Sky and Telescope - June 2018 - 26

The shoebox-size Mascot probe has its own camera, a
multi-color microscope to closely examine the asteroid's
surface materials, a radiometer to take Ryugu's temperature,
and a magnetometer. Its battery should power a 16-hour or so
investigation of the surface.
The Minerva-II rovers are direct descendants of the
Minerva deployed toward Itokawa from the original Hayabusa
but which unfortunately failed to actually reach the asteroid's
surface. There are two types of hopper: one larger, eight-sided
rover that's roughly 20 cm tall, and a second, hexadecagonal
pair, each just 10 cm tall, or roughly the size of the palm of
your hand. They contain their own cameras and other instru-

Figuring out just which types of meteorites come
from the various types of asteroids observed in
the main belt has been a Holy Grail of planetary
science for decades. We put asteroids into taxonomic groups based on their colors, albedos, and
the shapes and depths of absorption features in
the spectra of sunlight reǫected off their surfaces. We then attempt to interpret these spectral
properties in terms of actual mineral compositions and, in turn, connect the dots to link them
with the meteorite types we measure in our labs.
But because space weathering alters the spectral
properties of asteroids' surfaces, and because
some asteroid types show few clearly diagnostic
spectral features, this is easier said than done.
(In addition, the alphabetic designations and
groupings can vary depending on who is doing
the classifying.)
What we think we know is that the S-type asteroids are the source of the ordinary chondrites,
which are the most common stony meteorites
that fall on Earth. The primitive
carbonaceous chondrites
seem to be related to
the C-type asteroids
(and the other closely
related types of dark
asteroids like the
Bs, Fs, and Gs). The
predominate minerals we detect on
the M-type asteroids
appear to be metal,
making these bodies
the likely source of iron
meteorites.
26

J U N E 2 018 * SK Y & TELESCOPE

ments similar to those on Mascot. Each probe has two DC
motors inside that work together to "hop" the probes across
the surface. Combined with Mascot, the rovers will reveal
what the surface is like on a scale similar to what a human
explorer would experience poking around as a ﬁeld geologist,
nicely complementing the global-scale observations the main
spacecraft gathers.
Finally, the mother ship will descend toward the surface to
collect its prize. A meter-long, metal sampling horn extends
down from the spacecraft's underbelly. An aluminum contact
sensor and collapsible metal skirt will sense the touchdown
on Ryugu, setting off a sample collection process that will
shoot a 1-cm-size tantalum projectile into the bit of surface
inside the end of the sampling horn at a speed of 300 meters
per second (700 mph). The ejecta from the little impact will
travel up the horn and into one of three separate sample
containers inside the return capsule. The edge of the sampling horn is also folded back under to create an inner rim
designed to trap material that gets stuck in there when the
horn touches the asteroid, as a backup in case the projectile sampling mechanism fails to ﬁre (as happened on the
original Hayabusa mission). There's no time to linger: Just
one second after this sequence, the spacecraft will boost itself
away from the asteroid to avoid tipping over.
The entire descent, sample, and ascent sequence will be
repeated, up to a total of three times, ﬁlling all three sample
containers with about 100 milligrams of regolith particles.
The ﬁnal grab-and-go will be special, though. To uncover
material unaltered by space-weathering effects, Hayabusa 2
will ﬁrst shoot an explosive impactor toward the asteroid,
accelerating a copper projectile enough to excavate a crater
a few meters wide. The freshly exposed subsurface material
will serve as the sample site. A small camera subsatellite will
watch the cratering process unfold from about a kilometer
way in space, taking an image every second for later downlink to Earth while the mother ship hides behind Ryugu.
Once things are safe, the mother ship will again descend,
sample, and dart away.
Once all three samples have been acquired,
the sample container will be shut tight
inside the return capsule with an aluminum seal that will protect any volatiles
in the material from being vaporized by
exposure to space's vacuum.

Size of spacecraft
t SIZE AT A GLANCE Ryugu is
nearly twice as large as Bennu, and
both are more rotund than kidneybean Itokawa. The spacecraft icon
shows the approximate size of
Osiris-REX and Hayabusa 2.

RY UGU A ND ITOK AWA: JA X A , BENNU: OSIRIS-RE X TE A M / N ASA / GSFC

Asteroid Plundering

Sky and Telescope - June 2018

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