2019-07-01_Australian_Sky_&_Telescope

10 AUSTRALIAN SKY & TELESCOPE July 2019

BENNU: NASA / GSFC / UNIV. OF ARIZONA; RYUGU: JAXA / TOKYO UNIV. / KOCHI UNIV. / RIKKYO UNIV. / NAGOYA UNIV. / CHIBA INSTITUTE

OF TECHNOLOGY /

MEIJI UNIV. / AIZU UNIV. / AIST; PARTICLES: NASA / GSFC / UNIV. OF ARIZONA / LOCKHEED MARTIN

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 Osiris-

REX’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 designed

to retrieve particles less than 2 cm 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. 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.

hours, and it’s accelerating 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.

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

6. 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

Ryugu vs. Bennu: Updates from the asteroids

Ryugu Bennu

1,000metres 500 metres S Particles spewed

from Bennu on January 19.

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