Rip figured quickly. In accordance with Newton's Law, gravitational attraction increased rapidly on approaching a body. If he could put the asteroid even closer to the sun, the boat problem would become worse, until even a small velocity change in the wrong direction could leave a boat in the terrible position of not having enough thrust for a long enough time to keep from being drawn into the sun. But to change the asteroid's orbit was dangerous! It meant losing just enough velocity to be drawn closer to the sun, and then picking up a much higher velocity to get free again! Rip got his instruments and pulled out a special slide rule designed for use in space. He had Koa stand by with stylus and computation board and take down his figures. He recalculated the safety factor he had used when deciding how close to the sun to put the asteroid, 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 less speed would let them be pulled so far within the sun's field of gravity that, within an hour or so, small boats would venture into space 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, but also 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 forth-coming--until Dominico spoke up. "Sir, why don't we make two bombs from one?" "I wish we could," Rip said. "Do you know how?" "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 knew, sir." Rip grunted. If they had parts, he could assemble 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 behind this particular bomb design was simple. Two or more correctly sized pieces of plutonium or uranium isotope, when brought together, formed what was known as a critical mass, which would fission. The fissioning released energy and produced the explosion.