But that future may still be realizable, believe many scientists in and out of NASA who meet now and then to try to imagine the technology that could bring the space age to life. In their eyes, those first moon landings represented a false dawn of space exploration, just as the early Viking voyages to America were ahead of their time. Only when technology makes space flight cheap enough, as one astronomer put it, "to hock your socks and go," will the real space age begin. What will that age produce?
For starters, rockets will go the way of the dinosaurs. Future spacefarers will look back on the notion of sending people (or anything precious) aloft on huge, lumbering towers of flame and smoke as primitive, brutal and notoriously unreliable. Before the next millennium is very far along, humans will get their lift from space planes that take off and land like conventional jets but are powered by "scramjets" that, once aloft, will enable them to swoop into orbit or go halfway around the world in two hours. Cargo will be shot into orbit by electromagnetic rail guns that ramp up the sides of mountains, or will be flung upward by looping orbital tethers, sort of like David's slingshot.
Probes and people would sally forth into the deeper universe, propelled by thin sails filled by the feeble but inexorable pressure of sunlight or traveling on ion drives that get their boost by shooting high-energy electrified particles out of the rear of the vehicle. Other possible vehicles for space travel may be propelled by a series of tiny thermonuclear explosions using pellets of fuel mined on the moon, or by mass drivers employing electromagnetic fields to expel bucketloads of dirt from the back.
Once the transportation is ready, Mars will be a popular destination. According to scenarios worked out by a group of scientists who call themselves the Mars Underground, the first voyagers would be preceded by ferries carrying equipment for setting up a permanent base. Early visitors would face a hostile environment: a thin atmosphere of mostly carbon dioxide, temperatures that , fluctuate from -10 degrees F to -190 degrees F and hurricane winds. People would live chiefly underground, grow vegetables in greenhouses, wear space suits and explore the countryside in dirigibles.
After a few decades of familiarization and exploration had established that there was no indigenous Martian life, humans might be ready to undertake the ultimate real estate-development project: the greening of Mars. The first step, according to one recent study, would be to warm the planet by releasing large amounts of chlorofluorocarbons into the atmosphere. These gases would act like a greenhouse, trapping the sun's heat. As the planet warmed, the polar caps would begin to melt, releasing water vapor and carbon dioxide into the Martian air, thickening it and increasing the greenhouse effect. Eventually the permafrost, where most of Mars' water is locked up, would melt, and rivers and lakes -- if not oceans -- would flow across the Red Planet again.
The next task would be to oxygenate Mars' atmosphere and introduce life. The "gardeners" on Mars could import anaerobic organisms -- for example, the blue-green algae that flourished on earth billions of years ago. Other genetically engineered organisms would follow until one day, probably millenniums from now, the new Martians could breathe freely under clear skies.
Most of the work of investigating and colonizing the solar system (and perhaps beyond) would be done by robot probes smaller and smarter than those of today. Advances in computer technology and genetic engineering, predicts physicist Freeman Dyson of the Institute for Advanced Study in Princeton, New Jersey, will enable scientists to squeeze the capabilities of a Voyager spacecraft, say, into a 2-lb. package that is half machine, half organism. This he dubs the astrochicken. Launched as an "egg," the astrochicken would sprout solar-panel wings that would double as radio antennae during flight. Arriving at its destination, the craft would nibble on the ice in planetary rings and shoot around like a bombardier beetle exploring moons.
Robot probes no bigger than bacteria will eventually be possible. According to K. Eric Drexler, author of Engines of Creation, they will use nanotechnology to assemble devices atom by atom or molecule by molecule. His colleagues have already made motors smaller in diameter than a human hair. Drexler believes a bundle of nanorobots, weighing practically nothing, would be the perfect interstellar emissaries. Having arrived at a planet or asteroid around some distant star, perhaps in a solar sailship pushed to high speeds by a powerful laser beam from earth, they would go to work, antlike, building radio transmitters and other gear to report home for new instructions. They could also reproduce themselves and their ships in order to send off a new set of explorer robots.
Could humans follow their robots to the stars? Because people and their life-support systems are so massive, it would take gargantuan amounts of energy and time to get anywhere. By one estimate, a round trip to a nearby star at one-tenth the speed of light would take 500 times the energy the U.S. produces in a year. Many scientists argue that no society would ever find the trip worth it, unless perhaps the sun were threatened with imminent destruction -- an event not due for 5 billion years.
Nonetheless, within 500 years humans may be ready to pack their belongings into starships -- as the early Polynesians did into canoes -- and set off in search of new worlds. With a possible 1 trillion people spread out over the solar system by then, a trip into the galaxy beyond will not seem so daunting, contend Eric Jones, a physicist at the Los Alamos (New Mexico) National Laboratory, and his collaborator Ben Finney, an anthropologist and expert on migration at the University of Hawaii. In their scenario, robots will have transformed the planet Mercury into a giant solar-power station, beaming energy to the rest of the solar system in the form of microwaves, and the moon will be a mining and construction center.
A starship could be accelerated to interstellar-travel speeds by having one of those powerful microwave lasers on Mercury push against a vast, thin sail constructed perhaps of diamond fibers. At its destination, the starship would then hunt out an asteroid, upon which microrobots would descend and begin mining and constructing a colony. Perhaps in a few hundred or a few thousand years, the inhabitants of this new world would be ready to send a migratory ship even farther. In this way, Jones and Finney argue, humans could colonize the galaxy in a few million years.
But it might not be necessary to send people's bodies. The answer, says Konstantin Feoktistov, a former Soviet cosmonaut, could be the human fax. Feoktistov has pointed out that it might be possible someday soon to "download" the entire contents of a human brain into a computer, the way a file on a PC can be transferred onto a floppy disk, and broadcast it to a % robot in a remote star system. After a few days or years of exposure to this strange world, the surrogate brain would "fax" its new information back to earth and its original owner. Feoktistov suggests that human faxing would be even easier if we could contact some extraterrestrials and have them build receiving stations for us.
What about those extraterrestrials? In that regard, the future is already here: a new search for extraterrestrial intelligence (SETI to astronomical aficionados) is about to start. On Columbus Day, radio antennas in California and Puerto Rico are scheduled to begin a survey of the heavens, monitoring 8 million radio channels simultaneously for signs of life. By the end of the decade, if they get lucky, NASA's radio astronomers may discover the signal that ends mankind's loneliness. SETI theorists hypothesize that even advanced civilizations might find interstellar travel an expensive and time-consuming way, at best, to meet the neighbors, and would instead set up radio beacons to call out to one another -- a cosmic ham radio club.
The detection of an extraterrestrial signal would be one of the greatest events of this or any other millennium. Direct contact with aliens, who would probably be vastly more powerful than we ourselves, could have a demoralizing and destructive effect on human culture, much as white men destroyed Native American life. It might also pose a challenge to the world's religions.
Fortunately, perhaps, the odds against physical contact are, well, astronomical. Even if another civilization were in our own corner of the galaxy, it could be several hundred or a thousand light-years away. With signals propagating at the speed of light, the distances involved suggest that all communications would essentially be monologues. Frank Drake, a California radio astronomer and SETI pioneer, once said that the most likely signalers would be races of immortals because they could afford to wait almost forever for the return message.
Others speculate that humanity will tap into a galactic radio network, a cosmic encyclopedia in which the cultures and histories of civilizations -- some of them by now dead -- would be preserved and broadcast eternally. The development of radio astronomy technology would constitute the entrance fee for this cosmic lonely-hearts club. Since we have reached that level of technology only in the past 50 years, humans would almost by definition be the most junior members of an association of cultures thousands or even millions of years older.
Moreover, the human race would probably not even understand any signals from outer space. SETI people liken the task of decoding and understanding such signals to biblical scholarship or the deciphering of ancient hieroglyphics. The obstacles have led Philip Morrison, a Massachusetts Institute of Technology physicist who helped invent SETI, to call the task "the archeology of the future." How would humanity respond to signals from other beings? The writer and physician Lewis Thomas once proposed that we should send the music of Bach, acknowledging that it would be bragging but holding that we had a right to put our best foot forward.
And what if, after a millennium of listening and looking, there is only silence -- what if we still seem alone? If interstellar migration is as easy and inevitable as Finney and Jones have outlined, and if the galaxy, 10 billion years old, is populated by other advanced races, critics of SETI argue, E.T.s should have come calling by now. There is no scientific evidence that they have, and the lack of it has led some scientists to argue that there is no life out there at all. One answer to the dilemma, popular in SETI circles but not very flattering, is called the zoo hypothesis: extraterrestrial ethics would bar other creatures from interfering with quaint, developing species. Somewhere out beyond the orbit of Pluto there may be a sign bearing the astronomical equivalent of DON'T FEED THE BEARS