Space rocks tell Earth’ s story
ON Wednesday morning our time, a NASA spacecraft almost certainly obtained a sample of material from the surface of asteroid Bennu, marking a first for NASA and generating cheers and applause by all those involved in themission.
The spacecraft is called ORISIS-Rex, which is an acronym for Origins, Spectral Interpretation, Resource Identification, Security and Regolith Explorer. It was launched in September 2016, and arrived at Bennu in December 2018.
Bennu is not one of the ‘main belt’ of asteroids, which orbit mostly between Mar sand Jupiter. Its period of revolution around the Sun is just 1.2 years, not much more than that of Earth. It is about 500m across, making it the smallest object ever to be orbited by space-
craft. However, one cannot really call this a ‘diameter’, as Bennu has a rather unusual shape, rather like a spinning top with a bulge in the middle.
It was once thought that this shape developed over time, but analysis of the results sent back from Ben nu–in particular, the presence of craters in the equatorial‘ bulge’ –s how that it probably originally formed that way.
Although we think of the asteroids as rocky, Bennu is more like a‘ rubble pile ’. Indeed, that description has been given to many a steroids which, although appearing to be a solid chunk of rock, are quite porous. The average density of Bennu is not much more than that of water!
The sample-collection activity used the spacecraft’s TAGSAM (Touch-and-Go Sample Acquisition Mechanism) device, at the end of a robotic arm, which contacted the surface for just a few seconds.
Nitrogen was blasted around the device, dislodging material for collection.
An exciting discovery was that the surface material was crushed by the TAGSAM, meaning that it is material that is very loosely held together. This contrasts with meteorites found on Earth’s surface, most of which are also pieces of asteroids, having been broken off as a result of violent collisions.
Quite apart from that difference, meteorites found on Earth have all had to pass at high speed through our atmosphere. Bringing back a sample taken from an asteroid and sealed in a canister to protect it from the fiery atmospheric entry means that the sample will be exactly as it existed on the surface of the asteroid. It’s good science!
This weekend, an estimate will be made of the amount of material collected. This will be done by rotating the spacecraft with the robot arm extended, and measuring a quantity called the moment of inertia.
This has already been done with the TAGSAM empty, so the comparison will show approximately how much mass has been added. Results of this measurement are expected to be announced by about Tuesday our time.
If there is not enough material, another sample collection will be attempted in January, at a different location on the asteroid.
Enclosed in its canister, the sample from Bennu will arrive on Earth in September 2023, dropping safely through the atmosphere and finishing with a desert landing in Utah in the USA.
NASA will not be the first to return asteroidal material. Japan has already had success by returning a very small sample of the asteroid Itokawa, and in just a few weeks, material from the asteroid Ryugu will fall to Earth, landing in South Australia.
Although twice the diameter of Ben nu, Ryugui sanother‘ rubble pile’ a steroid, and it is possible that both it and Bennu are fragments of the same larger object that was disrupted in a collision long ago. However, they appear to have quite different amounts of water in their surface material. Samples from each will be shared for scientific investigation, and this will hopefully shed some light on the reasons for the differences.
Knowing the amount of water in asteroids, and the differences between them, are very important in understanding the source of Earth’ s water. In the past, it was thought to have arrived mainly from comets, but the tide of opinion has swung to asteroids being responsible.