Chandrayaan 3: Canadian astronaut explains why Vikram took ‘2 descent pauses’ just before Moon landing

Chandrayaan-3's successful touchdown on the south pole of the Moon on Wednesday – just days after Russia’s Luna-25 which was aiming for the same lunar region spun into an uncontrolled orbit and crashed – has captured the attention and admiration of the scientific community.

Leading scientists and experts said this monumental accomplishment not only marks India's indelible imprint on lunar exploration but also demonstrates the prowess of human collaboration, determination, and cutting-edge technology.

Chris Hadfield, a Canadian retired astronaut, took to X (formally Twitter) to share how ISRO's Vikram lander took “two descent pauses” to let the cameras find a flat spot on the lunar surface right before its historic landing.

“This is what landing on the Moon looks like - so many potholes to avoid! Note the 2 descent pauses to let the cameras find a flat spot, before success. Well done @isro!,” Hadfield wrote sharing a video clip of the lander.

How Vikram landing was executed

Chandrayaan-3’s descent started, as planned, at 5.45pm. The entire landing process largely comprised seven steps, with one additional step at the end that marks the release of the Pragyan rover.

During this 18-minute-long landing procedure, Chandrayaan would drop 30km in altitude, and it would fluctuate between speeds of 5,760 km per hour, to a near-still hovering at around 150 metres from the surface, to finally dropping down.

The first phase of the Chandrayaan-3’s descent was a phase called “rough braking”. As it was orbiting the Moon at an altitude of 30km, and around 750km away from the landing spot, all four main engines of the craft were activated, plummeting it down towards the lunar surface. In the next 11 minutes, Chandrayaan dropped nearly 23km and reduced around 4,500 kmph of horizontal speed (at which it was headed to the landing spot).

In the second phase, where altitude dropped from 7.4km to 6.8km, eight smaller thrusters fired on the spacecraft, tilting its orientation from 90° to 59° – giving it the ability to photograph the surface and identify its final landing spot.

Still barrelling towards the landing location at over 1,300kmph, the spacecraft entered its third stage, with the main rockets firing once again to reduce horizontal speed.

Meanwhile, the small thrusters worked to bring the orientation of the Vikram lander to near-straight. During this stage, the altitude of the craft dropped from 6.8km to 800 metres.

Stage four, known as “fine braking” stage, is where Chandrayaan-2 had struggled. As altitude drops from 800 metres and 150 metres, the craft uses its cameras to scrutinise an obstruction-free path to reach the selected spot to land – in the case of Chandrayaan-3, this was a 4km by 2.5km patch of highland between the Manzinus and Boguslawsky craters. Four years ago, the engines of Chandrayaan-2 ended up providing slightly more thrust than needed due to an error, which led to the craft to spinning around this time. Another issue with Chandrayaan-2 was that its target landing zone was much smaller – just 500m by 500m, giving the spacecraft very little room for error in final manoeuvre.

The Isro chief said that additions of new world-class sensors on Chandrayaan-3 helped them overcome the obstacles they struggled with the last time.

“The technology we have deployed in Chandrayaan-3 is no less complex and advanced than any other technology that goes to the Moon [by other countries]. We have the best of the sensors in the world, and we have used them in Chandrayaan-3. One of the main differences between Chandrayaan-2 and Chandrayaan-3, is an instrument has been added that is called laser doppler velocimeter. This is a world-class instrument developed by one of the labs of Isro and it is capable of measuring minute changes in velocity,” Somanath said.

Stage five was when most scientists could feel they were within seconds of history. In this stage, Chandrayaan successfully managed to drop to an altitude of 150 metres and then hovered for about half a minute. During this hover, it was able to make final adjustments and deviated to a slightly safer location to land.

In stages six and seven, the craft dropped to an altitude of 10 metres, from where thrusters powered down, dropping the lander on the lunar ground. In doing so, ISRO chairman S Somanath said Chandrayaan-3 managed a far safer speed than they were prepared for.

“We were able to achieve most of the optimal conditions required for landing. The final landing velocity we achieved was far less than 2 metres per second (around 7km per hour), which gives us a lot of confidence that the health of the craft will be very good. This also tells us that we will be able to roll out Pragyan and conduct our experiments as planned.”