Australian Sky & Telescope - June 2018

28 AUSTRALIAN SKY & TELESCOPE July 2018

ASTEROID PLUNDERING by Dan Durda

W

e’ve been to asteroids before. Several of them, in

fact — flybys, orbital rendezvous... we’ve even

touched down on two asteroids and brought

back samples from one of them. But this winter,

two robotic emissaries will usher in a new era of exploration

when it comes to these small Solar System bodies.

That’s because these spacecraft, the Japanese space

agency’s Hayabusa 2 and NASA’s Osiris-REX, will study

two dark, primitive, near-Earth asteroids unlike any we’ve

explored before. Many of the asteroids we’ve examined

close up have been of just one type — the so-called S-type

asteroids (S stands for silicaceous). These dominate the inner

asteroid belt and may be the source of the most common

stony meteorites we have in our museum collections, known

as ordinary chondrites. But they only make up one-sixth of

all the asteroids we know of. The majority belong to another,

very important class: the carbonaceous C-type asteroids.

C-type asteroids (and the alphabet soup of other, closely

related ‘dark’ spectral types) have, for the most part, escaped

our exploratory escapades. But these carbon-rich bodies

are some of the most pristine survivors we have from the

Solar System’s early days. What little we know about their

composition suggests that it’s similar to the Sun, albeit

without most of the hydrogen, helium and other easily

vaporised compounds. As such, we think they’ve changed very

little since the planets formed some 4½ billion years ago.

We’re pretty sure we have plenty of meteorites from these

types of primitive asteroids — the carbonaceous chondrites. But

meteorites we collect here on Earth have complicated pasts and

don’t usually come with ‘Made in X’ labels on them. Trying

to decipher their origins is a little like trying to understand

the detailed geologic history of eroded pebbles gathered from

the bed of a stream flowing from a distant mountain range.

Samples hand-picked directly from these asteroids, on the

other hand, would give us a rare look at the very material from

which our own planet accreted, without ambiguity about its

provenance and context. Planetary scientists thus hope that

the small caches Hayabusa 2 and Osiris-REX bring back to

TOUCHDOWN A brief touch is all NASA’s Osiris-REX needs

to gather priceless info on the Solar System’s building blocks.

Japan’s Hayabusa 2 will use a similar strategy.

DS

ennu s surfaces

are about as

black as toner

cartridge powder.