The Jupiter Theft

A twentieth-century scientist named Bussard once had calculated that an intake area of about 80 miles in diameter would be a sufficient-sized scoop for an interstellar ramjet feeding on clouds of ionized hydrogen. True, he was talking about a 1,000-ton spaceship, but he was also talking about ordinary hydrogen fusion.

Even at the final stages of their journey, with their Jovian giant shrunk to Earth size, they’d have a scoop with a diameter of 8,000 miles. And that was just the small end of a truncated cone—the impact area. The size of the large end would be anybody’s guess. It would depend on such things as gravitational attraction and the rate of ionization induced by the planet’s magnetic field. It could sweep an area hundreds of thousands of miles in diameter.

Jameson felt his cheeks burning with excitement. He wished he had Ruiz to talk this over with. Ruiz would be able to work out the math. But he was sure that he was right in his assumptions. It felt right! Hell, he could be wrong by a factor of ten and the principle would be the same. Burn 99 percent of a gas giant to get the remaining 1 percent—plus your piddling fifteen-mile ships—up to velocity. Then brake by burning 99 percent of the remaining 1 percent. Maybe they’d ridden here on a superjovian, ten or twenty times Jupiter’s mass. Maybe they’d have to limp out of the solar system at a mere 90 or 95 percent of the speed of light.

“The word is infinite,” Tetrachord said. He sounded like a cageful of twittering birds, all except the sound for “infinite,” which was a single sweeping glissando spanning two octaves, with a little turn at the top. Was it Jameson’s imagination, or was there an overtone of approval in the Cygnan’s voice, like a person patting a smart puppy on the head for doing a trick?

“But an infinite weight can never be reached,” Jameson responded quickly.

“No.” Again Jameson thought he heard approval. “But that-which-pulls will be heavy enough in…” There was a phrase for a unit of time.

That was the sticky point. The Cygnans, in their arrogance or self-sufficiency, had never bothered to give Jameson a scale involving Earth’s year or period of rotation. Perhaps they simply didn’t care to study the planets encountered during their transient pit stops. They used a Cygnan time scale, and to understand the timetable for this cosmic theft he was going to have to find out something about the Cygnan home planet. Perhaps it didn’t even exist any more, but it was still part of their cultural baggage.

Just how close to the speed of light could the little probe itself get in its flashing circuit, and how long would it take it to gain enough mass to move Jupiter? The probe didn’t have to accelerate any mass except itself; its fuel tank was external—Jupiter’s atmosphere. He frowned, trying to recall some of the theoretical scuttlebutt that had come out of the abandoned studies for a Centaurus probe. A twentieth-century rocket expert named Sanger had estimated that a respectable-sized spacecraft could attain 99.999,999,999,999,999,996 percent of the speed of light by annihilating a mass the size of the Moon. You could get there in a year at one g, in less than four days at 100 g’s.

The probe the Cygnans called “that-which-pulls” had been at it for more than six months. It must have brushed the speed of light within a few days. Now it was nibbling away at the remaining fraction, fighting an uphill battle against relativistic imperatives. The efficiency of its engines would be diminishing, from an outside observer’s viewpoint, by the penalty of an enormous time-dilation effect. But it was certainly getting there.

Now how the hell did the Cygnans keep the probe from flying off into space? At that speed, with its abnormal mass, and with its tight turns around Jupiter, the centrifugal force must be … unthinkable!

He was about to ask Tetrachord when he realized that the question was without meaning. What did it matter how much energy was diverted to tie the probe down in orbit? Just take whatever was needed out of all that kinetic energy stolen from Jupiter’s mass, no matter how mind-boggling the sum. Blow it out in a direction perpendicular to the orbit. Or change the attitude of the robot craft to provide a vector that would balance the enormous forces. The method didn’t even have to be particularly complicated. A simple feedback mechanism would do.

He took a closer squint at the cascade of repeated shapes frozen on the screens. There! He could make it out! A plume of light sprouting from the waist of the ship, driving it inward toward its primary.

He shuddered. What was that spray of light? A waste product of the drive, as heat is the byproduct of the work done by a mechanical engine? He suspected that the Cygnan exhaust was something akin to pure gamma rays. It wouldn’t do to let that stuff get too close to an inhabited planet, to say nothing of that terrible bow wave!