OSIRIS-REx marks the spot: NASA selects a landing site on asteroid ‘Bennu’ for its 2020 mission

NASA has selected the site for its asteroid sample collection mission from the four previously-proposed candidates after a year of study.

The spinning-top-shaped asteroid, ‘101955 Bennu’, is a 1,614 feet (492 m) wide near-Earth object with a cumulative 1-in-2,700 chance of hitting Earth from 2175–2199.

The chosen primary sample site — dubbed ‘Nightingale’ — is located in a young crater high up in the asteroid’s northern hemisphere.

The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer — or OSIRIS-Rex — craft has been analysing Bennu since December 2018.

If successful in its mission, OSIRIS-Rex will be the first US spacecraft to return samples of an asteroid to the Earth for analysis.

For NASA researchers, Bennu will act like a time-capsule from the birth of the solar system, containing information on its formation and evolution.

Its carbon-rich makeup will provide a window on the formation of the organic compounds that, on the Earth, ultimately enabled the origin of life.

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The OSIRIS-REx team have spent the last few months evaluating the potential of four candidate sampling sites on Bennu based on close-range data collected by the spacecraft.

Each of the four sites — which have been dubbed ‘Kingfisher’, ‘Nightingale’, ‘Osprey’ and ‘Sandpiper’ — had been initially put forward because they posed the fewest hazards to OSIRIS-REx’s safety while still affording great sampling potential.

The asteroid’s bird-based naming system stems from the name Bennu itself, which is taken from the ancient Egyptian bird deity that is linked with the sun, creation, and rebirth.

OSIRIS-REx will collect small (less than .8 inches / 2 cm in diameter) samples from Bennu by first slowly descending onto the asteroid’s surface.

A burst of nitrogen gas will then be used to blast the samples into a collection scoop at the end of the spacecraft’s robotic arm. 

‘After thoroughly evaluating all four candidate sites, we made our final decision based on which site has the greatest amount of fine-grained material and how easily the spacecraft can access [such],’ said principal investigator Dante Lauretta.

‘Of the four candidates, site Nightingale best meets these criteria and, ultimately, best ensures mission success,’ he added. 

The Nightingale site is located within a young crater on the north of Bennu that is around 460 feet (140 metres) wide.

Images taken of the crater show that its rocky surface material — the so-called ‘regolith’ — is dark in its colour, with its surface being relatively smooth. 

The low northern temperatures and young age of the crater mean that the surface material at Nightingale is well-preserved and relatively freshly-exposed.

This would allow OSIRIS-REx to take a close-to ‘pristine’ sample of the asteroid, from which scientists will be able to reconstruct Bennu’s geological history.

Although Nightingale may have been determined to be the best option from the four proposed sample sites, inherent challenges will still be present.

For example, the original OSIRIS-REx mission plan had called for a sample site of around 164 feet (50 metres) in diameter.

While the crater surrounding Nightingale has a diameter nearly three times that size, the safe area where the spacecraft can make its brief contact with the asteroid surface is in contrast around ten times smaller than planned.

In fact, the touchdown site will only be around 52 feet (16 metres) in diameter — which will call for highly accurate and precise targeting of the surface.

To further complicate matters, a building-sized boulder is also located on the eastern ring of the crater surrounding Nightingale, providing a hazard for OSIRIS-REx to avoid as it retreats from the site after samples are collected.

‘Bennu has challenged OSIRIS-REx with extraordinarily rugged terrain,’ said OSIRIS-REx project manager Rich Burns of NASA’s Goddard Space Flight Center.

‘The team has adapted by employing a more accurate, though more complex, optical navigation technique to be able to get into these small areas.’

‘We’ll also arm OSIRIS-REx with the capability to recognise if it is on course to touch a hazard within or adjacent to the site and wave-off before that happens.’

The spacecraft has been designed to be able to perform multiple attempts to collect rock samples, but there is the risk that successive tries in one location may end up becoming unfeasible.

The hazard-avoiding so-called ‘wave off’ manoeuvre requires the activation of the probe’s thrusters — the exhaust from which has the potential to disturb the surface of the collection site in the low gravity.

Such could potential prevent the craft from collecting a sample there on a subsequent attempt. 

Given this, the researchers have also selected Osprey as a backup sample-collection site, which the OSIRIS-REx spacecraft will visit in the event that it proves impossible to gather material from the Nightingale site.

With the primary and backup sample collection sites selected, the mission team will move in January to have OSIRIS-REx perform further reconnaissance flights over both Nightingale and Osprey — with such continuing throughout the spring.

When these have been completed, the probe will begin rehearsing its first sample collection manoeuvre, with the actual attempt scheduled for August 2020.

Once its sampling is complete, OSIRIS-REx will be departing from Bennu the next year and is expected to return to the Earth in the September of 2023.