Ground-based radar technique helps to spot the spacecraft

India’s first lunar probe, Chandrayaan-1, which was considered lost, is still orbiting the moon, NASA scientists have found by using a new ground-based radar technique.

The ISRO lost communication with Chandrayaan-1 on August 29, 2009, almost a year after it was launched on October 22, 2008.

Now, scientists at NASA’s Jet Propulsion Laboratory (JPL) in California have located the spacecraft still circling some 200 km above the lunar surface. “We have been able to detect NASA’s Lunar Reconnaissance Orbiter (LRO) and the Indian Space Research Organisation’s Chandrayaan-1 in lunar orbit with a ground-based radar,” said Marina Brozovic, a radar scientist at JPL and principal investigator for the test project.

“Finding LRO was relatively easy, as we were working with the mission’s navigators and had precise orbit data... Finding Chandrayaan-1 required a bit more detective work because the last contact with the spacecraft was in August of 2009,” said Brozovic. Chandrayaan-1 is very small and cuboid in shape, about 1.5 metres in length on each side. Although the interplanetary radar has been used to observe small asteroids several million miles from the earth, researchers were not certain that an object of this size could be detected as far away as the moon.

To find a spacecraft 380,000 km away, JPL’s team used NASA’s 70-metre antenna at NASA’s Goldstone Deep Space Communications Complex in California to send out a powerful beam of microwaves towards the moon. Then the radar echoes bounced back from lunar orbit were received by the 100-metre Green Bank Telescope in West Virginia.

Finding a derelict spacecraft at lunar distance that has not been tracked for years is tricky because the moon is riddled with mascons (regions with higher-than-average gravitational pull) that can dramatically affect a spacecraft’s orbit over time, and even cause it to have crashed into the moon.

JPL’s orbital calculations indicated that Chandrayaan-1 is still circling some 200 km above the lunar surface, but it was generally considered “lost.”

However, with Chandrayaan-1, the radar team utilised the fact that this spacecraft is in the polar orbit around the moon, so it would always cross above the lunar poles on each orbit.

On July 2 last year, the team pointed Goldstone and Green Bank at a location 160 km above the moon’s north pole and waited to see whether the lost spacecraft crossed the radar beam. 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.

The team used data from the return signal to estimate its velocity and the distance to the target. This information was then used to update the orbital predictions for Chandrayaan-1.

Chandrayaan operated for 312 days, as against the intended two years.