By Duncan Steel 06/09/2019

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All being well, the Indian space probe Chandrayaan-2 now in orbit around the Moon will drop its lander safely and softly onto the surface on Saturday morning. The lander (named Vikram) will then roll out its rover (Pragyan), which it is hoped will prowl around for the next two weeks before the cold of the lunar night closes it down. The parent orbiter should continue to function for the next year, mapping the lunar surface and making other measurements of the space environment. 

Life-off for Chandrayaan-2 on July 22nd. Photo: ISRO.

Following its launch on July 22nd (above), the Chandrayaan-2 mission (a follow-up to Chandrayaan-1, which flew to the Moon a decade ago) has been following a path that many would find peculiar. For a direct path to the Moon, a vast amount of energy is needed. A more efficient way to get there, though it takes far longer, is a series of rocket burns that gradually raise the probe’s apogee in its geocentric orbit, until it is close enough to the Moon for the lunar gravitational attraction to take over, pulling the spacecraft into orbit around itself. A series of rocket burns are then employed to reduce the size of its selenocentric path. Separation of the module being left in orbit around the Moon and the lander then occurs, with the latter being gradually slowed and dropped down to a landing on the surface, all being well.

Schematic of the path taken by Chandrayaan-2 to the Moon. This, along with the other graphics and photographs in this blog post, are courtesy the Indian Space Research Organisation (ISRO).

This lunar mission (for details see here), if successful, will make India the fourth nation to make a soft landing on the Moon’s surface, following the much-earlier accomplishments of the USA, the former Soviet Union, and more recently China, at the beginning of this year. International media are abuzz, though that is nothing like the excitement in India itself.


The Chandrayaan-2 mission has three physical components, as shown above (courtesy ISRO). The Orbiter is expected to continue to function for the next year or so, and then be abandoned in its selenocentric orbit, where its fellow Chandrayaan-1 orbiter still resides.

The lander module, called Vikram, is destined to settle down onto the lunar surface at a latitude near 70 degrees south, all being well. It will then fold out its ramp, and the small rover named Pragyan will roll down to the dusty surface on its six wheels. With a mass of just 27 kg, Pragyan is about the size of a child’s stroller.

Model of the Vikram lander and the Pragyan rover on the lunar surface.

If all goes according to plan Pragyan will travel up to 50 metres around its landing site, moving at up to one centimetre per second, until its electronic systems are anticipated to give out when it enters the cold of the dark lunar nighttime in about 14 days’ time.

Vikram and Pragyan in their assembly hall; note the hand-written paper signs saying “DO NOT DISTURB”.

What is different about this mission to the Moon (apart from being India’s first soft lander)? The chosen destination is far south, in the polar region. This area is of particular interest to planetary scientists because evidence has accumulated (since the first hints from the US Clementine mission in 1994, with which I was involved) that there may be ice just underground in parts of the lunar surface perpetually shadowed through being in deep craters or basins. Apart from being of immense scientific interest, such ice deposits might make more feasible the concept of an inhabited lunar base, the water being used for growing food, drinking and washing, but also as a source of oxygen.

Fingers crossed (not that I am superstitious) for a safe landing by Vikram, then, as we count down the hours and minutes to around 08:00 NZST on Saturday.

Image of the Moon’s northern polar region collected as Chandrayaan-2 approached in August. The Vikram lander’s intended destination is in the southern polar region.

Update, Saturday September 7th, 10:20 NZST: Damnation. I woke up this morning to the very unwelcome news that ISRO mission control lost contact with the Vikram lander when it was just 2 km above the lunar surface. We await further updates… I linked in the comments below to three news sites in India which are obviously well-positioned to keep people briefed on what has happened. Everyone interested in space exploration will be saddened by these events, whilst congratulating India and its space agency on getting so close to full success. The Chandrayaan-2 orbiter is still returning useful data from its orbit around the Moon.

When we try to do things that are so challenging, we must expect failures at times. The secret of life is not avoiding tripping up as we learn to walk, then run, then bound along; the secret is being able to get up on our feet, and try again.

Update, Monday September 9th, 12:45 NZST: Reports from ISRO (such as this one) indicate that Vikram had a “hard landing” on the lunar surface, but that the state of the spaceprobe has not yet been ascertained (i.e. whether it has been destroyed, or is perhaps still functional).

One can easily estimate the minimum speed with which Vikram struck the surface of the Moon, subject to various assumptions. Let me assume that there was no firing of retrorockets after contact was lost (i.e. no automated descent that could have braked its speed). Let me further assume that Vikram was essentially stationary when contact was lost when it was 2.1 km above the surface, with no velocity component in any direction (either vertically, or transversely); any motion at that juncture would lead to an impact speed greater than the minimum calculated below.

Under the above assumptions the final speed v at the surface can be calculated from the following equation for uniformly-accelerated motion:  =  + 2 a s.   We are assuming that the initial speed u = 0.   The distance moved is s = 2.1 km = 2100 metres.  The acceleration due to the Mo0n’s gravity is a ≅ 1.62 m/sec² (i.e. about one-sixth of Earth). Plugging in those values one derives v ≅ 82.5 m/sec, which is just below 300 kilometres per hour.

On that basis I would imagine that there is a new small crater on the lunar surface.

A similarly-based calculation to that described above indicates that the time taken to free-fall from an altitude of 2.1 km to the lunar surface is just less than 51 seconds.

A statement from ISRO has also said that the lander has been “located” on the lunar surface. Other reports have been a little more guarded, stating that it should be possible to identify the landing spot and how it looks now. If the (Chandrayaan-2) Orbiter High Resolution Camera (OHRC) really can zoom in to a pixel size of 30 cm from a 100-km orbit, then it should certainly be feasible to examine the crash site in some detail.

There are other satellites currently orbiting the Moon which should also be usable to identify and examine the Vikram crash site. In particular NASA’s Lunar Reconnaissance Orbiter (LRO) carries a Narrow Angle Camera (NAC) which delivers images with pixel size 50 cm from an altitude of 50 km above the lunar surface; for detailed information about LRO and the NAC see this NASA mission website.

Update, Monday September 9th, 19:30 NZST: Using information available on the web, I have taken a look at what may have happened to the Vikram  lander and posted another blog here.


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