Earthbound!
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 was certain to 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 calculations. They would be exposed 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 just 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.
Spaceships 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 spaceship," 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 one bomb for retrothrust, then
fire two 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. "After the asteroid is kicked out of orbit, it will
fall toward the sun. At our closest approach to the sun, we'll have
enough velocity to carry us past safely. Then we'll lose speed constantly
until we come into Earth's gravitational field and have to brake."
It was just space luck that Terra was on the other side of the sun from
the asteroid's present position. By the time they approached, it would
be in a good place, just far enough from the line to the sun to avoid
changing course. Of course, Rip's planned orbit was not aiming the
asteroid at Earth, but at where Earth would be at the end of the trip.
"That means more than three weeks of radiation, then," Corporal Santos
observed. "Can we take it, sir?"
Rip shrugged, but the gesture couldn't be seen inside his space suit. "At
the rate we're getting radiation now, plus what I estimate we'll get from
the nuclear explosions, we'll get the maximum safety limit in just three
weeks. That leaves us no margin, even if we risk getting radiation
sickness. So we have to get shielding pretty soon. If we do, we can last
the trip."