Australian Sky & Telescope - June 2018

30 AUSTRALIAN SKY & TELESCOPE July 2018

water, and Ryugu is a small, airless world without weather,

the possible presence of clays suggests that Ryugu’s larger

parent body was heated just enough to melt at least some

of the water ice it accreted along with rocky minerals as it

formed. Hayabusa 2 may thus bring us a sample of early Solar

System materials that have been altered by ancient water.

With a mean diameter of 492 metres, Bennu is about

half the size of Ryugu. The asteroid passes close to Earth

every six years, making Bennu simultaneously one of the

most potentially hazardous near-Earth objects and the most

accessible carbonaceous asteroid.

These close approaches have provided numerous

opportunities for ground-based optical and radar observations,

meaning Bennu is also one of the best-characterised near-

Earth asteroids, with detailed assessments of its size, shape

and spin state. Its rapid, 4.3-hour rotation and top-like shape

suggest that it’s been spun up by solar thermal effects and

possibly by tidal effects due to close encounters with Earth,

both of which would cause loose surface material to migrate

toward the equator. The radar observations also show evidence

for at least one house-size boulder on the surface. Based on

what the first Hayabusa spacecraft found at the near-Earth

asteroid 25143 Itokawa, scientists expect to find plenty of rocks

strewn across the landscape.

Long before Bennu evolved into its present orbit, it

likely began life as part of

larger, 100-km-scale par

asteroid somewhere in t

inner main belt 4.6 billi

ago. The mineralogy imp

by its spectral properties

suggests it has survived

largely unchanged by

geochemical processes s

its initial assembly durin

first several million year

Solar System history. Ba

detailed computer simul

the fragment that becam

Bennu was likely knocked from

its parent body between 700

million and 2 billion years ago

and gradually found its way

into near-Earth space through

a combination of rotation-

fed orbital drift and the giant

planets’ gravitational effects.

Grab and Go

The challenge for both missions

is to rendezvous with, touch, and

acquire samples from the surfaces

of very small Solar System bodies,

bodies for which we have very

little ‘hands on’ intuition. How do you design a sample-

collection strategy when you don’t know the surface’s makeup

or the size of its fragments, and in a microgravity environment

that can turn even the simplest of everyday field geology

activities into a quagmire of unexpected outcomes? Will you

be sampling fine dust, pea-size gravel, or the solid surface of

exposed bedrock? Do you plan to scoop up a loose aggregate,

drill into hardened rock, or blast the asteroid with a projectile

and fly through the cloud of lofted debris? And how do you do

any of these without endangering the spacecraft itself?

Fortunately, decades of observational and theoretical

studies of asteroids and the findings from the pioneering

missions that preceded Hayabusa 2 and Osiris-REX have taught

us at least a little of what to expect when we arrive at Ryugu

and Bennu. Every asteroid we’ve explored so far has had some

sort of fragmented regolith on its surface, a layer of pulverised

rock built up by millennia, even eons, of impacts by smaller

asteroids. So we expect the surfaces of Ryugu and Bennu to be

similarly littered with rocky samples just waiting to be nabbed.

Furthermore, because the asteroids are so small, their

surface gravities will be very weak, making operations there

more akin to zero-g docking at the International Space Station

than a hike across a lunar plain. These considerations rapidly

funneled mission planners toward sampling concepts that

involved a form of a ‘touch-and-grab-some-gravel’ approach.

XEXPLOSIVE

ARRIVAL On its

inal sampling run,

Hayabusa 2 will drop

an explosive impactor

toward Ryugu (1) then

dash to hide behind

the asteroid’s limb

before the impactor

detonates. As it lees,

the mother ship

will drop a camera

satellite to watch the

explosion (2). Once

the explosion-sped

projectile carves a

crater in the asteroid

(3), Hayabusa 2 will

return to the site and

briely touch down

(4-5). A projectile shot

from its collecting horn

will spray debris up

into the spacecraft,

where it will be caught

and cached. A second

later, Hayabusa 2 will

take off (6).

ofa

rent

he

on years

plied

s

ince

ngthe

rsof

sed on

lations,

me

ed from

Ion engine

Thrusters

Navigation cameras

Navigation camera

Laser altimeter

Explosive impactor

Target mar ker s

Rovers Thermal infrared imager

Sampler horn

Near infrared

spectrometer

Lander

Reentry

capsule

Star trackers

XHAYABUSA 2

INSTRUMENTS

ASTEROID PLUNDERING