McCauley plunged into the desperately necessary task of this moment. He had to determine his present course and speed. He could not take the time to look out of the ports at the immensity of Earth below him. Men in capsules, orbiting, had been as high as this, but they did not have to compute their height or guide their vehicles. McCauley had to do both.
The height was relatively simple. A radar screen, reduced to a vertical slot for economy of space and weight, told him the distance to whatever was below. A Doppler-effect velocity indicator would read off the change in frequency of a crystal-controlled radio signal which his speed produced. This substantially resembled the way an automobile horn changes pitch when two cars pass each other; the pitch drops swiftly at the moment of passing. But there was an observation which was simpler and more direct.
He spotted a bright star near the horizon ahead. He read off its angular distance from the world's edge. Looking aft, under the belly of the ship, he read another angle from the world's edge to another star. Minutes later, he repeated the observations. The star ahead was higher, the one behind was lower. If one star rose faster than the other sank, he would be gaining height. If one sank faster than the other rose, he would be falling. If one rose exactly as fast as the other dropped, he would be in a perfect circular orbit, neither rising nor falling. That was too good to be expected. But from even two sets of observations he could tell the line the ship was following, and hence its speed.
The ship did not have quite the speed necessary for a complete orbit. It needed more. He could guess how much.
He said curtly to Furness:
"We've got to have a two-second push, anyhow. Maybe more later. Get set."
Furness did not reply, but McCauley heard him reporting.
There was singularly little exultation in the small cabin. Furness' face was drawn and colorless behind his helmet plate. McCauley was busy.
Presently, after a warning gesture, he set the rocket timer and pressed the firing button. All the ghastly impact of high acceleration repeated itself. But, lasting only two seconds, it was not much worse than—say—falling from a second-story window down on a hard mattress. It lasted longer, but there was not much other difference. It did not build up to the torture of continued rocket thrust.
Then the ship floated on. There was utter silence. The vertical-slot altimeter indicated a height which seemed absolutely steady. The Doppler-effect velocity meter gave a reasonably satisfactory if not too precise message. McCauley was working intensively on his course when Furness said, with an effort: