planned to take its first sample in October, but
the boulders proved so difficult that sampling was
pushed to February 2019 at the earliest.
Hayabusa2 will sample three sites to capture as
much of the asteroid’s mineral diversity as possible. One of the samples will come from within a
several-meter-wide crater that doesn’t yet exist.
The spacecraft will create the hole by firing a two-kilogram copper projectile at the asteroid, then
hide on the other side of Ryugu to avoid debris
when the projectile hits. The aim is to see if the
asteroid’s interior is different from the surface.
It’s hard to imagine NASA approving such a
crazy maneuver, says Nakamura-Messenger,
who grew up in Japan. It’s too risky. “The NASA
way, the American way, is: The success rate has
to be really high,” she says. But she’s rooting for
Hayabusa2’s bold moves.
“In my heart, I’m Japanese,” she says. “Therefore,
I’m like, ‘Go for it!’ ”
Bound for Bennu
Still, Ryugu’s surprise boulder field made
Lauretta, Nakamura-Messenger and the rest of
the OSIRIS-REx team nervous about Bennu.
“I’ve been lying awake at night anticipating
Bennu,” Lauretta says. “It’s fascinating and fright-
ening all at once.”
Fitting with NASA’s cautious approach, the
OSIRIS-REx team knew a lot more about Bennu
than JAXA knew about Ryugu before the missions
launched. Bennu came close enough to Earth in
1999, 2005 and 2011 for radio telescopes to map
the asteroid’s shape (though not close enough to
reveal much detail).
“We compiled the most comprehensive data-base from astronomy for any asteroid in the solar
system,” Lauretta says of the team’s prework on
Those radio measurements allowed researchers to see how sunlight nudges the asteroid on its
orbit, a phenomenon called the Yarkovsky effect.
As asteroids tumble through space, they absorb
sunlight on one side and re-emit that energy as heat
later, when that side faces away from the sun. The
force of that radiating heat is enough to push the
asteroid around, making it difficult to predict the
asteroid’s orbit over the long term.
The Yarkovsky effect calculation yielded a worrying prediction: Bennu has a 1 in 2,700 chance of
hitting Earth in the late 22nd century, one of the
highest probabilities of any known asteroid.
That forecast makes OSIRIS-REx’s mission
even more urgent. Testing the returned samples
will give scientists a better understanding of how
Bennu’s surface material absorbs and emits heat.
That information will sharpen the researchers’
predictions of where the asteroid will go, and help
inform future missions to deflect asteroids that
come too close to Earth.
That’s only if Bennu is smooth enough for the
spacecraft to get a sample. The first images taken
as OSIRIS-REx approached Bennu on December 3
didn’t do much to quell the team’s fears. With the
naked eye, Bennu seems to have about as many
boulders as Ryugu, maybe a little fewer, says
planetary scientist Kevin Walsh of the Southwest
Research Institute in Boulder, Colo.
“Even if we convince ourselves that there’s
a site that’s boulder free, there’s still a chance
it could change later on. So we’ll have to see,”
says Walsh, who presented an early comparison of Bennu and Ryugu on December 11 at
a Washington, D.C., meeting of the American
Geophysical Union. “We have plenty of tools to find
the places with the least amount of hazards, even if
we can’t find a place that’s completely free of them.”
That is a relief, Nakamura-Messenger says. But
every mission so far has surprised her.
“I don’t make wild guesses anymore,” she says.
“Nature is wilder.” s
s Tomohiro Yamaguchi et al. “Hayabusa2-Ryugu
proximity operation planning and landing site
selection.” Acta Astronautica. October 2018.
s Dante Lauretta et al. “OSIRIS-REx: Sample
return from asteroid (101955) Bennu.” Space
Science Reviews. October 2017.
Drop and dodge
To sample Ryugu below
the surface, Hayabusa2
will launch a projectile,
then take a lap around
the asteroid to avoid
damage (panels 1–3).
Returning to the new
crater, the spacecraft
will come close to the
surface and shoot a
small bullet from its
sampling horn. The
bullet will splash on the
surface, sending dust
and sand into a catcher
in the upper part of the
horn (panels 4–6).
Hazard map OSIRIS-REx took this mosaic image of
Bennu in early December and has seen signs of liquid
water in the asteroid’s past (see Page 6).