"No. Can't figure velocity without any reliable data on how many atoms of matter exist per cubic meter out here." Cleveland was staring at the calculator. "It's constant, of course, at the value at which the friction of the medium is equal to our thrust. Incidentally, we can't hold it too long. We're running a temperature, which shows that we're stepping along faster than anybody ever computed before. Also, it points out the necessity for something that none of us ever anticipated needing in an open-space drive—refrigerators or radiating wall-shields or repellers or something of the sort. But to get back to our velocity—taking Throckmorton's estimates it figures somewhere near the order of magnitude of ten to the twenty-seventh. Fast enough, anyway, so that you'd better bend an eye on that plate. Even after you see them you won't know where they really are, because we don't know any of the velocities involved—our own, theirs, or that of the beam—and we may be right on top of them."