Sky & Telescope - November 2022 - 8

NEWS NOTES
Asteroid 101955 Bennu, as imaged by the
OSIRIS-REX spacecraft
explains principal investigator Dante
Lauretta (University of Arizona).
In practice, the jet blasted out
6,000 kilograms (13,000 pounds) of
dust and rock, creating an elliptical
crater 9 meters long. " What we saw was
a huge wall of debris radiating out from
the sample site, " Lauretta says. " We
were like, 'Holy cow!' "
What's more, TAGSAM didn't stop
when it touched the surface. It kept
on going, pushing another half-meter
down before the spacecraft reversed
course. Particles stuck to the spacecraft
as well as to the TAGSAM, overflowing
the collector, and even jamming the
cover until they were carefully shaken
out days later.
The new analyses show that there's
SOLAR SYSTEM
Asteroid Bennu
Almost Swallowed
Spacecraft Whole
WHEN NASA'S OSIRIS-REX took a
sample from the asteroid 101955 Bennu
on October 20, 2020, it revealed that
this little world is more rubble pile than
solid rock. New analyses in the July 7th
Science and Science Advances show
that when the probe touched down, it
punched right through the surface. If
the spacecraft hadn't lifted itself back
out a few seconds later, the asteroid
would have engulfed it.
On sampling day, a 3.35-meter (11foot)
articulated arm attached to the
probe unfolded to deploy an aluminum
collection head; together, these pieces
make up the Touch and Go Sample
Acquisition Mechanism (TAGSAM). The
probe approached the surface at a slow
10 cm/s (0.2 mph). The plan was then
for the TAGSAM head to touch the surface
and stay there for 5 seconds while
a jet blew nitrogen gas into the surface
material, swirling some of it up into the
collection head. Lab tests of the sampling
procedure " barely made a divot, "
no cohesive force holding together
Bennu's outer layers other than the
asteroid's feeble gravity. " The particles
making up Bennu's exterior are so
loosely packed and lightly bound to
each other that they act more like a
fluid than a solid, " Lauretta explains.
Other work, published July 11th in
Nature Astronomy, explains why Bennu's
surface is so rough and boulder-strewn,
rather than dust-covered as planetary
scientists had expected (S&T: July 2019,
p. 8). Close examination of other small
rubble-pile asteroids, such as 162173
Ryugu, has also shown little dust.
Hsiang-Wen Hsu (University of Colorado,
Boulder) and colleagues used lab
experiments to show that static electricity
on these small worlds' surfaces
makes dust jump like popcorn, ejecting
most small particles from km-size asteroids
within a few million years.
Observations of Bennu so far have
shown this near-Earth asteroid is a
strange world in its own right. Analysis
of the 250 grams that OSIRIS-REX is
carrying, to be returned to Earth late
next year, will reveal further details.
¢JEFF HECHT
Watch OSIRIS-REX encounter Bennu at
https://is.gd/RubblePile.
The spacecraft extended a robotic arm
toward Bennu's surface (left), then stirred up
particles for sample collection (right).
8
NOVEMBER 2022 * SKY & TELESCOPE
MOSAIC OF BENNU: NASA GSFC / UNIVERSITY OF ARIZONA; BEFORE/
AFTER IMAGES OF OSIRIS-REX TOUCHDOWN: NASA GSFC
https://www.is.gd/RubblePile
Sky & Telescope - November 2022

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