OSIRIS-REx to bring asteroid samples back to Earth

This weekend an exciting mission that began 7 years ago and aims to return to Earth samples from a nearby asteroid should - if all goes to plan - come to fruition.

NASA are hoping that a capsule containing pristine samples harvested from the asteroid “Bennu” will plop down in the Utah desert. There are a few reasons for visiting this cosmic near neighbour, one being that next century it might well end up on a collision course with us meaning we might need to move it out of the way to avoid a potential impact. To do that, we need to know what it’s made of. But finding that out also opens a door on the earliest history of our solar system, and even the question of where we all came from. David Rothery is a planetary geologist at the Open University and has been following the mission keenly…

David - The mission is called Osiris-Rex. It launched seven years ago in September, 2016. It took two and a bit years to rendezvous with a small asteroid, a bit less than half a kilometre diameter called Bennu. They orbited it, chose somewhere to briefly touch down and grab some sample, and now that's been brought back to Earth. And as the spacecraft flies past the earth, the sample capsule is going be dropped off and will land in the desert somewhere in Utah while the spacecraft sails on to have a future rendezvous with another asteroid.

Chris - Oh wow. Which asteroid and where?

David - It's going to rendezvous with an asteroid called Apophis. It won't bring any sample back, but it will be able to do a good job with its rendezvous, hopefully going into orbit around the asteroid if there's enough fuel. You can learn an awful lot from orbiting these bodies: you can get the density pinned down because you know their size and their mass, and there's an x-ray spectrometer which gives you the elemental abundances. Now this has already been done at Bennu, so we've got a good map of Bennu's composition and we can see how variable it is. If it's the same, that's great. If it's different, we're learning even more. So whatever we learn from Apophis will be great on top of what we've learned from Bennu. But Bennu by then would have samples in the laboratory where we will be able to really take it apart piece by piece and understand the history and whether the building blocks for life are already there.

David - We think that Bennu is made of much the same stuff as the Winchcombe meteorite that people might remember. This landed in Winchcombe in the Cotswolds at the end of February, 2021 when we were all locked down, but it landed on somebody's drive and scientists were there very quickly and it was collected very quickly. It had travelled through the Earth's atmosphere, been on somebody's tarmac drive and some bits were in fields, so slightly contaminated. We're hoping to get at least 60 grams from Bennu. We won't know quite how much the spacecraft has collected until the return capsules are opened up. So it's going to be very exciting when we open these canisters and send them out to the laboratories for analysis.

Chris - Some of the headlines which have documented this mission have said that one of the reasons for going to Bennu is because this poses a threat because it's almost on a collision course with the earth. So we need to know what it's made of so we can work out if we have to blow it up or something. Was that just to get some headlines, is it more that it's interesting studying asteroids and that's the primary purpose of the mission? Or is there genuinely a threat here from this potential impactor?

David - I think Bennu was chosen for multiple reasons. Scientifically, it's very interesting because of its richness in carbon and therefore in organic molecules. Its shape is intriguing, it's spinning so fast, it's in the shape of a spinning top. But also, as you said, it's a potential threat to the Earth. Bennu's orbit crosses the Earth's orbit. One day, it could collide with the earth. Now this size, half a kilometre across, not much of it would survive passage through the Earth's atmosphere. It wouldn't break apart because it's a collection of boulders. It's not a solid lump, so you wouldn't be forming a major dinosaur killing impact on the earth. But we do need to know about the physical composition, mechanical composition of these things because if a bigger one heads our way, we would like to be able to divert it. If it's going to fall apart too easily, you might turn one crater forming impact into several crater forming impacts, which is perhaps the last thing that you want.

Chris - Presumably though, it can tell us useful chemical things because, if this has been out there for billions of years and is a vestige of what our solar system was made from, it must contain the interesting recipes of where we all came from?

David - Yes. And it probably has materials which were delivered to planets like the Earth which enabled life to get going. Now we don't say asteroids carry life, we say they carry the potential building blocks for life. And we'd like to look at these in Bennu in the samples that have come back totally uncontaminated in a sealed container. Those materials will be exactly as they were in space.

Chris - And what's involved in the actual landing and recovery? How are they actually going to do this?

David - The parent spacecraft will fly past Earth on the way to its next destination, but it will drop off a reentry capsule. There's a heat shield on it to slow it down, parachutes will deploy and eventually it'll hit the ground in Utah. But I'm not sure how fast. You wouldn't want to be an astronaut onboard a capsule like that. It'd be a fair old jolt. Hopefully it will not break open at all, it will still be a sealed container when taken to the laboratory.