NASA’s OSIRIS-REx spacecraft has finally arrived at its destination: a lumpy, diamond-shaped space rock orbiting the Sun between Earth and Mars.
“We’ve been trying to catch up with the asteroid for two years,” said Heather Enos, the mission’s deputy investigator from the University of Arizona.
Launched in 2016, the bus-sized spacecraft has travelled more than 2 billion kilometres to reach the asteroid known as 101955 Bennu.
It’s taken the long way around to save energy, using a flyby past Earth to slingshot it into the asteroid’s orbit.
On its way, the spacecraft sent back its first images in August, and since then, the asteroid’s shape and surface have started to become clearer.
Now it’s arrived, the spacecraft will start work mapping the asteroid’s surface.
Over the next three weeks, hovering about 7 km above the surface, the craft will take high-resolution images and get preliminary data about the asteroid’s mass, spin rate and shape.
Then, for the next 18 months, it will explore every nook and cranny of Bennu, searching for an ideal spot between the rocks to collect a sample of space dust to bring back to Earth in 2023.
What’s so special about Bennu?
Bennu was chosen by NASA from more than 500,000 known asteroids.
To start off with, it had to be close enough to get to.
Bennu is one of 7,000 near-Earth objects. Not only is it close to us, it has an orbit that is similar to ours, which makes it easier for a spacecraft to go into orbit.
Around 500 metres in diameter, it is just one of 26 asteroids with the perfect size.
Asteroids less than 200 metres in diameter rotate so quickly that it would be difficult for the spacecraft to keep up.
“We will get up close and personal with Bennu and match its rotation state, so it’s important that it’s not rotating too quickly,” Ms Enos said.
There would also be less chance of scooping up material from the surface of a smaller asteroid, she added.
“[Bennu] should maintain much of its material because it’s not spinning so quickly,” she said.
“So size is a big deal.”
And its dark colour indicates it’s one of just five asteroids that may contain a lot of organic compounds.
“Bennu is like a time capsule of the elements and composition of things that were preserved in the early solar system,” Ms Enos said.
That not only makes it a record of our early solar system, but an interesting target for mining in the future.
“It could be an opportunity for agencies such as NASA or private industries to be able to determine whether or not they could go to such an asteroid and extract the resources necessary to create fuel, for example,” she explained.
Studying Bennu could also give us information about the threat posed by near-Earth asteroids.
Ms Enos said Bennu is classified as a potentially hazardous asteroid.
It’s too small to wipe out life, according to calculations by the team, but it would cause a major natural disaster with damage for hundreds of kilometres around the impact site.
The OSIRIS-Rex mission will measure how solar wind affects the asteroid’s trajectory, a phenomenon known as the Yarkovsky effect.
Ms Enos said direct observation of this effect will help scientists update the probability of Bennu, as well as any other potential asteroids, hitting the Earth.
How will the spacecraft collect space dust?
OSIRIS-REx is short for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer.
On board the spacecraft are a suite of cameras and an altimeter to provide detailed images of the asteroid’s surface, and tools that map chemical composition and temperature.
For the next month, OSIRIS-REx will do three flybys over Bennu’s poles and equator, measuring the asteroid’s mass, spin rate and shape down to a resolution of 75 centimetres.
Once NASA has this information pinned down, they will put the spacecraft in a series of orbits that match the rotation of the asteroid.
The spacecraft will inch closer and closer to the surface of the asteroid over time, as it zeros in on a target spot.
“We have to get a site that has the right tilt and has the right sample and grain size,” Ms Enos explained.
They will also need to find a spot that is no more than 75 degrees Celsius to ensure the chemical compounds in the sample are preserved in pristine condition.
In mid-2020, the craft will descend slowly towards the surface and stretch out a “touch and go” robotic arm that contains a camera and dust collector.
“OSIRIS-REx will very slowly descend to the surface about 10 centimetres per second, touch the asteroid for less than five seconds,” Ms Enos explained.
“As we are descending and doing our touch we’ll use a burst of compressed nitrogen gas to mobilise the asteroid surface.”
They’ll use the camera on the arm to decide whether the dust collector has sucked in enough space dirt.
Ideally, they’d like to get enough dirt in one go, she said, but they have enough nitrogen to try three times.
After the spacecraft collects the sample, it will need to wait another six months before beginning its two-year journey home.
So far, so good…
Already the team are happy with what they’ve seen.
“Bennu is not as rocky as Ryugu,” Ms Enos said.
Ryugu is another near-Earth asteroid, which is being orbited by the Japanese spacecraft Hayabusa 2.
Earlier this year the Japanese mission placed three robots on Ryugu, but plans for the spacecraft itself to touchdown on the asteroid’s rocky surface and collect a soil sample have been delayed.