OSIRIS-REx has recently arrived at the asteroid Bennu. The long name for the mission is NASA’s Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer. Heading to an asteroid and mining it could be a lucrative proposition when the technology matures and the economics work. Before that can happen we need to know we can get to an asteroid and safely bring some of it back to Earth. Though, of course, the motivation to do this is not just about the mining potential, it’s about understanding the birth of our solar system and what the early conditions were like through the examination of asteroid material. The Japanese have been quite successful with their Hayabusa 2 probe and its trip to Ryugu, and late last year the OSIRIS-REx probe, from NASA, got up close with Bennu and is slowly making its way through a mission to eventually return a small piece of that asteroid to earth in September 2023 (the featured image is Bennu taken by OSIRIS-REx from about 80km, credit: NASA).
Bennu is quite tiny and it is now the smallest object visited by a spacecraft at only 492m across. The mission of OSIRIS-REx is at least a year and aims to collect about 60-2000 grams of material to bring back to the Earth. This material will give scientists a few more clues as to how the solar system formed and give a better understanding of asteroids, what they are made of and what we might be able to do about them if one heads on a collision course with us. Bennu is in the class of asteroids that could be a problem for us so they also want to understand how these sorts of asteroids can have their orbits altered and end up on a collision course. Astronomers has predicted that Bennu has a 1 in 2700 chance of hitting the Earth sometime between 2175 and 2196, not big odds but enough to warrant a closer look. The challenge is that it is very difficult to predict the path of Bennu more than about four orbits into the future, as each path it takes near the Earth or Moon adjusts the next orbit slightly.
OSIRIS-REx arrived at Bennu on 4 December last year and entered orbit on the 31st. The spacecraft will end up in an orbit that brings it to within 1.25km of the asteroid and from February it will start mapping the surface to look for a spot to take a sample from. Part of this mapping is also to determine the differences in heat over the surface caused by the Sun heating the asteroid up. This causes what is known as the Yarkovsky Effect. This is basically an effect of solar radiation on the asteroid and how it can influence its path through uneven heating. Scientists want to test if the heating of the sunny side causes a drift to the cooler side through the scattering of photons over a long period of time. This is the perfect opportunity to test the models of the Yarkovsky Effect as there has not been an opportunity to do this before. Understanding this effect will improve the models used to predict orbits of asteroids that may threaten the Earth.
The spacecraft has already found evidence of water inside clays on the asteroid using data from its spectrometers. The surface is a mix of relatively smooth areas and groups of larger boulders, including one that is about 50m in height. Once the spacecraft has finished mapping the surface it will leave orbit again and start a series of flypasts to survey more detail and ultimately find a place to sample. The sampling will occur by blowing nitrogen at the surface to stir up boulders and debris that the spacecraft will collect. There’s enough nitrogen to make three attempts at sample collection. OSIRIS-REx will begin its return to Earth in March 2021, if all goes well.
