NASA’s OSIRIS-REx mission returns to Earth with pristine asteroid samples

What happens when you combine a rock star, cutting-edge engineering, and almost a billion dollars? An out of this world mission of scientific discovery.

Shortly before 4pm UK time on 24 September 2023, OSIRIS-REx’s sample return capsule touched down on Earth for the first time in seven years. Inside, more than 250 grams of the asteroid Bennu have been preserved in pristine condition, waiting to be analysed by scientists.

This is around 50 times more material than has previously been brought back from an asteroid, allowing scientists a much greater opportunity to find out more about how our solar system formed than ever before.

Professor Dante Lauretta, the leader of the OSIRIS-REx mission, says, ‘Sample return is the gift that keeps on giving. When we explore the solar system, most of the time we just send robotic space probes to go into orbit or fly by. However, you’re very limited in the kinds of chemical analyses that you can perform in those situations.’

‘The amount of Bennu material returned to Earth will allow us to harness the analytical capabilities of the entire planet Earth not just today, but well into the future.’

The team is made up of a wide variety of researchers, including the Natural History Museum’s own Professor Sara Russell as well as astrophysicist and Queen guitarist Sir Brian May.

Now the sample is safely on Earth, the team hope to address some of the biggest questions about the origins of our solar system, our planet, and perhaps even ourselves.

‘I think we would all dream of seeing whether or not the seeds of life were actually in these asteroids,’ Brian says. ‘Looking at samples from Bennu might help us get deeper into solving the puzzle of where life came from.’

Why go to Bennu?

The end of the OSIRIS-REx mission is the culmination of decades of work. Following the first-ever return of an asteroid sample by Japan’s Hayabusa probe in 2010, the NASA mission was announced in 2011 with the goal of retrieving at least 60 grams of material.

The first step on this journey was to identify which asteroid to visit. While over 30,000 near-Earth asteroids have been discovered to date, OSIRIS-REx’s target had to fall within a series of strict criteria.

Firstly, it had to be close enough to Earth for the probe to visit, collect a sample and return it. The asteroid also needed to be relatively large, as bigger asteroids spin more slowly and so are more likely to have loose surface material that a probe can easily collect.

Another key constraint was its tilt, relative to Earth. If an asteroid is too high above the direction of the planet’s orbit, then any sample would return too steeply through Earth’s atmosphere and burn up.

‘When we went through all of those different constraints, we had five objects that were left,’ Dante says. ‘We eventually settled on Bennu, primarily because of its chemistry. We wanted something that was carbon rich to help us understand if these kinds of asteroids may have delivered the building blocks of life to Earth.’

Knowledge about the chemistry of asteroids mainly comes from fragments which have landed on Earth, known as meteorites. The Natural History Museum has samples of over 2,000 meteorites, which researchers like Sara study.

‘The brilliant meteorite collection here at the Natural History Museum shows that asteroids are incredibly diverse in their composition,’ Sara says. ‘However, because they’re chemically altered by Earth’s atmosphere, they’re limited in what they can tell us.’

‘That’s why it’s so important to collect samples from asteroids, which can reveal so much more.’

Star power

Following the mission’s launch in 2016, the OSIRIS-REx team turned their attention to studying their new subject. However, as the probe got closer, they discovered that what had appeared as a smooth surface from Earth was actually quite different.

‘Bennu turned out to have a rough and rugged surface strewn with boulders, which was really different from what we had anticipated,’ Dante says. ‘This meant that we needed to get a better idea of what was on the surface.’

Enter, from stage left, rock star Brian. He has a long-standing passion for producing stereoscopic images, which involves creating two slightly different views of the same object. When the brain perceives these images through different eyes, it gives the illusion of depth, allowing the viewer to see a 3D image.

In their spare time, Brian and his collaborator Claudia Manzoni had created some images of Bennu using publicly released photographs. As the probe didn’t have two cameras pointing in the same direction, Brian and Claudia used pairs of photos taken in quick succession to produce the images.

‘While a simple photo can tell you a lot, it doesn’t let you know if a crater is flat enough to be able to land a spacecraft on,’ Brian says. ‘A stereoscopic picture gives you this extra insight, and when I offered some stereoscopic interpretations of the data that Dante already had he was quite shocked to see the extra amount of information that you get.’

What had been seen as a relatively flat surface suddenly revealed prominent features like Gargoyle, a distinctive rock sticking out of the surface. Colliding with any rocks of this size would have destroyed OSIRIS-REx, so Brian and Claudia were brought on to the team to help narrow down the possible landing sites.

‘With Brian’s stereo images, we very quickly came to realise that the finely grained material which our sampling mechanism could collect was concentrated in very small craters of about 10 to 20 metres in diameter,’ Dante says. ‘As our accuracy was only 50 metres at the time, we had to redesign our guidance system mid-flight to ensure we could hit our target.’

From a variety of possible options, the team eventually selected Nightingale crater as their preferred landing site. The date was set for 20 October 2020.

Sampling Bennu

While Nightingale was the most promising site for collecting samples from Bennu, it wasn’t without risk.

A large and potentially dangerous boulder, nicknamed Mount Doom, sat on the edge of the crater, while the crater itself was full of smaller rocks around a metre in diameter.

‘OSIRIS-REx is about the size of a van, and we had to park it in an area the size of four parking spaces,’ Dante explains. ‘If we hit one of the larger boulders, OSIRIS-REx would tip over and, when the rockets fired, fly straight into Mount Doom and destroy the spacecraft.’

‘We couldn’t control OSIRIS-REx’s descent ourselves, as it took about 18 minutes for our signals to reach it from Earth. We had to make it autonomous enough to do the sampling itself, or back away if it wasn’t safe.’

Once the signal to go was sent, all the OSIRIS-REx team could do was wait nervously to find out whether the craft would still exist in 18 minutes’ time. Meanwhile, 100 million miles away, OSIRIS-REx began its descent.

While the descent went exactly as planned, the touchdown wasn’t quite as expected. The surface was much softer than the team had anticipated, with the impact of OSIRIS-REx’s sample-collecting probe producing an eight-metre-wide crater, having sunk around half a metre into the asteroid.

Though the spacecraft had survived its descent, its sample container was leaking. A stone was jammed in the mechanism, allowing Bennu fragments to drift out into space. Further movement of the spacecraft was stopped while the team stowed the remaining asteroid sample away, ready to return to Earth.

On 24 September, this sample return capsule was finally released from OSIRIS-REx. While the main spacecraft headed back into space to study a different asteroid, called Apophis, the capsule sped towards Earth over 10 times faster than a bullet.

When it hit the atmosphere, most of the energy was dissipated through its heat shield, and once the capsule was slow enough, parachutes deployed to lower it down into the Utah desert in central USA.

It was then picked up by helicopter and taken to a temporary clean room before being flown directly to Johnson Space Center in Houston, Texas, where scientific studies will begin.

What could Bennu reveal?

Over the past three years, there has been plenty of speculation on what Bennu’s samples will reveal. Following its journey to the Johnson Space Center, the work to find out will begin as the capsule is carefully peeled open, and initial analyses carried out on small grains of Bennu.

One of the scientists who will be among the first to work on the sample is Sara. When the initial research is complete, portions of Bennu will be sent all over the world, with Sara co-ordinating the study of this material in the UK.

‘OSIRIS-REx is the biggest NASA mission since Apollo, and there are hundreds of scientists who are eager to learn more about Bennu,’ Sara says. ‘Collaboration is so important in science, and by bringing in scientists from all over the world you can draw on a huge amount of expertise and additional instrumentation that can’t be found in just one organisation.’

Some of Bennu will come to the Natural History Museum, whose scientists specialise in studying the chemical makeup of asteroids. Members of the team like Ashley King, Helena Bates, Paul Schofield, Catherine Harrison and Natasha Almeida will compare samples of Bennu with the meteorite collections to understand how best to research them, and what that might reveal.

‘The mineralogy of the samples will tell us everything about Bennu’s formation,’ Sara explains. ‘It can tell us about the conditions of the early solar system, how the asteroid evolved over time, and perhaps more about how our planet formed.’

Other researchers will examine different aspects of Bennu to answer some of the deepest questions we have about the solar system.

‘We're trying to tell the entire history of our solar system from the few hundred grams that we're bringing back from this asteroid,’ Dante says. ‘For instance, how did the protoplanetary disk, which the planets formed out of, come together in the first place? Primitive asteroids like Bennu can answer that.’

Analysis of Bennu is set to continue for decades to come, with 75% of the sample being held back for future generations to study with new scientific techniques. In the meantime, plans are already underway for more sample return missions to other objects in outer space.

‘The Japanese space agency has planned a mission to Phobos, one of Mars’s moons, as no one really knows how it formed,’ Sara explains. ‘It’s due to launch next year, and if all goes as planned, it will return a piece of the moon to us by 2029.’

‘In the longer term, I’d love to see an analysis of an interstellar asteroid which has come from outside our solar system. If we could capture one of those and find out what it’s made of, it will tell us about not only our solar system, but what's beyond it.’

With a variety of manned missions planned for the Moon, Mars and beyond, the study of space is set to only increase in the coming decades, showing that the sky is by no means the limit for science.