“Do we want to go to the moon or not?” asked Dr. Houbolt.

Since the 1950s, Dr. Houbolt, who was 95 when he died on April 15 in Scarborough, Me., had been arguing for a smaller, lighter and less expensive option — a Chevrolet, not a Cadillac, he liked to say — that was called lunar orbit rendezvous. According to this method, a rocket launched from Earth would send a spacecraft into orbit around the moon that would then deploy another vehicle, known as a “bug” or lunar module, to the lunar surface.

The module would carry two men who, after exploring the moon, would travel in the module back to the orbiting spacecraft and then return to Earth. It, too, was complicated, but it did not require the kind of massive rocketry the other approaches did — technology that did not yet exist.

“Why is Nova, with its ponderous ideas, whether in size, manufacturing, erection, site location, etc., simply just accepted, and why is a much less grandiose scheme involving rendezvous ostracized or put on the defensive?” Dr. Houbolt wrote to Dr. Seamans. “I fully realize that contacting you in this manner is somewhat unorthodox, but the issues at stake are crucial enough to us all that an unusual course is warranted.”

Until then, lunar orbit rendezvous had been dismissed as far-fetched. In 1961, no American had even orbited Earth — John Glenn would do so the next year — and there were broad concerns that the proposed sequence of events posed too many risks. It required multiple vehicles and complicated maneuvers high above the moon’s surface.

“Do not be afraid of this,” Dr. Houbolt urged Dr. Seamans, assuring him that he was not “dealing with a crank.”