When the OSIRIS-REx probe arrived at the asteroid Bennu, it found a body that looked and behaved very differently than scientists expected.
When NASA began planning its first mission to capture a asteroid sample, the space rock science community was abuzz with excitement over another asteroid mission: Japan’s Hayabusa. In 2010, for the first time in history, that mission was accomplished triumphantly Earth a fragment of an asteroid, a space rock called Itokawa. A few years earlier, Hayabusa had mapped the entirety of Itokawa, revealing a boulder-strewn landscape but also characterized by smooth beach-like plains, or ponds, of gravel and sand.
It was these images of Itokawa that guided the design of NASA’s OSIRIS-REx mission. But as it turned out, despite some superficial similarity, the asteroid towards which OSIRIS-REx was headed turned out to be completely different.
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“The strategy for planning with OSIRIS-REx was to take Itokawa and all the observations of asteroid Bennu that we had done previously,” said Kevin Walsh, a planetary scientist at the Southwest Research Institute and lead scientist for the Regolith Development Working Group of the OSIRIS-REx mission, told Space.com. “So we would look at it in a different way [the two asteroids] they reflect light and the different way they reflect radar, and everything indicated that Bennu would have more fine grain ponds than Itokawa.”
It wasn’t until OSIRIS-REx arrived the asteroid Bennu, two years after its 2016 launch from Kennedy Space Center in Cape Canaveral, Florida, the mission team discovered that their assumptions were “totally wrong,” Walsh said. Instead of broad plains of sand and gravel interspersed with accumulations of boulders, the spacecraft’s cameras revealed a “hellscape of boulders” that had none of the smooth, open areas on which they imagined OSIRIS-REx would land and collect its sample.
The mission’s lead scientist Dante Lauretta told Space.com in a previous interview the team feared that sample collection might not be possible at all.
“When we designed the spacecraft, we had a precision-focused design [for the landing] of approximately 50 metres [164 feet]”Lauretta said. “The thermal properties, even the radar properties [of Bennu], it really looked like a smooth surface. So when I first saw it [the surface was completely different]I really thought we might be in trouble there.”
As the team grappled with the question of whether their precious spacecraft could land safely among the towering boulders rising against Bennu’s weak ship severity at heights never seen on Earth, they received support from an unexpected source. Legendary rock band Queen guitarist and noted astronomy enthusiast Sir Brian May contacted Lauretta to express her interest in the mission. May, who holds a doctorate in astronomy, completed after a 30-year hiatus imposed by Queen’s rise to fame in the 1970s, is also known for his interest in stereoscopic imaging. It was this ability that he offered to the OSIRIS-REx team, which at the time was struggling to find an area sufficiently clear of boulders on which to land the spacecraft.
Stereoscopic imaging replicates the ability of human eyes to perceive surrounding space in three dimensions. Dedicated stereo cameras help Martian rovers navigate their exploration site. But the OSIRIS-REx spacecraft was not equipped with a stereo camera. May, however, was able to solve this problem by selecting images of various points on Bennu taken from different angles and processing them for 3D viewing.
“Once you have a stereo image of that particular potential landing site, you can really make a gut judgment about whether or not things are going to work.” May told Space.com in a previous interview. “You see that there is this boulder, how much slope there is, how dangerous it is to go up and down.”
With May’s help, the OSIRIS-REx team finally identified a crater clear enough to attempt sample collection. However, the team had to remotely reprogram the spacecraft to accomplish the feat. Instead of the originally planned 50-meter-wide landing side, the van-sized spacecraft had to squeeze into Nightingale Crater just 33 feet wide (10 meters).
“When we launched, we planned to use a laser altimeter for guidance to the asteroid because we were expecting these big smooth areas,” Lauretta said. “We just thought we needed to make sure we were descending at the right speed towards the surface. Instead, we had to completely change strategy, using onboard cameras and running an extensive mapping campaign, sometimes mapping features as small as a couple of centimeters to be inserted into the spacecraft’s memory so it can make real decisions and guide itself to safety.”
The descent was smooth. But when OSIRIS-REx’s sample collection device pressed down on the asteroid’s surface, something unexpected happened. Contrary to expectations, the surface behaved almost like a swamp. Within seconds, the spacecraft sank 50 cm deep inside Bennu. As the sample collection head sucked in the sample and the spacecraft’s rear thrusters fired, a huge a wall of debris rose from the craterengulfing the ascending spacecraft.
The OSIRIS-REx team only learned what happened when images from onboard cameras reached Earth. The researchers later admitted that the kicked-up gravel could have damaged the retreating spacecraft.
Walsh described the landing as “scientifically interesting, although operationally challenging”. Just as the team misjudged Bennu’s surface area, it turned out they had misjudged its density as well. The surface layer was unexpectedly soft, behaving more like water than solid material, something that analysis of Bennu’s orbit measurements did not indicate.
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“When we did our calculations, we initially took the density of all of Bennu, which is 1.1 grams per cubic centimeter,” Walsh said. “But our models later showed that to be able to compress the surface so much and push the tag head so deep into the surface, the surface density would have to be about 0.4 grams per cubic centimeter. And so it was less than half the density as the whole body.”
Scientists still don’t know why Bennu’s surface has this water-like quality. Walsh believes that smaller sand-like particles may have seeped through the spaces between the larger rock fragments inside the asteroid, leaving a lot of empty space in the asteroid’s surface layer. This would explain the unexpectedly low density of the surface, but also the overall density of the asteroid which appears to be much higher than that of the surface.
Despite the challenges, OSIRIS-REx has collected much more material from the asteroid than the mission expected, and the spacecraft will drop this cargo on Sunday, September 24. Lauretta hopes to publish the first scientific results from the sample analysis by the end of this year. And it is likely that Bennu will surprise researchers again.