nt 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.

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. I