Sky & Telescope - May 2020 - 16

Bennu

Mercury
Venus

Earth

Ryugu
Mars
p NEAR-EARTH ASTEROIDS Both Bennu and Ryugu cross Earth's
orbit as they go around the Sun, making them potentially hazardous asteroids. The two have probably been in the inner solar system for several
million years. Jupiter and Saturn's gravitational effects on the main belt
drive asteroids inward, where they may survive for 10 million years or so
before hitting something or being kicked out. Positions are for May 1st.

and its Japanese counterpart, Hayabusa 2, which recently
left asteroid 162173 Ryugu after more than a year of exploration. Both projects are sample-return missions, designed to
brieﬂy touch down on the asteroids' surfaces, nab handfuls of
debris, and bring the bits back to Earth for study (S&T: June
2018, p. 22). And both have revealed wondrously rubbly and
perplexing worlds.

Formidable Terrain
Asteroids are not glamorous spacecraft targets. They don't
have swirling cyclones like Jupiter or ancient, dried-up deltas
like Mars. But these unassuming space rocks are precious
to planetary scientists, because they're time capsules. Their
qu ITOKAWA Below: The target of the
first Hayabusa mission, 25143 Itokawa
is a near-Earth asteroid with few craters
and patches of smooth-looking terrain. Its
longest dimension is 540 meters, about the
same size as Bennu. Right: This close-up
shows the smooth region around part of
Itokawa's neck. Most boulders shown are a
few meters wide.

M AY 2 0 2 0 * S K Y & T E L E S C O P E

chemical, structural, and geological makeups record a playby-play of the solar system's early years. Therefore, in order
to understand the origins of our planet and the compounds
that make up our bodies, scientists turn to these chunks of
planetary detritus.
Spacecraft have visited several asteroids, and we also have
meteorites fallen to Earth that, based on their composition,
we think come from different kinds of asteroids. We've even
brought back samples from one: 25143 Itokawa, visited by the
ﬁrst Hayabusa spacecraft some 15 years ago. Itokawa proved
a dead ringer for the most common type of stony meteorite,
called ordinary chondrites.
But ordinary chondrites aren't the most pristine planetary
crumbs out there. For that, we need carbonaceous chondrites.
Scarce on Earth, these meteorites are the closest chemical
matches to the Sun and look like the majority of the asteroids
we observe. So to acquire the least tainted bits of the solar
system's building blocks, scientists decided to snatch rocks
from two carbonaceous asteroids whose orbits cross that of
Earth: Ryugu and Bennu.
Small asteroids such as these - Bennu is about 500 meters
wide, Ryugu 1 km - are rubble piles, conglomerations of
debris from larger asteroids that broke up. Astronomers didn't
know much about these two particular worlds when they
picked them, only what they could infer from ground- and
space-based telescopes. Because the surfaces of Ryugu and
Bennu heat and cool quickly as they spin from daylight to
darkness, scientists expected the two asteroids to have relatively smooth, beach-like regions like those found on Itokawa,
and not to be covered in large rocks, which should take longer
to warm and chill. OSIRIS-REX in particular was designed
with a beach landscape in mind.
Nature had other ideas.
The spacecrafts' cameras revealed that both asteroids'
surfaces are a sea of shards. Boulders ranging from about a
meter to 100 meters wide dominate the landscape, with more
big boulders near the poles than at the equator. It's as though
someone took meteorites and
strewed them everywhere -
there's not a smooth region to
be found. Upon seeing Bennu's
surface, "My impression was
'Yikes! Where are we going to
go on this asteroid?'" Lauretta
said last September at a planetary science conference in
Geneva, Switzerland.
A range of rock types
populates the asteroids, which
are about as dark as fresh
asphalt. Many rocks appear
to be rubble piles themselves,
mosaics of fragments fused
together. There are boulders
with a crumbly, cauliﬂower

ORBITS: LE A H TISCIONE / S&T;
ITOK AWA A ND CLOSE-UP: JA X A

Asteroid Recon

Sky & Telescope - May 2020

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