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[music]

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NASA’s OSIRIS-REx spacecraft is
equipped with a suite of

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instruments designed to study
its target: a near-earth

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asteroid called Bennu. One such
instrument is called REXIS – The

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Regolith X-Ray Imaging
Spectrometer. That complex name

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matches its complex task. "REXIS
is something completely

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different than the rest of the
instruments aboard OSIRIS-REx.

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And that’s because REXIS will
detect elements – the individual

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atomic elements that make up
this asteroid. Everything else

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is mapping the geology or the
interior or even the mineral

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structure, and we are at the
most fundamental level looking

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at what this asteroid is made
out of." REXIS works off of how

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the Sun’s X-ray light interacts
with the surface material

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covering Bennu. Atoms on the
asteroid absorb these X-rays,

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causing their electron levels to
temporarily change and emit

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their own X-rays. These
re-emitted X-rays have a

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specific energy that tells us
about the atom that they came

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from. This process is called
fluorescence, but you can think

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of it like a glow. REXIS is a
telescope that images that X-ray

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glow which allows scientists to
create a map of the different

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elements present on Bennu’s
surface. And REXIS has the

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ability to do all this based on
its design and its position on

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the spacecraft. "REXIS is about
the size of a shoebox. It’s

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located on the main instrument
deck of the OSIRIS-REx

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spacecraft – basically this is
the side that’s always facing

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the asteroid." "The pieces of
the REXIS instrument are
composed

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of two primary parts. We have
the main REXIS telescope, which

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views the asteroid Bennu – it’s
consisting of four CCDs that

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look through a coded aperture
mask. The coded aperture mask is

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just a stainless steel plate
with a number of holes that are

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known to us. The second part of
the X-ray instrument is called

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the Solar X-ray Monitor, or the
SXM. The SXM is viewing the Sun

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at all times so that we can
monitor the X-ray spectrum, the

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solar x-ray spectrum incident on
the surface of the asteroid."

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"The Solar X-ray Monitor let’s
us calibrate out the day-to-day,

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even the hour-to-hour,
variations in the X-ray flux

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that comes from the Sun." So
like your digital camera or

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camera phone, the main
spectrometer uses a light
sensor, known as a

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CCD or charge-coupled device. In
this case there are four of

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them, and they’re only sensitive
to X-ray light. Before the X-ray

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photons from Bennu are detected
by the CCDs, however, they pass

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through a mask with a random
pattern of open and closed

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holes. By analyzing how the
shadow of the mask pattern is

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shifted on the CCDs, scientists
can determine the location of

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the X-rays coming from the
asteroid’s surface. This is how

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REXIS images Bennu. And one
surprising aspect of the REXIS

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instrument is the team that
built it: students from MIT and

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Harvard. "The students are
fantastic to work with, they’re

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so eager to learn. It’s been
great sort of watching them as

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they grow in this experience.
And I think they’ve come with

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some interesting ideas and some
ways of looking at it that we

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may not have had had we been
with someone who’s been doing

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this for, you know, years and
years and years." "Other
missions

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have had x-ray spectrometers
onboard, but not nearly the

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level of sensitivity and
sophistication of REXIS. And

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this really is a new generation,
a next-generation in X-ray

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imaging for planetary science,
and it’s all being done by

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students. 

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[beeping]