If all goes according to plan for Chandrayaan-2, its lander Vikram will soft-land in the South Pole Aitken basin, a site of ancient lava flows caused by an asteroid impact that excavated material from its mantle, this September. The lander will then allow the Pragyan rover across a ramp and the rover will move, very slowly, at 1 cm per second, eventually to 500 m. Powered by solar panels, both will have an expected mission life of about 14 earth days, by which time the experiments are expected to be terminated.
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The mission duration, after all the intricate manoeuvring and the around Rs 1,000-crore mission cost, has a reason. The moon takes about 29 days for a full rotation, which means it has half that, around 350 hours, of night. In the harsh, cold temperatures of the lunar night, the scientific instruments and electronics become unusable, batteries cannot charge or discharge, and communication systems fail.
The moon has extremes of both hot and cold temperatures. Near the its equator, daytime temperatures reach 120 degrees Celsius, while at night it is about -130 degrees Celsius. “The temperature falls like a waterfall,” says Ron Creel, a former NASA thermal engineer. Creel worked on the Apollo missions since 1969, particularly the moon buggies of the later J-missions, beginning with Apollo 15 in 1971.
Where Vikram lands, it is going to be -180 degrees Celsius in the night, which is way colder than the coldest night on Antarctica. Most of the solid-state electronics in sensors and cameras stop working well above that temperature, according to Narendra Bhandari, who had started Isro’s planetary sciences and exploration programme around 2001. Bhandari was a key member of the Chandrayaan-1 science team and played an advisory role to complete Chandrayaan-2’s mission readiness.
Only three missions to the moon have survived the lunar night: the Apollo Alsep (Apollo lunar surface experiments package, 1969-77) experiments, the former USSR’s Lunokhods I and II rovers (1971 and 1973, respectively), and China’s Yutu (or Jade Rabbit) rovers (in 2013 and 2019). All of these used radioisotopes to keep warm in the cold lunar night. India, however, and so far, has decided not to use nuclear-powered systems in its space missions.
Apart from lunar night survival, such radioisotope systems allow for space exploration beyond Mars. However, this does not affect Indian Space Research Organisation’s (ISRO’s) planned missions: the Shukrayaan mission around Venus (in 2023), Aditya mission around the sun (2019-20), or, of course, the Gaganyaan (2021). “We can manage all these missions in solar (power),” says Bhandari. It gets increasingly difficult to use solar power from Mars onwards, which is, on an average, 1.5 times the distance the earth is to the sun.
The abbreviated mission life due to the lunar night will be a consideration for the upcoming private players, who have tied up with NASA, for commercial landing services through its Artemis programme. Three companies — Astrobotic, Intuitive Machines and Orbit Beyond — have been contracted to carry NASA payloads. Orbit Beyond’s lander will be developed by the Bengaluru-based Team Indus.
“Eight or 10 earth days of science is somewhat limiting,” says Sharad Bhaskaran, mission director for the Pittsburgh-headquartered Astrobotic. The company’s focus at the moment is on developing its Peregrine lander and completing its first mission, which will not have survive-the-night capability.
But it is finding technologies for the purpose, though not based upon radioisotopes. A reason is risk avoidance – the regulatory issues are not clear for private use of nuclear fuel. “Not knowing if that approval is going to come and ultimately planning entire mission around that uncertainty could be a bit risky,” says Bhaskaran.
Science-journal papers and conferences indicate that ISRO has been looking at passive survivability during the lunar night to provide more mission capability. Design Framework of a Configurable Electrical Power System for Lunar Rover submitted to the IEEE (Institute of Electrical and Electronics Engineers), New Jersey, in 2017, looks at an electronics system where the power requirement is 50 watts and the rover weight 25 kg, more or less corresponding to the Pragyan rover. At a November 2018 conference in Maryland, US, the power systems group at ISRO presented on passive survivability of lithium-ion batteries. ISRO did not respond to confirm whether any of this will be tested in the Chandrayaan-2 mission.
Such an approach, where the rover or lander hibernates or is in low-power mode during the lunar night, has been used most recently by China for its Yutu and Yutu-2rovers. But that, too, had a nuclear-power source.
Bhandari, meanwhile, does think the ISRO will try to “wake up” the Indian rover and lander after the lunar night. Neither he nor others actually believe it will work though, particularly since there don’t appear to be any heating mechanisms for even passive survival.
By waking up, Bhandari means warming and bringing them in the range of working temperature and re-establishing contact. The machines have to be tested after the freezing and heating cycle. Often, “they don’t behave in the normal way,” he says. This was certainly true of the Yutu or Jade Rabbit (2013), which stopped moving after the second lunar night.
Moon dust is the other main challenge to long-term functioning of machines. As the moon does not have weathering, the dust particles — that Creel calls “insidious” — are sharp. It is like a beach all over and the sand gets through. A 2005 Nasa technical report states how it “obscured vision on landing, clogged mechanisms, abraded the Extravehicular Mobility Suits, scratched instrument covers, degraded the performance of radiators, compromised seals, irritated the eyes and lungs.”
Precious astronaut time had to be spent on clearing moon dust on the Apollo Lunar Rover Vehicles, whose battery radiators wouldn’t cool down. With dust on them, their solar absorption increased several multiples.
“It’s not like, say, in your car, that if its radiator started to overheat,” says Creel, “you put water in it so that the temperature went down, and you can keep driving.”
Creel believes the dust simply has to be avoided, at the moment, technical solutions to remove it are still not at the mission level of capability. He adds that the dust may be as prevalent on the South lunar pole where Vikram will land.
Of course, the immediate challenge for any moon mission is the landing. The privately-owned Israeli lander, Beresheet, was unable to slow down and crashed on the moon this April. For the Chandrayaan-2 mission, too, it is a perilous descent. Ejected from 100 km above the moon’s surface, the systems will take photographs during the descent from about 30 km onwards to decide exactly where to land — autonomously.
“The prime concern is that it should be smooth, not rocky,” says Bhandari. Considerations like the lander not being highly tilted where it lands come into play, and that it “stands on four legs rather than two”.
Virat Markandeya is a Delhi-based science journalist. This headline appeared in the print edition with the headline ‘The biggest challenge for Chandrayaan-2: Surviving days and nights on the moon’
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