Secrets of a celestial traveller

SOME time ago I wrote of the launch of NASA’s Osiris-REX mission to the asteroid Bennu.

A major aim of the mission is to return a sample of the asteroid to Earth, and all is going well so far. The spacecraft is on its final approach to Bennu, and is now obtaining images of the little body, which currently appears simply as a point of light.

Bennu is a relatively small asteroid, orbiting the sun every 1.2 years. It was discovered in 1999, and is about 500m across. Radar observations have shown it to have a quite round shape. I found this to be quite interesting in itself, as the smaller asteroids often have quite irregular shapes.

Bennu has been classified as a “near Earth asteroid” (NEA), which has a chance of one day colliding with the Earth. But that is only one reason for it being an object worthy of investigation. One of the important aspects of the study of asteroids is the connection between them and meteorites.

Meteorites are rocky objects that survive their fiery passage through the atmosphere and reach the ground. Bennu’s composition is thought to be related to meteorites called carbonaceous chondrites, which give us information about the composition of the solar system in its very early period of existence.

There were many other criteria that led to Bennu being chosen for this study. I was interested a few years ago to look at the way in which asteroids were being eliminated from the list of candidates.

An initial list of over 7000 NEAs was quickly whittled down to only 192, based on the need for a sample-return mission and the capability to achieve this aim based on the orbit of the object.

Then, the smallest asteroids were eliminated because they tend to be very irregular and rotate rapidly, neither of which was a suitable condition for the success of the mission. This reduced the number to 26.

The composition of each asteroid, inferred from Earthbased observations, then left only five possibilities. Bennu then won because of those five, it was the asteroid about which we already had the most extensive Earth-based observations.

This procedure was an interesting one and it made me think of the process of selecting a prospective employee for interview, based on the way in which each applicant has addressed the selection criteria!

The point about having such an extensive set of observations from here is very significant. Using that as an important criterion is an example of performing excellent science: comparing these with the observations made by the spacecraft.

The reason for the importance of this is that the vast majority of astronomical observations are still made from the ground. I wrote about an excellent example of this recently, when I could not contain my excitement about the best picture of Neptune we have seen since the 1989 Voyager 2 encounter with the planet — taken using an Earthbased telescope in Chile called the Very Large Telescope.

So on one hand we have detailed knowledge of the characteristics of asteroids obtained by spacecraft in close proximity to them; on the other, we have data gathered from our far more distant point of observation (the Earth’s surface).

Because of the huge number of these Earth-based remote observations, the more we can learn about what they are actually telling us, the more valuable these will become.

It will be some time before really useful science results are obtained. For now, Bennu appears just a few pixels across to Osiris-REX’s camera, but it will appear to become increasingly bright as the spacecraft closes in.

There will surely be a lot of smiles right now as the mission continues to go well. But I think the biggest one is likely to be on the face of Michael Puzio, who entered and won a competition to see who could choose the best name for the asteroid. Bennu was a being from Egyptian mythology that was depicted as a bird, often a heron. When he suggested that name, he was just nine years old! Martin George is manager of the Launceston Planetarium (QVMAG).