Chandrayaan-3. Wishing India’s moon lander a happy touchdown

In the wee hours of September 7, 2019, India’s moon lander Vikram met its end on the lunar surface, hurtling down at 58 m per second when it was supposed to descend gently at 2 m per second.

The disappointment left Dr K Sivan, the then chairman of India’s space agency ISRO, in tears, prompting a consoling hug from Prime Minister Narendra Modi at the mission control room.

Sreedhara Panicker Somanath, ISRO’s current chief, would for sure prefer a pat on the back rather than a comforting hug at the end of the upcoming ₹615-crore Chandrayaan-3 mission.

Landing (not crashing) on the moon is the central objective of ISRO’s planned third lunar exploration mission, slated to launch on July 14. It is the most important of the mission’s three elements of technology demonstration — reaching the lunar orbit, soft-landing a lander, and getting a rover to slide out of it and crawl on the moon surface.

The lander in the previous mission, Chandrayaan-2, failed due to a software glitch, which has since been fixed; ISRO says other precautions have also been taken this time around to ensure a successful landing.

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The lander is slightly heavier this time — 1,752 kg, compared with 1,471 kg previously, despite having only four engines in place of the five earlier. (These engines are meant to provide an upward thrust to the descending lander, in order to slow it down.) The removal of one engine is presumably to offset the added weight of the stronger legs in the new lander. Somanath has said that some redundancies have been built in, to ensure a safe touchdown. The lander has a number of sensors, including an accelerometer, altimeters (Ka-band and laser), Doppler velocimeter, star sensors, inclinometer, touchdown sensor, and a suite of cameras for hazard avoidance and positional knowledge.

The side-mounted solar panels of the Chandrayaan-3 lander are designed to provide more power — 738 W compared with 650 W earlier, though this is not a factor in the landing.

Soft-landing power

Apart from these — and perhaps luck — there is little difference between the Chandrayaan-3 and Chandrayaan-2 lander-rovers. A touchdown on the moon surface on August 23 or 24 would make India the fourth country to demonstrate lunar soft-landing (after the US, the former Soviet Union and China.)

All the scientific instruments of Chandrayaan-3 are the same as those of its predecessor — they will probe the lunar regolith to determine which elements are present in it by burning the mud into a plasma and analysing the plumes; check how heat is conducted on the moon surface near the polar region by drilling a probe 10 cm into the ground; and study the gas and plasma environment of the moon. But the main objective of the mission is to master the technology of soft-landing.

However, there is one crucial difference with respect to an instrument that will not land on the moon but remain with the ‘propulsion module’, the vehicle that will take the lander across the distance between the earth and the moon. The LVM-3 rocket will take the propulsion module plus the lander from the Sriharikota launch station to an elliptical earth orbit of 170 km (nearest to earth) and 36,500 km (farthest from earth). At this point, whatever remains of the LVM-3 rocket (the upper stage with a cryogenic engine, after the other parts have fallen off into the sea) will bid goodbye to the propulsion module and wander off into space. The propulsion module, having thanked the LVM-3 for ferrying it upto there, will circle the earth five times in elliptical loops, each bigger than the previous one, before gathering enough velocity to shoot off on a month-long journey towards the moon.

Earthy spectrum

As it closes in, it will circle the moon in 5-6 elliptical orbits, each time getting closer, until it reaches 100 km above the lunar surface and divests itself of the lander. At this point, an interesting instrument on board the propulsion module will come to life — one that was not part of the Chandrayaan-2 mission.

The Spectropolarimetry of HAbitable Planet Earth (SHAPE) will look at the earth from the moon’s neighbourhood to see what kind of spectrum is generated by the light emerging from earth. Light is affected by the type of gases in an atmosphere, because different elements absorb or deflect different wavelengths of light. So, if you know what the earth spectrum is like, then you can look for similar spectra from exoplanets (planets of other stars) — if they match, you might want to believe that the exoplanet could, like earth, hold life.

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