L.O.R. had been considered so unrealistic in 1961 because of the maneuver that was now about to take place for the first time: a launch-to-rendezvous by a spacecraft 240,000 miles from home. The first reason why that prospect had been so forbidding is that spacecraft cannot move sideways any great distance. They can move up, down, forward, or backward, but for practical purposes they are locked into the plane in which they have been launched—sideways movement against that momentum is extremely costly in propellants. Therefore the first constraint of space rendezvous is that the planes of the
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