Scientists search for lunar leavings

Search for Lunokhod
One nifty exercise that's now being considered is attempting to bounce a laser beam off a lost-on-the-moon Soviet rover: Lunokhod 1. That eight-wheel robotic lunar buggy is outfitted with laser "retroreflectors" and rolled about within the moon's Sea of Rains for nearly a year after landing in November 1970.

Lunokhod 1 may still have value as a laser ranging target. That's if it can be located precisely.

The Soviet machinery is sitting silent at the most northerly of all landings on the moon to date, "and would be a vital part of understanding the moon's exact rotation for geophysical purposes," Stooke said.

However, the robot's position is uncertain by at least 3 miles (5 kilometers), and stabs at getting laser returns all failed since early in the Lunokhod 1's mission. 

"We don't know if it is still clean or oriented suitably ... if not, there's no hope. But if it is, we still need a good position," Stooke said.

Tantalizing target
"The recovery of Lunokhod 1 is on my wish list," said Jim Williams of the Jet Propulsion Laboratory in Pasadena, Calif. He and fellow investigators are part of a Lunar Laser Ranging data analysis group at JPL.

Williams told Space.com that there are four laser retroreflector arrays on the moon that get ranged: the Apollo 11, 14, and 15 sites, plus the other Russian moonbuggy, Lunokhod 2. "As a result of the ranging, we know those positions to better than one-meter accuracy" or roughly 3 feet, he said.

The arrays of 14 prisms on Lunokhods 1 and 2 were built by the French. The Lunokhod 2 retroreflector has been ranged, so the design works, Williams noted. The Lunokhod would rove during daylight, but was parked at night so that the array was oriented properly for Earth-to-moon laser ranging.

Lunokhod 1 is a tantalizing target, Williams said.

"A few laser observations were reported early in the mobile Lunokhod 1's journey and later after it stopped moving, but those observations have never been publicly distributed or processed with a data analysis program of modern accuracy," Williams explained. "It is unclear whether the effectiveness of the Lunokhod 1 array has been compromised or whether position uncertainty and weak signal conspired to prevent its acquisition by standard ranging stations ... but a modern search is in order."

Apache Point Observatory
The sole U.S. lunar laser-ranging station in the early 1970s tried to range Lunokhod 1, Williams said, but did not find it. 

"The task is more difficult than it sounds," Williams said. "The return signal should be very weak and only a limited spread of distance and angle could be searched ... for a sequence of shots. The position may have been too poor, or the Lunokhod 1 retroreflector may be obscured somehow."

There is a new lunar laser-ranging station now undergoing checkout at Apache Point Observatory in New Mexico.

"Once the new station is operational and returning data, then a Lunokhod 1 search will be practical," Williams said. This new equipment at the facility will be a good instrument for the search, he added, but will still require a major effort.

Location uncertainties
Meanwhile, Stooke is on the lunar lookout trail.

Early Apollo Moon landing craft and the moonwalkers themselves scuffed up areas of the lunar surface. These disturbances are visible in orbital imagery taken by later Apollo missions.

A number of discarded Apollo rocket stages and lunar module ascent stages were tracked to narrow the search area. Those which fell in areas subsequently imaged were identified fairly easily, Stooke reported in his conference paper.

Philip Stooke / Defense Department This imagery from the Pentagon's Clementine probe appears to show the dark impact site of the Ranger 6 spacecraft, indicated by the arrow in the picture.

Ranger 6's impact area may now have been found in images taken by the Defense Department's moon-orbiting Clementine spacecraft in 1994, Stooke stated. On the other hand, many impact craters made by orbiters at the end of their missions — or by failed landers — will be difficult to find because of location uncertainties.

NASA Lunar Orbiter impact sites, for example, are uncertain by hundreds of kilometers and would be very difficult to distinguish from natural fresh impacts.

On the other hand, craters formed by camera-carrying Ranger probes were found easily, because images taken during their plunge toward the lunar terrain allowed impact sites to be predicted. Two of the three successful kamikaze-like Ranger missions imaged their impact points for comparison with later images.

Hiten, a Japanese lunar orbiter, should be located easily using Earth-based tracking data and observation of the impact, Stooke said.

Protection of sites
In his lunar work, Stooke noted that several challenges remain.

Can the site of Russia's Luna 2 impact be located? If it is, will visual observers be vindicated after years of doubt? Can Luna 9 or Surveyor 5 be located at last? Can the craters caused by the hits of spent rocket stages from the Apollo 15, 16 and 17 missions be found?

"The scientific value of this work includes placing lander data in better context, refining seismic results from Apollo [rocket stage] impacts, or planning future visits to old sites for study of old hardware, as Apollo 12 did for Surveyor 3," Stooke says. "If in the future it is decided to confer special designations or protection on these sites, finding them is essential."

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