In April 2010, the location of Lunokhod 1 was discovered by the Lunar Recon Orbiter. The Apache Point Observatory Lunar Laser-ranging Operation team at UC San Diego then tried shining a laser beam at that location, because they knew it was fitted with corner-cube prisms that return any incident light back to the direction from which it came. That returning light can then be "caught" and analyzed. The team was astounded by what happened next. From NASA:
"We got about 2,000 photons from Lunokhod 1 on our first try. After almost 40 years of silence, this rover has a lot to say," notes Murphy.
Back in the late 1960s and early 1970s, Apollo astronauts placed three other retroflectors on the Moon to allow laser ranging of the Moon's orbit. Assisted by a fourth reflector on Lunokhod 2, a twin of Lunokhod 1 that landed in 1973, these mirrors constitute the only Apollo science experiment still operating.
Eric Silverberg, now retired from the University of Texas, was in charge of the lunar laser ranging activities at the McDonald Observatory from 1969 until 1982. "During that time," he recalls, "we successfully ranged all three of the Apollo corner reflectors and the Lunakhod 2 reflector. We also tried to range on the first lunar rover but had only one possible (but not definite) detection on Dec 31, 1970. Our lack of knowledge of the location of the rover and the pressures of keeping up with the Apollo program caused us eventually to lose interest in Lunakhod 1."
"When I read that Tom Murphy had discovered returns from the lost rover I was very surprised and elated," says Silverberg.
Murphy's initial reaction was disbelief: "The signal was so strong, my first thought was that our detector was acting up! I expected the rover's reflector to be degraded and dull after all this time, so I thought, 'this couldn't possibly be it.' But it was."
"This reflector is even strong enough to let us get measurements in lunar daylight – a first for this experiment!"
Silverberg continues: "The fact that Lunokhod 1's reflection is now stronger than that of its twin is a mystery."