Sky & Telescope - July 2019 - 8

NEWS NOTES

Ryugu

Bennu

1,000 meters

500 meters

SOLAR SYSTEM

SCIENTISTS WITH NASA'S Osiris-REX
and the Japan Aerospace Exploration
Agency's Hayabusa 2 spacecraft provided preliminary results at the annual
Lunar and Planetary Science Conference regarding their respective explorations of near-Earth asteroids 101955
Bennu and 162173 Ryugu. A slew of
papers in Nature, Nature Astronomy,
Nature Geoscience, and Science accompanied the presentations.
Except for size, Bennu and Ryugu
are hard to tell apart in photos. Both
are so-called "rubble piles," collections of debris weakly bound by gravity.
Their low densities imply Swiss-cheese
interiors. On the surface, both terrains
are dark (reﬂecting about 4.5% of incident sunlight) and strewn with large
boulders. And, despite being around for
more than 100 million years, both bodies have few small craters, suggesting
that something - perhaps shaking from
bigger impacts - is ﬁlling them in.
Because of the hazardous surface
terrain, NASA is still planning OsirisREX's descent. Bennu's regolith - the
loose gravel and rocks on the surface
- might also present a problem. There
are no large patches of ﬁne regolith,
but sample collection systems were

J U LY 2 0 1 9 * S K Y & T E L E S C O P E

designed to retrieve particles less than
2 cm (0.8 inch) in diameter. Another
potential difﬁculty is that the surface
reﬂectivity varies more than predicted,
which poses a challenge for the spacecraft's laser-based navigation systems.
Bennu's surface thus presents a scenario
"beyond the spacecraft design speciﬁcations," notes the Osiris-REX team.
Nevertheless, the scientists are conﬁdent they can work with what they have
within the current schedule.
The Japanese spacecraft is a bit ahead
of its American counterpart, having
arrived at Ryugu roughly ﬁve months
earlier. Hayabusa 2 has already successfully deployed three rovers that snapped
pictures as they hopped around the
asteroid's surface. The mother ship also
skirted boulders to collect a sample of
regolith (S&T: June 2019, p. 10). And
on April 5th, Hayabusa 2 shot a copper
cannonball at Ryugu to create an artiﬁcial crater, with the aim of studying the
newly exposed ejecta.
Early observations have also revealed
the asteroids' differences. Over time,
rapid rotation has led to the formation
of ridges along their equators, but their
spins point to unique pasts. Bennu
rotates every 4.3 hours, and it's acceler-

p Particles spewed from Bennu on January 19th.

ating with time. Its period shortens by
a second every 100 years due to uneven
solar heating of its surface, known as
the YORP effect (named for scientists
Yarkovsky, O'Keefe, Radzievskii, and
Paddack). On the other hand, Ryugu's
shape suggests that it has slowed from
spinning as fast as every 3.5 hours to its
current rotation period of 7.6 hours.
The asteroids also have unexpectedly different water content. There are
hydrated minerals on both objects,
and Osiris-REX found evidence of
past interactions with water on Bennu
(S&T: Apr. 2019, p. 10). However, Ryugu
appears drier than expected; its parent
body might not have had much water.
The biggest surprise, though, is the
discovery of rocky plumes that occasionally spew from Bennu. The ﬁrst
plume was detected on January 6th. The
team has detected a total of 11 events
so far, three of them substantial enough
to eject dozens to hundreds of particles.
Most of these particles move relatively
slowly and eventually make their way
back to the asteroid's surface, but some
particles reach escape velocity. The
probability of any of these rocks hitting
the spacecraft is very low.
■ JAVIER BARBUZANO

BENNU: N ASA / GSFC / UNIV. OF A RIZON A; RY UGU: JA X A / TOK YO UNIV. / KOCHI UNIV. / RIKK YO UNIV. / N AGOYA UNIV. / CHIBA INSTIT U TE OF TECHNOLOGY / MEIJI UNIV. / AIZU UNIV. /
AIST; PA RTICLES: N ASA / GSFC / UNIV. OF A RIZON A / LOCK HEED M A RTIN

Ryugu vs. Bennu: Updates from the Asteroids

Sky & Telescope - July 2019

Table of Contents for the Digital Edition of Sky & Telescope - July 2019