INTO THE UNKNOWN

INTERSTELLARTRAVEL

CLASSIFICATION BEYONDTHE SOLARSYSTEM

AT A PRICEA ton of antimattercould send a ship tothe stars, but scientistscan only make it anatom at a time. Rightnow, antimatter costs

. in the region of S 10trillion an ounce.

SLOWPOKEThe Saturn 5 rocketthat put men on theMoon could have sentan Apollo craft to the'stars. But the one-waytrip would have takenat least 50,000 years.

LOOKING AHEAD'In 1997, NASA held athree-day conferenceon interstellar travelfor some of itsscientists andengineers.

The void between the stars is vast beyond theimagination. Light itself takes more than four years toreach Alpha Centauri, one of our nearest neighbors in

the galaxy and the nearest Sun-like star in the sky. To crosssuch immense distances, we will need astronauts who areready to devote their lives to traveling, and technologies thatare far in advance of those we have today. Scientists andengineers already have designs on their drawing boards.With luck, the new century should see some of them built.

SEE ALSO HOW ROCKETS WORK, TRAVELING FASTER THAN LIGHT

THREE WAYS TO GO THE DISTANCE

ROAD TOTHESTARSTo propel a spaceship on a journey to another star, today's

chemical rockets are almost useless. They simply cannotburn enough fuel to reach the speeds that we will need to

cross interstellar space in less than a human lifetime.Nuclear-powered rockets are potentially many times more

powerful. Chemical rockets mix fuel with on-board oxygen tocreate the "burn" that provides the hot gas that pushes therocket along. A nuclear rocket would use atomic reactions tovaporize its propellant-liquid hydrogen, perhaps, or just plainwater. This, in turn, would provide far more push than hot gas.

NASA has already experimented with rockets that usenuclear fission to heat propellant, but to accelerate a ship to asignificant fraction of light-speed, even fission is not enough.Fusion might be an improvement. But to power a workingstarship, we may have to master the art of making antimatter,which combines with ordinary matter in a blaze of pure energy.

Even an antimatter drive would still need vast amounts ofpropellant to reach the stars. The alternative is to develop oneof the ingenious designs shown here, which either leave theirfuel behind, or pick it up en route.

R~ICI •.iV"i~'s Ti~~-Squ~~z:~Not even supertechnology can deliver unlimited speed.According to the laws of relativity, nothing can go fasterthan light. But if a spaceship managed to get close to thespeed of light, the same laws predict that, among the weirdthings that would happen, time on board would slow down.

At 99.999% of light-speed, a year on Earth would pass inless than 40 hours of ship-time. If the ship could nudge itsway up to 99.9999%, then, in theory, the crew could makethe 30,000 light-year voyage to the center of the galaxy-and return-in only 50 years of their own personal time.

Unfortunately, the crew would find that the Earth hadaged 60,000 years in their absence. Friends and family, andperhaps the human race itself, would be long dead. In anycase, even with the help of relativistic time compression,one lifetime would be far too short to explore verymuch of a galaxy. Interstellar explorers may justhave to accept that they are not coming back.

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••• WE WORK OUT HOW TO REACH THE STARS?Young people today may live to see the launch of the

first-unmanned-interstellar mission. Given ourfast-developing knowledge of microelectronics, we willsoon be able to cram a huge number of sensors-alongwith the artificial intelligence to guide them-into verysmall robot probes. Before long we should also be able tobuild at least some of the drive systems planned today.

Unfortunately, there will still be a long wait to learnthe secrets of the stars. Even a probe to the relativelynear star Epsilon Eridani could easily take half a centuryto make the one-way trip; and its signals, traveling atlight-speed, will need almost another 11 years to makethe return journey to Earth.

If the information from such a probeis exciting enough-for example, ahabitable planet, or better still, one thatis already inhabited-a serious attemptto send humans across the void couldbegin. It would mean building a shipwith thousands of tons of payload,rather than the few pounds or ounces ofa robot probe, and the cost would bestaggering. Yet if the world's economiescontinue to grow richer, money will notbe a problem: After all, a single satellitelaunch today costs more than the entireUS was worth 200 years ago.

A 21st-century interstellar robot spacecraftreaches its target star system and launches acluster of microprobes to investigate a newfoundplanet. Thanks to advanced micro technology, theprobes are smaller than gnats. Their mother shipitself is only a little larger than a human thumb.

Of course, there may be other, technologicallyinclined beings out there in the galaxy who havesomething to say about spacefaring humans. It is easy toimagine sophisticated aliens being more annoyed thanimpressed when they see one of our ships approachtheir solar system. The proposed Bussard ramjet, forexample, would appear as a massive object closingdangerously fast, with a white-hot, fiercely radioactiveexhaust plume trailing for thousands of miles behind it.Then again, they could take pity on the primitive life-forms piloting the ship and show them a more elegantway to make such an awesome journey.