From the beginning of history, the "wandering stars" that we call "planets" were little more than points of light in the nighttime sky. That all began to change forty years ago this next week, when the American space probe Mariner 2 passed 22,000 miles from the planet Venus after its launch 3 1/2 months earlier. During its 42-minute flyby of Venus on December 14, 1962, Mariner 2 discovered, among other things, that Venus possesses an extremely hot surface (approximately 900 deg F, due to a runaway greenhouse effect).

During the four decades that have elapsed since then almost all the planets, and many of the other bodies, in our solar system have been visited by various space probes, completely revolutionizing our knowledge of these objects, and turning them from points of light into unique and individual worlds. With the forty-year anniversary of the Mariner 2 flyby upon us, perhaps it is time to examine the various space probes that have told us so much about our fellow planets.

MERCURY. The only space probe to fly by the solar system's innermost planet is the American Mariner 10 mission, which made three flybys of Mercury during 1974 and 1975, returning to us many photographs of a heavily cratered world. Meanwhile, the planned Mercury-orbiting MESSENGER spacecraft is slated for launch in 2004.

VENUS. Mariner 2 was followed by the successful flyby of Mariner 5 in 1967. The Soviet Union initiated a successful series of Venera missions to Venus in the mid-1960s, which included several soft-landings upon Venus' surface (beginning with Venera 7 in 1970) and the deployment of balloons into Venus' atmosphere during the VEGA missions in 1985. More recent American missions include Pioneer Venus, which arrived at Venus in 1978, and Magellan, launched in 1989 to perform a comprehensive radar-mapping survey of Venus' surface from orbit.

MARS. The red planet has been the focus of the strongest planetary exploration efforts, beginning with flybys from the American Mariner 4 mission in 1965 and Mariners 6 and 7 in 1969, and the Mars-orbiting Mariner 9 probe in 1971. These were followed by the immensely successful Vikings 1 and 2 missions launched in 1975, which soft-landed two probes onto Mars' surface, and also placed two probes into orbit which carried out detailed surface mapping over the next several years.

More recently, the Mars Pathfinder mission deployed a rover, named Sojourner, onto Mars' surface in 1997, and Mars Global Surveyor (launched in 1996) and Mars Odyssey, (launched in 2001) have performed detailed studies of the Martian surface from orbit.

Several Mars missions are planned during forthcoming years, including both NASA and European Space Agency (ESA) probes to be launched next year that will deploy landers and rovers.

JUPITER and SATURN. The first Jupiter flybys were performed by the twin Pioneers 10 and 11 spacecraft in 1973 and 1974, respectively, with Pioneer 11 continuing on to Saturn in 1979. These were followed by the extremely successful twin Voyagers 1 and 2 spacecraft, which performed flybys of Jupiter in 1979, then of Saturn in 1980 and 1981, respectively, returning, among other data, spectacular photographs of both planets along with their moons and ring systems.

The Galileo spacecraft has been in orbit around Jupiter since late 1995, and among other studies has dropped a probe into Jupiter's atmosphere and has performed multiple flybys of all Jupiter's large moons. Meanwhile, the Cassini mission, launched in 1997 and due to arrive at Saturn in July 2004, will go into orbit around that planet in a mission similar to that of Galileo (including the deploying of a probe, christened Huygens, into the atmosphere of the large and enigmatic moon Titan).

A mission to Jupiter's moon Europa (which, from data obtained by the Voyagers and Galileo, is believed to possess a subsurface ocean of liquid water underneath an icy crust, thus possibly harboring life) has been planned for launch in 2008 but presently isn't being funded.

URANUS, NEPTUNE, and PLUTO. Following its flybys of Jupiter and Saturn, Voyager 2 went on to equally successful flybys of Uranus in 1986 and Neptune in 1989. There have been no missions so far to Pluto, but a mission dubbed New Horizons, which would fly by that planet as well as one or more of the Kuiper Belt objects that have been discovered recently, is currently being planned for launch around 2006.

COMETS and ASTEROIDS. The first cometary encounter was performed by the International Cometary Explorer (ICE) spacecraft (a sun-studying probe given a second life), which flew by comet Giacobini-Zinner in 1985. This was followed by an international armada of spacecraft that flew by Halley's Comet in 1986, a group which included the Soviet VEGA probes, the Japanese probe Suisei, and the ESA probe Giotto (which subsequently went on to encounter another comet, Grigg-Skjellerup, in 1992). The Deep Space 1 spacecraft performed a successful flyby of Comet Borrelly in 2001.

Comet Wild 2 (arrow), which the Stardust spacecraft will fly by in early 2004 Image courtesy Alan Hale

The first flybys of asteroids were performed by Galileo enroute to Jupiter, these being the asteroids Gaspra in 1991 and Ida in 1993 (with Galileo images showing a one-mile-diameter moon around the latter object). Other asteroid flybys include Mathilde by the Near-Earth Asteroid Rendezvous (NEAR) mission in 1997, Braille by Deep Space 1 in 1999, and Annefrank by Stardust last month. The highlight asteroid mission was NEAR, which went into orbit around the asteroid Eros in early 2000 and performed a detailed reconnaissance of that object before executing a soft-landing onto its surface.

Stardust is scheduled to fly by Comet Wild 2 in early 2004 and then return collected dust samples to Earth. Future planned missions include the Japanese MUSES-C probe, scheduled for launch next year to the asteroid 1998 SF36; Dawn, slated for launch in 2006 to the asteroids Vesta and Ceres; Deep Impact, to be launched in 2004 to Comet Tempel 1; and the ESA probe Rosetta, slated for launch next year to Comet Wirtanen.