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This is Bennu: a near-Earth asteroid, a remnant
from the origins of the solar system, and the target of NASA’s
OSIRIS-REx mission.
When OSIRIS-REx arrived in December 2018, it brought Bennu
into focus, confirming early radar images that suggested the
asteroid was shaped like a spinning top.
But the close-up images also brought a big surprise.
Before OSIRIS-REx arrived, scientists had expected Bennu’s
surface to consist largely of fine-grained material, like a
sandy beach.
Sand heats up quickly during the day, and cools off quickly at
night.
In contrast, solid objects like rocks and boulders heat up and
cool down more slowly.
Infrared telescope observations had shown that as Bennu rotates,
its surface rapidly heats and cools – much like a sandy beach.
But OSIRIS-REx was greeted by a rocky world littered with
boulders – the size of cars, the size of houses, the size of
football fields.
This unexpected roughness confronted the mission with a
major challenge.
The main science goal of OSIRIS-REx is to briefly touch
down on Bennu, and grab a sample of fine-grained material for
return to Earth.
To protect the spacecraft, the original plan called for
touchdown in a boulder-free zone with a diameter of at least 164
feet.
But as it turns out, boulder-free sites of that size
don’t exist.
Following arrival, mission planners began looking at safe
zones that are just a few parking spaces wide, with enough
loose material to provide a sample.
In mid-2019, they identified four candidate sites, and named
them after birds that can be found in Egypt: Osprey,
Kingfisher, Nightingale, and Sandpiper.
Mission planners evaluated each site based on its safety to the
spacecraft, the ease of getting to the site, the amount of
sampleable material that it contains, and the science value
of the material itself.
After carefully evaluating these criteria, they chose Nightingale
as the primary sample collection site and Osprey as the backup.
Nightingale is located near Bennu’s north pole.
It sits inside a small crater measuring 66 feet in diameter.
Nightingale contains mostly fine-grained material, and has
multiple areas for sample collection.
It is also the darkest of the candidate sites, and has high
color variation, suggesting a diverse mineralogy.
Osprey is also located within a 66-foot-wide crater. just north
of Bennu’s equatorial bulge, and may contain rock types from both
the northern and southern hemispheres.
Osprey has the strongest signal for carbon-rich materials of all
four sites, and contains a dark patch at its center that is of
high scientific interest to the mission.
Now that the primary and backup sites have been chosen,
OSIRIS-REx is performing additional reconnaissance
flights to prepare for the sample collection event.
Later in 2020, the spacecraft will descend to Bennu’s surface,
briefly touch down, and collect up to four-and-a-half pounds of
loose material.
After collection, OSIRIS-REx will carefully stow the sample
and make the long journey back to Earth.
In late 2023 it will return the sample, delivering pristine
material from the origins of the solar system that will be
studied on Earth for decades to come.
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