An artistst impression of the spacecraft OSIRIS-REx descending to the rocky surface of Bennu, with its collecting arm outstretched and ready for contact.

OSIRIS-REx will spend four and a half hours descending to the surface of Bennu, but will only touch down for a maximum of 16 seconds before lifting off again ©NASA

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NASA's OSIRIS-REx spacecraft to make history in asteroid Bennu touch down

Spacecraft OSIRIS-REx made history as it  collected a sample from the asteroid Bennu by briefly touching down on the rocky surface. 

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 was 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.'

A composite picture showing the asteroid Bennu, with a grey rocky surface on a black background of space.

The asteroid Bennu was selected out of more than a million asteroids whizzing around the solar system ©NASA

How will OSIRIS-REx sample asteroid Bennu?

The spacecraft completed a Touch-And-Go (TAG) sample collection on Tuesday 20 October. The mission involved three separate manoeuvres.

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

This involved 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 touched the asteroid.

When OSIRIS-REx was just 125 metres above the surface, it initiated the Checkpoint manoeuvre. This adjusted the spacecraft’s position and speed as it then descended more steeply towards the asteroid. 

'I can’t believe we actually pulled this off,' said lead scientist Dante Lauretta of the University of Arizona. 'The spacecraft did everything it was supposed to do.'

A picture looking down on the surface of Bennu, showing a flat dusty target area surrounded by a ring of bigger rocks and rubble.

The target site for OSIRIS-REx is the Nightingale crater on Bennu's surface ©NASA

After around 11 minutes, the final Matchpoint burn manoeuvre slowed the spacecraft's decent and matched the asteroid's rotation.

OSIRIS-REx touched Bennu for less than 16 seconds, and grabbed some of the surface material. The spacecraft then fired its thrusters and moved back to a safe distance.

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. 

An artists impression of OSIRIS-REx, with its two solar panels held in a 'v' shape, and collector heading pointing directly down at the asteroid Bennu.

It has taken four years for OSIRIS-REx and NASA to get to this point, and the mission will not be completed until the spacecraft returns to Earth in 2023 ©NASA

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.

OSIRIS-REx will now start its long journey home and will hopefully touch back down on Earth in 2023.