Artists impression of OSIRIS-REx orbiting Bennu.

OSIRIS-REx will 'touch' Bennu to collect a sample of its surface © NASA Goddard Space Flight Center

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OSIRIS-REx has snapped the first images of asteroid Bennu

After almost two years of hurtling 1.8 billion kilometres through space, NASA's spacecraft OSIRIS-REx has finally caught a glimpse of its target, the asteroid Bennu. 

This weekend the OSIRIS-REx mission entered its Approach Phase, getting close enough to snap pictures of its target. From now until the spacecraft reaches Bennu in under a hundred days, it will be taking three images per week of Bennu.

The mission team will spend the next few months analysing the images beamed back. OSIRIS-REx will also use a host of other instruments to gather as much information as possible about its target.

The team want to understand everything they can about Bennu, including its size, shape, surface features and surroundings, before the spacecraft arrives there and attempts to land. After touching down and taking a sample of the asteroid's surface, it will return to Earth so that the material can be analysed.  

Prof Sara Russell, a Museum researcher interested in the formation of the solar system and member of the OSIRIS-REx Science Team, says, 'It's super exciting to see OSIRIS -REx approach its target asteroid after its long, two-year journey.

'It's really amazing to see the first images of Bennu and I can't wait to find out more about this asteroid.'

The first image of asteroid Bennu taken from OSIRIS-REx

OSIRIS-REx’s First Image of Asteroid Bennu © NASA/Goddard/University of Arizona

Bennu is thought to be a fragment left over from the formation of the solar system 4.6 billion years ago. It is hoped that studying the asteroid can offer a glimpse of when our own planet was developing.        

Currently, all we have to work with are the remote observations of asteroids using telescopes, and the meteors that have made it to Earth. Scientists are trying to match asteroids to the meteors they originated from.

By landing on Bennu, taking a physical sample and then returning it to Earth, researchers will have the best picture yet as to where the meteors and their parent asteroids originated.

Sample return

Currently, OSIRIS-REx is still 2.2 million kilometres from Bennu. Travelling at up to 30,000 kilometres per hour, it is expected that the spacecraft will catch up with the asteroid in under 100 days' time.

Once there, it will begin a series of manoeuvres in order to study the celestial body in as much detail as possible.

Within the first month, OSIRIS-REx will perform flybys of Bennu's north pole, equator and south pole, dipping from between 19 to seven kilometres from the surface. This will give researchers a much better picture of the asteroid's surface, as well as allow them to make the first direct measurements of the Bennu's mass.

The spacecraft will continue surveying and mapping the surface extensively. This will give the mission team the time needed to identify two potential landing sites.

Rich Burns, the project manager for OSIRIS-REx, shares some of the main obstacles of this project. 'Bennu's low gravity provides a unique challenge for the mission,' says Burns. 'At roughly 500 metres in diameter, Bennu will be the smallest object that any spacecraft has ever orbited.'

Once a suitable site has been selected, the spacecraft will touch Bennu to collect a sample of its surface material, known as regolith. This is expected to occur in early July 2020, before OSIRIS-REx turns around and head back to Earth along with it its cargo, landing back around September 2023. 

The spacecraft OSIRIS-Rex launching from Cape Canaveral.

OSIRIS-REx launched from Cape Canaveral in 2016 © NASA/United Launch Alliance

Here's looking at Bennu

There are a number of reasons Bennu was selected out of the roughly 250,000 asteroids that are whizzing around in the solar system.

For a start, Bennu is unusual in that unlike most asteroids, its orbit is much closer to Earth's. While the majority of asteroids are found within the asteroid belt between Mars and Jupiter, Bennu's orbit actually crosses over Earth's, making it much easier to get to.

It has also been studied by astronomers ever since it was discovered in 1999. This means that compared to a lot of the asteroids out there, we think we understand quite a lot about the physical and chemical properties of Bennu. It is a type thought to be particularly primitive, made of components formed in the earliest stages of solar system formation, so can tell us more about this early era.

By actually visiting and sampling the asteroid it will allow researchers to see if their predictions are correct, which in turn will mean they can be more confident about analysing other asteroids that are harder to sample.  

The peculiar orbit of Bennu will also help scientists to understand what is known as the Yarkovsky effect. This is a phenomenon that is related to how small, dark objects in space absorb the heat from the Sun and then radiate it, influencing their rotation and thus the direction in which they travel.

This could help explain why Bennu's orbit is drifting, and will improve our ability to predict the directions of other asteroids as they come towards Earth.