RTHBOUND!

There was something else that Rip didn't add, although he knew the
Planeteers would realize it in a few minutes. Probably some of them
already had thought of it.

To move the asteroid into a new orbit, they were going to fire nuclear
bombs. Most of the highly radioactive fission products would be blown into
space, but some would be drawn back by the asteroid's slight gravity. The
craters would be highly radioactive and some radioactive debris would be
scattered around, too. Every particle would add to the problem.

"Is there anything we can do, sir?" Koa asked.

Rip shook his head inside the transparent bubble. "If you have a good luck
charm in your pocket, you might talk to it. That's about all."

Nuclear physics had been part of his training. He read the gamma meter
again and did some quick mental calculations. They would be exposed to
radiation for the entire trip, at a daily dosage of--

Koa interrupted his train of thought. Evidently the sergeant-major had
been doing some calculations of his own. "How long will we be on this
rock, sir? You've never told us how long the trip will take."

Rip said quietly, "With luck, it will take us a little more than three
weeks."

He could see their faces faintly in the dim sunlight. They were shocked.
Space ships blasted through space between the inner planets in a matter of
hours. The nuclear drive cruisers, which could approach almost half the
speed of light, had brought even distant Pluto within easy reach. The
inner planets could be covered in a matter of minutes on a straight speed
run, although to take off from one and land on the other meant
considerable time used in acceleration and deceleration.

The Planeteers were used to such speed. Hearing that it would take over
three weeks to reach earth had jarred them.

"This piece of metal isn't a space ship," Rip reminded them. "At the
moment, our speed around the sun is just slightly more than ten miles a
second. If we just shifted orbits and kept the same speed, it would take
us months to reach Terra. But we'll use two bombs to kick the asteroid
into the orbit, then fire one to increase speed. The estimate is that
we'll push up to about forty miles a second."

Koa spoke up. "That's not bad when you think that Mercury is the fastest
planet and it only makes about thirty miles a second."

"Right," Rip agreed. "And when we really have the sun's gravity pulling
us, we'll increase speed. We'll lose a litt