Poised and ready for a date with Bennu

The OSIRIS-REx spacecraft is poised to make a daring landing on asteroid 101955 Bennu, briefly touching down in a tiny crater on its 490-metre surface to scoop up a sample of as much as two kilograms of dust and gravel for return to Earth for analysis.

The action is scheduled for 9:12am on Wednesday, Australian Eastern Daylight Time, and if all goes well it will be a first for NASA.

The Japanese have now done it twice, most recently with their Hayabusa2 mission, whose sample from another near-Earth asteroid, 162173 Ryugu, is scheduled to parachute to Earth near Woomera, South Australia, on 6 December.

However, NASA’s sample – due home in September 2023 – could be vastly larger than Hayabusa’s minimum target of 100 milligrams , not to mention giving scientists a chance to peer at material from another asteroid whose rocks date back to the dawn of the Solar System and might shed light on the formation of our own world.

And if there’s a prize for high drama in snagging its sample, OSIRIS-REx is the clear winner – assuming it survives the attempt.

Launched in 2016, the spacecraft was designed to find a smooth landing site at least 50 metres across, touch down, and gently lift off, in a maneuver describe as TAG (Touch-And-Go), in which only a sample-collection chamber about the size and shape of an automobile air filter would make contact, and that for a mere 10 seconds.

During those seconds, the spacecraft planned to blow compressed nitrogen gas into the loose materials of the asteroid’s surface, hoping to stir up bits the size of dust, sand and small gravel and suck them into its sample chamber.

Then it got to Bennu, and discovered that suitable landing sites were in short supply.

Bennu proved to be boulder-studded minefield on which the best landing site proved to be a tiny crater dubbed Nightingale in which the landing needed to be done within an eight-metre circle.

Worse, it was surrounded by 10-metre spires, the largest of which has been dubbed Mt Doom and is a serious threat to the spacecraft’s two-metre-long solar panels if they happen to clip it on landing or ascent.

Michael Moreau, the project’s flight dynamics system manager, of NASA’s Goddard Space Flight Centre, compares it to trying land in a space about the width of a couple of parking places, just two to three parking spots away from a two-to-three storey building. And on autopilot, 320 million kilometres away.

The mission’s project manager, Rich Burns, also from Goddard, notes that other engineers have called it “bullseye tag”.

Originally, he says, the plan for the landing was akin to throwing a dart at a dartboard and simply hitting the circle. “Now, we’re going to try to hit the centre of the bullseye.”

If it works, the spacecraft will secure its sample and prepare for its three-year return to Earth. But if it doesn’t succeed, and the spacecraft has to bail out to avoid catastrophe, all isn’t lost.

Not only does it have fuel to make additional landing attempts – either at Nightingale or elsewhere – but it has enough nitrogen gas to make three efforts at collecting a sample, in case the first one (or two) don’t succeed.

It even has an ingenious method of weighing its sample, once collected, to see if it meets the minimum goal of at least 60 grams: it can extend its sample-collection arm and spin, determining just how much the mass in it affects the spin rate – much like an ice skater spinning in the Olympics.

“By far, the most likely outcome is [that] we will come away with a large sample,” says Dante Lauretta, a planetary scientist and cosmochemist at the University of Arizona’s Lunar and Planetary Laboratory, and the mission’s principal scientist.

“But Bennu has thrown us a number of curve balls already, which is why we are fully prepared to TAG at the backup site if that becomes necessary.”

For those who want to watch the action live, it will be broadcast on NASA TV beginning at 8am AEDT and lasting for about 90 minutes.