Zero hour arrived. Again Jonner pulled levers and again the faint glow appeared around the tail of their distant tug. Across space the exhaust of the _Marsward XVIII_ flared into blinding flame. In a moment, it began to pull ahead visibly and soon was receding like a meteor. Near the _Radiant Hope_, the space station seemed not to have changed position at all. "The race is not always to the swift," remarked Jonner philosophically. "And we're the tortoise," said T'an. "How about filling us in on this jaunt, Jonner?" "Is should, Jonner," agreed Qoqol. "T'an know all about crazy new engine, I know all about crazy new orbit. Both not know all. You tell." "I planned to, anyway," said Jonner. "I had figured on having Serj in on it, but he wouldn't understand much of it anyhow. There's no use in waking him up." Serj was the ship's doctor-psychologist and fourth member of the crew. He was asleep below on the centerdeck. "For your information, Qoqol," said Jonner, "the atomic engine produces electrical energy, which accelerates reaction mass. Actually, it's a crude ion engine. T'an can explain the details to you later, but the important thing is that the fuel is cheap, the fuel-to-cargo ratio is low and constant acceleration is practical. "As for you, Tan, I was surprised at your not understanding why we'll use low acceleration. To boost the engine power and give us more Gs, we'd either have to carry more fuel or coast part of the way on momentum, like an ordinary rocket. This way's more efficient, and our 63-day margin over the _Marsward_ each way is more than enough for unloading and loading more cargo and fuel." "With those figures, I can't see how Marscorp expects to win this competition," said T'an. "We've got them, flat, on the basis of performance," agreed Jonner. "So we'll have to watch for tricks. I know Marscorp. That's why I arranged to take aboard that G-boat at the last minute. Marscorp controls all the G-boats at Marsport, and they're smart enough to keep us from using them, in spite of the Space Control Commission. As for refueling for the return trip, we can knock a chunk off of Phobos for reaction mass." The meteor alarm bells clanged suddenly, and the screen lit up once with a fast-moving red line that traced the path of the approaching object. "Miss us about half a mile," said Jonner after a glance at the screen. "Must be pretty big ... and it's coming _up_!" He and T'an floated to one of the ports, and in a few moments saw the object speed by. "That's no meteor!" exclaimed Jonner with a puzzled frown. "That's man-made. But it's too small for a G-boat." The radio blared: "All craft in orbit near Space Station 2! Warning! All craft near Space Station 2! Experimental missile misfired from White Sands! Repeat: experimental missile misfired from White Sands! Coordinates...." "Fine time to tell us," remarked T'an drily.