Sky & Telescope - May 2020 - 21

p TOUCHDOWN Hayabusa 2 executed its second touchdown on July 11, 2019, near Ryugu's equator. The images capture 4 seconds before, the
moment of, and 4 seconds after touchdown.

nism is at work, or perhaps they work together. Thermal
stress could weaken the surface, "and then if a micrometeorite comes in and hits it, it's going to respond in a spectacular
way," Lauretta speculates. If thermal fracturing is the root
cause, then all near-Earth asteroids should be doing this,
he adds. But if water is the key factor, then only Bennu and
other hydrated asteroids will launch particles.
Since space agencies aren't likely to send a ﬂeet of spacecraft to check dozens of near-Earth asteroids for this activity, the OSIRIS-REX team has found a clever alternative: The
researchers partnered with SETI scientists to look for meteors
that might be from Bennu's debris. So far, nothing. But after
a couple of years, they hope to have enough observations to
identify any potential links between meteor populations and
near-Earth asteroids.

Landward, Ho
Hayabusa 2 has already ﬁnished its investigation of Ryugu,
leaving the asteroid last November with two samples safely
stowed. It will drop these in Australia during an Earth ﬂyby
at the end of 2020. With the extra propellant onboard, the
spacecraft might continue on to whizz by another asteroid.
OSIRIS-REX, which arrived at its asteroid about ﬁve
months after Hayabusa 2 did, intended from the beginning to
take more of a tortoise pace with its sampling. But the jagged
surface hasn't done the team any favors, either. The original
plan to monitor the spacecraft's descent with lidar doesn't
offer the guidance accuracy necessary to avoid potential

hazards. Instead, the researchers will track features using a
catalog they'll upload to the craft in advance.
OSIRIS-REX has a primary and a backup sample site, both
announced in December 2019. The ﬁrst, Nightingale, is a
relatively smooth spot in a 70-meter-wide crater high in the
northern hemisphere. Scientists think the crater and the
debris it unearthed are fairly fresh. The backup site, dubbed
Osprey, sits in a much smaller, equatorial crater surrounded
by several types of rocks.
The spacecraft will do multiple recon passes of both sites
in the ﬁrst half of 2020 before touching down, hopefully in
late August. It has until early 2021 to snatch its sample, then
the craft will start the trip back to Earth. The cargo should
drop in Utah in September 2023.
The retrieved rocks won't be in perfect condition. Both
missions' grab-and-go strategies involve some rather violent
jostling, and then there's the atmospheric entry. "It's coming
in at 12.4 km/s, so it's going to shake up a bit on that ride,"
Lauretta says. "No getting around that. I tried."
Still, scientists expect to learn much from Hayabusa 2 and
OSIRIS-REX's samples. The asteroid pieces, shaken up though
they may be, will be treasures of chemical and geological
insight, providing glimpses of how everything we see in the
solar system - including the delightful array of carbon-based
life around us - came to be.

¢ Science Editor CAMILLE M. CARLISLE gasped when she
first saw Ryugu's rugged landscape in a rover surface image.

JA X A (3)

Asteroid Prospecting?
Both scientists and starry-eyed entrepreneurs speak of
mining asteroids for water and metals. But although these
space rocks may someday provide valuable resources, we
have a long way to go before that day comes. "We're talking about the space economy of the 22nd century," says
asteroid expert Richard Binzel (MIT).
Water will likely be the first resource utilized, he says.
But first we have to learn how to recognize which asteroids

have minable water - something that, given the surprise
of a wet Bennu and dry Ryugu, we clearly need to work
on. The samples OSIRIS-REX and Hayabusa 2 bring back
will tell us how much water the asteroids' minerals contain
and potentially indicate how much future spacefarers could
extract globally.
"But I don't predict it for this century," Binzel says. "I
would love to be wrong."

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

http://www.skyandtelescope.org

Sky & Telescope - May 2020

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