Astronomy - September 2015

48 ASTRONOMY • SEPTEMBER 2015

Southwest Research Institute in Boulder, Colo rado,
and Alice’s principal investigator, in a June statement.
“It shows us the value of going to comets to observe
them up close, since this discovery simply could not
have been made from Earth or Earth orbit with any
existing or planned observatory.”
Scientists also have learned that Comet 67P’s
weak gravity still has the power to hold onto com-
etary debris. At first blush, the jets spewing from the
comet’s surface seem strong enough to shoot any
dust particles well beyond the nucleus’ sphere of
inf luence. After all, the dust tail forms when the
weak pressure of solar radiation pushes away these
ejected grains.
But Rosetta’s main camera system, the Optical,
Spectroscopic, and Infrared Remote Imaging System
(OSIRIS), targeted the motions of cometary particles
located in a 12° by 12° area of sky. It tracked four
debris pieces ranging from 6 inches (15cm) to 20
inches (50cm) across for which mission scientists
could calculate orbits. Three of the particles appar-
ently orbit 67P’s nucleus and likely were ejected dur-
ing the comet’s last perihelion passage in 2009, while
the fourth definitely is on an escape trajectory.

An icy demeanor?
Sublimation of 67P’s ices drives away many, but
not all, dust particles — the comet’s dark surface
proves that. Some of the dust remains on the surface
as ices underneath sublimate. But much more gets
released slowly and then falls back somewhere else,

covering the surface with a thin veneer and leaving

little exposed ice. “A 1-millimeter-thick [0.04 inch]

layer of dark dust is sufficient to hide the layers

below from optical instruments,” said Holger Sierks,

OSIRIS principal investigator at the Max Planck

Institute for Solar System Research in Göttingen,

Germany, in a June statement.

But scientists using OSIRIS’ narrow-angle cam-

era did discover water ice in more than 100 patches

on the comet’s surface. These areas are typically

several feet (a few meters) across and up to 10 times

brighter than the surface average. Some occur in

isolation, often on boulders that seem to have been

transported from elsewhere. Others appear in clus-

ters and usually lie in debris fields at cliff bases.

These might be regions exposed by recent erosion

or a cliff ’s collapse.

The researchers claim the bright areas must be

water ice because that substance would be stable

under the Sun’s illumination at the observation time,

while both carbon dioxide and carbon monoxide

ices would have sublimated under the same sunlight.

The bright patches should shrink or even disappear

as the comet approaches perihelion, but new ones

could appear in recently exposed areas.

Second chances and longer looks

The next few months should provide a fascinating

look at the changes a comet undergoes as it nears the

Sun. Perihelion occurs in mid-August at a distance

of 116 million miles (186 million km). With any luck,

Philae should be operating throughout this period.

After waking up June 13, the lander contacted

Rosetta a half dozen times in the following 10 days,

though the two craft still were trying to establish a

reliable communications link.

Scientists know Philae’s solar panels were receiv-

ing nearly three hours of sunlight each day and pro-

viding the probe with 24 watts of peak power, enough

to raise its internal temperature to –31° F (–35° C).

Those values should improve as the comet nears the

Sun and the probe experiences more daylight hours.

Assuming that the communications problem gets

ironed out, the science team will activate Philae’s

instruments in a specific order. The first to awaken

will include ones that don’t move and drain little

power, such as those that measure temperatures and

magnetic fields. Next up are ones that require a bit

more power, which include the camera systems.

Power-hungry instruments — in particular the two

ovens that analyze the chemistry of comet samples

— belong to the final group.

Philae scientists also may try to revive the drill if

they can do so without upsetting the lander, which

came to rest on its side. But some researchers think

GASEOUS PLUMES

VENT from the

comet’s nucleus June

15, just two months

prior to perihelion.

ESA/ROSETTA/NAVCAM

SEVERAL JETS

ERUPT from the

comet’s small lobe

just after the Sun

set over the region.

ESA/ROSETTA/MPS/OSIRIS TEAM

Philae came to rest next to

a cliff (topographic model,

above) that blocked sun-

light from reaching its solar

panels. The lander man-

aged to image part of the

wall (left). ESA/ROSETTA/PHILAE/

CNES/FD; INSET: ESA/ROSETTA/PHILAE/CIVA