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Most of us can remember playing tag as a kid. It was an easy game to learn:
identify your target, run around, touch, and get out of there.
Pretty simple, if you're on Earth. Now, how would you play this game in space, with
little gravity, and with an asteroid? That's actually part of the mission of
OSIRIS-REx; a NASA spacecraft that will study the asteroid Bennu in 2018. As
leftover debris from the solar system formation process that began over four
billion years ago, asteroids can teach us a lot about the history of the Sun and
planets. This is why OSIRIS-REx plans on collecting a sample of Bennu for us to
analyze. And to do that, it will play a more sophisticated version of tag,
with an instrument on board called the Touch-and-Go Sample Acquisition
Mechanism, or TAGSAM. The TAGSAM system consists of a robotic arm with an
attached sampler head. Think of this as the spacecraft's arm and hand. But unlike
the game we played as kids, where speed was a goal, the spacecraft needs to
approach Bennu carefully to avoid damage; moving about 10 centimeters a second
slower than a human's walking pace. When TAGSAM touches the surface of Bennu,
it will release a burst of nitrogen gas, causing loose rocks and soil to be
stirred up and directed into a collector on the sampler head. The entire process
will take about five seconds. There's enough nitrogen on board to support up
to three sampling attempts if necessary, but hopefully the first time's a charm.
As a backup, special contact pads on the bottom side of the sampler head are
designed to trap fine grained, dust like material, up to a few millimeters across.
After the spacecraft measures the mass of the sample, the TAGSAM head will then
be stowed in a Sample Return Capsule for the journey home. Just before reaching
our planet, OSIRIS-REx will release the capsule for
collection on Earth, where scientists will be able to study the sample of the
Bennu for decades to come. The TAGSAM feature was chosen, in part, to avoid some
of the difficulties that landing on an asteroid would present. Since the
gravitational force at the surface of Bennu is only a fraction of what we
experience on Earth, an astronaut trying to dig with a shovel there could easily
wind up launching himself into space.
OSIRIS-REx would have the same challenge. Anchoring the entire spacecraft to Bennu
wouldn't be any easier, and would add more technological complexity to OSIRIS-REx's
design. Plus, the asteroids surface is hot and extended exposure could cause
instruments on the spacecraft to overheat. So playing tag with Bennu is
actually one of the simplest and safest ways to get a sample. It's not child's
play, but when scientists considered their options, they said, "TAGSAM you're it."
Beep, Beep, Beep