Sky & Telescope - May 2020 - 17

the right asteroid?'" Sugita jokes. "It was
kind of a weird atmosphere."
Craters mottle both of the asteroids'
surfaces, with large ones on the ridges
that girth their equators. The pockmarks
look soft-edged, not sharp like many on
the Moon or Mars. Ryugu's big craters are
about as densely packed as those on Bennu,
but both appear to be short on craters
smaller than 50 meters or so, implying
something has erased them.
Such resurfacing might be connected
to the strange die shape, Sugita suggests.

u A BEACH OF ROCKS Hayabusa 2 deployed
two of its Minerva-II rovers to Ryugu's surface in
September 2018. Rover-1B took these shots on
September 23rd.

texture and others that look smooth and
sharp-edged. Some are distinctly brighter
than others. It's unclear how many of the
differences are due to composition, as
opposed to different degrees of exposure to
space weathering and other processes.
Scientists on both teams suspect the
rocks were able to masquerade as smoother
surfaces when seen from afar because
they're extremely porous - so porous that
Hayabusa 2 scientists can't see the difference between Ryugu's ﬁne-grained regolith
and its boulders when they look at the
surface in infrared, says the navigation cameras' science team
leader Seiji Sugita (University of Tokyo). On both Ryugu and
Bennu, the whole surface heats up at more or less the same
rate. "That's the surprising discovery," he says. "We are really
scratching our heads."
Another Hayabusa 2 experiment also suggests the asteroids are extraordinarily porous. Before its second and ﬁnal
touchdown on Ryugu, the spacecraft launched an explosive
projectile to blast a hole in the surface, digging up material
previously protected from space weathering. The scientists
expected the resulting crater would be a few meters wide.
Instead, the explosion dug a 13-meter-wide pit. Probably the
rocks' low porosity, paired with the asteroid's weak gravity,
explains the peculiarly large hole, mission manager Makoto
Yoshikawa (Japan Aerospace Exploration Agency) said at the
Geneva conference.

Hiding Their Ages (For Now)

RY UGU'S SURFACE: JA X A (2); CAULIFLOWER
ROCK: N ASA / GODDA RD / UNIV ERSIT Y OF
A RIZON A; MISSION TIMELINE: TERRI DUBÉ / S&T

Bennu and Ryugu are eerily similar. Both have the same
diamond shape, the same
density, same albedo. That's
not what scientists expected:
The two teams intended to
go to different asteroids and
then compare them. "When
we got there, to Ryugu, we
were like, 'Did, did we get to
u CAULIFLOWER This strange,
hummocky rock near Bennu's
equator is one of the darkest
boulders on the asteroid. The
small, light-colored rock perched
on it is 1.9 m (6.2 ft) long, about
the size of a person. The backup
touchdown site, called Osprey, lies
directly behind the boulder from
this perspective.

Mission Timeline
Hayabusa 2

2018

OSIRIS-REX

JUN 27: ARRIVAL AT
ASTEROID RYUGU
SEPT 21: MINERVA-II1 ROVERS
DEPLOYED TO SURFACE
OCT 3: MASCOT ROVER DEPLOYED
FEB 22: TOUCHDOWN 1

DEC 3: ARRIVAL AT BENNU

2019

APR 4: BLASTED ARTIFICIAL
CRATER USING EXPLOSIVE SMALL
CARRY-ON IMPACTOR (SCI)

DEC 31: ENTERED ORBIT
AROUND BENNU
FEB 28: DETAILED
SURVEYING BEGAN

JUL 11: TOUCHDOWN 2,
ABOUT 20 M FROM SCI CRATER
OCT 3: MINERVA-II2
ROVER DEPLOYED

COMPLETED

NOV 13: DEPARTURE
FROM RYUGU

2020
JAN 21: SITE RECON
FLYOVERS BEGAN

PLANNED
AUG 25: TOUCHDOWN
DEC: SAMPLE DROP
IN AUSTRALIA

2021
MAR: DEPART BENNU

2022

2023

SEPT: SAMPLE DROP IN UTAH

2024

s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 0

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Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020