Asteroid Bennu is much stranger than researchers expected
The landing of the OSIRIS-REx spacecraft on the asteroid Bennu resulted in a pile of debris.
When the OSIRIS-REx probe arrived at the asteroid Bennu, it discovered a celestial body that looked and behaved completely differently than scientists expected.
As NASA began planning its first mission to sample rock from an asteroid, the space rock science community was excited about another asteroid mission, Japan’s Hayabusa. In 2010, this device triumphantly delivered a fragment of a cosmic rock called Itokawa to Earth for the first time in history. A few years earlier, Hayabusa had mapped all of Itokawa, revealing a boulder-strewn landscape and smooth beach plains, or ponds, of gravel and sand.
It was these images of Itokawa that formed the basis for the design of NASA’s OSIRIS-REx mission. But, as it turned out, despite the external similarity, the asteroid to which OSIRIS-REx was supposed to be heading turned out to be completely different.
“The planning strategy for the OSIRIS-REx mission was to take Itokawa and all the observations of asteroid Bennu that we had previously made,” he said. Space.com Kevin Walsh, a planetary scientist at the Southwest Research Institute and lead scientist on the OSIRIS-REx mission regolith working group. “We looked at how differently [эти два астероида] reflect light and reflect differently on radar, and everything pointed to Bennu having more fine grain ponds than Itokawa.”
It wasn’t until OSIRIS-REx arrived at the asteroid Bennu, two years after its 2016 launch from Kennedy Space Center in Cape Canaveral, Florida, that the mission team discovered that their assumptions were “completely wrong,” Walsh said. Instead of sweeping plains of sand and gravel punctuated by clusters of boulders, the spacecraft’s cameras showed a “hellscape of boulders” with none of the smooth open areas where they expected OSIRIS-REx to land and collect a sample. .
The chief scientist of the spacecraft, Dante Lauretta, in his interview with the site Space.com reported that the team had concerns that collecting samples would not be possible at all.
“When we designed the spacecraft, we aimed for a landing accuracy of about 50 meters,” Lauretta said. “According to thermal and radar characteristics [Бенну] really looked like a smooth surface. So when I first saw that [поверхность совершенно другая]I thought we might have problems.”
A stereoscopic image of a rocky outcrop on the surface of the asteroid Bennu.
While the team was deciding whether their precious spacecraft could land safely among the huge boulders that, resisting Bennu’s weak gravity, rose to heights unprecedented on Earth, they received support from an unexpected source. Legendary Queen guitarist and noted astronomy enthusiast Sir Brian May contacted Lauretta to express his interest in the mission. May, who holds a PhD in astronomy, which he famously completed after a 30-year hiatus following Queen’s rise to fame in the 1970s, is also known for his interest in stereoscopic photography. It was this skill that he offered to the OSIRIS-REx team, which at the time was having difficulty finding an area sufficiently clear of boulders to land the spacecraft.
Stereoscopic photography reproduces the ability of the human eye to perceive the surrounding space in three dimensions. Special stereo cameras help rovers navigate the terrain. But the OSIRIS-REx spacecraft was not equipped with a stereo camera. However, May knows how to get around this problem: he collected images of various points on Bennu, taken from different angles, and processed them for three-dimensional viewing.
“Once you get a stereo image of a particular potential landing site, you can instinctively judge how well it will work out,” May said in interview website Space.com. “You can see that there’s a boulder there, how big the slope is, how dangerous it is to get on and off.”
With May’s help, the OSIRIS-REx team eventually identified a crater clear enough of obstacles to begin collecting samples. However, this required remote reprogramming of the spacecraft. Instead of the originally envisaged landing site 50 m wide, the spacecraft had to squeeze into the Solovyiny crater, which is only 10 m wide.
Asteroid Bennu turned out to be completely different from what scientists expected.
“At launch, we planned to use a laser altimeter to target the asteroid because we expected to see large, smooth areas,” says Lauretta. “We just thought we would need to know we were descending at the right speed to the surface. Instead, we had to completely change strategy, use on-board cameras and conduct an extensive mapping campaign, sometimes mapping objects just a couple of centimeters in size, to record them into the spacecraft’s memory so it can make real decisions and guide itself down to safety.”
The descent went smoothly. But when the OSIRIS-REx sample collection device pressed against the asteroid’s surface, something unexpected happened. Contrary to expectations, the surface behaved almost like a swamp. After a few seconds, the spacecraft plunged 50 cm deep into Bennu. As the sample collection head sucked in the sample and the craft’s reverse thrusters ignited, a huge wall of debris rose from the crater, engulfing the ascending spacecraft.
The OSIRIS-REx team learned about what had happened only after images from onboard cameras reached Earth. Researchers later acknowledged that the disturbed gravel could have damaged the retreating vehicle.
Walsh described the landing as “scientifically interesting, although operationally challenging.” It turned out that, as with the assessment of Bennu’s surface, the team was mistaken in estimating its density. The surface layer turned out to be unexpectedly loose, more like a liquid than a solid material, which was not shown by analysis of measurements from Bennu’s orbit.
“When we did the calculations, we initially used the density of the entire Bennu, which is 1.1 grams per cubic centimeter,” Walsh said. “But then our models showed that in order to compress the surface so much and drive the tag head so deep into the surface, the surface density would have to be about 0.4 grams per cubic centimeter. Otherwise, it was less than twice as dense than the whole body.”
Scientists still don’t know why Bennu’s surface is so similar to water. Walsh believes that small particles such as sand may have seeped into the asteroid’s interior through gaps between larger rock fragments, leaving a lot of empty space in the asteroid’s surface layer. This explains the unexpectedly low surface density, as well as the overall density of the asteroid, which appears to be much higher than that at the surface.
Despite all the difficulties, OSIRIS-REx collected much more asteroid material than planned, and on Sunday, September 24, the spacecraft will deliver this cargo to Earth. Lauretta hopes that the first scientific results from the samples will be published by the end of this year. And there is a possibility that Bennu will surprise researchers more than once.