Cheers as spacecraft tags asteroid

In a brief moment that its principal scientist Dante Lauretta described as “transcendental”, NASA’s OSIRIS-REx spacecraft successfully played tag with an asteroid.

The manoeuvre, officially called TAG (touch-and-go), saw the intrepid spacecraft touch town perfectly in one of the few safe landing zones on an asteroid 300 million kilometres away. 

It then activated its sample collection mechanism, and safely backed away, all without hitting any of the dangerous boulders flanking the tiny collection site, whose safe zone was a mere eight metres in diameter.

“I can’t believe we actually pulled this off,” Lauretta said as the control centre exploded in cheers and pantomimed high-fives. 

“The spacecraft did everything it was supposed to do,” he added, noting that the emotion of the success “is almost hard to process… Everything went just exactly perfect.”

The landing, which occurred right on schedule at 9:12am Australian Eastern Daylight Time on 21 October, is the culmination of the spacecraft’s four-year, multi-billion-kilometre journey to asteroid 101955 Bennu, from which it hopes to return a sample for study on Earth in 2023.

In the minutes leading up to the landing, the spacecraft used its array of 28 thrusters to inch down toward the landing zone and match velocity with the asteroid’s rotation, eventually touching down only 1.7 metres from its target with a vertical velocity of only 10.2 centimetres per second and a lateral drift of a mere 0.2 millimetres per second.

To put that in perspective, says mission scientist Beau Bierhaus, of Lockheed Martin, “[that’s] 15 times slower than walking speed”.

Such slow speeds are necessary because Bennu is so tiny – a mere 490 metres across – that its surface gravity is only 1/100,000th that of Earth’s.

“Everything kind of happens in slow motion when you’re acting in a microgravity environment like Bennu,” Lauretta says.

Navigation was also tricky. Not only did it have to be done automatically, since the asteroid is so far away that it takes more than 18 minutes for radio signals to travel there from Earth, but the large number of jagged boulders that needed to be dodged meant it couldn’t be done by the type of radar navigation conventionally used for such landings. 

Instead, says Mike Moreau, of NASA’s Goddard Space Flight Centre, it had to be done optically, with the spacecraft repeatedly calculating its position by taking images of surface features and comparing them to extremely high-resolution maps of the asteroid’s surface. That’s much like how people navigate, but it had never before been done for an operation like this.

At this point, all that NASA knows for sure is that the landing went perfectly and the sample collector operated as intended. 

Whether the spacecraft actually got a sample or not, however, is unknown. Any of a number of factors could have stopped it from doing so. It could, for example, have landed on a patch of large rocks too big for its sample head to ingest. Or, it might have landed partially on a large rock that tilted the sample head and kept it from making firm contact with the ground. 

The first inkling of whether it got what it was after will arrive at mission control, when the spacecraft reorients itself to point its high-gain antenna at Earth. It will then start beaming high-resolution images of the landing back home. 

“[These] images will tell us a lot,” Lauretta says – including what happened when the spacecraft fired the jet of nitrogen gas that the sampling mechanism uses to waft dust and gravel into its collection head. “I want to know how the surface responded,” he says.”

Later, the spacecraft will reorient the sampling arm so that it can peer at the sample head with its onboard camera. If that shows signs of material in the sampling head, or even clinging to its base, Lauretta says, that will indicate that the spacecraft collected at least something.

Finally, Lauretta says, the flight engineers will use “an incredibly clever physics technique” to see how much energy it takes to rotate the spacecraft with its sample arm extended at full reach. 

That had been measured earlier, with the chamber empty. And it’s possible to measure any change in it very accurately. “The precision is phenomenal,” Lauretta says. 

In that way, he says, it’s possible to weigh the sample to within tens of grams, from hundreds of millions of kilometres away.

The goal, he says, is to have collected at least 60 grams. But if there’s less than that? No problem. OSIRIS-REx doesn’t have to depart Bennu anytime soon for its return to Earth. And it has enough nitrogen gas in its sampling mechanism to make two more attempts to fill the chamber.

Related reading: The ambitious missions to bring asteroid rocks back to Earth