It’s indeed Rocket Science!! Use of gravity to propel Chandrayaan-2 spacecraft for over 3 lac kms

Bengaluru, Sep 06: In around six weeks since the launch on July 22, Chandrayaan-2 has traveled close to 384,400 km and ISRO's moon lander 'Vikram' is all set to make a soft-landing near the Lunar South Pole at 1:55 am IST on 7 September. On September 2, the lander 'Vikram' separated from the orbiter and entered an orbit of 100 km x 30 km around the Moon. Making Vikram soft land vertically (Upright) on pads of four legs is technically the most complex part of the entire mission.

How did a system weighing 3,877 kg travel through space for over 3 lakh kms and what strategy did ISRO use to make it happen? Surely, the Chandrayaan-2 spacecraft did not head straight for the Moon by burning its engines all the way for 384,400 kms. That would have been impossible as it is not possible to carry so much fuel to keep the engine burning for over 3 lakh kilometers.

ISRO used series of orbital maneuvers to attain this. Upon launch on July 22, the Chandrayaan-2 craft was injected in an elliptical orbit of 169.7 x 45,475 km around earth. In space missions and astronomy parlance, the nearest point to Earth in an elliptical orbit is called perigee while the farthest is called apogee. Once in the elliptical orbit, the spacecraft's engines fired in a planned manner and performed multiple orbit-raising maneuvers.

The first earth bound orbit raising was done on July 24, when the Chandrayaan-2 spacecraft's onboard propulsion system was fired for 48 seconds and it entered new orbit of 230 X 45163 km. After that four more orbit-raising maneuvers were done until it took the craft within the Moon's gravitational influence.

Once Chandrayaan 2 was within the Moon's gravitational influence, the spacecraft fired its engines which gave it a thrust to break free from earth's orbit. When thrusters helped it escape earth's gravitational influence, the craft was hurtled in such a manner that it came within the Moon's gravitational influence. Once Moon's gravity captures it, the craft would begin orbiting the Moon. This is the basic theory behind spacecraft having traveled lakhs of kilometers with minimal fuel spent.

Before being hurtled towards Moon's orbit, the spacecraft went around the earth five times. It must be noted that no fuel is needed to make Chandrayaan-2 craft revolve around earth (in a given orbit), it does so due to the earth's gravitational pull. Fuel is spent only when thrusters are activated to make it jump from one orbit to another.

Chandrayaan-2 spent nearly 23 days revolving around earth, and on August 14 left earth's orbit when ISRO put the spacecraft into a lunar transfer trajectory. Precision and timing to make spacecraft break free from earth's orbit is of utmost importance. It was done at a precise moment so that craft broke free from earth's gravity and the momentum hurtled it towards the moon.

Since space is a vacuum and a body in motion continues to remain in motion, once the craft is free from earth's gravitation pull, the momentum gained during orbit revolution around the earth propels it towards the moon.

Once Chandrayaan 2 is within the Moon's gravitational influence, and getting closer, the orbiter fired its engines in a direction opposite to its motion - like applying the brakes. This manoeuvre ensured that the craft was captured by Moon's gravity. Chandrayaan 2 was then in an elliptical orbit around the Moon. The closest point of the orbit was 100 km from the lunar surface and the farthest point was several thousand kilometres away.

Chandrayaan-2 then spent the next few days adjusting its orbit around the moon and lowered itself steadily to a circular path of 100 km passing over the moon's poles.This is the final orbit for the rest of its mission. On September 2, Vikram, which also houses rover Pragyan, separated from the orbiter. For its descent, Vikram's onboard propultion would be used to slow it down. It will touch down near the South Pole of the moon at 1:55 am IST on 7 September. The rover Pragyan will then be deployed on the lunar surface by 5:55 am IST the same day.

ISRO is essentially making use of gravitational pulls of celestial bodies (in this case earth and Moon) to make its spacecraft traverse lakhs of kilometres in space.