This image shows a view across asteroid Bennu’s southern hemisphere and into space, and it demonstrates the number and distribution of boulders across Bennu’s surface. The image was obtained on Mar. 7, 2019 by the PolyCam camera on NASA’s OSIRIS-REx spacecraft from a distance of about 3 miles (5 km). The large, light-colored boulder just below the center of the image is about 24 feet (7.4 meters) wide, which is roughly half the width of a basketball court.  Credits: NASA/Goddard/University of Arizona

The composition of the particle plumes is under investigation,
but not yet known and will take some time to collect observations and interpret
the data, PI Dante Lauretta told me at the media briefing. 

This series of images taken by the OSIRIS-REx spacecraft shows Bennu in one full rotation from a distance of around 50 miles (80 km). The spacecraft’s PolyCam camera obtained the thirty-six 2.2-millisecond frames over a period of four hours and 18 minutes.  Credits: NASA’s Goddard Space Flight Center/University of Arizona

Here are additional details from NASA:


“Shortly after the discovery of the particle
plumes on Jan. 6, the mission science team increased the frequency of
observations, and subsequently detected additional particle plumes during the
following two months. Although many of the particles were ejected clear of
Bennu, the team tracked some particles that orbited Bennu as satellites before
returning to the asteroid’s surface.


The OSIRIS-REx team initially spotted the
particle plumes in images while the spacecraft was orbiting Bennu at a distance
of about one mile (1.61 kilometers). Following a safety assessment, the mission
team concluded the particles did not pose a risk to the spacecraft. The team
continues to analyze the particle plumes and their possible causes.


“The first three months of OSIRIS-REx’s up-close
investigation of Bennu have reminded us what discovery is all about —
surprises, quick thinking, and flexibility,” said Lori Glaze, acting director
of the Planetary Science Division at NASA Headquarters in Washington. “We study
asteroids like Bennu to learn about the origin of the solar system.
OSIRIS-REx’s sample will help us answer some of the biggest questions about
where we come from.”


OSIRIS-REx launched in 2016 to explore
Bennu, which is the smallest body ever orbited by spacecraft. Studying
Bennu will allow researchers to learn more about the origins of our solar system,
the sources of water and organic molecules on Earth, the resources in
near-Earth space, as well as improve our understanding of asteroids that
could impact Earth.


The OSIRIS-REx team also didn’t anticipate the
number and size of boulders on Bennu’s surface. From Earth-based observations,
the team expected a generally smooth surface with a few large boulders.
Instead, it discovered Bennu’s entire surface is rough and dense with
boulders. 


The higher-than-expected density of boulders
means that the mission’s plans for sample collection, also known as
Touch-and-Go (TAG), need to be adjusted. The original mission design was based
on a sample site that is hazard-free, with an 82-foot (25-meter) radius.
However, because of the unexpectedly rugged terrain, the team hasn’t been able
to identify a site of that size on Bennu. Instead, it has begun to identify
candidate sites that are much smaller in radius.


The smaller sample site footprint and the
greater number of boulders will demand more accurate performance from the
spacecraft during its descent to the surface than originally planned. The
mission team is developing an updated approach, called Bullseye TAG, to
accurately target smaller sample sites.


“Throughout OSIRIS-REx’s operations near Bennu,
our spacecraft and operations team have demonstrated that we can achieve system
performance that beats design requirements,” said Rich Burns, the project
manager of OSIRIS-REx at NASA’s Goddard Space Flight Center in Greenbelt,
Maryland. “Bennu has issued us a challenge to deal with its rugged terrain, and
we are confident that OSIRIS-REx is up to the task.” 


The original, low-boulder estimate was derived
both from Earth-based observations of Bennu’s thermal inertia — or its ability
to conduct and store heat — and from radar measurements of its surface
roughness. Now that OSIRIS-REx has revealed Bennu’s surface up close, those
expectations of a smoother surface have been proven wrong. This suggests the
computer models used to interpret previous data do not adequately predict the
nature of small, rocky, asteroid surfaces. The team is revising these models
with the data from Bennu. 


The OSIRIS-REx science team has made many other
discoveries about Bennu in the three months since the spacecraft arrived at the
asteroid, some of which were presented Tuesday at the 50th Lunar and Planetary
Conference in Houston and in a special collection of papers issued by the
journal Nature. 


The team has directly observed a change in the
spin rate of Bennu as a result of what is known as the
Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. The uneven heating and
cooling of Bennu as it rotates in sunlight is causing the asteroid to increase
its rotation speed. As a result, Bennu’s rotation period is decreasing by about
one second every 100 years. Separately, two of the spacecraft’s instruments,
the MapCam color imager and the OSIRIS-REx Thermal Emission Spectrometer
(OTES), have made detections of magnetite on Bennu’s surface, which bolsters
earlier findings indicating the interaction of rock with liquid water on
Bennu’s parent body.”