Space Probe

A new program recently initiated by NASA, the Discovery program, has its objective to find cheaper ways to explore the solar system. It was largely inspired by the dramatic failure of the $1-billion Mars Observer mission in 1993, which exploded on arrival at Mars and is supposed to supplant large, expensive, infrequent missions with relatively small, inexpensive, frequent. It was Discovery's original goal to increase mission frequency to one every 12 to 18 months and to provide for a more continuous accumulation of diverse scientific information on asteroids, planets, and the Sun. In the frame of the Discovery program, the Mars Pathfinder and the Near-Earth Asteroid Rendezvous (NEAR) missions were launched in 1996. The Pathfinder lander mission, which landed successfully on Mars in 1997, included a low-power, low-mass instruments, and a small six-wheeled rover (named Sojourner) to analyze rock composition on the Martian surface. NEAR journeyed through the asteroid belt, flying by the asteroid Iliya in 1996, and after a gravity boost from Earth, NEAR encountering near-Earth object 433 Eros in December 1998. NEAR was completely successful in its mission to study 433 Eros at close range, and even managed to make a soft touchdown on its surface (an add-on mission for which it had not been originally designed).

However, the pace of the Discovery series of missions slowed drastically after the failure of two consecutive Mars probes in 1999 (Mars Orbiter) and 2000 (Mars Polar Lander). Critics charged that NASA had allowed its new "better, cheaper, faster" philosophy to compromise its engineering standards, and NASA, in the face of two consecutive catastrophes, agreed. Its missions have subsequently become less cheap and less fast. Since the twin disasters of 1999 and 2000, NASA has successfully orbited one craft around Mars (Mars Odyssey 2001). Its next major lander mission to Mars, Mars Exploration Rover, will feature duplicate probes much like the successful (and expensive) Viking landers of the 1970s. Each Mars Exploration Rover probe will, if successful, deploy a sophisticated rover onto the surface of Mars in 2004, exploiting technologies tested during the Pathfinder landing of 1997. Another orbiter, Mars Express, is due to arrive at Mars in 2003. Mars Express will deploy a small lander as well.

Possible future missions include a Mercury orbiter, a Pluto mission, a Venus environmental satellite, comet life history investigation, and NEARS (Near-Earth Asteroid Returned Samples), which would be equipped with a special "shooter" for firing sample tubes into an asteroid's surface. After collecting up to 21 oz (600 g), of sample material, the probe would return it to Earth. The realities of Earthly politics, however, make it difficult to fund interplanetary missions; each must be fought for by the scientists who believe in its value, and a valuable mission may be definitively canceled (or launched) only after years of vacillation at the political level.

The United States launches the vast majority of interplanetary probes, and will probably continue to do so, but other nations are also beginning to do so. Japan launched its Planet B orbiter toward Mars in 1998; it is expected to assume an orbit around that planet in January 2004.