This is Why Vikram Lander Crash-landed on the Moon. ISRO Answers to Government of India. Chandrayan 2 Update!

  This visualization shows how Chandrayaan-2's Vikram lander planned to land on the moon.

   (Image: © ISRO)

Chandrayan 2 was launched on 22nd July'19 on heavy lifting rocket GSLV Mark III which weights 6,40,000 kgs. It was the most powerful rocket India has ever built. Once the rocket has palced the Chandrayan 2 unit (Orbiter, Lander and Rover) into parking orbit, multiple orbit-raising maneuvers were performed from 24th July to 6th August, each time raising the orbit to get close to the moon. We can send a spacecraft on a direct path to the moon, but it requires a powerful rocket which would be expensive. Once a satellite or a spacecraft is placed into an orbit around a planet or moon, it almost doesn't require any energy to stay in the orbit. It will orbit for thousands of years. Once, the Chandrayan 2 was put it into parking orbit, scientists have performed 5 orbit-raising maneuvers around the earth using rocket boosters onboad to raise it into highly eccentric orbits. On 13th August, the spacecraft was injected into moon's trajectory which was called "Trans Lunar Injection" as you can see in the picture below.

On 20th August, when the spacecraft was near the moon at an altitude of around 100 km above the moon, it was inserted into lunar orbit. A series of 5 orbit-lowering maneuvers were performed on Chandrayan 2 from 20th August to 1st Sep to lower the altitude. On 2nd September, Vikram lander seperated from the Orbiter and started lowering it's altitude on 2 deorbiting maneuvers by 6th Sep. At this stage, Vikram lander was travelling at a speed of around 27,000 kmph. On 7th September, at 1:40 am the Vikram lander started it's powered descent to soft land on the south pole of the Moon.  From 30 km altitude above the moon, the lander had to reduce it's speed from 6058 kmph to 7kmph in just 15 minutes. The planned landing was at 1:55 am on 7th Sep.

How Vikram Lander Reduces it's Speed?

Vikram lander was travelling at speed of 27,000 kmph as of 6th September. Vikram lander had 4 thrusters (booster rockets) on the backside which fire in the direction of the motion to reduce the speed. According to Newton's 3rd law, every action has equal and opposite reaction. So when you fire booster rockets on-board spacecraft in the direction of the motion, the spacecraft slows down. The trick here is that all the four thrusters has to work in unison. Everything went well until 2km above the moon and then there was no signal. There was a silence in the control room. You could see the trajectory of the vikram lander in the below picture from the ISRO control room. The communication was broken at 2 km above the moon.

What went wrong? Why Vikram Crash-landed?

The first phase of the descent went well where Vikram lander lowered it's altitude from 30 km to 7.4 km above the moon. This is called Rough-Braking-Phase where lander reduced it's speed from  6058 kmph to 525 kmph. Then the Fine-Braking-Phase started for soft landing. During this phase the lander had to reduce it's speed from 525 kmph to 7kmph. But at the start of the fine braking phase the speed of lander was lower than designed parameters. Due to this difference in the values, the conditions were different for the software due to which Vikram deviated from its path and crash-landed 500 meters of the designated landing site. ISRO acknowledged in a letter to Loksabha that braking thrusters were the problem. 

However, softlanding was only a technological demonstration. But all 8 state-of-the-art science instruments on the orbiter. The mission is 96% successful except softlanding. After a series of meetings over the last few weeks, ISRO said they are planning to land on the moon by Nov'2020. But this time, there will be only lander and rover as the orbiter is in good shape and now orbiting the Moon.

Sreekanth Panjala

Science Popularizer | Writer