Bennu asteroid’s history comes to light days before epic spacecraft landing

An OLA scan that was taken over 5.5 minutes and contains 3,342,748 measurements. Shadows are in areas that were not visible from the perspective of OLA. Credit: NASA/University of Arizona/Canadian Space Agency/York University/MDA.

Just days ahead of a spacecraft landing on Bennu, a newly released model of the asteroid shows a complex history – including the fact that its surface regolith might have expanded and contracted at least once.

The information comes from data gathered on the Canadian-led OSIRIS-Rex Laser Altimeter (OLA). The instrument’s high-resolution laser gathered more than 3 billion data points on Bennu before failing last year after taking an order of magnitude more measurements than expected. (A lower-resolution laser is still functioning for things such as camera focusing support.)

One of the jobs of OLA is assisting with a touch-and-go by the spacecraft, whose acronym stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer. On Oct. 20, if all goes to plan, the NASA spacecraft will scoop a small sample of Bennu for return to Earth in September 2023.

The new paper, however, uses OLA data to better understand the formation and evolution of Bennu. Only a handful of spacecraft have ever visited an asteroid, and OLA provides the best look ever at the overall shape of such a small world, using the laser ranging method. Previous missions mainly relied on using data such as stereo imagery coupled with illumination models, said paper lead author Michael Daly, director of York University’s Centre for Research in Earth and Space Science, in an interview.

“This is the first time that we’ve presented the underlying data that we got [from OLA] last year, in July and August,” he said. “We built a 20-centimeter resolution of the asteroid using data from OLA, which was not published until now.”

The power of the dataset shines in a new movie released by the team, showing a global model of Bennu based on data from the Light Detection and Ranging (LIDAR) technique OLA used to probe the asteroid. Lidar is used to measure ranging distances from the asteroid to the spacecraft, providing an idea of the asteroid’s shape as well as the height of various features such as rocks. The movie shows a spectacular view of Bennu using the high-resolution data.

Underlying the data is valuable science to better understand the formation history of Bennu, which may be applicable to other asteroids. Scientists study these small worlds to better understand the evolution of our solar system, which once was filled with asteroids, comets and planetesimals (the beginnings of planets). While much of the early material eventually coalesced into the planets and moons we see today, quite a number of small worlds still exist – providing a valuable time capsule for studying how the solar system came together.

OLA has revealed two key aspects of Bennu that never showed before, either through previous spacecraft examinations or through observations with facilities such as the Arecibo Telescope in Puerto Rico, which obtained low-resolution radar images of Bennu prior to OSIRIS-REx’s launch in 2016.

The first aspect is surface asymmetry between the north and south of Bennu. Further, boulders in these regions are holding back finer material from migrating to the equator, which would be expected otherwise from what scientists predict, based on the asteroid’s spin about its axis. 

“Those finescale processes relate to the age of the surface,” Daly said, adding that the age of the surface may not be the true age of the asteroid as the surface is subject to a lot of change. More information (and a possible age range) is likely forthcoming with future releases of OLA data, but for now, Daly said the age of the asteroid is still being studied.

OLA-derived albedo registered to a shape model slightly shaded to indicate topography - Dark (green and blue) areas are more prevalent in the southern hemisphere, with dark “spokes” present between the longitudinal ridges (approximate locations shown in white). The sides of some rocks do not have meaningful OLA lidar albedo data owing to observation angle limitations and so appear gray in some views.
OLA-derived albedo registered to a shape model slightly shaded to indicate topography – Dark (green and blue) areas are more prevalent in the southern hemisphere, with dark “spokes” present between the longitudinal ridges (approximate locations shown in white). The sides of some rocks do not have meaningful OLA lidar albedo data owing to observation angle limitations and so appear gray in some views. Credit: NASA/University of Arizona/Canadian Space Agency/York University/MDA.

The other new finding relates to some ridges found on Bennu, primarily in the northern regions but with some others shown in the south. Bennu, like many asteroids, is built a little like a rubble pile that is loosely held together by gravity. The ridges show that at least once in Bennu’s history, the rubble pile expanded into space and then contracted back again, not wholly tearing the asteroid apart but still, in Daly’s terms, “breathing out” a little bit.

This event may have happened due to something called the YORP (Yarkovsky–O’Keefe–Radzievskii–Paddack) effect, which happens when a rotating asteroid absorbs solar radiation and then re-emits it as energy. This process can speed up an asteroid’s spin, making the asteroid rotate faster. That said, the “breathing” at Bennu is unique among the handful of asteroids studied up close, Daly said – which makes it difficult to predict how common it is among the asteroid population more generally.

Daly said it’s also possible that Bennu has periodic breathing episodes, but that would be difficult to predict except through modelling. “Unfortunately, when [these] disruptions occur, they reset the whole asteroid,” he explained, meaning that the surface becomes disrupted to such an extent that any evidence of past disruptions is erased. “All we can really do is look at the last version of the asteroid, and put constraints on that disruption,” he explained.

Canada’s OLA, which is supported by the Canadian Space Agency, has been a powerful technique to get a sense of Bennu’s shape and size. OLA also ended up playing a key role in planning for the landing after Bennu’s surface was found to be more complex than predicted, Daly said, which meant the team took on extra, unexpected duties for landing support. “It’s been exceptionally busy for my team at York,” he said, and that’s even before taking into account the need to operate remotely from the main spacecraft team in Tucson, Arizona due to novel coronavirus pandemic quarantine restrictions.

OLA has been a boon for the asteroid study, he added. It isn’t a perfect instrument – sometimes it underestimates the heights of boulders, and it tends to perform a little less well in areas with “poor geometry” to analyze, such as at the poles. But overall, OLA has given scientists the full shape of the asteroid and eventually will provide a map of features to, in places, an incredible 5 cm in resolution.

One area that will really benefit from this fine detail will be examining the physical properties of craters nearer to the surface. Crater counting will also give a sense of the frequency distribution, which in turn should (with further iterations of the data) lead to an estimate for the surface age of Bennu, Daly said.

A paper based on the research was published Oct. 8 in Science Advances.

About Elizabeth Howell

Elizabeth Howell
Is SpaceQ's Associate Editor as well as a business and science reporter, researcher and consultant. She recently received her Ph.D. from the University of North Dakota and is communications Instructor instructor at Algonquin College.

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