In 1969, three NASA astronauts abroad Apollo 10 flew incredibly close to the Moon. They had a lunar module a working descent engine but weren't allowed to land

In May 1969, three NASA astronauts flew to the Moon and came within roughly nine miles of its surface. They had a lunar module, a working descent engine and a trajectory designed to simulate a Moon landing. But they were never supposed to touch down.

Thomas Stafford and Eugene Cernan descended in the lunar module Snoopy, while John Young remained in the command module Charlie Brown, orbiting above them. Their mission was Apollo 10 — the final full rehearsal before NASA attempted to put humans on the Moon.

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Apollo 10 launched on May 18, 1969, just eight months after Apollo 7 and less than two months before the historic Apollo 11 landing. NASA needed one more test of the complicated machinery and procedures required for a lunar landing.

The mission was designed to do almost everything Apollo 11 would do, except the final touchdown.

After reaching lunar orbit, Stafford and Cernan entered Snoopy and separated from Young's command module. They fired the lunar module's descent engine and followed a flight profile that simulated the approach to the Moon's surface. At their lowest point, they came within about 47,000 feet — roughly nine miles — of the lunar surface.

They also flew over the Sea of Tranquility, the area NASA had selected for the upcoming Apollo 11 landing, photographing and surveying the site. The astronauts tested the lunar module's landing radar and practiced the maneuvers that would be required during an actual landing.

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But there was an important reason they stopped short.

Apollo 10 was a test flight, not the landing mission. NASA wanted to verify that the lunar module could descend, navigate near the Moon, communicate with Earth, and then rendezvous and dock with the command module. The lunar module used on Apollo 10 was not equipped to complete an actual lunar landing.

There was also no room for the crew to improvise a historic first landing. The mission's job was to prove that the equipment and procedures worked so that Apollo 11 could attempt the touchdown with greater confidence.

And Apollo 10 came remarkably close to going wrong.

During the lunar module's descent, Snoopy experienced a sudden gyration caused by a problem involving its guidance system. Stafford took manual control and recovered the spacecraft. NASA later described the incident as a momentary gyration caused by a faulty switch setting.

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After completing their low passes, Stafford and Cernan fired the ascent engine and climbed back toward Young's command module. The two spacecraft rendezvoused and docked, completing one of the most important rehearsals of the Apollo program.

Apollo 10 returned to Earth on May 26, 1969. The crew had spent eight days in space and had completed the mission's primary objectives. Three months after launch, Apollo 11 would follow the path Apollo 10 had tested and attempt the first human landing on the Moon.

Apollo 10 therefore occupies an unusual place in NASA history. Its astronauts flew closer to the Moon than almost anyone had before them, saw the future landing site from a few miles above the surface and practiced nearly every step of a lunar landing.

But they had to turn around.

The next crew — Neil Armstrong and Buzz Aldrin — would be the ones allowed to go the final nine miles.