NASA’s OSIRIS-REx spacecraft arrived at asteroid Bennu, an unexpected discovery
by the mission’s science team revealed that the asteroid could be active, or
consistently discharging particles into space. The ongoing examination of Bennu
– and its sample that will eventually be returned to Earth – could potentially
shed light on why this intriguing phenomenon is occurring.
The OSIRIS-REx team
first observed a particle-ejection event in images captured by the spacecraft’s
navigation cameras taken on Jan. 6, just a week after the spacecraft entered
its first orbit around Bennu. At first glance, the particles appeared to be
stars behind the asteroid, but on closer examination, the team realized that the
asteroid was ejecting material from its surface. After concluding that these
particles did not compromise the spacecraft’s safety, the mission began
dedicated observations in order to fully document the activity.
This animation illustrates the modeled trajectories of particles that were ejected from Bennu’s surface on January 19. After ejecting from the asteroid’s surface, the particles either briefly orbited Bennu and fell back to its surface or escaped away from Bennu and into space.
Bennu’s many surprises, the particle ejections sparked our curiosity, and we’ve
spent the last several months investigating this mystery,” said Dante
Lauretta, OSIRIS-REx principal investigator at the University of Arizona in
Tucson. “This is a great opportunity to expand our knowledge of how
the results of the observations, the mission team released their findings in a Science
paper published Dec. 6. The team observed the three largest particle-ejection
events on Jan. 6 and 19, and Feb. 11, and concluded that the events originated
from different locations on Bennu’s surface. The first event originated in the southern
hemisphere, and the second and third events occurred near the equator. All
three events took place in the late afternoon on Bennu.
The team found
that, after ejection from the asteroid’s surface, the particles either briefly
orbited Bennu and fell back to its surface or escaped from Bennu into space. The
observed particles traveled up to 10 feet (3 meters) per second, and measured from
smaller than an inch up to 4 inches (10 centimeters) in size. Approximately 200
particles were observed during the largest event, which took place on Jan. 6.
investigated a wide variety of possible mechanisms that may have caused the
ejection events and narrowed the list to three candidates: meteoroid impacts,
thermal stress fracturing and released water vapor.
impacts are common in the deep space neighborhood of Bennu, and it is possible
that these small fragments of space rock could be hitting Bennu where
OSIRIS-REx is not observing it, shaking loose particles with the momentum of
The team also
determined that thermal fracturing is another reasonable explanation. Bennu’s
surface temperatures vary drastically over its 4.3-hour rotation period. Although
it is extremely cold during the night hours, the asteroid’s surface warms
significantly in the mid-afternoon, which is when the three major events
occurred. As a result of this temperature change, rocks may begin to crack and
break down, and eventually particles could be ejected from the surface. This
cycle is known as thermal stress fracturing.
may also explain the asteroid’s activity. When Bennu’s water-locked clays are
heated, the water could begin to release and create pressure. It is possible
that as pressure builds in cracks and pores in boulders where absorbed water is
released, the surface could become agitated, causing particles to erupt.
But nature does
not always allow for simple explanations. “It could be that more than one
of these possible mechanisms are at play,” said Steve Chesley, an author
on the paper and Senior Research Scientist at NASA’s Jet Propulsion Laboratory
in Pasadena, California. “For example, thermal fracturing could be chopping
the surface material into small pieces, making it far easier for meteoroid
impacts to launch pebbles into space.”
fracturing, meteoroid impacts or both are in fact the causes of these ejection
events, then this phenomenon is likely happening on all small asteroids, as
they all experience these mechanisms. However, if water release is the cause of
these ejection events, then this phenomenon would be specific to asteroids that
contain water-bearing minerals, like Bennu.
activity presents larger opportunities once a sample is collected and returned
to Earth for study. Many of the ejected particles are small enough to be
collected by the spacecraft’s sampling mechanism, meaning that the returned
sample may possibly contain some material that was ejected and returned to
Bennu’s surface. Determining that a particular particle had been ejected and
returned to Bennu might be a scientific feat similar to finding a needle in a
haystack. The material returned to Earth from Bennu, however, will almost
certainly increase our understanding of asteroids and the ways they are both
different and similar, even as the particle-ejection phenomenon continues to be
a mystery whose clues we’ll also return home with in the form of data and
further material for study.
collection is scheduled for summer 2020, and the sample will be delivered to
Earth in September 2023.
Space Flight Center in Greenbelt, Maryland provides overall mission management,
systems engineering, and the safety and mission assurance for OSIRIS-REx. Dante
Lauretta of the University of Arizona in Tucson, is the principal investigator,
and the University of Arizona also leads the science team and the mission’s
science observation planning and data processing. Lockheed Martin Space in
Denver built the spacecraft and is providing flight operations. Goddard and
KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft.
OSIRIS-REx is the third mission in NASA’s New Frontiers Program, which is managed
by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s
Science Mission Directorate in Washington.
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