Source: Natural History Museum

The NASA spacecraft OSIRIS-REx has spent the last four years speeding its way 334 million kilometres (207 million miles) to, and then entering orbit around, the asteroid Bennu.

Along with the Japanese spacecrafts Hayabusa and Hayabusa2, it’s the first sample return mission to an asteroid.

The goal is to collect at least 60 grams of material from the surface of Bennu, before turning tail and heading back to Earth where the sample can be fully analysed in laboratories.  

With the huge distances involved, it takes around 18 and a half minutes for signals to be sent from Earth to OSIRIS-REx. This means that the entire sampling mission has to be done autonomously, with the team on Earth uploading all the commands beforehand, and then simply letting it run.

Prof Sara Russell is a researcher at the Museum who is a member of the OSIRIS-REx science team.

‘This is a super exciting mission that will bring back more rock than any other space mission since the Apollo missions in the 1960s and 1970s,’ says Sara. ‘We think Bennu may be similar to some of the meteorites we have in our collection at the Museum but we won’t know for sure until the mission returns to Earth in 2023.’

Follow the action on the NASA website, including live coverage of the mission from 10pm BST on Tuesday 20 October.

How will OSIRIS-REx sample asteroid Bennu?

The spacecraft will attempt a Touch-And-Go (TAG) sample collection on Tuesday 20 October. The mission will involve three separate manoeuvres that are expected to take four and a half hours to complete.

The first of these is the orbit departure manoeuvre, in which the spacecraft exits the orbit it is currently maintaining around the asteroid and begins its 770-metre descent. During the four hours that this takes, the spacecraft will begin reconfiguring its hardware.

This involves extending its sampling arm, known as the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) into the correct sampling position. At the end of this arm is the sampling head, which is the only part of the spacecraft which will touch the asteroid.

The solar panels of OSIRIS-REx will also fold back to protect them from any potential knocks on the asteroids surface, while the centre of gravity of the spacecraft will be shifted to make sure that it is directly over the TAGSAM collector head.

When OSIRIS-REx is just 125 metres above the surface, it will initiate the Checkpoint manoeuvre. This will adjust the spacecraft’s position and speed as it then descends more steeply towards the asteroid. 

After around 11 minutes, the final Matchpoint burn manoeuvre will occur, which will slow the spacecrafts decent and match the asteroids rotation for when touch down occurs.

OSIRIS-REx will only touch Bennu for less than 16 seconds, during which the TAGSAM will fire a bottle of compressed nitrogen gas to lift up some of the surface material. This will then be collected in an air filter-like device on the collector head, before the spacecraft fires its thrusters and moves back to a safe distance.

It is carrying three canisters of nitrogen, meaning that if the first attempt is not successful then it will have another two goes at collecting a sample.

Why is OSIRIS-REx collecting an asteroid sample?

The asteroid Bennu was selected out of over a million asteroids that can be found hurtling through our solar system, in part due to its orbit crossing over our own.

It has also been known to scientists and studied since 1999, meaning that they have a pretty good understanding of what it is made from. This means that by actually visiting the asteroid itself, scientists can see how accurate their observations have been. 

This could prove vital when studying other, more distant asteroids, allowing researchers to be more confident about the physical and chemical properties they have observed on these rocky bodies.

‘Rocks from asteroids like Bennu are also thought to date from around the time the Solar System was born, and so can tell us about how the Sun and planets formed and evolved,’ explains Sara. ‘Bennu may contain water and organic material, and we think similar asteroids impacted the early Earth to seed the materials it needed for life to flourish.’ 

Finally, studying Bennu in such fine detail will help scientists to understand what is known as the Yarkovsky effect, which is related to how small, dark objects in space absorb and then radiate out the heat from the Sun. This can affect their orbit in detail and help us understand if any asteroids are on a collision course with Earth.

Once OSIRIS-REx has collected its samples, it is expected to then start its long journey home and will hopefully touch back down on Earth in 2023. 

Source: Natural History Museum