OSIRIS-Rex marks its spot
NASA decides exactly where on Bennu the sample-seeking mission will land.
And the nominations are: the four possible landing sites that made the short list.
By Richard A Lovett
NASA has picked the landing site on the asteroid Bennu for its sample-return mission OSIRIS-REx.
The target, the mission team announced to a meeting of the American Geophysical Union, is a 20-metre crater nicknamed Nightingale, ringed by jagged rocks, one of which is a 10-metre spike the scientists jokingly refer to as Mt Doom.
Landing there to collect the sample won’t be easy, however.
Michael Moreau, the project’s flight dynamics system manager from NASA Goddard Space Flight Centre, compares it to trying to land in a tightly packed urban neighbourhood.
“We are attempting to navigate the spacecraft down to a space that’s the width of a couple of parking places, just two to three [parking] spots away from a two-to-three storey building,” he says.
“It’s a tight fit,” adds the mission’s principal investigator, Dante Lauretta, from the University of Arizona.
Originally, the spacecraft wasn’t designed for such a hazardous landing; the scientists had expected to find broad enough areas of smooth terrain that they would have at least a 25-metre margin of error.
But the boulder-studded terrain of Bennu, the asteroid on which it will be landing, defeated that plan.
Now, if it’s going to complete its mission, OSIRIS-REx has to set down in a safety zone that allows only an eight-metre margin of error.
Nightingale is not the only possible landing site, but of the options studied it’s the most interesting, Lauretta says. “What really drove me is its scientific value,” he says.
Nightingale is interesting, in part, because it appears to be largely covered in fine-grained materials — important because the spacecraft’s sample collector can’t scoop up particles bigger than two centimetres.
But it’s also important, Lauretta says, because it’s located at 55-degree-north latitude and is relatively cool compared to sites near the equator.
That, he says, means that organic materials and hydrated minerals exposed there should be better preserved than those in warmer regions.
To land, Moreau says, it was necessary to reprogram the spacecraft’s landing automated landing protocols in order to increase their precision.
The team also created a high-resolution hazard map, so the spacecraft knows which areas should and shouldn’t be safe.
If the spacecraft finds itself heading for a dangerous area — or if the its onboard sensors see something they don’t like - it will back off and try again.
If all else fails, Lauretta says, there’s a backup site near the equator called Osprey, though it isn’t as interesting, scientifically. “But it has fewer hazards,” Lauretta says, “meaning it’s a good choice for the backup.”
Not that the landing will occur soon. First, the spacecraft will do two close flybys, coming as close as 250 metres to the surface in order to help the scientists refine their maps to the greatest possible level of detail.
Then it will make two practice landings without actually touching down. The first will come to within 50 metres of the surface, the second to within two metres.
Only then will it attempt an actual landing: though “landing” isn’t a perfect term, because only the spacecraft’s sample-collection arm will touch the surface, and that only for a few seconds.
Then it will fire its thrusters to escape back into the safety of space.
The target date, Moreau says, is 25 August 2020. But if that fails, there’s plenty of time until spacecraft’s return-to-Earth orbital window, in March-May 2021.
“That gives us several opportunities,” Moreau says.