Prakash Chandra
Had India’s first lunar lander, Vikram, successfully soft-landed on the Moon, it would have been icing on the cake for the Indian Space Research Organisation (ISRO)’s 50th anniversary celebrations. However, Vikram lost contact with mission control just minutes away from touch-down on the lunar surface and, although it was later located from orbit, its revival is unlikely. There is no instance in the history of spaceflight when an unmanned spacecraft that malfunctioned while landing on another world was heard from again.
The lunar surface, in particular, is strewn with the remains of crashed unmanned probes. The Soviet Union’s Luna 2 (the first spacecraft to reach the Moon), launched September 12, 1959, flew a direct path to the Moon, its journey taking just two days. On reaching the Moon, without any thrusters to slow it down, it slammed into the lunar surface. Over the years, four Luna missions, two US Surveyor landers and four Ranger probes, India’s Chandrayaan 1 impactor and, more recently, the Israeli Beresheet lander found their graveyard on the Moon.
The failure-analysis committee set up by ISRO to find out what went wrong with Vikram during the final moments of its descent “will investigate the propulsion technology that controlled Vikram’s landing pattern and the navigation system meant to identify the best landing site,” said a top ISRO official. Since ISRO developed these advanced technologies only recently, he said, a “faulty thruster” might have possibly pitched Vikram upside down momentarily and caused an increase in velocity in its descent. “This could have made the spacecraft deviate from the curved band of lunar surface within which it should have landed,” he added.
Mechanical failure aside, the lunar environment is still a mystery to scientists and several other factors could have led to Vikram’s ‘hard landing’. Despite advances in space technology, very little has changed in the 60 years since Luna 2, and remote controlling an autonomous spacecraft to soft-land on the Moon involves reckoning with many unknowns.
During simulations on Earth, engineers try to second-guess these uncertainties to obtain as precise a projection as possible of a spacecraft’s landing sequences. These same manoeuvres, however, could face unexpected challenges on the Moon that cannot be predicted or simulated on Earth. For instance, closer to the lunar surface, the Moon’s powdery regolith could ‘blind’ a lander’s navigation sensors to the presence of craters or rocks.
Then there is the Moon’s irregular gravitational field, which is a clear and present danger to spacecraft. Concentrations of mass called ‘mascon’ underneath some of the large lunar basins exert a stronger gravity pull on probes, potentially perturbing their trajectories. It is possible that Vikram descended near — or even over — a mascon and the sudden change in gravity field hastened its plunge to the surface.
Indeed, such uncertainties hold true for landing on any celestial body. Twenty years ago, NASA’s Mars Polar Lander disappeared as it was trying to set down near the Red Planet’s south pole. Investigators later found that when the craft’s landing legs unfolded, instruments misread the vibrations as the probe touching down and shut off the engine, which sent the lander plummeting to its doom. This could have been avoided if the computer programme for the spacecraft’s landing had included instructions for sensors to restart the engine if the probe was found to be descending too fast.
Space engineers learn from their failures and are getting better at orbital mechanics, even landing probes successfully on fast moving asteroids. Earlier this year, the Japan Aerospace Exploration Agency (JAXA)’s Hayabusa 2, in a remarkable piece of remote-controlled navigation, rendezvoused with an asteroid, landed on it, drilled its surface to collect samples and then launched Earthwards again — all this with the duo hurtling through space at 30 km per second! This is comparable to something like, say, a fisherman in Kochi (in Kerala) using the maths of probability to snap up fish from a lake in Connecticut (in the US) without using a line.
The good news for ISRO is that in spite of losing Vikram, Chandrayaan 2’s orbiter is safely running rings around the Moon. Thanks to the fuel the spacecraft managed to save on its way to the Moon, the orbiter is now expected to be in lunar orbit for up to seven years. This will help the mission do a lot of science and its data will help not just ISRO but other international efforts that are looking to return people to the Moon. NASA has already acknowledged that data from Chandrayaan 2 would come in handy for Artemis, which plans to land astronauts (including the first female moonwalker) on the lunar south pole.
ISRO’s next Moon-shot will be a collaborative effort with JAXA scheduled for 2024. It will use JAXA’s cutting-edge technology called SLIM (Smart Lander for Investigation of Moon) to soft-land on the Moon. Although JAXA is a leader in space navigation, data from Chandrayaan 2 is expected to provide vital inputs for the mission to chart precision imagery of the lunar surface. With several exciting missions to the Sun, Venus and asteroids lined up on the horizon, ISRO can look forward to doing bold science again.
Prakash Chandra is a science writer. Views are personal.
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