The OSIRIS-REx spacecraft was launched with a clear mission:
collect a sample from an asteroid called Bennu, and send
it back to Earth. But with the number of known asteroids in our
solar system reaching into the hundreds of thousands, why was
Bennu chosen? What made it the best target? The first set of
selection criteria was proximity to Earth. The most accessible
asteroids are located around 74 to 153 million miles away. And
in that group, the most ideal targets are ones with an
Earth-like circular orbit and a low degree of tilt, due to
spacecraft maneuverability. At the time of selection in 2008,
that brought the number of known near-Earth objects down from
seven-thousand to one-hundred ninety-two. From there, the size
of the asteroid became a factor. Bigger meant better in this
case, because asteroids with smaller diameters rotate faster,
and material on their surface are more easily ejected. Not
ideal conditions for safely grabbing a sample. So, mission
scientists wanted the target to have a diameter larger than 200
meters. That brought the candidates down from one-hundred
ninety-two to twenty-six. The chemical composition of the
asteroid was the next factor, because we want our return
sample to help us learn more about the history of our solar
system and life on Earth. That dropped our target list to
twelve asteroids with a known composition. And from there,
only five of those were ideal for sampling because they are
known from Earth observations to be carbon-rich. This matters,
because a carbon-rich asteroid may contain organic molecules,
volatiles, and amino acids that may have been the precursors to
life on Earth. So now it’s decision time. Of these five
contenders, why Bennu? Quite simply, it’s because there was
more information about it available to scientists. It was
the only object of the five that had been observed with radar.
This provided not just an accurate measurement of its
size, dimensions, and shape, but also suggested it has a smooth
surface with an ample amount of material to sample, and few
large boulders to get in the way. We didn’t have this level
of detail with the others, which put the ability to safely get a
surface sample in doubt. Sorry fellas! The material we gather
from asteroid Bennu will help generations of scientists better
understand how the planets formed and the source of the
organic materials and water that helped spawn life on Earth. And
beyond learning about the past, studying Bennu can help us plan
for the future – everything from space travel, to utilizing
natural resources, and better understanding the risks of
asteroids. And this is why we chose Bennu.