Two unmanned probes lost in space have been located by NASA's Jet Propulsion Laboratory in Pasadena, California. Too small to be seen with optical telescopes, NASA's Lunar Reconnaissance Orbiter (LRO) and the Indian Space Research Organization's Chandrayaan-1 spacecraft were found by ground-based radar stations using a pioneering radar technique that could help in planning future missions to the Moon.

According to JPL principal investigator Marina Brozovic the experimental hunt for the two derelict spacecraft involved two levels of detective work. The first spacecraft found was the LRO, which is still active and is transmitting location information back to mission control in Pasadena, so it wasn't actually "lost" so much as considered so for test purposes. That may seem a bit like cheating, but it's a common engineering practice in testing to eliminate as many variables as possible before moving on to the more difficult levels.

The second spacecraft, Chandrayaan-1, went silent in 2009 and though the specifications of its orbit were well known, finding it was another matter. The orbiter is a cube about five ft (1.5 m) on each side, which makes it a very tricky target. Bouncing a radar beam off the Moon is easy and was achieved during Project Diana in 1946 by the US Army Signal Corps, and asteroids are often tracked by radar today, but finding a satellite smaller than a Mini Cooper is another thing entirely.

Computer-generated image depicting the Chandrayaan-1's location at time it was detected by the Goldstone Solar System radar(Credit: NASA/JPL/Caltech)

Worse, the Moon itself complicated the tasks because of its uneven gravitational field. These are caused by regions of high density, called mascons, under the lunar surface, and can act like shifting currents do on a ship to send orbiting satellites off course as they revolve around the Moon. According to the best calculations, Chandrayaan-1 was still in lunar orbit at an altitude of about 124 mi (200 km) with a period of about two hours and eight minutes, but exactly where was unknown, so it was officially classified as lost.

Using the 70-m (230-ft) antenna at NASA's Goldstone Deep Space Communications Complex in California, the JPL team fired a tight, high-powered radar beam at the Moon that was strong enough to send back meaningful signals across 237,000 mi (380,000 km) of space. These reflected signals were then captured by the 100-m (330-ft) Green Bank Telescope in West Virginia.

To improve the odds, the JPL team didn't just aim the radar beam anywhere. Chandrayaan-1 was last seen in a polar orbit and odds were that it was still in the vicinity, so on July 2, 2016, the beam was aimed 100 mi (160 km) above the Moon's north pole. During the observations, an object crossed the beam twice in four hours, indicating that this was Chandrayaan-1. This was confirmed by seven more tracks over three months as well as independent observations using the more powerful radar of the Arecibo Observatory in Puerto Rico.

Radar imagery acquired of the Chandrayaan-1 spacecraft as it flew over the moon's south pole on July 3, 2016(Credit: NASA/JPL/Caltech)

"It turns out that we needed to shift the location of Chandrayaan-1 by about 180 degrees, or half a cycle from the old orbital estimates from 2009," says Ryan Park, the manager of JPL's Solar System Dynamics group.""But otherwise, Chandrayaan-1's orbit still had the shape and alignment that we expected."

The JPL team says that tracking of the two spacecraft by ground radar opens up new possibilities for lunar exploration with such stations providing future missions to the Moon with a new tool to asses the danger of collisions and for an extra level of safety for spacecraft that experience issues with navigation or communication.

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