since 1976, several new Earth crossing asteroids, i.e., those which cross the orbit of the Earth, far from the inner boundary of the asteroid belt, have been discovered. Among them was Ra-Shalom, discovered in 1979.

Most asteroids have been found to very dark, perhaps a symptom of carbon content, and chemically bound water has been detected on some of the darker surfaces.

photo of a small boy with the allenede meteorite
Sample from the stars: the Allende, Mexico meteorite and a puzzled Earthman. (Photograph courtesy of Brian Mason, National Museum of Natural History, Smithsonian Institution.)

The mass of the large asteroid Vesta has been determined, and it has been found that the surface composition of Vesta resembles that of basalt lava.

In 1979, a minor planet (Chiron) was discovered between the orbits of Saturn and Uranus, far outside the known limits of the asteroid belt.

Laboratory study of meteorites reveals that they typically have spent only 10 million to 100 million years exposed to the space environment, a finding that suggests that relatively recent collisions have occurred in the asteroid belt.

Some meteorites are composed of lava, indicating that early melting and volcanic eruptions occurred on their parent asteroids.

Daughter products of extinct, primordial , radioactive isotopes, such as iodine-129, plutonium-244, and aluminum-26, have been found in meteorites. These isotopes, when still present in the parent objects of the meteorites, may have served as important heat sources.

Amino acids have been found in meteorites (see Appendix section on Exobiology).

White inclusions of minerals formed at high temperatures have been found in the Allende meteorite. These may be samples of the first material to solidify in the original cloud from which the solar system was born.

Elemental anomalies found in the white Allende inclusions indicate that some matter may have been introduced into the solar system cloud from another star, presumably a supernova.

The formation ages of meteorites date back 4.6 billion years, providing a firm estimate for the age of the solar system.

Gases from the solar wind have been found to be trapped and preserved in meteorites.

The rates at which iron meteorites cooled from the molten state within their parent objects have been determined; they range from 1 to 20 degrees centigrade per million years. This cooling is too rapid to have occurred within the iron core of a large planet.


In 1970, the existence of huge hydrogen clouds around the comets Tago-Sato-Kosaka (1969 IX) and Bennett (1970 II) was discovered with the OAO-2 satellite. The clouds, also observed by OGO-5, are detectable only in ultraviolet light. The existence of such clouds had been predicted on the basis of the icy conglomerate or "dirty iceball" model of comets, and thus the discovery helped to confirm the theory.

The predicted presence within the cometary hydrogen cloud of high velocity material, with speeds of about 20 kilometers per second (12 miles per second), was found to be consistent with the intensity distribution of the hydrogen clouds mapped by OAO-2 and subsequent spacecraft. It was recently confirmed by spectroscopic observations from the Copernicus satellite.

The hydrogen clouds provide direct evidence for a rate of mass flow from the nucleus of a comet that is adequate to account for the so-called "nongravitational force" that disturbs the motions of comets.

Radio telescopes have observed the hydroxyl (OH) radical in several comets and have made apparent de tections of the substances CH3CN, HCN, and CH in Comet Kohoutek (1973 XII).

The first definite detections of carbon and oxygen atoms in a comet were made by ultraviolet spectroscopy from two sounding rockets launched to study Comet Kohoutek in 1974.

Infrared measurements made from the ground have revealed the presence of silicate dust grains in several comets.

Radio observations of Comet Kohoutek and Comet West (1975n) revealed the presence of transient microwave emission apparently produced by the icy-grain halo, a structural feature of comets that had been predicted to exist.

Observations of Comets Seargent (1978m) and Bradfield (19791) with the IUE satellite revealed the presence of atomic sulfur and CS in the coma and showed that these species are produced from an extremely short lived (less than about 100 seconds) parent molecule, perhaps CS2. The forbidden neutral oxygen line at 2972 A was identified; it is thought to come from photodissociation of water and thus provides a means of determining the spatial distribution of water vapor in the coma.

Using IUE, the water vapor production rate of Comet Bradfield was determined over a range of heliocentric distance from 0.7 to 1.5 astronomical units, the first time this has been measured for a comet. The variation was inconsistent with the idea that the vaporization might depend only on the input of heat from the Sun.

Interplanetary Dust

Pegasus 1, in 1965, and other space craft showed that interplanetary dust particles are about 10,000 times less abundant than had been indicated by early space experiments.

Pioneer 10 and 11, in 1973 and 1974, found that Mars sweeps up the interplanetary dust near its orbit.

Pioneer observations also showed that the zodiacal light is produced by dust that orbits the Sun at great distances from the Earth.

The first definite samples of interplanetary dust have been collected by high-altitude research aircraft since 1978.

magnified photo of meteorite chondrule
A tiny bead (chondrule) in a meteorite displays a jewel-like arrangement of microscopic crystals and glass. (Photograph courtesy of Laurel Wilkening, University of Arizona.)

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