The Chandrayaan 2 mission will be classified as a partial success. The most ambitious component– a soft landing in an unknown, un-surveyed spot followed by exploration via a rover vehicle – has failed, for reasons unknown at time of writing. But the orbiter is functioning fine, and it will continue to carry out its scientific experiments.
In many respects, a soft landing on the moon is a very challenging task. Any soft landing requires the lander to gradually lose speed with respect to the moon as it comes closer to the surface. This requires tricky calculation since the moon has much lower gravity and no atmosphere to speak of.
That’s only part of the challenge. Much of the moon is covered with a thick layer of dust and it’s not easy to figure out how deep that dust is. A lander could sink metres under the surface. Picking a spot where there is firm ground is, in itself, quite hard.
It’s even harder to do this on the uncharted “far side”. The ISRO mission deliberately chose a spot not studied by earlier missions. Think of the moon as a ball, which orbits the Earth once every 27.3 days. That ball also rotates, turning around in almost exactly the same period of 27 days. This tidal locking means that it always presents the same side to an Earth-bound observer and just about 59 per cent of the moon can be observed from the Earth.
The Chandrayaan 2 mission aimed to land at the Lunar South Pole on the far side.
That’s an area we know very little about. This is precisely why it was chosen. The plan was for the Vikram Lander-Pragyan Rover combination to survey local rocks, study their chemical composition, and above all, look for water.
The last news before the communication link broke showed that Vikram was moving at around 50 metres/ second at 2.1 km above the surface. A successful landing as planned, would have required speed reduction to about 2 metres/ second in the final touchdown.
The first phase — “rough braking” — to reduce speed from 1680 m/s seemed to go well. But the next fine braking phase to take it down to near ground-level and cut speed to 2 m/sec is when communication broke down. The breakdown suggests that this phase didn’t complete as scheduled and the lander crashed.
The orbiter may be able to see Vikram at some stage or perhaps, data analysis will throw light on what happened. ISRO must go back to the drawing board and figure out what went wrong before it makes another attempt at a moon mission.
There will be a next attempt at some stage for sure, though we can’t say when. Meanwhile, the orbiter continues to orbit the moon in a near-circular orbit about 100 kms above the surface. Over the next year, the orbiter will carry out the surveys and experiments it was designed for.
The orbiter has eight scientific instruments (“payloads”) which will be used to map the lunar surface and study its atmosphere. This includes terrain mapping cameras, an x-ray spectrometer to search for chemical elements, an x-ray monitor to study solar radiation, high-resolution cameras to zoom in and map the surface, an infrared spectrometer to look for water and minerals, radar for depth and high-resolution mapping, a tool to map the composition of the exosphere (upper atmosphere) and an instrument that uses dual-frequency signals to study electron density in the lunar ionosphere.
This suite of experiments will certainly add to the datum of knowledge about Earth’s largest satellite. The orbiter has a designed operating life of one year, so we should see updates coming in for quite a while.
The Vikram lander had a seismometer designed to log moon-quakes, a gauge to measure the thermal properties of the lunar surface, and other instruments to map the atmosphere (which is very thin), as well as a mirror-instrument (a laser retroreflector) which was to very precisely measure the distance between Earth and Luna. Unfortunately those will have to be written off.
It’s important to note that the odds were against anybody pulling off a soft landing at the first attempt. Just three nations have managed soft lunar landings and an Israeli mission failed earlier this year. ISRO should be proud of what it has achieved and this will surely aid the next mission.Twitter: @devangshudatta