Space is vast, so when some piece of human technology goes missing outside of Earth's atmosphere it's very difficult to find it again. And that task gets especially hard when the object is very small and isn't emitting any kind of signal.
In October 2008, the Indian Space Research Organisation (ISRO) launched its first lunar probe called Chandrayaan-1. It successfully entered a lunar orbit in November 2008, but in August 2009 things started to go wrong. Technical problems began to surface including sensors failing and thermal shielding not working effectively. On August 29, 2009 contact was lost.
Chandrayaan-1 remained lost until now, some eight years later, and ISRO has NASA to thank for finding it again. NASA discovered Chandrayaan-1 maintaining a lunar orbit 124 miles above the surface using a new technological application of interplanetary radar developed by NASA's Jet Propulsion Laboratory.
NASA actually located two spacecraft using this new technique and ground-based radar. The first was the agency's own Lunar Reconnaissance Orbiter (LRO), which was easy to find as the mission's navigators were on hand with precise orbit data. For Chandrayaan-1 however, it was a much more difficult task.
The challenge posed by Chandrayaan-1 was that nobody knew where it was coupled with the fact this is a very small cube spacecraft measuring only 1.5 meters wide. NASA's scientists were looking at the moon from Earth (a distance of 237,000 miles) attempting to find this tiny spec of an object.
Chandrayaan-1 was located using NASA's Goldstone Deep Space Communications Complex in California and a beam of microwaves. The radar echoes bounced back were received by the Green Bank Telescope in West Virginia. The only lead they had to go on was the spacecraft's last known orbit from 2009, which was a polar orbit, so the beam was focused on the moon's north pole hoping the spacecraft would pass by. Sure enough it did, and multiple detections over a three month period allowed NASA to confirm the object definitely is Chandrayaan-1.
It is hoped that this new method of object detection using multiple ground-based radar antennas can be put to good use in the future. As well as being able to detect very small spacecraft, the radar system could collision map the moon before robots or even humans visit there again.