When NASA's OSIRIS-REx spacecraft arrived at its destination, asteroid Bennu, on Dec. 3, it found a tiny world that in many respects looks like what the mission team had expected based on ground-based observations, but holds quite a few surprises, too.

In a series of eight scientific papers published March 19 across several Nature Publishing Group journals, the members of the University of Arizona-led OSIRIS-REx mission present their first up-close observations of asteroid Bennu since the spacecraft's arrival.

While the spacecraft's observations confirmed many of the measurements obtained by ground-based observations, Bennu turns out to be a more challenging target than what the mission was originally designed for.

Its surface is rougher and more varied in brightness than expected, prompting mission engineers and scientists to re-evaluate some of the approaches designed around the mission's primary goal: collecting a sample of surface material, or regolith, and returning it to Earth in 2023.

"Despite these newly found challenges, our team is confident that we will meet the science objectives," said Dante Lauretta, OSIRIS-REx principal investigator and professor of planetary science at the UA's Lunar and Planetary Laboratory.

"The OSIRIS-REx team designed the mission with flexibility and capabilities to deal with the unknown, and we are prepared to answer the challenges Bennu has given us."

Coinciding with these publications, mission members presented some of the findings made by the spacecraft upon arrival at the asteroid at the 50th Lunar and Planetary Sciences Conference in Houston on March 19.

The following provides the key findings published in the scientific papers whose principal authors are UA researchers - Lauretta, Daniella DellaGiustina, Carl Hergenrother, and Heather Enos, all of the UA's Lunar and Planetary Laboratory, or LPL. Josh Emery is an alumnus of the LPL.

One of the Darkest Objects in the Solar System with Rougher-Than-Expected Terrain
The OSIRIS-REx team developed its sampling strategy around what was known about Bennu when the mission was designed, promising plentiful patches of relatively smooth surface with gravel and pebbles less than 1 inch in size that expand over at least 55 yards (50 meters).

Instead, the spacecraft observed only a small number of regions, each between 5.4 and 22 yards (5 and 20 m), that are devoid of large boulders that pose a hazard to the spacecraft when it touches down to collect its sample.

Instead of having the equivalent of half a football field to navigate in, the spacecraft will have to operate within the confines of sampling sites that are half the size of a basketball court or smaller.

High-resolution images made with the OSIRIS-REx camera suite, or OCAMS, designed and built at the LPL, reveal a surface packed with more than 200 boulders larger than 33 feet (10 m) in diameter and many more that are 3 feet (1 m) or larger.

The largest boulder measures 63 yards (58 m) across. OCAMS measurements confirm that Bennu is one of the darkest objects in the solar system, reflecting only 4 percent of sunlight. One of the surprises turned out to be a larger-than-expected variability in surface albedo.

In other words, Bennu's surface features vary greatly from one another with respect to their brightness. The high variability in albedo presents a challenge for the laser of the spacecraft's lidar system, designed to guide the sample acquisition approach.

A Strange Rubble Pile
The asteroid has the spinning-top shape already evident in radar observations made in 1999 and 2005. Estimated to be between 100 million and 1 billion years old, the asteroid's surface is older than expected, but shows evidence of more recent activity. High-standing ridges run from Bennu's north to south pole and appear to direct the flow of surface material.

Features such as infill of large craters, fractured boulders and a deficiency in small impact craters hint at a dynamic surface with ongoing changes. Bennu is a rubble pile with a lot of void space, up to 60 percent total porosity, but its shape indicates that it has interior "stiffness" - enough internal friction or cohesion to allow the surface to crack.

A Watery Past
Spectral data obtained with the OSIRIS-REx Visible and InfraRed Spectrometer, or OVIRS, confirm Bennu's classification as a primitive carbonaceous chondrite and indicate that it most closely resembles aqueously altered CM chondrites.

Hydrated minerals are ubiquitous across the surface of the asteroid. There is evidence for molecules that contain oxygen and hydrogen atoms bonded together, known as hydroxyls.

The team suspects that these hydroxyl groups exist globally across the asteroid in water-bearing clay minerals, meaning that at some point, the rocky material interacted with water. While Bennu itself is too small to have ever hosted liquid water, the finding does indicate that liquid water was present at some time on Bennu's parent body, a much larger asteroid.

Spinning Faster
Bennu's rotation is accelerating steadily at about one second per 100 years. This is thought to be due to the so-called YORP effect, a phenomenon in which differences in reflectivity and temperature across an asteroid's surface result in a faster spinning rate over time, which in some cases can ultimately lead to an asteroid breaking apart.

Because Bennu's rotation rate has been changing, mission scientists infer that its surface slopes have been changing over the past million years.

In a surprise find, navigational images taken by the spacecraft upon arrival revealed particles in the vicinity of Bennu which will be investigated in more detail during the upcoming site selection campaign.

References:

* Lauretta and DellaGiustina et al., "The unexpected surface of asteroid (101955) Bennu," 2019 Mar. 19, Nature.

* DellaGiustina and Emery et al., "The regolith of (101955) Bennu from OSIRIS-REx imaging and thermal analysis," 2019 Mar. 19, Nature Astronomy.

* Barnouin et al., "Shape of (101955) Bennu indicative of a rubble pile with internal stiffness," 2019 Mar. 9, Nature Geoscience.

* Walsh et al., "Craters, boulders and regolith of (101955) Bennu indicative of an old and dynamic surface," 2019 Mar. 19, Nature Geoscience

* Hamilton et al. "Evidence for widespread hydrated minerals on asteroid (101955) Bennu," 2019 Mar. 19, Nature Astronomy

* Hergenrother et al., "Operational environment and rotational acceleration of asteroid (101955) Bennu from OSIRIS-REx observations," 2019 Mar. 19, Nature Communications

* Scheeres et al., "The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements," 2019 Mar. 19, Nature Astronomy

Related Links
OSIRIS-REx at NASA
Asteroid and Comet Mission News, Science and Technology

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OSIRIS-REx images close in on Bennu's northern hemisphere
Greenbelt MD (SPX) Mar 15, 2019
This trio of images acquired by NASA's OSIRIS-REx spacecraft shows a wide shot and two close-ups of a region in asteroid Bennu's northern hemisphere. The wide-angle image (left), obtained by the spacecraft's MapCam camera, shows a 590-foot (180-meter) wide area with many rocks, including some large boulders, and a "pond" of regolith that is mostly devoid of large rocks. The two closer images, obtained by the high-resolution PolyCam camera, show details of areas in the MapCam image, specifica ... read more