Thrown away from it, the centers of their orbits changed. In theory,
the center of the Platform's orbit was the center of Earth. But the
centers of the orbits of the thrown-away objects were pushed a few
miles--twenty--fifty--a hundred--away from the center of Earth.

The returning space ships also had the orbit and speed of the Platform.
They wanted to shift the centers of their orbits by very nearly 4,000
miles, so that at one point they would just barely graze Earth's
atmosphere, lose some speed to it, and then bounce out to empty space
again before they melted. Cooled off, they'd make another grazing
bounce. After eight such bounces they'd stay in the air, and the stubby
fins would give them a sort of gliding angle and controllability, while
the landing rockets would let them down to solid ground. Or so it was
hoped.

Meanwhile they headed out instead of in toward Earth. They went out on
their steering-rockets only, using the liquid fuel that had not been
needed for course correction on the way out. At 4,000 miles up, the
force of gravity is just one-fourth of that at the Earth's surface. It
still exists; it is merely canceled out in an orbit. The ships could
move outward at less cost in fuel than they could move in.

So they went out and out on parallel courses, and the Platform dwindled
behind them. Night flowed below until the hull of the artificial
satellite shone brightly against a background of seeming sheer
nothingness.

The twilight zone of Earth's shadow reached the Platform. It glowed
redly, glowed crimson, glowed the deepest possible color that could be
seen, and winked out. The ships climbed on, using their tiny steering