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trajectories that carry them across Earth’s orbit and occasionally within a few million miles of the Earth. Many asteroids are mainly carbonaceous—with large quantities of organic materials and chemically bound water. The organic matter is thought to have condensed in the very earliest stages of the formation of the solar system from interstellar gas and dust, some 4.6 billion years ago, and their study and comparison with cometary samples would be of extraordinary scientific interest. I do not think that materials from a carbonaceous asteroid are likely to be criticized in the same way that the Apollo returned lunar samples were—as being “only” rocks. Moreover, a manned landing on such an object would be an excellent preparation for the eventual exploitation of resources in space. And finally, landing on such an object would be fun: because the gravity field is so low, it would be possible for an astronaut to do a standing high jump of about ten kilometers. These Earth-crossing objects, which are being discovered at a rapidly increasing pace, are called—by a name selected long before manned spaceflight—the Apollo objects. They may or may not be the dead husks of comets. But whatever their origin, they are of great interest. Some of them are the easiest objects in space for humans to get to, using only the Shuttle technology, which will be available in another few years.

THE SORTS of missions I have outlined are well within our technological capability and require a NASA budget not much larger than the present one. They combine scientific and public interest, which very often share coincident objectives. Were such a program carried out, we would have made a preliminary reconnaissance of all the planets and most of the moons from Mercury to Uranus, made a representative sampling of asteroids and comets, and discovered the boundaries and contents of our local swimming hole in space. As the finding of rings around Uranus reminds us, major and unexpected discoveries are waiting for us. Such a program would also have made the first halting steps in the utilization of the solar system by our species, tapping the resources on other worlds, arranging for human habitation in space, and ultimately reworking or terraforming the environments of other planets so that human beings can live there with minimal inconvenience. Human beings will have become a multi-planet species.

The transitional character of these few decades is evident. Unless we destroy ourselves, it is clear that humanity will never again be restricted to a single world. Indeed, the ultimate existence of cities in space and the presence of human colonies on other worlds will make it far more difficult for the human species to self-destruct. It is clear that we have entered, almost without noticing it, a golden age of planetary exploration. As in many comparable cases in human history, the opening of horizons through exploration is accompanied by an opening of artistic and cultural horizons. I do not imagine that many people in the fifteenth century ever wondered if they were living in the Italian Renaissance. But the hopefulness, the exhilaration, the opening of new ways of thought, the technological developments, the goods from abroad, and the deprovincialization of that age were then apparent to thoughtful men and women. We have the ability and the means and—I very much hope—the will for a comparable endeavor today. For the first time in human history, it is within the power of this generation to extend the human presence to the other worlds of the solar system—with awe for their wonders, and a thirst for what they have to teach us.

PART IV

THE FUTURE

CHAPTER 17

“WILL YOU WALK

A LITTLE FASTER?”

“Will you walk a little faster?” said a whiting to

a snail,

“There’s a porpoise close behind us,

and he’s treading on my tail.”

LEWIS CARROLL,

Alice in Wonderland

FOR MUCH OF human history we could travel only as fast as our legs would take us—for any sustained journey, only a few miles an hour. Great journeys were undertaken, but very slowly.