Science & Technology

Chandrayaan 2 in Moon’s orbit, now gets ready for ‘15 minutes of terror’

On September 7, the lander will use its four thrusters and central engine to attempt to soft land on the lunar surface

By Akshit Sangomla
Published: Tuesday 20 August 2019
Chandrayaan 2 has had to change its trajectory from what was originally planned. Photo: ISRO
Chandrayaan 2 has had to change its trajectory from what was originally planned. Photo: ISRO Chandrayaan 2 has had to change its trajectory from what was originally planned. Photo: ISRO

Chandrayaan 2 successfully entered the lunar orbit on August 20, 2019 at 9:02 am. It is currently in a 114 kilometres x 18072 km around the moon.

When the process of the spacecraft’s insertion into lunar orbit took place, the Moon was at its farthest distance from the Earth which means the Earth’s gravitational pull on it was minimum. This made it easy for it to slip into the gravitational influence of the satellite planet.

The process lasted for around 30 minutes. “The manoeuvre that made this feat possible was crucial as we would have lost the craft otherwise,” K Sivan, Indian Space Research Organisation’s (Isro) chairman, told the media at a press conference.

“A higher than expected approach velocity would have bounced off the spacecraft into deep space, while a slow approach would have made the Moon's gravity pull Chandrayaan 2 and crash it on the lunar surface. The approach velocity had to be just right and the altitude over the moon rather precise,” he added.

If the orbit is not what it should be then the subsequent processes of the mission cannot be carried out. Chandrayaan 2 has had to change its trajectory from what was originally planned. This way it will land on the Moon on the designated date and time in spite of the launch getting delayed by a week.

Tough times ahead

But the toughest phases for India’s second lunar mission start now and will end with the first soft landing attempt at the south pole of the Moon on September 7. In the first of these phases, from today (August 20, 2019) till September 1, the spacecraft will reduce its orbit with the help of seven minor manoeuvres done by selective and timely burning of its engine thrusters.

Some of the major manoeuvres will be done on August 21, August 30 and finally on September 1. At the end of this, the spacecraft will be placed in a circular orbit, 100 kms above the Moon’s surface.

The next phase will start on September 2 when Vikram, the lander, will separate from the orbiting spacecraft to begin its five-day journey to the lunar surface. “The focus of our concern will then shift to the lander rather than the orbiter,” said Sivan.

During its journey, Vikram will be on its own and not micro managed by ground control. “This is a learning from the earlier attempts of soft landing on the Moon, only 37 per cent of which have been successful,” said Sivan.

There have been many other improvements in navigation and sensor characterisation technologies. On September 3, a small de-orbit manoeuvre of the lander lasting three seconds will ensure the normalcy of all the onboard systems.

The next day, the lander will reduce its orbit to 97 km x 35 km and then all the systems will be self-checked onboard for the subsequent three days.

Final descent

On September 7, at 1:40 am, Vikram’s power systems will be activated for the final descent. The lander will first reach a position perpendicular to the lunar surface, then it will start capturing images of the Moon with its onboard camera.

It will compare these images with the simulated image that it has carried from Earth to ascertain the exact landing spot.

The landing area needs to have an inclination of less than 12 degrees so the craft does not topple upon landing. A suitable spot has already been chosen by Isro’s engineers.

Once the landing spot is identified, Vikram will use its four thrusters along with a central engine to attempt the soft landing on the Moon. This will take approximately 15 minutes. The crucial bit is that the orbit of the lander in this entire time has to remain at a 90 degree angle to the lunar surface, which is a necessary condition for landing on the poles, according to Sivan.

This is not required in the equatorial regions where soft landings have been attempted before. Sivan called this relatively easy recalling the landing of the Chinese lunar craft — Chang’e 4 — in January 2019 on the far side of the Moon.

Every correction in the orbit of Vikram will come with a penalty of wasted fuel which cannot be then used for other processes. But the lander does carry 60-70 kilogrammes extra fuel for such exigencies.

This challenge has made Sivan call the last leg of Vikram’s journey “15 minutes of terror”. After Vikram lands, Pragyan, the rover, will get activated at 3:55 am and at 4 am touch the lunar surface.

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