"Not quite, Pederson," Rip corrected. "Koa is on the right track. The pull of the sun is pretty strong. But I don't think it's strong enough to capture boats."

He had figured the asteroid's...o...b..t to pa.s.s as close to the sun as possible while maintaining a margin of safety. He had wanted to use the sun's gravity to pick up speed. His regular star sightings had told him several days before that the sun was dragging them.

But Koa had started a train of ideas running through Rip's head. If they could get close enough to the sun so small boats would be unable to break free of its gravity, the Connie wouldn't dare send a landing force. The powerful engines of a cruiser could break loose from Sol's pull, but not the chemical jets of a cruiser's boats.

Rip got his instruments and pulled out a special slide rule designed for use in s.p.a.ce. He had Koa stand by with stylus and computation board and take down figures as he called them off.

He recalculated the safety factor he had used when deciding how close to put the asteroid to the sun, then took quick star sights to determine their exact position. They were within a few miles of perihelion, the point at which they would be closest to Sol.

Rip tapped gloved fingers on his helmet absently. If they could blast out of the orbit and drive into the sun ... he estimated the result. A few miles per second of extra speed would put them so far within the sun's field of gravity that, within an hour or so, small boats would venture into s.p.a.ce only at their peril.

He reviewed the equipment. They had tubes of rocket fuel, but the tubes wouldn't give the powerful thrust needed for this job. They had one atomic bomb. One wasn't enough. Not only must they drive toward the sun, they must keep reserve power to blast free again. If only they had a pair of nuclear charges!

He called his Planeteers together and outlined the problem. Perhaps one of them would have an idea. But no useful suggestions were forthcoming until little Dominico spoke up. "Sir, why don't we make two bombs from one?"

[Ill.u.s.tration: "Sir, Why Don't We Make Two Bombs From One?"]

"Sir, Why Don't We Make Two Bombs From One?"

"I wish we could," Rip said. "Do you know how, Dominico?"

"No, Lieutenant. If we had parts, I could put bombs together. I can take them apart, but I don't know how to make two out of one." The Italian Planeteer looked accusingly at Rip. "I thought maybe you know, sir."

Rip grunted. If they had parts, he could a.s.semble nuclear bombs, too. Part of his physics training had been concerned with fission and its various applications. But no one had taught him how to make two bombs out of one.

The theory of nuclear explosions was simple enough. Two or more correctly sized pieces of plutonium or uranium isotope, when brought together, formed what was known as a critical ma.s.s, which would fission. The fissioning released energy and produced the explosion.

But there was a wide gap between theory and practice. A nuclear bomb was actually pretty complicated. It had to be complicated to keep the pieces of the fissionable material apart until a chemical explosion drove them together fast and hard enough to create a fission explosion. If the pieces weren't brought together rapidly enough, the ma.s.s would fission in a slow chain reaction and no explosion would result.

Rip was trained in scientific a.n.a.lysis. He tackled the problem logically, considering the design of a nuclear bomb and the reasons for it.

Atomic bombs had to be carried. That meant an outer casing was necessary.

Probably the casing had a lot to do with the design. Suppose no casing were required? What would be needed?

He took the stylus and computation board from Koa and jotted down the parts required. First, two or more pieces of plutonium large enough to form a critical ma.s.s. Second, a neutron source-some material with the type of radioactivity that produced neutrons-to start the reaction. Third, some kind of neutron reflector. And fourth, explosive to drive the pieces together.

Did they have all those items? He checked them off. Their single five KT bomb contained at least enough plutonium for two critical ma.s.ses, if brought together inside a good neutron reflector. Each ma.s.s should give about a two kiloton explosion. And they did have a good neutron reflector-nuc.l.i.te. There wasn't anything better for the purpose.

"What have we got for a neutron source?" he asked aloud. He was really asking himself, but he got a quick answer from Koa.

"Sir, some of the stuff left in the craters from the other explosions gives off neutrons."

"You're right," Rip agreed instantly. A small piece from one of the craters, when combined with half of the neutron source in the bomb, should be enough. As for the explosive, they had exploding heads on their attack rockets.

In other words, he had what he needed-except for a method of putting all the pieces together to create a bomb.

If only they had a tube of some sort that would withstand the chemical explosion-the one that brought the critical ma.s.s together!

He told the Planeteers what he had been thinking, then asked, "Any ideas for a tube?"

"How about a tube from the snapper-boat?" Santos suggested.

Rip shook his head. "Not strong enough. They're designed to withstand the slow push of rocket fuel, not the fast rap of an explosion. When I say slow, I mean slow-burning when compared with explosive. Who has another idea?"

Kemp, the expert torchman, said, "Sir, I can burn you a tube into the asteroid."

Rip grabbed the Planeteer so hard they both floated upward. "Kemp, that's wonderful! That's it!" The details took form in his mind even as he called orders. "Dominico, tear down that bomb. Santos, remove two heads from your rockets and wire them to explode on electrical impulse. Kemp, we'll want the tube just a fraction of an inch wider than a rocket head. Get your torch ready."

He took the stylus and began calculating. He talked as he worked, telling the Planeteers exactly what they were up against. "I'm figuring out where to put the charge so it will do the most good, but my data isn't complete.

If our homemade bomb goes off, I don't know exactly how much power it will give. If it gives too much, we'll be driven so close to the sun well never get free of its gravity."

Bradshaw, the English Planeteer, said mildly, "Don't worry, Lieutenant.

We're caught either way. If it isn't the solar frying pan, it's Connie fire."

A chorus of agreement came from the other Planeteers. What a crew! Rip thought. What a great gang of s.p.a.ce pirates!

He finished his calculations and found the exact spot where Kemp would cut. A few feet away from the spot was a thick pyramid of thorium. That would do, and they could cut into it horizontally instead of drilling straight down. He pointed to it. "Let's have a hole straight in for six feet. And keep it straight, Kemp. Allow enough room for a lining of nuc.l.i.te. Koa, pull a sheet of nuc.l.i.te out of the cave and cut it to size."

Kemp's torch already was slicing into the metal. Rip asked, "Can you weld with that thing, Kemp?"

"Just show me what you want, sir."

"Good." Rip motioned to Trudeau. "Frenchy, we'll need a strong rod at least eight feet long."

The French Planeteer hurried off. Rip consulted his chronometer. Less than ten minutes had pa.s.sed since the call from Terra base.

He went over his plan again. It had to work! If it didn't, asteroid and Planeteers would end up as subatomic particles in the sun's photosphere, because he had calculated his blast to drive the asteroid past the limit of safety. It was the only way he could be sure of putting them beyond danger from Connie landing boats or snapper-boats. The Connie would have only one chance-to bring his cruiser down on the asteroid.

If he tried that, Rip thought grimly, he would get a surprise. The second nuclear charge would be set, ready to be fired. The Connie cruiser was so big that no matter how it pulled up to the asteroid, some part of it would be close enough to the charge to be blown into s.p.a.ce dust. No cruiser could survive an atomic explosion within five hundred yards, and the Connie would have to get closer to the nuclear charge than that.

Dominico reported that the bomb had been dismantled. Rip went to it and examined the raw plutonium, being careful to keep the pieces widely separated.

This particular bomb design used five pieces of plutonium which were driven together to form a ball. Rip made a quick estimate. Two were enough to form a critical ma.s.s. He would use two to blast into the sun and three to blast out again. He would need the extra kick.

There was only one trouble. The pieces were wedge shaped. They would have to be mounted in thorium in order to keep them rigid. Only Kemp could do that. They had no cutting tool but the torch.

Santos appeared, carrying a rocket head under each arm. They had wires wound around them, ready to be attached to an electrical source.

Rip hurried back to where Kemp was at work. The private was using a cutting nozzle that threw an almost invisible flame five feet long. In air, the nozzle wouldn't have worked effectively beyond two feet, but in s.p.a.ce it cut right down to the end of the flame. Kemp had his arm inside the hole and was peering past it as he finished the cut.

"Done, sir," he said, and adjusted the flame to a spout of red fire. He thrust the torch into the hole and quickly withdrew it as pieces of thorium flew out. A stream of water hosed into the tube would have washed them out the same way.

Rip took a block of plutonium from Dominico and handed it to Kemp. "Cut a plug and fit this into it. Then cut a second plug for the other piece.

They have to match perfectly, and you can't put them together to try out the fit. If you do, we'll have fission right here in the open."

Kemp searched and found a piece he had cut in making the tube. It was perfectly round, ideal for the purpose. He sliced off the inner side where it tapered to a cone, then, working only by eye estimate, cut out a hole in which the wedge of fission material would fit. He wasn't off by a thirty-second of an inch. Skillful application of the torch melted the thorium around the wedge and sealed it tightly.

Koa was ready with a sheet of nuc.l.i.te. Trudeau arrived with a long pole he had made by lashing two crate sticks together.