The three massive communication facilities spread across the globe aren’t just used for talking with distant spacecraft. They can also be used to find them. Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California have come up with a new way to use interplanetary radar. With it, the scientists found two spacecraft orbiting the moon. One still in the midst of its mission (Lunar Reconnaissance Orbiter). The other, silent for nearly eight years (Chandrayaan-1).
Spotting NASA’s Lunar Reconnaissance Orbiter (LRO) was a piece of cake. The folks from JPL reached out to the satellite’s navigators and gathered the exact orbit data. Armed with this data, the JPL team used NASA’s 230-foot antenna located at the Goldstone Deep Space Communications Complex to send a burst of microwaves 237,000 miles away towards the moon. The radar echoes bounced off the LRO and were received by the even larger 330-foot Green Bank Telescope in West Virginia. Nothing too it.
Now, finding a spacecraft that last sent signals to Earth almost eight years ago isn’t as easy. Besides the obvious problem with the amount of time between receiving a signal, the moon doesn’t make it easier.
Mascons pepper the moon’s surface. These are features were denser material below the surface caused a local bump in gravitational pull. While local, mascons can have a big effect on a spacecraft’s orbit over time.
The JPL team crunched the numbers and figured India’s now silent Chandrayaan-1 is still regularly circling the moon 124 miles above its surface. This particular satellite’s orbit helped narrow down the search. Because it’s in a polar orbit (which also helps with the mascons problem) around the moon, the JPL team could always count on it crossing above the lunar poles on each orbit (once every two hours). Still, finding a satellite about half the size of a tiny smart car would be a challenge.
Last July, the JPL team trained the Goldstone and Green Bank antennas at a location about 100 miles above the moon’s north pole and waited. Sure enough, the radar signature detected an object about the size of Chandrayaan-1 twice over a four-hour span.
Using this data, scientists updated the orbital predictions for the silent satellite.
“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. “But otherwise, Chandrayaan-1’s orbit still had the shape and alignment that we expected.”
Radar echoes from the spacecraft were gathered seven more times over three months and lined up perfectly with the new orbital predictions.
This lunar scavenger hunt isn’t just fun science. It could be applied to future missions to the moon. Imagine if a future spacecraft runs into communications issues. While engineers figure out the problem, this new radar application could keep tabs on it and make sure its orientation and orbit is correct.
As for Chandrayaan-1? It’ll keep circling the moon for the foreseeable future. And next year, Chandrayaan-2 is expected to join it around the moon.