Random Walk
Looking kind of like a bathtub on wheels, the Lunokhod 1 is 2.3 meters long and 1.5 meters tall. The reflectors sit in the tray jutting out on the left.
Shoot the Moon, Hit a Rover
A Caltech alum has found a lunar rover that’s been lost for 40 years. On November 17, 1970, Lunokhod 1 rolled off a ramp from the Russian spacecraft Luna 17 and became the first remote-controlled robot to land on another world. The eight-wheeled vehicle, about the size of a riding mower, explored Mare Imbrium, the Sea of Rains, covering 10.5 kilometers and traveling for 322 days before its handlers lost contact with it. The second Lunokhod (Russian for “moonwalker”) landed on January 15, 1973, and covered 37 kilometers over four months before it overheated.
The twin Lunokhods had French-built laser reflectors on their backs, and since Lunokhod 2’s exact location was known, scientists have been shooting laser pulses at it to measure the distance to the moon with extreme accuracy. On the other hand, Lunokhod 1’s coordinates were known only to within five kilometers—to hit it with laser pulses, you would need to know where it is within better than 100 meters. “It’s good enough to put a push-pin into a map, but not nearly good enough to make a laser search likely to succeed,” says Tom Murphy (MS ’97, PhD ’00), associate professor of physics at UC San Diego and the principal investigator for the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO).
The project is recording every tilt, tip, and wobble of the moon in order to gauge its precise orbit and test Einstein’s theory of gravity, general relativity. APOLLO uses the 3.5-meter telescope at the Apache Point Observatory in New Mexico to shoot lasers at the Lunokhod reflectors or one of the other three reflectors planted on the surface by Apollos 11, 14, and 15. By timing how long it takes the laser pulse to return to the telescope, researchers know exactly how far it is to that point on the moon’s surface. Each reflector consists of an array of three mutually perpendicular mirror segments arranged like the inner corner of a cube. These so-called corner reflectors bounce the light beam directly back toward its source, regardless of its direction. Apollos 11 and 14 have 100 reflectors in their arrays, Apollo 15 has 300, and the Lunokhods have 14 larger ones.
Although they had four targets, Murphy and his colleagues wanted to find Lunokhod 1 because it sits nearer to the edge of the lunar disk. The moon’s axial tilt and precession cause it to wobble, and the outer part of the disk—called the limb—moves the most toward or away from Earth, which is what laser ranging measures well. Figuring out the limb’s motion would thus give a more accurate measurement of the total wobble. Since the team had a rough guess as to where Lunokhod 1 was, give or take five kilometers, they hoped to hit the rover’s mirrors with laser pulses. The laser beam, which leaves the telescope about three meters wide, stretches to two kilometers by the time it hits the moon. Still, nothing ever came back. “It almost seemed like a waste of telescope time,” Murphy says.
Meanwhile, NASA’s Lunar Reconnaissance Orbiter (LRO) had been snapping shots, at a resolution of one meter, of all the lunar-landing sites, from those of Apollo to those of the Russian Luna missions. In March, LRO took a picture of the area around Luna 17’s landing site, and spotted the missing rover. It was four kilometers from where Murphy and his team thought it was, and without LRO’s help, they would’ve never had a chance to find it, he says. LRO was able to pinpoint the rover’s coordinates to within 100 meters, giving APOLLO a target to work with.
Hitting a mirror with a reflective area of 489 square centimeters from 384,400 kilometers away is like trying to hit a grain of rice in New York City from Los Angeles. The researchers shoot 20 pulses per second, and with every pulse, they blast 1017 photons at the target. On a good night, maybe one photon per pulse bounces back and reaches the telescope. Because the signal is so weak, the detector has to amplify every photon it receives. As a result, it is only turned on for 100 nanoseconds per pulse, to avoid picking up background photons that would flood out the signal. Since light travels at almost 300,000,000 meters per second, this means the distance to Lunokhod 1 had to be known to within about 15 meters, although the team was later able to improve their technique and widen the window to 90 meters. Using LRO’s altimeter, called LOLA, the researchers determined the elevation of the Sea of Rains to within five meters, and on April 22, when they started firing, photons came piling in—the first time any signal has come from Lunokhod 1 in four decades.
Lunokhod 1 was so reflective that it shocked Murphy and his team. After sending about 10,000 pulses, the team had gotten about 2,000 photons back—a bounty compared with the best-ever 750 photons that Lunokhod 2 had returned from 5,000 pulses. Overall, Lunokhod 1 is five times brighter than its twin. Since the Lunokhods are identical, and one would expect both mirrors to have endured similar degradation from dust and tiny meteorites, why the twin rovers are so different is a mystery.
The newfound rover also outshines the mirrors left by Apollo 11 and 14, making it the second brightest reflector on the moon. “Its position near the limb, combined with the fact that it’s so strong, means that it will become, after Apollo 15, our most valuable target,” Murphy says. APOLLO can measure distances to within a millimeter, and the team is now trying to pin down Lunokhod 1’s location to that degree of accuracy. This will take about a year, after which they’ll be able to get equally accurate numbers for how the moon wobbles, leading to a better fix on the center of the moon’s mass and therefore the shape of its orbit. Isaac Newton’s 300-year-old equations describe orbits quite well, but when you zoom in to a scale of around five meters, general relativity gives different numbers. APOLLO’s data will test Einstein’s theory to an accuracy of one part in 10,000.
Other than testing general relativity, knowing the precise motions of the moon will also help scientists glean details about the moon’s interior structure and composition. “Imagine that you walk along a sidewalk and run into a trash can,” Murphy explains. “If the can falls over, it’s empty. If it wobbles, it’s full of stuff.” Scientists still don’t know for sure whether the lunar core is entirely liquid or solid, or a combination of both, nor do they fully understand the interaction between the core and the mantle. Knowing more about lunar interior structure will tell us more about how the moon and solar system formed—and ultimately, how we all came to be. —MW
The final location of the Lunokhod 1 rover.
