The basic answer is that NASA's Mars program in 1996 is smaller, faster, and cheaper than its counterpart of a few years ago. The contrast is evident by looking at the Mars Observer spacecraft, which in 1993 ceased all communications with Earth just before entering Mars orbit. This question was recently examined by a National Research Council committee that we led. First, it is important to be clear about the reasons for studying Mars. If missions do not lead to a substantial improvement in understanding of the planet, they cannot be justified.
Scientists agree on the important topics to be addressed.
The Mars Surveyor program is designed to address these and other questions to varying degrees. Mars Global Surveyor, scheduled for launch in November, will use copies of many of Mars Observer's instruments to investigate the planet's atmosphere, surface, and magnetic field from orbit. Mars Pathfinder, scheduled for launch in December, will land a science package, including a small roving vehicle, on Mars next July. Subsequent orbital missions in 1998 and 2001 will continue the global characterization of the martian environment, while the accompanying small landers will begin studying a variety of unexplored terrains. With the announcement of possible signs of martian life, NASA and Congress face great temptation to reorient the Mars Surveyor program to begin searching for life right away.