Washington. -- In the last three decades, NASA's robotic spacecraft have visited every planet from torrid Mercury to frigid Neptune.
The data returned by these Pioneers, Mariners, Vikings and Voyagers have revolutionized our understanding of the solar system. These achievements rank among the greatest accomplishments of the late 20th century.
But where do we go from here? How can we justify future missions? Can we afford to continue dispatching robotic emissaries to the farthest reaches of the solar system?
The scientific potential is virtually unlimited. We could send spacecraft to roam across Mars' surface, return pieces of a comet to Earth, explore Pluto or orbit Mercury.
Unfortunately, neither the United States nor any combination of international partners has the personnel, infrastructure or financial resources to realize all of our scientific dreams. Priorities must be set.
Political, economic and social factors will help determine what missions actually fly. But scientific priorities should also be a major consideration.
We have to ask how future missions will answer two fundamental questions:
How do planetary systems and life originate?
And how do planets work? For example, why are the Earth's atmosphere and climate so different from those of our nearest neighbors, Mars and Venus?
Progress in answering these questions requires several simultaneous approaches.
On the ground, research and analysis with telescopes and theoretical models can improve our understanding.
In space, small spacecraft like the Discovery series can pursue limited objectives while large ones like the Cassini mission to Saturn aim at comprehensive goals.
Still, the question remains: Which missions are the most important scientifically?
We have recently been heading a National Research Council committee that sought to answer this question. Our conclusion: The top scientific priorities should be the study of comets, Mars and Jupiter, and the search for planets around other stars.
Comets are the best-preserved remnants of the material from which the solar system formed. These primitive bodies may also contain the biogenic elements and compounds with which the primordial Earth was endowed. Comets, and especially their pristine interiors, have received little detailed scrutiny so far, despite their potential to shed light on the origin of the solar system.
Mars holds special interest because it may help unlock many secrets of our own world. Although the planet is cold and dry today, its atmosphere seems to have been denser and wetter eons ago. Indeed, its past climate may have been so benign that the planet could provide a unique perspective on the origins of life.
Also, Mars is perhaps the easiest planet to explore and would be the next logical destination for human exploration.
As the largest planet in the solar system, Jupiter displays a dazzling variety of phenomena.
It has a complex weather system driven not by the Sun's heat (as with the Earth), but by the planet's own internal energy. Jupiter also has intense Van Allen radiation belts and a mini-solar system of moons, including the bizarre Io with its erupting volcanoes.
NASA's Galileo spacecraft will reach Jupiter next December and -- despite a jammed main antenna -- will return much valuable data. But its 1970s-era technology and safe equatorial orbit will leave much interesting exploration for later.
Looking for planets around other stars has a broader motivation. To understand our star system, it must be viewed from a cosmic perspective. Is the Sun's retinue of planets, moons, comets and asteroids unique in the galaxy, or is it commonplace?
Detecting a single planet around a Sun-like star elsewhere in the galaxy would be a momentous discovery. But to understand our system's origins, we need basic information about a significant number of planets around other stars. With recent advances in telescope and detector technology, this field is ripe for progress.
We have many reasons for exploring the solar system. Our journeys into space answer important questions about ourselves and our world, stimulate technological progress, spark interest in science and technology among the young, and challenge the explorer in everyone. If we select directions now and stay on track, we can satisfy all these goals while writing the next chapter in one of humanity's greatest adventures.
Joseph A. Burns, engineering and astronomy professor at Cornell University, chairs the National Research Council's committee on planetary and lunar exploration. David H. Smith, of the council, directs the work of the committee.