MARS is the Greek god of war. The planet probably got this name due to its red color; Mars is sometimes referred to as the Red Planet. Mars is much like Venus-- it's very bright and therefore easily spotted in the night sky. Because of this, we don't know who exactly discovered Mars. We do know it was named after the Roman god of war, because its reddish color reminded people of blood. Mars has been known since prehistoric times. Of course, it has been extensively studied with ground-based observatories since then, but even very large telescopes find Mars a difficult target, because of its small size.

In 1659, Christian Huygens discovered a strange feature on the surface of the Red Planet. It was later called the Syrtis Major. We have been scared of Martians ever since. In 1802, one scientist was so convinced there was life on Mars that he wanted to draw huge figures in the snow to signal the Martians!

In 1971, Mariner 9 visited Mars and sent back images of enormous volcanoes and vast canyons. It discovered Olympus Mons, now the most famous volcano not on Earth. This massive volcano could cover the state of Missouri, and reaches 15 miles above the surface! Mariner 9 also found evidence that water once flowed on Mars. However, there were no sightings of Schiaparelli's famous canals.

In 1975, two spacecraft named Viking I and II landed on Mars to study its surface. They analyzed the rocks and soil of the planet while providing us with information about its atmosphere and weather patterns. Even today we are exploring Mars. The Mars Global Surveyor made a map of the planet in 1997.

Jove, Zeus, the King of Gods are also referred to as JUPITER. He was the ruler of Olympus and the patron of the Roman state. Jupiter may not actually be the King of the Gods, but it sure is the largest planet in the solar system and the fourth brightest object in the sky. It has been known since prehistoric times as a bright "wandering star". But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter. This was the first discovery of a center of motion not apparently centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets (along with other new evidence from his telescope: the phases of Venus and the mountains on the Moon). Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition. Today Galileo's theories are accepted and treasured.

JUPITER was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiter for eight years. It is still regularly observed by the Hubble Space Telescope.

In Roman mythology, SATURN is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday." Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).

SATURN was first visited by NASA's Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA / ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four years.

The Greeks call him Poseidon, and the Romans refer to him as NEPTUNE, god of the sea. Discovered on September 23, 1846, Neptune was the first planet discovered by Johann Gottfried Galle, a mathematical prediction rather than regular observation. Perturbations in the orbit of Uranus led astronomers to deduce Neptune's existence. In1613, more than two centuries earlier, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days. To this day Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL and the California Institute of Technology.

In Roman mythology, PLUTO (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell. Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet, and now many objects similar to Pluto are being discovered outside our solar system. Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.