As Chandrayaan-3 and Luna 25 prepare to land on Moon, two questions

What determines the landing time of the two spacecraft?

Luna 25 rode on a powerful rocket to reach lunar orbit in just six days after launch on August 10. Chandrayaan-3 took 23 days after launching on July 14, because ISRO still does not have a powerful enough rocket to go directly to lunar orbit. Chandrayaan-3’s circuitous route, however, helped save energy and costs.

Now that both spacecraft are in lunar orbit, Luna 25 has no particular advantage over Chandrayaan-3 that facilitates its early landing. It is not as though Luna 25 can descend to the lunar surface faster than Chandrayaan-3. The choice of the landing date is dictated by other factors.

August 23 is the beginning of daytime on the Moon. One lunar day is equivalent to about 14 days on Earth, when sunlight is continually available. The instruments on Chandrayaan-3 have a life of just one lunar day, or 14 Earth days. That is because they are solar-powered instruments, and require sunlight to remain operational.

The Moon gets extremely cold during night time, well below minus 100 degree Celsius. Electronics not specifically designed to operate at such low temperatures can freeze and become non-functional.

To maximise the time for observations and experiments, it is crucial for Chandrayaan-3 to land at the beginning of the lunar day. If for some reason, it is unable to attempt the landing on August 23, another attempt is supposed to be made the next day. If that too is not feasible, it would wait for one full month — about 29 days —for the lunar day and lunar night to get over.

Simply put, Chandrayaan-3 cannot land before August 23, and would not like to land after August 24.

Luna 25 has no such restrictions. It too is solar powered, but it also has an onboard generator to provide heat and power to the instruments during night time. It has a life of a year, and the choice of its landing date is not dictated by how much Sun is shining on the Moon.

How far apart will the Indian and Russian missions land?

Though it is being said the landings will be near the “South Pole”, the landing sites are not exactly in the polar region on the Moon. The selected site for Chandrayaan-3 is about 68 degree South latitude while that of Luna 25 is closer to 70 degree South.

But these are still much farther to the south than any other landing on the Moon. All landings have so far happened in the equatorial region, mainly because this area gets the most sunlight.

The actual distance between the landing sites of Chandrayaan-3 and Luna 25 could be several hundred kilometres on the lunar surface.

The polar region of the Moon is expected to get busier in the future, with several upcoming missions seeking to explore this part, mainly because of the greater potential of finding frozen water.