Asteroid Bennu is turning out to be a hazardous place for the NASA spacecraft to sample: It has uneven, rugged terrain and occasionally even hurls rocks into space.

NASA’s Osiris-REX has revealed a few surprises about the surface of asteroid 101955 Bennu, where the spacecraft is supposed to retrieve a sample in July 2020. While preliminary studies and en-route imaging had predicted a smooth surface covered by fine-grained materials, close-up analyses conducted since the craft reached the asteroid on December 3, 2018, have revealed a bumpier reality. The analyses appear in a slew of papers in the journals Nature, Nature Astronomy, and Nature Geoscience.

Asteroid Bennu
Asteroid 101955 Bennu. Click to see the movie of its rotation
NASA / Goddard / Univ. of Arizona

Challenges to Sample Retrieval

Big boulders cover Bennu’s surface, 208 of them more than 10 meters (30 feet) in diameter, leaving few safe spots for the spacecraft to land safely. The slopes are irregular, too. In addition, the regolith – the loose gravel and rocks that cover the surface – includes more varied sizes than predicted, rather than the large patches of fine grains that scientists had expected based on ground-based measurements. That's problematic for sample collection systems designed to retrieve particles less than 2 centimeters (0.8 inch) in diameter.

Another potential difficulty for the landing is that the reflectivity of surface materials has a wider range than expected. Bennu is among of the darkest objects in the solar system, reflecting on average only 4% of incoming sunlight. However, some parts of the rocky surface reflect more than 15% of the light. This variability poses a challenge for the spacecraft’s laser-based navigation systems.

Osiris-REX principal investigator Dante Lauretta (University of Arizona) and his colleagues conclude in Nature that the asteroid’s varied reflectivity, range in particle size, and overall roughness of terrain “are beyond the spacecraft design specifications.” That’s the technical way of saying that they’ve got their work cut out for them. Nevertheless, the team is confident that they can work with what they have and keep within their current schedule, even if that means adjusting their original plans for how they’ll descend to the asteroid’s surface.

Candidate sample site on asteroid Bennu
As OSIRIS-REx surveys asteroid Bennu in detail, the mission team is beginning to identify potential locations for the spacecraft to collect a sample, such as this candidate site in the asteroid’s northern hemisphere. The candidate landing sites are smaller in radius than originally planned, requiring more precise spacecraft operations.
NASA / Goddard / Univ. of Arizona

Meet Bennu: An Ancient but Active Rubble Pile

Even before the spacecraft lands, it has already learned a great deal about the asteroid’s characteristics and history.

Bennu is slightly wider than the Empire State Building is tall, and researchers describe its shape as that of a spinning top; though, as Dave Dickinson puts it, it also bears an uncanny resemblance to a 10-sided die typical of table games. The asteroid’s shape is also strikingly similar to that of 162173 Ryugu, the target of Japan Aerospace Exploration Agency’s Hayabusa 2.

Based on Osiris-REX’s images of the asteroid’s surface, Barnouin (Johns Hopkins University Applied Physics Laboratory) and colleagues have determined that Bennu is a so-called rubble pile, with a porous, sponge-like interior. However, its shape — especially the presence of a series of ridges that run from pole to pole — also indicates an internal stiffness that helps hold the asteroid together, at least for now.

Currently, Bennu rotates with a period of 4.3 hours, but its period is shortening by about a second every 100 years. This is due to the something called the YORP effect (Yarkovsky-O'Keefe-Radzievskii-Paddack), where the asteroid gradually spins up as sunlight heats its surface unevenly. Mission scientists think that this YORP effect is driving changes in Bennu’s surface. Despite its estimated 100 million to 1 billion years old surface, Bennu doesn’t always show its age — it has large, ancient-looking craters but few small ones, hinting that its surface is slowly evolving.

Researchers had classified Bennu as a primitive carbonaceous chondrite, many of which have water and carbon-based compounds on their surfaces. Osiris-REX has now taken spectra of the asteroid’s surface, revealing hydrated minerals as well as evidence of hydroxyls, molecules that contain hydrogen and oxygen atoms bonded together. Researchers think that these molecules are part of water-bearing clay materials, which point to a past interaction with water, most likely before Bennu broke off from a much larger parent asteroid.

Rocky Plumes

Osiris-REX has also witnessed the asteroid spew rocks and dust into space at irregular intervals. “I would say [these events are] one of the biggest surprises of my science career,” said Lauretta on March 19th at the Lunar and Planetary Science Conference. The team still isn’t sure what’s causing these ejections. One idea is that, now that the asteroid is at perihelion — the closest it gets to the Sun in its orbit — the increased heating of its surface might play a role.

Particle plumes from Bennu
This view shows asteroid Bennu ejecting particles from its surface on January 19th. The image is a composite of two images taken by the NavCam 1 imager onboard NASA’s Osiris-REX spacecraft: a short-exposure image (1.4 ms), which shows the asteroid clearly, and a long-exposure image (5 sec) that shows the particles clearly.
NASA / Goddard / University of Arizona / Lockheed Martin

The first plume event was detected on January 6th; since then, the team has detected a total of 11 episodes. During three of the most substantial events, the asteroid ejected dozens or even hundreds of particles into the space around it. The particles range in size from centimeters to tens of centimeters in diameter.

Most of these particles move slowly relative to the asteroid’s escape velocity. While some of them assume short-lived orbits that are expected to last a few weeks or maybe months, they eventually make their way back to the asteroid’s surface, Lauretta said. But some particles reach velocities of several meters per second, fast enough to escape to interplanetary space.

The probability of any of these rocks hitting the spacecraft is very low.

At the press conference, Lauretta speculated about the possibility that the debris these events leave behind could result in meteor showers visible from Earth, when our planet crosses Bennu’s orbit in the future. “That would happen in late September every year,” he says. “We’re working with our colleagues at the SETI Institute in California to set up some monitoring cameras and see if there actually is a Bennu meteor shower.”


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