WEBVTT FILE

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[Music]

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One of Earth's closest
neighbors is a dark, jumbled

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mass of rocks and boulders
known as asteroid Bennu.

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Bennu is ancient, a rugged
survivor of the solar system's

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chaotic past that may hold clues
to the origins of life.

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In October 2020,
a NASA spacecraft called

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OSIRIS-REx touched down on Bennu
and collected a sample

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for return to Earth.

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Scientists had expected
that this Touch-And-Go event,

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or TAG, would have little impact
on the asteroid.

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After a slow descent,
the sampler head would briefly

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make contact, inject

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a puff of gas, and capture
a handful of material.

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Perhaps it would also

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leave a small divot
at the sample site...a subtle

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footprint in the soil...or
so it was thought.

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When images of the TAG event
beamed back to Earth,

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they were far more dramatic
than anticipated.

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Despite its slow touchdown,
OSIRIS-REx had punched

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through the surface and set off
an explosion of loose material.

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Tons of rocks
and pebbles were ejected,

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radiating outward
in a wall of debris.

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The pictures were stunning,
but why did Bennu's surface

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behave so unexpectedly?

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[Music]

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The answer involves cohesion,

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an attractive force that can
bind molecules together.

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Cohesion gives water its surface
tension and keeps

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droplets together

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even in a microgravity
environment,

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like the International Space
Station.

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Granular materials like wheat
flour, cocoa, and dust can also

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exhibit cohesion, which pulls
individual grains into clumps.

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On Bennu, scientists
had expected cohesion

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to act like a bit of glue
between the rocks,

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making its loose surface
more solid.

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But the TAG event showed

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that Bennu's uppermost
layers are nearly cohesionless,

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deforming under stress
like a fluid.

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A good analogy is a ball pit.

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Although the plastic balls
are solid,

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they easily slide past one
another (and past

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boisterous children),
behaving en masse like a fluid.

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[Music]

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Thanks to OSIRIS-REx,
we now know

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Bennu's surface is not held
together by cohesion,

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but by gravity...or,
microgravity, with a minute tug

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less than 100,000th
the pull of Earth.

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On the Moon, gravity is 16%
as strong as it is on Earth,

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and more than 16,000 times
stronger than it is on Bennu.

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As a result, loose material
in the lunar subsurface

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is packed together more tightly,

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making the Moon's surface
relatively firm.

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If a 50 kilogram mass of solid
iron were to hit the Moon

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at the same speed
as the TAG event,

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it would sink into the ground
by only half a centimeter.

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Repeating this experiment

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at Bennu would yield
a dramatically different result.

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Though the mass would strike
with the same force,

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it would plunge 17 centimeters

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before stopping - over 30 times
deeper than at the Moon.

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[Music]

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Bennu has consistently defied
scientists' expectations,

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as each new finding reveals
another facet of this small

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but surprising world.

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Using data from OSIRIS-REx,
we now have the ability

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to look back and accurately
recreate thirty seconds

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on asteroid Bennu.

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On October 20th, 2020,

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OSIRIS-REx made its
final descent

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to a sample site
called Nightingale.

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With its TAGSAM arm
outstretched,

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it approached the surface

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at ten centimeters per second -
the walking pace of an insect.

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One second after contact,

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it released a canister
of pressurized nitrogen,

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detonating
an explosion of particles

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and driving material

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into the TAGSAM head
for sample collection.

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Six seconds after contact,

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while it was still sinking
into Bennu, OSIRIS-REx fired

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its thrusters
to begin the back away maneuver.

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The engine burn lasted
for 24 seconds, continuously

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pushing against the spacecraft
and rapidly slowing its descent.

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Flying debris from the thrusters
and the gas release

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pelted the science instruments,
clogging them with dust.

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Nine seconds after contact,

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when OSIRIS-REx had sunk
nearly half a meter into Bennu,

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it reversed course and began
to rise.

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At sixteen seconds,
the TAGSAM head reemerged

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from the subsurface,

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as the spacecraft
continued to accelerate.

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Thirty seconds after contact,
OSIRIS-REx shut off

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its thrusters and drifted away
with its sample of Bennu.

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[Fade music]

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Almost six months later,

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on April 7th, 2021,
the spacecraft returned

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for one last flyover
to observe its footprint.

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At the point of impact
was a new crater, averaging

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eight meters across and reaching
68 centimeters in depth.

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Thruster marks overlapped
with this "TAG crater" in an X

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pattern, increasing its volume
by as much as 40%.

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A ridge of ejected material
that had been kicked up

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during sample collection
and then fallen

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back to the surface, circled
the crater like a campfire ring.

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With a puff of gas
and an engine burn, OSIRIS-REx

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had displaced 12 cubic meters
of granular material...

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six tons of loose rock
that may have been

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packed together as lightly
as a bowl of popcorn!

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[Music]

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After a final departure
maneuver in May 2021,

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OSIRIS-REx began a two-year
journey back to Earth.

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Stowed on board were about
250 grams of asteroid

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Bennu - a bounty of
scientific treasure

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destined for future discoveries.

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[Music full]

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[Music fades]