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." Private Dominico saluted and moved forward. "Sir, may I ask a question?" Rip turned to face the Planeteer, still worrying over the problem. He nodded and said, "What is it, Dominico?" "Sir, I think we can't worry too much about this radiation, eh? You will think of some way to take care of it. What I want to ask, sir, is when do we let go the bombs? I do not know much about radiation, but I can set those bombs like you want them."