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NASA’s OSIRIS-REx spacecraft is equipped with a suite of
instruments designed to study its target: a near-earth
asteroid called Bennu. One such instrument is called REXIS – The
Regolith X-Ray Imaging Spectrometer. That complex name
matches its complex task. "REXIS is something completely
different than the rest of the instruments aboard OSIRIS-REx.
And that’s because REXIS will detect elements – the individual
atomic elements that make up this asteroid. Everything else
is mapping the geology or the interior or even the mineral
structure, and we are at the most fundamental level looking
at what this asteroid is made out of." REXIS works off of how
the Sun’s X-ray light interacts with the surface material
covering Bennu. Atoms on the asteroid absorb these X-rays,
causing their electron levels to temporarily change and emit
their own X-rays. These re-emitted X-rays have a
specific energy that tells us about the atom that they came
from. This process is called fluorescence, but you can think
of it like a glow. REXIS is a telescope that images that X-ray
glow which allows scientists to create a map of the different
elements present on Bennu’s surface. And REXIS has the
ability to do all this based on its design and its position on
the spacecraft. "REXIS is about the size of a shoebox. It’s
located on the main instrument deck of the OSIRIS-REx
spacecraft – basically this is the side that’s always facing
the asteroid." "The pieces of the REXIS instrument are composed
of two primary parts. We have the main REXIS telescope, which
views the asteroid Bennu – it’s consisting of four CCDs that
look through a coded aperture mask. The coded aperture mask is
just a stainless steel plate with a number of holes that are
known to us. The second part of the X-ray instrument is called
the Solar X-ray Monitor, or the SXM. The SXM is viewing the Sun
at all times so that we can monitor the X-ray spectrum, the
solar x-ray spectrum incident on the surface of the asteroid."
"The Solar X-ray Monitor let’s us calibrate out the day-to-day,
even the hour-to-hour, variations in the X-ray flux
that comes from the Sun." So like your digital camera or
camera phone, the main spectrometer uses a light sensor, known as a
CCD or charge-coupled device. In this case there are four of
them, and they’re only sensitive to X-ray light. Before the X-ray
photons from Bennu are detected by the CCDs, however, they pass
through a mask with a random pattern of open and closed
holes. By analyzing how the shadow of the mask pattern is
shifted on the CCDs, scientists can determine the location of
the X-rays coming from the asteroid’s surface. This is how
REXIS images Bennu. And one surprising aspect of the REXIS
instrument is the team that built it: students from MIT and
Harvard. "The students are fantastic to work with, they’re
so eager to learn. It’s been great sort of watching them as
they grow in this experience. And I think they’ve come with
some interesting ideas and some ways of looking at it that we
may not have had had we been with someone who’s been doing
this for, you know, years and years and years." "Other missions
have had x-ray spectrometers onboard, but not nearly the
level of sensitivity and sophistication of REXIS. And
this really is a new generation, a next-generation in X-ray
imaging for planetary science, and it’s all being done by
students.
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