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By Root 338 0

of half a million. Only the five giants—Ceres, Pallas, Juno, Eunomia and Vesta—were more than two hundred kilometres in diameter; the vast majority were merely oversized boulders that would fit into a small park. Almost all moved in orbits that lay beyond Mars; only the few that came far enough sunwards to be a possible danger to Earth were the concern of SPACEGUARD. And not one in a thousand of these, during the entire future history of the solar system, would pass within a million kilometres of Earth. The object first catalogued as 31/439, according to the year and the order of its discovery, was detected while still outside the orbit of Jupiter. There was nothing unusual about its location; many asteroids went beyond Saturn before turning once more towards their distant master, the sun. And Thule II, most far-ranging of all, travelled so close to Uranus that it might well have been a lost moon of that planet. But a first radar contact at such a distance was unprecedented; clearly, 31/439 must be of exceptional size. From the strength of the echo, the computers deduced a diameter of at least forty kilometres; such a giant had not been discovered for a hundred years. That it had been overlooked for so long seemed incredible.

Then the orbit was calculated, and the mystery was resolved—to be replaced by a greater one. 31/439 was not travelling on a normal asteroidal path, along an ellipse which it retraced with clockwork precision every few years. It was a lonely wanderer between the stars, making its first and last visit to the solar system—for it was moving so swiftly that the gravitational field of the sun could never capture it. It would flash inwards past the orbits of Jupiter, Mars, Earth, Venus and Mercury, gaining speed as it did so, until it rounded the sun and headed out once again into the unknown.

It was at this point that the computers started flashing their 'Hi there! We have something interesting' sign, and for the first time 31/439 came to the attention of human beings. There was a brief flurry of excitement at SPACEGUARD Headquarters, and the interstellar vagabond was quickly dignified by a name instead of a mere number. Long ago, the astronomers had exhausted Greek and Roman mythology; now they were working through the Hindu pantheon. And so 31/439 was christened Rama.

For a few days, the news media made a fuss of the visitor, but they were badly handicapped by the sparsity of information. Only two facts were known about Rama—its unusual orbit, and its approximate size. Even this was merely an educated guess, based upon the strength of the radar echo. Through the telescope, Rama still appeared as a faint, fifteenth magnitude star—much too small to show a visible disc. But as it plunged in towards the heart of the solar system, it would grow brighter and larger, month by month; before it vanished for ever, the orbiting observatories would be able to gather more precise information about its shape and size. There was plenty of time, and perhaps during the next few years some spaceship on its ordinary business might be routed close enough to get good photographs. An actual rendezvous was most unlikely; the energy cost would be far too great to permit physical contact with an object cutting across the orbits of the planets at more than a hundred thousand kilometres an hour.

So the world soon forgot about Rama; but the astronomers did not. Their excitement grew with the passing months, as the new asteroid presented them with more and more puzzles.

First of all, there was the problem of Rama's light curve. It didn't have one.

All known asteroids, without exception, showed a slow variation in their brilliance, waxing and waning within a period of a few hours. It had been recognized for more than two centuries that this was an inevitable result of their spin, and their irregular shape. As they toppled end over end along their orbits the reflecting surfaces they presented to the sun were continually changing, and their brightness varied accordingly.

Rama showed no such changes. Either it was not spinning at