The 90 degree challenge for Chandrayaan-2

New Delhi: As Chandrayaan-2 prepares for its historic soft landing on the moon on 7 September, one of the biggest challenges for it would be to ensure that its orbit inclination is precisely on the mark, before it attempts touchdown on the moon.

It is for the first time that a space mission is attempting to land near the south pole of the moon, where no space mission has ever been before. The most recent moon mission by China landed near the equator on the far side of the moon, while the one from Israel could not achieve a successful landing.

“The orbit of the spacecraft needs to achieve an inclination of 90 degree, only then it can execute a successful soft landing on the lunar surface on 7 September. Even if there is a slight variation, we could miss the landing at the targeted site,” ISRO chairman Dr K Sivan said after the spacecraft successfully entered the moon’s orbit.

Chandrayaan-2 was injected into the moon’s orbit early on Tuesday and would undertake four more manoeuvres to reach the final orbit of 100kms x 100 kms around the moon. The next crucial step would be performed on September 2, when Lander Vikram would separate from the orbiter. It will carry out a 3-second manoeuvre the next day to test if all systems onboard the Lander are normal. De-orbiting would be done on September 4 to bring down the Lander to an orbit of 35 X 97 kms.

Over the course of next three days, Isro will keep monitoring the functioning of the Lander. At 1:40 am on September 7, it will begin its powered descent. While its coming down, it will screen the landing site and compare it with the Isro images onboard.

“The Lander is autonomous and intelligent. It would hover over surface looking for the earmarked site and decide where to land. If all goes as planned, it should make touchdown near the south pole of the moon between the two craters at 1:55 am. After two hours, the ramp would open for Pragayan Rover to come out and touch the lunar surface,” Dr Sivan said while addressing the media.

However, the Lander Vikram will have to wade through several challenges before it lands on the moon. If the landing takes place on an inclined surface where the inclination of the slope is more than 12 degrees, it could topple.

Isro is using a new technology for soft landing using the five thrusters of the Lander for throttling the engine onboard. The Lander would have to decrease its velocity from 6km/second to zero in a controlled, but autonomous manner. All the image and altitude sensors have to work precisely to allow the Lander to make proper decisions.

Learning from Israel’s failed moon mission this April, Isro has also upgraded the sensor characterization onboard and made the module more autonomous to ensure less ground control to avoid any false decisions.

Lunar dust is another major concern as it could cover the Lander and impair its functions. To avoid this, scientists have automated the system to switch off all the four thrusters during landing with only the central thrusters on. This way, the plumes of dust would not cover the Lander and it can land smoothly.

As Chandrayaan-2 inches closer to the moon, the anxiety levels are building up among the scientists at Isro. The success rate for space missions for soft landing has only been 37% so far. But Isro is confident of pulling it through.

“We have done enough tests, simulations at all levels, including sensor and thruster levels. We are confident because we have done whatever was humanly possible," said Dr Sivan, "but this phase of the mission (soft landing) is new to us."

The landing near the South Pole would be critical, as countries across the world are vying with each other to fly their flags at the site. Chandrayaan-2 would also provide inputs to US-based National Aeronautics and Space Administration (NASA) for Artemis — its upcoming manned moon mission to the south pole of the moon.