When satellites die, their signals fade into the void, leaving them as nothing more than tiny specks of bonded atoms floating in an infinite abyss. After that happens, they can be pretty hard to find. But thanks to a new interplanetary radar technique, NASA just found an old space probe that no one had seen since 2009.
Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California were testing the new system of ground-based microwave radar on the agency’s Lunar Reconnaissance Orbiter, a small robotic spacecraft orbiting the moon and sending back data for the agency’s future manned missions. But while spotting the LRO was cool in and of itself, JPL then pulled off something even more exciting: it found Chandrayaan-1, an Indian Space Research Organization probe that has been dead for well over 7 years, coasting silently in an endless loop around the moon.
“Finding LRO was relatively easy, as we were working with the mission’s navigators and had precise orbit data where it was located,” Marina Brozovic, a radar scientist at JPL and principal investigator for the test project, said in a release. “Finding India’s Chandrayaan-1 required a bit more detective work because the last contact with the spacecraft was in August of 2009.”
Finding small objects — Chandrayaan-1 is a five-foot cube of metal — near the moon is extremely difficult. Sunlight reflecting off the surface of the moon makes optical telescopes pretty much useless, and microwave radar-based systems require operators to point a beam of energy in just the right spot to find what they’re looking for. Essentially, NASA had to shoot a dime out of the air with a rifle, and only knew vaguely where it would be thrown.
Visualization of the little satellite's orbit.
But the scientists at JPL realized they did know where Chandrayaan-1 would be. Though the spacecraft was technically “lost,” in an unpredictable orbit thrown off by the moon’s inconsistent gravitational pull, scientists did know two spots it would most likely pass over: the moon’s two magnetic poles. Chandrayaan-1 was in an orbit over both of the moon’s poles doing 3D mapping and other imaging processes, pursuing the hunch that one of the two poles would have frozen water hidden in its dusty gray plains. The JPL team figured Chandrayaan would do a lap of the moon at least once every two hours and eight minutes, so they pointed the biggest microwave radar they had — a 230-foot antenna at NASA’s Goldstone Deep Space Communications Complex in California — about a 100 miles above the moon’s north pole, and waited. If anything was out there, it would bounce the signal back to the 330-foot Green Bank Telescope in West Virginia. Sure enough, during a four-hour window, something small and reflective bounced back the radar beam twice, and Chandrayaan-1 was lost no more.
“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,” said Ryan Park, the manager of JPL’s Solar System Dynamics group who relayed the good news to the radar team. “But otherwise, Chandrayaan-1’s orbit still had the shape and alignment that we expected.”
Over the next three months, JPL verified the results using an even bigger radar, the National Science Foundation’s Arecibo Observatory in Puerto Rico. It’s a huge step for the future — now that NASA knows how to use this technique, it has the capability to track almost infinitely small objects (relative to the expanse of space) in lunar orbit using ground-based radar. NASA says this will be essential for “collisional hazard assessment” and for helping spacecraft navigate in the event that they lose communication with the ground or their internal navigation systems.
