THE Magellan radar mapper is sending back pictures of Venus. Galileo, now on its way to explore Jupiter and its moons, will ``eyeball'' the asteroid Gaspra in October. Mars Observer is being readied for a September 1992 launch on a mission to map the Red Planet. A mission, called Cassini, to revisit the Saturn planetary system is under design. In short, the United States planetary exploration program has picked up momentum, and no one is more pleased than former Voyager mission chief scientist Edward Stone. ``In the eighties, the whole space science program was suffering because of the problem of launching things,'' he says.
But, he adds: ``I think that's past now. I really think we're back in business.''
This puts zest into Dr. Stone's new job as director of the California Institute of Technology's Jet Propulsion Laboratory (JPL), which manages this planetary exploration for the National Aeronautics and Space administration. Reflecting recently on the planetary challenges JPL faces as he takes up its directorship, Stone has his eye on a new millennium less than a decade away.
He notes that the job in hand - the job to be done in this decade - is important. Yet, he explains, the time scales for planetary missions are such that ``you also have to be thinking about what you're going to be doing after the turn of the century.''
His wish list includes items favored throughout the planetary science community:
A mission to Pluto, the only planet left unexplored.
A revisit of the Neptune system with a spacecraft that orbits the planet and makes a detailed study of the giant moon Triton.
Extensive Mars exploration.
Research to tie all the new knowledge together into a better understanding of the nature and evolution of the solar system.
Speaking of Mars, Stone says he's not thinking in terms of human exploration - at least not for an indefinite time. He foresees ``a lot of steps along the way and a lot of things to be learned'' before astronauts walk the Martian sands.
``Once human beings get there,'' he explains, ``they are going to want to do something. The question is, what do they do? What questions are they trying to answer?''
One important question, for example, asks whether Mars had life around three billion years ago when there was abundant water. If it did, where would one look for fossils? ``Where is the Olduvai Gorge?,'' Stone asks.
To decide whether that is a sensible question and, if it is, how to answer, it will take much preliminary unmanned research. Stone says he believes ``it is a very important thing to be carrying out over the next decade - the next two decades - to sort of provide that kind of context and focus.''
Missions to other planets and their moons - such as revisiting Jupiter and Saturn - are also important to put together a more complete knowledge of the solar system. Stone notes, for example, that it now is clear that collisions have been very important in solar system evolution. Many bodies are heavily cratered like our moon. The rings around the outer planets probably are debris from colliding bodies. Even some of the moons seem to be fragments of larger bodies.
``That's a general theme one would like to explore,'' Stone says. He notes that, while new theoretical understanding is needed, spacecraft studies are crucial. ``There's a great deal more to be learned about the ring systems themselves than we could possibly have gotten from the Voyager flybys,'' he says.
Then there is the intriguing presence of organic materials elsewhere than on Earth. In the outer solar system, there are bodies (moons) that are 50 percent or more water ice.
The farther out they lie, the darker they appear. They seem to have a dark gray or black carbon-rich coating. Even Comet Halley turned out to be a dark object, covered with some kind of organic material.
Stone points out that this raises a basic question of the relationship of this carbon-rich material to solar-system evolution. To what extent was it primordial? Was it part of the interstellar medium from which the solar system formed?
To what extent is it produced in place by irradiation of simpler carbon-bearing compounds such as methane?
Stone considers these ``really important questions in terms of trying to understand the role the reservoir of organic material may or may not have played in the origin of life.''
He says it will take much creativity and ingenuity to plan and carry out the exploration needed to answer such questions.
So, although planetary exploration is unlikely to be a growth industry in coming decades, he does see job opportunity. ``That's where bringing young people in who are full of energy and ideas can make all the difference,'' he says.