With the OSIRIS REx mission, NASA will do something that it has never done before: collect
and return a bit of dust from one of the solar system’s small asteroids.
It’s a rocky relic named Bennu. Bennu resembles two giant rocky cones that
have been smashed together, creating an asteroid that’s roughly the height of the Empire
State Building. It carries what may be the very dust that made the planets and the very
molecules that led to life on Earth. NASA’s spacecraft - OSIRIS-REx - plans to
collect at least 60 grams of the asteroid’s dusty surface to ship back to Earth.
But to get even a sample from a small body like Bennu is not easy.
The first challenge is gravity. Gravity on a fairly big body like the Moon is a fraction
of Earth’s. In the Moon’s case, it’s about one-sixth. But that’s still a heavy
tug compared with comets and asteroids, which can be 1/80,000th of Earth’s gravity.
Bennu’s gravity is 1/10,000th that of Earth’s.
To stay in orbit or touch the surface, OSIRIS-REx can’t move quickly. In fact, it needs to
move only a few times faster than a slithering snail.
When OSIRIS-REx heads toward the surface, it can gently glide down, sample and, and
with a slight push, go. But the second and bigger challenge is finding
a safe place to sample.
Many small bodies, including Bennu, are “rubble piles” – heaps of rocks that pulled together
from their mutual gravity. Everything from microscopic dust grains to
house-sized boulders cover the asteroid, leaving almost no flat surfaces for the spacecraft
to touch down. So how do you safely collect a sample from
these solar system lightweights? For OSIRIS-Rex, NASA turned to the Small Body
Mapping Tool. It’s a program designed at the Johns Hopkins Applied Physics Laboratory
in Maryland. The program can take images and other surface
data collected by the spacecraft and project them onto a 3D object that’s shaped like
the asteroid. In the case of Bennu, the program combined
3D maps of the asteroid’s surface that were made using two techniques.
The first technique looks at how shadows and surface brightness change on a crater, boulder
or other surface feature on the asteroid during the day. That information can be used to determine
a feature’s shape, curvature and slope. It can even find cracks or pebbles that are
less than 1 inch long. The other method comes from a laser instrument
called the OSIRIS-Rex Laser Altimeter, or OLA.
OLA scans sections of Bennu’s surface dozens of times, determining the height of each feature
by how long it takes a laser beam to return from that spot.
Scientists at APL and York University in Canada used these scans to triangulate points on
the surface.
They then turned these points into a 3D cloud shaped like the surface.
Both of these techniques can be used to distinguish large rocks that the spacecraft needs to avoid, from
smooth areas it can sample.
When combined in APL’s Small Body Mapping Tool, these maps create a complete 3D reconstruction
of the asteroid’s surface.
Every bump, pebble and crevice can be seen within inches of its actual location.
This is how NASA identified the few places safe enough for OSIRIS-REx to sample, including
Nightingale, the crater where the spacecraft will briefly touch down.
But the 3D maps were also key to helping tell the spacecraft how to get to the sample site
- without any instruction.
Scientists and engineers typically rely on gravity to help steer a spacecraft to the
correct position when landing on a planet.
But on Bennu, where gravity is weak, that method just wouldn’t fly.
Instead, engineers identified waypoints on the surface of Bennu.
APL scientists, with colleagues from the Planetary
Science Institute and York University, built 3D maps of these landmarks. These maps
are embedded in OSIRIS-Rex.
When the spacecraft flies over Bennu, it can compare the surface below with its onboard
model. As it finds the waypoints, the spacecraft automatically adjusts itself until it flies
to the sample site.
OSIRIS-REx will be the first spacecraft to ever use such autonomous technology to collect
a sample.
Scientists have found a clever way to navigate - and safely land - on one of the solar system’s
lightweights.
In doing so, they may uncover how both the solar system - and life itself - came to be.