OSIRIS-REx approaches the Nightingale landing site at an altitude of about 131 feet (40 meters) in this sequence of images from the August 11 TAG rehearsal. The large boulder that the craft passes over is two stories tall and nicknamed “Mount Doom”.
NASA/GSFC/University of Arizona
To get to Nightingale, the van-sized spacecraft will have to navigate around a two-story boulder that the team dubbed “Mount Doom,” aiming for a target area barely the size of a couple of parking spaces.
OSIRIS-REx's sample collection head dangles off the bottom of the craft. When it contacts the surface, it will release a burst of pressurized nitrogen gas that will stir up dust and gravel. If the head manages to find clean contact with the surface, it will funnel those particles into a collector inside its rim. Seconds later, the craft will fire its retro thrusters and zip away from the asteroid — and the rubble-cloud it kicked up — to safety.
“Years of planning and hard work by this team are essentially coming down to putting the TAGSAM into contact with the surface for just 5 to 10 seconds,” said Moreau.
If it doesn't work — for instance, if the collection head catches on a rock and fails to make flush contact — the team is prepared to try another TAG attempt in January at a backup site, nicknamed Osprey. But it doesn't have unlimited tries: TAGSAM is equipped with a total of only three nitrogen canisters. One way or another, the craft is scheduled to depart Bennu in March 2021 and deliver its samples to Earth on September 24, 2023.
On a roll
Already, OSIRIS-REx has returned an impressive amount of science in the nearly two years since it arrived at Bennu. On October 8, mission researchers published six papers in the journals Science and Science Advances covering a wide range of Bennu's unique characteristics and mysteries.
They include another surprise that greeted OSIRIS-Rex when it arrived: the asteroid has an active surface, with eruptions of coin-sized pebbles that fly off into space, ejected by causes that remain undetermined. “It was a little scary at first,” said Lauretta. But, he added, it wound up being a “real bonanza” for studying Bennu's gravitational field.
“They get accelerated by solar radiation pressure — kind of like a light sail — and they launch into orbit, and they go around the asteroid for multiple revolutions, before re-impacting back on the surface,” explained Lauretta. “And because we have hundreds of them, this allowed us to probe the gravity field of Bennu at a scale we never thought possible when we were designing the mission.”
The gravity measurements indicate that Bennu's density isn't constant throughout, though. Instead, it's clumpy, with denser regions towards the poles and less dense material at its core and equator, “It's as if there is a void at its center, within which you could fit a couple of football fields,” said study leader Daniel Scheeres of the University of Colorado Boulder in a press release.
Another completely unexpected find was that Bennu's surface ages differently from other asteroids and the Moon: typically, those bodies turn dark and red as they are bombarded by cosmic rays and micrometeorites. But Bennu turns blue as it weathers and becomes brighter. “This is an exciting finding because it tells us that something about Bennu is quite different from planetary surfaces we've observed,” Daniella DelleGuistina, a research scientist at the University of Arizona who led the work, tells Astronomy.
DelleGuistina hopes to further investigate this behavior with samples returned from Bennu. The Nightingale target site is relatively pristine, so researchers could try to recreate the space weathering process in the lab, using high energy lasers to shock and irradiate the samples, then observe what chemical changes cause the bluing, she says.
On top of that, it appears that some of Bennu's boulders come from another world — the asteroid Vesta. In a study published in Nature Astronomy on September 21, DelleGuistina and her colleagues reported some unusually bright boulders appearing among the dark fields of Bennu. Observations from OSIRIS-REx revealed that they are full of pyroxene, a signature of material that originated from Vesta.