THE INNER PLANETS Mercury and Venus, orbiting the Sun within Earth's path around the central luminary of the Solar System, have been known from ancient times. Early man thought that there were four of these wandering stars, attendants to the Sun-two in the morning skies and two others in the evening skies.
Ancient Greeks were familiar with the dull-white star that shone steadily across the clear skies of the Aegean Sea in the warm glow of dawn. They called it Apollo. In Egypt the horoscopus priests of Thebes looked across the Nile toward Karnak and recognized it as the evil star of Set fleeing upward before Amun-Ra at dawn to be vanquished and disappear in the brilliance of the rising sun god.
Both Greeks and Egyptians thought the morning star different from another star seen close to the Sun after sunset. The Greeks named the evening star that lingered in the sunset glow across the Ionian Sea Hermes, the winged messenger of the gods, while the Thebans recognized it as Horus, the vanquisher of Set and follower of Amun-Ra.
By about 350 B.C., the time of Plato, the Greeks acknowledged the morning and evening stars as being one planet. The modern name, Mercury, is the Roman name for Hermes, the messenger of the gods. The Greek Hermes is still used as the adjective Hermian-of or relating to Mercury. Similarly, ancient astronomers did not recognize Venus as one planet. When east of the Sun and seen in the western sky after sunset, the planet was called Hesperus. When west of the Sun and rising before it, the planet was called Phosphorous. About the 12th century B.C., Homer mentions Venus but considers it as two objects. Pythagoras is said to have recognized the single identity of Phosphorous and Hesperus about 500 B.C.
The confusion between the morning and evening stars is reflected even as late as the writings of Eudoxus about 400 B.C., probably the earliest Greek astronomer, who is believed to have derived his knowledge of the planetary movements from Egypt. Although he stated the periodic times of the planets Mars, Jupiter, and Saturn quite accurately, he was much in error with times for Mercury and Venus. This contrasted greatly with his statements about the synodic periods: that is, the times between the reappearances of planets in the same configuration in Earth's sky. His synodic periods were quite accurate for Venus and Mercury as well as for the outer planets. Thus he showed accurate knowledge of the times when the evening and morning stars would appear, but seemed ignorant of their true motions around the Sun.
Planets of the Solar System ( Fig. 1-1) are today known to consist of three distinct types: small,  dense, inner planets with solid surfaces (Mercury, Venus, Earth, Moon, and Mars), large, predominantly gaseous, outer planets (Jupiter and Saturn), and large, ice-giant, outer planets (Uranus and Neptune ). Additionally there is a small, outermost planet, Pluto, large and small planetary satellites of various types, a group of minor planets concentrated between the orbits of Mars and Jupiter, many comets, meteor streams, and general debris.
Mercury is the innermost planet of the Solar System; Venus orbits the Sun between the orbits of Earth and Mercury. Both planets not only were confusing to the ancients but continued to confuse modern astronomers, although in other ways. For many years the rotation periods of these planets on their axes were unknown, and neither planet revealed any definite surface markings, even to the best of Earth-based telescopes.
Mercury's surface could not be observed because of the planet's small size, its distance from the Earth, and closeness to the Sun, while Venus was shrouded in mystery because of a dense atmosphere with thick clouds that only showed markings in photographs taken by ultraviolet light.
Mercury's distance from the Sun averages 5 8 million kilometers (36 million miles), which is about 38 percent of Earth's distance, while Venus, at 108 million kilometers (67 million miles), is about 72 percent of Earth's distance from the Sun.
Since the planet Mercury is so close to the Sun and moves along its orbit 1.5 to 2 times faster than Earth, it flits from side to side of the Sun so as to be seen only just before sunrise or just after sunset. Its mothlike rapid motion and brief appearances and disappearances are probably why the ancients associated it with the wingfooted messenger of mythology. By contrast, Venus comes closer to Earth, moves farther from the Sun in the evening and morning skies, appears placid and brilliant, and is perhaps the most beautiful object in the skies. " Mistress of the Heavens" said the Babylonians, while the Romans associated the planet with the goddess of beauty, Venus.
Apparitions of Inner Planets
 It is instructive to look at the inner planets Mercury and Venus from the standpoint of the earlier astronomers. At the beginning of recorded history men watched the motions of the planets against the background of stars, but it was many centuries before they deduced that the planets, including the Earth, move around the Sun in almost circular orbits. This awareness was slow in acceptance because early philosophers, later backed by the Christian Church, accepted an Earth-centered dogma. It was not until after the invention of the telescope in the early 1600's that the dogma was dispelled and a Sun-centered Solar System was accepted generally.
Galileo first discovered that Venus exhibits phases like the Moon. Cautiously he laid claim to his discovery in an anagram, published in 1610, which translated into English reads: "The mother of the loves (Venus) emulates the phases of Cynthia (the Moon)." Galileo used this observation of the phases of Venus as a strong argument for the truth of the Copernican theory that the Solar System is centered on the Sun, not the Earth.
Because Mercury and Venus orbit the Sun within Earth's orbit, they are termed inferior planets. As seen from the Earth, inferior planets appear to move close to the ecliptic (the apparent yearly path of the Sun relative to the star sphere, which is also the plane of the Earth's orbit projected against the stars), and to move backward and forward, oscillating to either side of the Sun and never far from it in the sky. The maximum angular distance to east or west of the Sun is termed elongation. At eastern elongation, Mercury and Venus are seen in the evening sky as evening stars because they appear to follow the Sun in its daily motion across Earth's sky owing to the rotation of the Earth (Fig. 1-2). At western elongation, they are ahead of the Sun and are seen as morning stars before sunrise.
Because the orbits of these inferior planets are completely contained within the Earth's orbit, both Mercury and Venus periodically pass between Earth and Sun. This is termed inferior conjunction (Fig. 1-3). At other times, when the planets are on the far side of the Sun from Earth, they pass through superior conjunction. Because the orbits of the Earth and the two planets are not exactly in the same plane-they are tilted....
....slightly with respect to each other like crossed hoops-Mercury and Venus normally pass through conjunction above or below the Sun. Infrequently, the orbits line up so that the planets cross the face of the Sun in a transit or behind the  Sun in an occultation. Occultations are not observable because of the brilliance of the Sun, but transits are.
Transits of Venus take place very rarely: the most recent occurred in 1882; the next are not due until the beginning of the next century-June 7, 2004, and June 5, 2012 (they occur in close pairs). The first recorded transit of Venus across the face of the Sun was observed by Jeremiah Horrocks and William Crabtree in Manchester, England, on December 4, 1639. In 1769, a search for a place from which to observe one of the next pair of transits of Venus led Captain Cook to visit the newly discovered Tahitian Islands and later to discover New Zealand.
Transits of Mercury occur much more frequently. The first recorded observation was by the philosopher critic Pierre Gassendi, at Aix, France, on November 7, 1631. The most recent was visible from the East Coast of the United States on November 11, 1973. The next transit will take place on November 12, 1986.
Mercury revolves around the Sun in a period of 88 days; Venus in a period of 225 days. But their visibility in Earth's skies depends also upon Earth's movement around the Sun. So Venus repeats its apparitions (elongations and conjunctions) in a synodic period of approximately 584 days. This period was known to within 14 days by the ancient Egyptians. Mercury repeats approximately every 116 days. The ancient Egyptians were even closer to this period-they recorded it as 110 days. Since Mercury's orbit varies much more from a true circle than does that of Venus, or Earth, the repetition of Mercury's positions relative to the Sun in Earth's skies varies too. The angular distance of Mercury from the Sun in the sky at elongation also varies, from only 18 degrees to as much as 27 degrees.
Mercury is intrinsically a relatively dark object. Like the Moon it does not reflect much of the sunlight falling upon it-it is said to have a low albedo-so it does not appear very bright in the sky. Moreover, Mercury can rise before or set after the Sun by only 2.5 hours at the maximum, so it is rarely seen in the dark sky, but usually only in the twilight glow. Because of its rapid orbital motion the planet cannot be seen for much longer than two weeks around the time of each elongation. The average interval between Mercury's appearance as an evening and a morning star is 44 days.
By contrast, Venus moves as much as 47 degrees from the Sun, so that it is seen in the late evening or early morning skies as the brightest starlike object. Because Venus reflects a large proportion of the Sun's light falling upon it-it has a high albedo-the planet appears very bright in the skies of Earth. When at its brightest, about one month before and after inferior conjunction, Venus casts distinct shadows. At this time a telescope shows it as a fat crescent shape.
Venus can be observed for many months at each elongation; it has even been seen through binoculars as it passes above or below the Sun at closest approach. It is also clearly visible in daylight if an observer knows where to look. For example, when Venus appears close to the Moon in the sky, the Moon can be used as a guide to finding the planet. Venus passes from greatest elongation as an evening star to greatest western elongation as a morning star in about 140 days, and from morning star back to an evening star in about 430 days.
As the inferior planets move around the Sun, their phases as seen from Earth (Fig. 1-4) are comparable to those of the Moon. When Mercury  and Venus are on the far side of the Sun from Earth, they appear fully illuminated like a full moon, but because of their great distance then, they are unfavorably placed for observation and show relatively small discs. At eastern and western elongations, the two planets appear about half illuminated. Then as they move between Earth and Sun, Venus and Mercury display a narrowing crescent phase to Earth-based observers until, if they cross the disc of the Sun, they appear as black spots upon it. Most times they pass either above or below the solar disc and....
....thus, in a telescope, they can be seen as a very fine crescent all the way through inferior conjunction.
Planets of the Solar System probably formed four to five billion years ago when hosts of small rocky particles and clouds of gases collected together by their own gravity. Gravity is a universal property of matter, as a result of which every particle, irrespective of size, attracts every other. Thus, left to themselves in space, individual particles tend to collect together into larger masses.
After the Sun condensed from the primordial nebula, planets of different sizes and probably different compositions accreted from concentrations of matter present at various distances from the Sun. Evidence for this process of accretion is presented in the cratered surfaces of planetary bodies ranging from small satellites such as Deimos and Phobos (Fig. 1-5) to planets such as Mars. These craters are believed to have been produced by falling bodies some time after the main stage of planetary formation.
The major differences among the terrestrial planets may have arisen because these planets formed at different distances from the Sun and thus consisted of different materials from the beginning. For example, Mercury might have formed from materials rich in iron, whereas Venus formed from silicate-rich materials. Earth may have accreted in a region of the primordial nebula where there were water-containing materials, while Venus did not.
Scientific information about conditions on other planets is important to increased understanding of the evolution of the Solar System and therefore our own planet Earth. Each spacecraft visiting a distant planet for flyby or landing adds more to this basic store of human knowledge. A number of spacecraft had already visited Venus; two successful flybys had been made by Mariners, and five Soviet Venera spacecraft had flown by, orbited or landed capsules on the surface. From these missions, combined with many decades of Earth-based observations using visible, ultraviolet, microwave, and spectroscopic techniques, Venus was known to possess a high surface temperature  of around 475°C (887°F) and a pressure at the base of the massive atmosphere about equal to that at a depth of 400 fathoms in the Earth's oceans. Venus was revealed as a hot, dry planet with only traces of water vapor in its predominantly (95%) carbon dioxide atmosphere. Spectroscopic studies had indicated the presence of sulfuric acid droplets high in Venus's atmosphere, which had been shown to possess distinct layering, both above and below the light-obscuring visible clouds.
Venus, virtually Earth's twin in diameter- 12,104 km (7521 mi) vs 12, 657 km (7926 mi)- in mass, and in density, is distinctive from its nearest planetary neighbor, Earth, in terms of its atmospheric composition and slow, retrograde spin. How these differences arose remains a central question in planetary science.
Observations by a new mission were not expected to answer this fundamental question, but rather to provide some additional pieces of evidence toward solving the great puzzle. Particular attention was placed on designing a program of systematic observation of the mysterious ultraviolet markings discovered years before in telescopic observations of the planet. Although a telescope reveals virtually no visible details on the brilliant surface of Venus (Fig. 1-6 ), some observers recorded faint and elusive markings on photographs obtained with light in the nearultraviolet region of the spectrum. These ill-defined, shadowy markings appeared to move around the planet in a period of a few days in the same direction as Venus's slow retrograde spin, established in 1961 by radar techniques to have a period of approximately 243 days.
Closeup observations of the ultraviolet markings were desired to define their fine-scale morphology and verify their apparent rapid rotation rate. It was hoped that information of importance in developing an understanding of the dynamics of the upper atmosphere could be obtained. Detailed knowledge of the present state of Venus's atmosphere and the physical mechanisms operating within it are prerequisite to unravelling its evolutionary history. Further, such knowledge will aid significantly in developing a deeper perception of the fundamental mechanisms acting within the Earth's complex and dynamic atmosphere.
In addition to the major differences in atmospheric pressure and composition and spin rate, Venus's lack of a sensible magnetic field sets it apart from its sister planet. The nature of Venus's interaction with the solar wind, wherein the plasma impinges directly upon a dense atmosphere not enclosed in a "magnetic bottle," had been investigated by earlier spacecraft, but much remained to be learned by a new mission to Venus, particularly regarding the nature of the region far downstream which had not been probed by the earlier spacecraft.
Mercury, smallest of the planets except possibly Pluto, has an equatorial radius of 2d39 km (1516 mi), making it intermediate in size between the Moon and Mars and smaller than two of Jupiter's satellites (Fig. 1-7). Until recently, astronomers thought that the closeness of Mercury to the Sun caused it to turn one hemisphere eternally sunward, just as the Moon turns one hemisphere  toward the Earth. However, radio astronomers discovered in 1965 that Mercury rotates on its axis in 58 days. Coupled with the planet's 88-day period of revolution around the Sun, this rotation gives Mercury a solar day of 176 Earth days. Thus, one day on Mercury occupies two years of Mercury time.
Through a large telescope the planet presents a yellowish color broken by indistinct greyish patches (Fig. 1-8). On the basis of optical and infrared studies of Mercury, astronomers had long inferred that the planet would be cratered and without any appreciable atmosphere. Its density was known to be much more than the Moon, but rather close to that of Earth.
The innermost planet's importance to planetary science was known to be disproportionate to its size. Earth-based observations at radar, visible, and infrared wavelengths had strongly suggested the absence of any atmosphere, and it was therefore hoped that a primordial surface, upon which was written a history of the early events in the inner Solar System, unerased by the action of wind and water, awaited a spacecraft's cameras. The opportunity to study this anticipated record was eagerly awaited because, should Mercury's primordial surface remain, a valuable comparison with similar surfaces on the Moon and Mars could be made, providing an insight into the distribution of the planetesimals whose impacts on these other two bodies had left a record in their cratered terrains. Furthermore, the probable location of the source of these bodies could be better understood by a careful analysis of the cratering densities on the three bodies.
Mercury's importance to planetary science was not restricted to the expected record of late stage formational events, however. The planet's high density, equivalent to that of the Earth, has long been of interest to theoreticians concerned with the formation of the inner planets. Because of Mercury's small size, the high density must reflect a metal-rich composition. What clues could Mariner provide as to the internal composition of Mercury? In particular, is the planet chemically differentiated? If so, when did this differentiation occur? The answers to these question would be significant with regard to theories of chemical evolution of the terrestrial planets.
Clues regarding Mercury's internal composition were expected to be obtained from the several instruments designed to study its interaction with the solar wind, the details of which depend upon gross planetary properties like atmospheric composition and pressure, the presence or absence of a magnetic field, bulk conductivity, and so on. Further information was expected from a study of the composition and structure of the planet's presumably tenuous atmosphere, expected to be less than a thousandth of the Earth's total pressure.
Thus, a mission to Venus and Mercury would have as its goal the acquisition of previously unavailable fundamental information on both planets-a few more pieces of the puzzle of how the planets, including the Earth, formed and....
 ....evolved to their present states. The questions posed for Venus were sharply focussed, based upon a considerable body of knowledge. Those applied to Mercury were broad, exploratory, firstorder: a reflection of what little was known of that tiny planet, so small and difficult to observe that the best estimate of generations of telescopic observers regarding its rotation rate had been proven wrong by radar experimenters only eight years earlier.