In 2009, a lunar orbiter launched by India went quiet and was never heard from again. Fast-forward eight years and NASA say it's spotted it using an Earth-based radar.
The Indian Space Research Organisation's Chandrayaan-1 orbiter was supposed to spend two years on its mission, but after 312 days its communications systems conked out.
As NASA explains, it couldn't simply extrapolate the little orbiter's last-known orbit because the moon is “riddled with mascons (regions with higher-than-average gravitational pull) that can dramatically affect a spacecraft's orbit over time”.
The agency's orbital calculations suggested, however, that the refrigerator-sized spacecraft was most likely around 200 km from the surface, and because it was in a polar orbit, the scientists knew it would always pass the poles.
NASA scientists therefore beamed microwaves from the 70m dish at the Goldstone Deep Space Communication complex in California, aimed at a point about 160 km above the moon's north pole. The 100m dish at the Green Bank Telescope in West Virginia was used as the receiver.
“Chandrayaan-1 was predicted to complete one orbit around the moon every two hours and eight minutes. Something that had a radar signature of a small spacecraft did cross the beam twice during four hours of observations, and the timings between detections matched the time it would take Chandrayaan-1 to complete one orbit and return to the same position above the moon's pole.”
As a proof-of-concept, the mission scientists had previously used the radar technique to spot NASA's Lunar Reconnaissance Orbiter, a much easier task since that craft's navigators knew precisely where to look.
Finding Chandrayaan-1 isn't merely an intellectual exercise – it's part of NASA's efforts both in asteroid-spotting and lunar exploration.
“Ground-based radars could possibly play a part in future robotic and human missions to the moon, both for a collisional hazard assessment tool and as a safety mechanism for spacecraft that encounter navigation or communication issues”, the NASA statement adds.
Collision hazards are also an issue a long way farther out: there are around 14 spacecraft assumed to be in orbit around Mars, but eight of them failed in one way or another and don't respond to contact.
NASA's Maven spacecraft, which last week had an orbit adjustment to dodge the risk of a collision with Phobos, had a near-miss with the Mars Reconnaissance Orbiter in January 2015. And that's a known hazard.
Since Mars is beyond the reach of Earth-bound radars, NASA wants better protocols so everyone knows where their Mars spacecraft are. ®