Curious craters explained

IF you have a pair of binoculars or a telescope, take a look at the moon tonight.

Along with what appears to be its right (eastern) edge, you will have a fine view of some of its features, especially some of the larger craters.

Recently, two new craters were formed on the moon, but you have no hope of seeing them: they are relatively small, and in any case they are on the far side of the moon – the side we don’t see from Earth.

The vast majority – close to 100 per cent – of lunar craters were formed by impacts on to its surface, and with very rare exceptions they have been caused by natural objects hitting our satellite.

However, in this case, humans are responsible. The impacting object is thought to have been an upper stage from a Chinese Long March 3C rocket, from the Chang’e 5 T1 mission. The spacecraft flew past the moon in 2014.

In times gone by, we would probably know nothing of such an event, the approach of the hardware to the moon, and especially the result of its impact on the lunar surface.

We have such images today because of a very capable spacecraft in orbit around the moon: the Lunar Reconnaissance Orbiter, or LRO. It is a NASA craft that, for many years, has been a high-resolution “eye’’ to examine the moon from above, so it can detect changes in the appearance of its surface.

Because LRO passes “behind’’ the moon as seen from Earth, it is perfectly placed to help us study lunar features that we never see from here.

The collision has produced a pair of craters, one about 18m across and the other 16m. It is interesting that there are two, of quite similar sizes. Both are certainly new: examination of images of the impact site taken earlier show no evidence of either. This seems to be an indication of a concentration of mass at two different places on the booster.

Natural impacts dominate over those involving artificial – that is, human-made – objects. However, the recently detected collision is far from being the first on to the surface of the moon for which humans are responsible. Indeed, there have been many.

The first was as long ago as 1959, when the Soviet probe called Luna 2 collided with the moon. More well known was that in the same year, Luna 3 returned images of the moon’s far side, a photograph that is one of the most remarkable ever taken by a spacecraft when one considers that it was not long after the beginning of the space age. (Luna 3 did not crash on to the moon – it is thought to have eventually burned up in the Earth’s atmosphere.)

Probably the most famous of lunar impacts were caused by the deliberate collisions of the ascent stages of lunar modules from NASA’s Apollo crewed lunar landing program half a century ago. The general aim was the test the seismometers left on the moon. However, less well known were the impacts of the Apollo SIV-B boosters used in the Apollo program, the results of which, in addition to the landing sites of the Apollo missions, have also been imaged by LRO.

No harm will come to the moon from these impacts, and we can be quite confident that there is no life there to be compromised. The story would be quite different if the moon were a place that could potentially harbour life.

An example of an object to avoid contaminating is Jupiter’s moon Europa, for which we have strong evidence of a subsurface ocean. It has been speculated that this ocean may contain living creatures, something that Galileo Galilei could hardly have expected as he drew the positions of Jupiter’s four brightest moons in the early 1600s.

When it comes to rockets hitting the moon, my thoughts often turn to the famous 1902 film Le Voyage dans la Lune by Georges Melies (1861-1938). In this story, the moon, represented by a face, is hit in the “eye’’ by the capsule. It is regarded as the first science-fiction film. I wonder if Melies anticipated that some day people would make a craft that could, and would, hit the moon.

Martin George is principal astronomer at the Ulverstone Planetarium.