Indian Moon landing followed by radio telescope

This year 2019 is a busy year for lunar exploration. On January 3, China successfully landed a probe and rover on the backside of the Moon, and on April 11, Israel attempted to land Beresheet, a small lander. Unfortunately that landing failed, as we observed with the Dwingeloo radio telescope.

On September 6, India was the third country to attempt a lunar landing this year. The Chandrayaan-2 mission included the Vikram lander, which was aimed to land near the lunar South Pole. Fortunately, the Moon was above the horizon, and the Dwingeloo radio telescope was ready to track the landing attempt. Simon Bijlsma, Michel Arts, Tammo Jan Dijkema and Cees Bassa used the 13 cm horn antenna to listen to radio transmissions from the Vikram lander at 2284 MHz.

The Dwingeloo signal chain was tested around 18:20 UTC (20:20 CEST), just before the Vikram lander disappeared behind the Moon for its final orbit. At 19:18 UTC, as predicted, the lander reappeared from behind the Moon and quickly got in contact with the Madrid Deep Space Network station; the radio signals received by the Dwingeloo telescope showed that Vikram locked to Madrid and began downlinking telemetry.

The landing burn started at 20:08 UTC when Vikram was at 30 kilometers altitude. The effect of the burn was immediately visible as a sharp kink in the Doppler curve, shortly followed by the announcements from the Indian livestream, which had a delay of about half a minute. The landing burn consisted of two phases: a ten minutes long rough braking phase to reduce the velocity as quickly as possible, followed by a two minutes fine braking phase to reduce errors in the trajectory. With the start of this fine braking phase at 20:18 UTC, when Vikram was at 7 kilometers altitude, the Doppler curve as received by the Dwingeloo telescope started to show unexpected small oscillations, which grew during the next two minutes, with the signal disappearing at 20:20UTC, three minutes before Vikram was planned to land on the Moon.

The oscillations in the Doppler curve, and the loss of signal three minutes before the expected landing time, suggests that Vikram had a problem during the final phases of the descent, and probably crashed onto the lunar surface. It is not clear how hard Vikram crashed and if the lander survived. About a day later the Indian Space Research Organization (ISRO) managed to image the lander on the lunar surface using the orbiter of the Chandrayaan-2 mission. So far ISRO has not managed to communicate with the lander.