Nasa's Osiris-Rex probe successfully touches asteroid to bring back surface dust

The spacecraft will stow the sample in a capsule which it will release when near the Earth again in September 2023

An artist's impression of Nasa’s Osiris-Rex spacecraft descending towards asteroid Bennu to collect debris from its surface. AFP

In a daring mission, a Nasa spacecraft has successfully landed on an asteroid to bring rock and dust from its surface back to Earth.

Just after 3am UAE time on Wednesday, Nasa’s Osiris-Rex probe skimmed over the surface of a small asteroid orbiting 330 million kilometres away from Earth.

Mission controllers hope the spacecraft has grabbed up to a kilogram of rock and dust from its surface - the largest sample ever brought back from another world so far from Earth.

The probe is now preparing to head back to Earth, dropping off its cargo for study in 2023.

In December 2019, the Osiris-Rex team announced the chosen landing site, which they called Nightingale.

But the site was only the size of a tennis court and at its edge was a boulder as big as a two-story building. The researchers called this boulder, Mount Doom - which the probe successfully dodged during its descent.

Why the interest in asteroids?

First discovered around 200 years ago, asteroids are the rocky debris left over from the formation of the solar system around 4.5 billion years ago.

While some are hundreds of kilometres across, most are much smaller and have remained largely unchanged since their formation – making them of intense scientific interest.

But asteroids are increasingly attracting attention for two other reasons: as a source of valuable metals including gold, and the threat they pose by colliding with the Earth. The Osiris-Rex mission is expected to cast light on all these issues.

What is Osiris-Rex?

Roughly the size of a van, Osiris-Rex was launched from Cape Canaveral in September 2016 on a two-year flight to a 500m-wide asteroid named Bennu.

Discovered in 1999, Bennu was chosen as it shows signs of being made of virtually pristine primordial material dating back billions of years.

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But Bennu is also on a path that could lead to a collision with the Earth around 150 years from now with the violence of a thousand thermonuclear weapons.

Since arriving at the asteroid in 2018, Osiris-Rex has been scanning the object to determine its shape, size and composition to better understand both its origin and the threat it poses to our planet.

But its principal goal is to grab and return a sample of the asteroid to Earth for detailed study.

How will Osiris-Rex get the sample back?

The probe used a 3.3metre-long sampling arm extended beneath it, fitted with a nozzle designed to blow fragments of rock off the surface and into on board traps.

Called the Touch-And-Go Sampler (Tagsam), it was designed by an engineer at Lockheed Martin using a plastic cup and air compressor, and was first tested on a gravel driveway.

The device may have retrieved as much as a kilogram of debris. That would be the biggest sample of another world retrieved since the Apollo moon missions half a century ago - and the most ever from so far away.

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While a Japanese probe is already heading back to earth with a sample of another asteroid, it retrieved just one tenth of a gram of material.

How tricky was it?

It was far trickier than anyone expected.

Before the mission set off, earth-bound studies of Bennu suggested its surface was relatively smooth and sandy.

But as Osiris-Rex approached the asteroid, its cameras revealed the surface to be far more rugged.

With the probe designed to orbit just 1km above the surface and then dip down to just a few metres to grab the sample, the discovery shocked mission designers.

They spent months mapping out the surface in meticulous detail, looking for sites that would pose the least threat to the probe as retrieved its sample.

The chosen site was a crater containing debris small enough for Tagsam to cope with.

But Osiris-Rex had to achieve its mission - including dodging Mount Doom - with no help from Earth.

The probe is so far away that any radio commands take 18 minutes to be received – far too long to avert disaster.

It did have the ability to abort the landing if it looked too dangerous.

What happens next?

Osiris-Rex will weigh and stow the sample in a capsule which it will release when near the Earth again in September 2023.

After re-entry, the capsule will land in the Utah desert and its contents will be taken off for detailed study.

But tests aboard Osiris-Rex are expected to reveal how much material it has grabbed within the next few days.

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