NOW that Voyager 2 has moved beyond Neptune, some planet explorers are thinking again about the inner solar system. Specifically, their eyes are fixed on Mars. The National Aeronautics and Space Administration's much-delayed Mars Observer is due to head for the red planet about two years from now. Planning to meet that schedule is in full stride here at the Jet Propulsion Laboratory (JPL), which is managing the mission for NASA.
Budget cuts and the 1986 Challenger accident caused the Mars Observer to miss several scheduled launch dates, including one set for 1990. But this time, project manager David Evans says, ``I have every reason to expect we will be funded at the level it takes to launch'' in September 1992.
The Mars Observer is important for those studying the planet, especially Soviet scientists, who plan to send a probe to Mars in 1994. The mission is also vital to the United States space effort as a whole. The mission is in the direct line of studies needed to prepare for a Mars expedition - a long-range, albeit controversial, goal for the US and the Soviet Union.
The mission is to study the planet from orbit during all four seasons of the Martian year, which is 669 Mars days, or 687 Earth days. It will involve assaying the chemical makeup of the surface and defining its topography more accurately than before. It also includes tracking atmospheric circulation and movement of dust and other materials.
The budget cuts that bumped the launch date from 1990 to 1992 also eliminated from the Mars Observer one of its most important and expensive instruments - the visual and infrared imaging spectrometer (VIMS). Mission scientists ``certainly lost some of the science data that they would have gotten with the VIMS,'' Mr. Evans says. Yet, he adds, ``It is a very solid mission. It will do a top-notch characterization of the planet Mars - its atmosphere, its dust storms, its surface composition.''
This knowledge will be welcomed by the international space science community, for whom Mars is a prime object for study in the 1990s. Recently, Mars was an leading topic for discussion at the International Conference on Solar System Exploration held here by the American Institute of Aeronautics and Astronautics and JPL during Voyager's Neptune flyby.
The Soviet Union, in particular, is determined to go ahead with extensive Mars studies in spite of its recent disappointing loss of two spacecraft sent to survey the Martian moon Phobos. It still plans to send its own Mars-orbiting spacecraft in 1994. Then it will try ``to finish the unfinished mission to Phobos,'' Valery Barsukov of the Vernadsky Institute of Geochemistry and Analytical Chemistry in Moscow told the conference.
Professor Barsukov cautioned that the Soviet Union, like the US, was having to manage space research money carefully. He said the goal of human exploration of Mars should be seen realistically as the next generation's task. And, he explained, the Soviets would like to coordinate lunar and planet research with the US.
The Mars Observer provides a small example of such cooperation. The Soviets' 1994 mission will send instrument-carrying balloons into the Martian atmosphere. It is a cooperative project with France and the Planetary Society - a Pasadena-based private organization. The Mars Observer will carry a French-supplied radio link to serve as a backup for relaying the balloon data.
Evans sees these kinds of missions as building a knowledge base for eventual human expeditions. But planners for such expeditions would probably want to follow up with even more detailed and higher resolution studies, he says. The Mars Observer will pick out details no smaller than about a meter across, about desk-top size. To pick landing sites, Evans says planners would probably want to see details 10 times smaller. ``If you set your mind to it, there's no reason you can't do some high resolution things'' like that, he says.