"It's a long stretch from Jupiter to any of the planets where we have power-plants, all right--particularly now, when Mars and Tellus are subtending an angle of something more than ninety degrees at the sun, and Venus is between the two, while Jupiter is clear across the sun from all three of them. Even when Jupiter is in mean opposition to Mars, it is still some five hundred and fifty million kilometers away, so you can form some idea as to how far it is from our nearest planet now. No, if we expect to get back under our own power, we've got to break away pretty quick--these lifeboats have very little accumulator capacity, and the receptors are useless above about three hundred million kilometers...." "But it'll take us a long time to go that far, won't it?" "Not very. Our own ships, using only the acceleration of gravity, and both plus and minus at that, make the better than four hundred million kilometers of the long route to Mars in five days. These birds are using almost that much acceleration, and I don't see how they do it. They must have a tractor ray. Brandon claimed that such a thing was theoretically possible, but Westfall and I couldn't see it. We ragged him about it a lot--and he was right. I thought, of course, they'd drift with us, but they are using power steadily. They've got _some_ system!" "Suppose they could be using intra-atomic energy? We were taught that it was impossible, but you've shattered a lot of my knowledge today." "I wouldn't want to say definitely that it is absolutely impossible, but the deeper we go into that line, the more unlikely intra-atomic energy power-plants become. No, they've got a real power-transmission system--one that can hold a tight beam together a lot farther than anything we have been able to develop, that's all. Well, we've given them quite a lot of time to get over any suspicion of us, let's see if we can sneak away from them." * * * * *