The Soviets tried to beat Apollo 11. They crashed a spacecraft on the moon instead.

As Neil Armstrong walked on the lunar surface and marveled at the "fine, sandy particles" that crunched under his boot, he and the rest of the Apollo 11 crew were not alone.

A Soviet spacecraft, Luna 15, had beat them to orbit days before, circumnavigating the moon in a final Cold War showdown race to land on another celestial body and return home.

The unmanned spacecraft's mission would be an epic coup: Get to the moon, scoop up rocks and jettison back toward Earth before the Americans returned with their own samples.

That did not happen. Luna 15 plummeted toward the moon on July 21, 1969, crashed into a mountain and cratered near the aptly named Sea of Crises - before Armstrong and Buzz Aldrin even left the surface.

NASA had worried Luna 15 would interfere with radio transmissions and present a safety risk with Apollo 11, prompting high-level officials to cross the divide in an unprecedented level of cooperation.

It signified something else. The competition between Apollo 11 and Luna 15 to land on the moon and return to Earth did not exactly finish when "the Eagle" landed, as most believe.

"The race to the moon ends when Luna 15 crashes," William P. Barry, NASA's chief historian, told The Washington Post.

The Soviet exploration timeline was aggressive and, at turns, tragic. Cosmonaut Vladimir Komarov melted during re-entry, along with the Soyuz 1 capsule in which he was riding, in 1967, all the while "cursing the people who had put him inside a botched spaceship."

In the next year, two Soviet tortoises became the first Earth beings to circle the moon.

Luna 15's design and launch time frame was incredible for its time, Barry said. Plans were drawn up for a robot designed to land, collect samples and scurry back to Earth.

That was complete in about six months as the Soviets raced to preempt the United States, including an unsuccessful June launch with a mission to grab moon rocks and study the lunar gravitational field. That rocket never left orbit.

On July 15, the rocket delivering Luna 15 roared toward the moon three days ahead of the Apollo 11 mission. The race had begun.

"I'm sure that the original plan was to beat Apollo 11 back to Earth with their sample," Barry said.

The launch puzzled NASA and surprised the Apollo 11 crew, who only knew about its existence en route to the moon and "did not know about Luna 15 or its goal," Armstrong said in 2009.

No one exactly knew where it was going or how it could interfere with the three Americans heading for the same place.

The moon's surface is about the size of Africa, Barry said, but orbital dynamics suggested landing spots around the moon's equator were best - potentially limiting the distance between the Columbia command module, the Eagle landing craft and Luna 15.

There was only one unlikely solution to all of this: Get the flight details from the Soviets themselves.

During the Cold War.

In the midst of the moon race.

And yet, Apollo 8 astronaut Frank Borman had met Soviet space official Academician Mstislav V. Keldysh. He later called on him to provide details on Luna 15 and assure it would not interfere with Apollo 11.

What came next was unprecedented in American-Soviet space relations, Barry said. Keldysh telegraphed the orbital details for Luna 15 and said it would be a safe transit for the U.S. astronauts, though he never divulged the mission details.

Meanwhile, as the Apollo 11 rocketed toward the moon, Houston ground control kept the crew informed about the whereabouts of Luna 15. It had entered orbit on July 17, Houston told the crew, according to flight logs. Both spacecrafts made orbital adjustments as ground control in both nations nervously watched.

Soviet engineers worried over rugged terrain of the Sea of Crises landing site, NASA has said, and delayed its planned landing for hours.

That opened the window for Apollo 11 to land. As Armstrong and Aldrin took photos, collected samples and marveled at the view of Earth, Luna 15 tumbled in orbit.

The Soviets realized they were running out of time, Barry said, and a day later, on July 21, they decided to make an effort to land - which surprised British astronomers listening to Soviet transmissions. They were unaware it was designed to do so, Discover magazine reported.

Luna 15 descended, cushioned by retro rockets. But its trajectory was off, sending the spacecraft careening into a mountain at 298 miles an hour, and finally, plummeting to the moonscape.

It crashed about 350 miles from the U.S. landing site at 15:50 UTC - a full two hours and four minutes before the Eagle began its flight back to the Columbia module.

"I say, this has really been drama of the highest order," a British astronomer said afterward.

The Post reported the crash the next day. "Thus, by a fluke of moon geography, a space flight riddle 'wrapped in a mystery inside an enigma' ended in an irony," Howard Simons wrote. Scientists speculated it was deliberately crashed to "remove its carcass from lunar orbit," he added, which was also a method the United States practiced.

The Soviets never fully admitted Luna 15 was an effort to beat the United States to the moon and back, Barry said, though those details emerged after the Soviet Union collapsed. And its return trajectory after the delays made it unlikely it would even beat the American mission back.

Luna 15 may have failed its mission, Barry noted, but what scientists learned from the experience became clear in the next decade.

More attempts were made for a Luna mission to collect moon rocks. Five in total failed, until 1970, when Luna 16 shoveled 100 grams of dust and returned it to Earth - the first time an unmanned spacecraft did so from any nation. Later missions yielded more payloads.

The bones from the doomed spacecraft may never be found, Barry said. "Distinguishing a crater caused by Luna 15 and a small meteorite would be pretty hard."