The moon is the final resting ground for scads of landed and crashed spacecraft, many of which have been pinpointed recently by sleuthing scientists.
Using observations by NASA's sharp-eyed Lunar Reconnaissance Orbiter, for example, researchers have located and imaged Apollo moon landing leftovers, old Soviet-era spacecraft and, more recently, the impact locales of NASA's twin Grail spacecraft that were deliberately driven into a mountain near the moon's north pole.
But the search is ongoing to find the exact location of several pioneering moon landers.
"We are still looking for [the Soviet Union's] Luna 9 and 13," said Jeff Plescia, a space scientist at the The Johns Hopkins University's Applied Physics Laboratory in Laurel, Md.
"Those were the small 'beach ball' shaped spacecraft," Plescia told SPACE.com "The beach ball might be hard to find, but it made a descent on a larger vehicle which then popped the beach ball off."
Plescia said he had assumed that it would be possible to find the landing sites of Luna 9 and 13 by spotting albedo marks — a change in the lunar surface brightness made by their descent engines.
Plescia is joined in the hunt by Mark Robinson of Arizona State University, principal investigator for the Lunar Reconnaissance Orbiter Camera, or LROC.
"We've both looked, but no luck so far." Plescia said.
Scientists hope to find several other lost lunar spacecraft, including NASA's Surveyor 4. Controllers lost contact with this unmanned probe during its descent to the moon on July 14, 1967.
"There are two options," Plescia said. "It completely failed and then just crashed, which would have formed a crater, or it just lost communications and continued to operate and landed. In the latter case, the spacecraft would be on the surface in one piece. The two potential sites have different locations. I made some preliminary searches a while ago, but need to finish it up."
Also on Plescia's to-do list is searching for Luna 18, a 1971 Soviet sample-return mission that failed. Again, communications were lost; it may have landed safely or crashed.
Craters from various fallen orbiters are possible targets as well. The holes gouged out by older missions may prove impossible to find. But coordinates exist for probes such as Europe's SMART 1 and Japan's Kaguya, but no serious search has been mounted yet, Plescia said.
Why all the focus on moon junk?
"There may be several kinds of value here," said Philip Stooke, associate professor in the Department of Geography at the University of Western Ontario in Canada. "One is a detective story," he said, stressing that images may help resolve the fates of failed spacecraft.
A second value has to do with protecting "heritage" sites, especially if the teams participating in the Google Lunar X Prize — which has spurred a privately funded robotic race to the moon — get to some of these sites.