NASA’s OSIRIS-REx Spacecraft Discovers Erupting Particle Plumes at Asteroid Bennu

This view of asteroid
Bennu ejecting particles from its surface on January 19, 2019 was created by
combining two images taken on board NASA’s OSIRIS-REx spacecraft. Other image
processing techniques were also applied, such as cropping and adjusting the
brightness and contrast of each image.  Credits:
NASA/Goddard/University of Arizona/Lockheed Martin

Ken Kremer &
– 23 March 2019

In a major and unexpected finding, NASA’s
OSIRIS-REx spacecraft discovered plumes
of particles and rocks actively erupting from Asteroid Bennu just days after
arriving and achieving orbit on Dec. 31.

Furthermore in another major surprise the surface of the near
Earth asteroid Bennu is far rougher than expected and covered with boulders as
big as 10 meters is size.

leading the OSIRIS-Rex mission revealed the mystifying results at a media
telecon on March 19.

“The discovery of plumes is one of the biggest
surprises of my scientific career,” said Dante Lauretta, OSIRIS-REx principal
investigator at the University of Arizona, Tucson, at the March 19 media

“And the rugged terrain went against all of our
predictions. Bennu is already surprising us, and our exciting journey there is
just getting started.”

Asteroid Bennu is delivering surprise after surprise to the
scientists who now will have to scrutinize the tiny body in even greater detail
and even more than expected as they simultaneously search for a safe spot to
snatch a small sample of soil and rocks from the regolith for a return trip to
Earth in 2023.

achieved orbit around Bennu at 2:43 p.m. EST on December 31, 2018 at a distance
of 70 million miles (110 million kilometers) away from Earth.

NASA’s Origins, Spectral
Interpretation, Resource Identification, Security-Regolith Explorer
(OSIRIS-REx) spacecraft thereby broke a space exploration record as Bennu
became the smallest object ever to be orbited by a spacecraft
from Earth at a range of approximately 1 mile (1.57 km) from the asteroid’s

Bennu is estimated to be about 0.3 miles (510 meters) tall,
just a bit longer than the Empire State Building in New York City which is 443
meters tall.

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. 
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

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.” 


The spacecraft will extensively survey the
asteroid before the mission team identifies two possible sample sites. Close
examination of these sites will allow the team to pick one for sample
collection, scheduled for early July 2020. After sample collection, the spacecraft
will head back toward Earth before ejecting the Sample Return Capsule for
landing in the Utah desert in Sept. 2023.    

OSIRIS-Rex was launched on Sept. 8,
2016 on a United Launch Alliance Atlas V rocket from Cape Canaveral Air Force Station,

OSIRIS-Rex counts as NASA’s first mission to visit a near-Earth asteroid,
survey the surface, collect a sample and deliver it safely back to Earth. 

Watch for Ken’s
continuing onsite coverage of NASA, SpaceX, ULA, Boeing, Lockheed Martin,
Northrop Grumman and more space and mission reports direct from the Kennedy
Space Center, Cape Canaveral Air Force Station, Florida and Wallops Flight
Facility, Virginia.

Stay tuned here for Ken’s continuing Earth and
Planetary science and human spaceflight news: – – twitter @ken_kremer
– email: ken at

Dr. Kremer is a research scientist and journalist based in the
KSC area, active in outreach and interviewed regularly on TV and radio about
space topics.


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Ken Kremer

Watch for Ken’s continuing onsite coverage of NASA, SpaceX, ULA, Boeing, Lockheed Martin, Northrop Grumman and more space and mission reports direct from Kennedy Space Center and Cape Canaveral Air Force Station in Florida and Wallops Flight Facility in Virginia. Stay tuned here for Ken's continuing Earth and Planetary science and human spaceflight news. Dr. Kremer is a research scientist and journalist based in the KSC area, active in outreach and interviewed regularly on TV and radio about space topics. Ken’s photos are for sale and he is available for lectures and outreach events.

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