‘Lost’ Chandrayaan-1 orbiting Moon: NASA

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, even with the world’s most powerful radars.

Chandrayaan-1 proved the perfect target for demonstrating the capability of this technique.

To find a spacecraft 380,000 km away, JPL’s team used the 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.

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, 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.

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