OSIRIS-REx - Tickling an asteroid for clues to the formation of the Solar System

Tuesday 19th January, 2021

Our speaker on Zoom was Prof Neil Bowles from Oxford University and his talk had the somewhat lengthy title - "OSIRIS-REx - Tickling an asteroid for clues to the formation of the Solar System". He introduced himself by saying that he is part of a team of scientists at Oxford University working on NASA's OSIRIS-REx spacecraft mission to asteroid Bennu but also builds space instrumentation. He joined the OSIRIS-REx mission in 2011.

He began by saying that the aim of the OSIRIS-REx mission is to look for the material that acted as the building blocks of our Solar System. He explained that the major phases of the formation of the Solar System were well understood but much of the details are still unknown. The theory is that a cloud of rotating cold gas and dust collapses and due to the cloud's angular momentum the material forms itself into a disc. Slowly the material in the disc clumps together due to both gravitational and electrostatic forces.

At some stage enough material has gathered at the centre of this disc for a protostar to form and fusion reactions to begin at its core. A stellar wind from this new star pushes lighter material and volatiles out of the inner regions and so rocky planets form nearer the star with icy planets further out. For various reasons not all the material will manage to form large objects such as planets and their moons. Large planets such as Jupiter have such a gravitational pull on even small objects that it disrupts the tendency for small bodies to amalgamate. Also, in the outer reaches of the Solar System there is literally more space between objects and so gravity is unable to bring them together.

As these smaller bodies, such as asteroids, have stayed unchanged from the early days of the Solar System's formation they contain within themselves the earliest history of the Solar System, which is why scientists wish to analyse them.

Sometimes meteorites that come from asteroids do find their way down to Earth but the problem is that they end up with contamination from our atmosphere and after impacting the surface. This is why NASA and other space agencies wish to use spacecraft to visit asteroids and obtain pristine samples.

NASA chose asteroid Bennu as it was one of the easier asteroids for a spacecraft from Earth to visit. Bennu has an Earth-like orbit with low eccentricity (fairly circular) and low inclination (its orbital plane is similar to that of Earth). It also comes to within 300,000 km of Earth, which is relatively close compared to other asteroids. It was also selected due to it being what is termed a "primitive B-type" asteroid where the "B" refers to it appearing somewhat blue in infrared spectra measurements, and "primitive" due to its unaltered composition and richness in carbon compounds.

The spacecraft set off for Bennu in 2016 and arrived two years later. Its first task - using its laser altimeter, infrared spectrometer, and cameras - was to map the asteroid so that scientists back on Earth could choose the best sample site. This was not an easy task as, to the scientists' surprise, the whole of Bennu's surface was covered in boulders, and they even spotted a couple of meteorites from other asteroids on its surface. The mapping was also hindered by the spacecraft having to avoid large particles, 1 to 10 cm in diameter, being shed from Bennu's surface.

Eventually, out of a number of proposed sites, one was chosen, named Nightingale that was in the centre of a 10-metre crater near the asteroid's north pole. The spacecraft would have to autonomously edge towards the surface and, using a blast of nitrogen from its sample instrument, dislodge material from the crater before backing away and stowing the material safely away for its journey home.

It successfully gathered a sample on 20th October 2020. Its sample instrument went 50 cm below the asteroid's surface and gathered so much material that a large rock wedged open part of the sample device and some of the dust and rock chunks ended up escaping back into space. Fortunately, the spacecraft engineers commanded it to complete the sample stow earlier than scheduled.

The spacecraft will complete one last flyover of the Nightingale sample site to examine it on 7th April 2021. Then on May 10th, 2021 it will leave Bennu, arriving back at Earth on 24th September 2023. A capsule containing the sample will land, using a parachute, in a military area in the Western Desert in Utah.

For more information on the OSIRIS-REx mission see: https://www.asteroidmission.org/