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One of Earth's closest neighbors is a dark, jumbled
mass of rocks and boulders known as asteroid Bennu.
Bennu is ancient, a rugged survivor of the solar system's
chaotic past that may hold clues to the origins of life.
In October 2020, a NASA spacecraft called
OSIRIS-REx touched down on Bennu and collected a sample
for return to Earth.
Scientists had expected that this Touch-And-Go event,
or TAG, would have little impact on the asteroid.
After a slow descent, the sampler head would briefly
make contact, inject
a puff of gas, and capture a handful of material.
Perhaps it would also
leave a small divot at the sample site...a subtle
footprint in the soil...or so it was thought.
When images of the TAG event beamed back to Earth,
they were far more dramatic than anticipated.
Despite its slow touchdown, OSIRIS-REx had punched
through the surface and set off an explosion of loose material.
Tons of rocks and pebbles were ejected,
radiating outward in a wall of debris.
The pictures were stunning, but why did Bennu's surface
behave so unexpectedly?
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The answer involves cohesion,
an attractive force that can bind molecules together.
Cohesion gives water its surface tension and keeps
droplets together
even in a microgravity environment,
like the International Space Station.
Granular materials like wheat flour, cocoa, and dust can also
exhibit cohesion, which pulls individual grains into clumps.
On Bennu, scientists had expected cohesion
to act like a bit of glue between the rocks,
making its loose surface more solid.
But the TAG event showed
that Bennu's uppermost layers are nearly cohesionless,
deforming under stress like a fluid.
A good analogy is a ball pit.
Although the plastic balls are solid,
they easily slide past one another (and past
boisterous children), behaving en masse like a fluid.
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Thanks to OSIRIS-REx, we now know
Bennu's surface is not held together by cohesion,
but by gravity...or, microgravity, with a minute tug
less than 100,000th the pull of Earth.
On the Moon, gravity is 16% as strong as it is on Earth,
and more than 16,000 times stronger than it is on Bennu.
As a result, loose material in the lunar subsurface
is packed together more tightly,
making the Moon's surface relatively firm.
If a 50 kilogram mass of solid iron were to hit the Moon
at the same speed as the TAG event,
it would sink into the ground by only half a centimeter.
Repeating this experiment
at Bennu would yield a dramatically different result.
Though the mass would strike with the same force,
it would plunge 17 centimeters
before stopping - over 30 times deeper than at the Moon.
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Bennu has consistently defied scientists' expectations,
as each new finding reveals another facet of this small
but surprising world.
Using data from OSIRIS-REx, we now have the ability
to look back and accurately recreate thirty seconds
on asteroid Bennu.
On October 20th, 2020,
OSIRIS-REx made its final descent
to a sample site called Nightingale.
With its TAGSAM arm outstretched,
it approached the surface
at ten centimeters per second - the walking pace of an insect.
One second after contact,
it released a canister of pressurized nitrogen,
detonating an explosion of particles
and driving material
into the TAGSAM head for sample collection.
Six seconds after contact,
while it was still sinking into Bennu, OSIRIS-REx fired
its thrusters to begin the back away maneuver.
The engine burn lasted for 24 seconds, continuously
pushing against the spacecraft and rapidly slowing its descent.
Flying debris from the thrusters and the gas release
pelted the science instruments, clogging them with dust.
Nine seconds after contact,
when OSIRIS-REx had sunk nearly half a meter into Bennu,
it reversed course and began to rise.
At sixteen seconds, the TAGSAM head reemerged
from the subsurface,
as the spacecraft continued to accelerate.
Thirty seconds after contact, OSIRIS-REx shut off
its thrusters and drifted away with its sample of Bennu.
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Almost six months later,
on April 7th, 2021, the spacecraft returned
for one last flyover to observe its footprint.
At the point of impact was a new crater, averaging
eight meters across and reaching 68 centimeters in depth.
Thruster marks overlapped with this "TAG crater" in an X
pattern, increasing its volume by as much as 40%.
A ridge of ejected material that had been kicked up
during sample collection and then fallen
back to the surface, circled the crater like a campfire ring.
With a puff of gas and an engine burn, OSIRIS-REx
had displaced 12 cubic meters of granular material...
six tons of loose rock that may have been
packed together as lightly as a bowl of popcorn!
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After a final departure maneuver in May 2021,
OSIRIS-REx began a two-year journey back to Earth.
Stowed on board were about 250 grams of asteroid
Bennu - a bounty of scientific treasure
destined for future discoveries.
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