"It's been silent for almost 40 years, and nobody knew if it was even viable anymore," said Tom Murphy, a physicist at UC San Diego. Murphy led the team that identified the location of the reflector.
The reflector, which is attached to the rover, contains a series of glass prisms. If physicists direct a laser beam at the reflector, the prisms bounce the light back. Based on how and when the light returns — and how many other reflectors are available to provide data — physicists can learn about the structure and movement of the moon.
There are four functioning reflectors on the moon already. Three were flown up by the U.S. Apollo 11, 14 and 15 missions from 1969 to 1971. The fourth was left on the moon by Lunokhod 2, the second rover sent up by the Soviets.
Data from one reflector, Murphy said, can show the approximate distance to that point on the lunar surface. Data from three reflectors can tell researchers where the center of the moon lies. And adding a fourth reflector's data helps researchers assess the impact of gravitational forces on the moon.
Having a fifth reflector in place should put a finer point on the data, including composition of the moon's fluid core.
Murphy's team, which studies the shape of the lunar orbit, had been using the four reflectors to gauge gravitational forces. The physicists were looking for discrepancies in Einstein's general theory of relativity, which explains the laws of gravity.
Some scientists believe the theory falls short. If gravitational distortions in the moon didn't match the theory's predictions, Murphy reasoned, the old theory could potentially be overturned.
In April 2008, the scientists began to make the occasional search for the Lunokhod 1 rover, in the hopes that turning up that fifth reflector would allow them to collect more detailed information.
Retired physicist Eric Silverberg said he was shocked when he heard Lunokhod 1 had been found. He too used lasers in his lunar research as head of the McDonald Observatory in Texas; that observatory had the only working laser station in the early days of space exploration.
The fact that the reflector works at all is probably the result of pure dumb luck, he said.
"We had learned many years ago that the Russian driver operating the Lunokhod did not know it had to be parked at a certain angle to use the reflectors, so we had assumed all these years that it had been lost to us," Silverberg said. "He would have had to park it within 10% of the right angle … and he did that accidentally."
But if Murphy's team had not been looking for the reflector, he added, it "wouldn't have been found."
Images taken in March from the NASA Lunar Reconnaissance Orbiter, which has been surveying the moon's surface, showed the rover. Researchers with NASA sent Murphy estimated coordinates of the rover's location.
On April 22, the scientists used a telescope-based laser beam, sent from the Apache Point Observatory in New Mexico, to pinpoint the location of the reflector. It was bouncing back a remarkably strong signal.
That's surprising, given that the signal from the other Soviet reflector is far weaker than it used to be, Murphy said.
The return signal "just came blazing through, incredibly strong — five times stronger than I expected," Murphy said. "So that's a new mystery we'll be chewing on — why are the two behaving so differently? What happened to Lunokhod 2?"
The reflector's location — close to the visible edge of the moon — allows scientists to get a much more sensitive reading when the moon wobbles due to gravitational distortion.
There was no word yet on the Russian take on the discovery, said James Williams, a senior research scientist at JPL who sent Murphy estimated coordinates of the rover's location in March. Williams has been in contact with Russian researchers in the past.
"I don't know if they know about it yet," Williams said. "They're very interested when you talk to them. They're proud of what they did — and they should be." The then-Soviet Union's exploration of the lunar surface, like that conducted by the United States, required enormous amounts of money, resources and expertise. "It was quite an accomplishment at the time," he added.
For those who remember the original Lunokhod launch, Silverberg added, the discovery of the long-lost rover and reflector borders on miraculous.
"Any way you look at it, it was a magnificent find, one of the most phoenix-like moments I've seen. … It was presumed dead for 39 years. Now it's suddenly alive and well."