Australian Sky & Telescope — July 2017

http://www.skyandtelescope.com.au 25

SOURCE: ESA / ROSETTA / NAVCAM & OSIRIS TEAMS

its full instrument suite for more than two years, sometimes

getting as close to the surface as about 7 km early in the mission

— and down to 2 km during its end-of-mission orbits. Yet at

times the spacecraft had to stand off at distances of 300 to

400 km or more because the comet became so active that the

resulting blizzard of dust blinded Rosetta’s star trackers. Two

distant excursions, 1,000 to 1,500 km away, enabled studies of

the comet’s tail and its interaction with the solar wind.

Why go to all this trouble and expense? Because we were

seeking some fundamental understanding about comets,

which are made from the primitive materials present

during the formation of the Solar System. So by studying

one up close, we stood to learn about the conditions and

constituents that formed the Sun’s planets.

Since comets spend most of their time in the distant Kuiper

Belt or Oort Cloud — very difficult or impossible to study

with spacecraft — we observe the ones that venture inward as

escaped examples of those regions of the Solar System. And

because comets sometimes also hit the inner planets (including

Earth) and deliver water, other volatiles and organics, studying

them up close can tell us how they might have influenced

conditions here, including those important to the origin of life.

First and foremost, how do we explain the dramatic shape

of Rosetta’s target? Two-lobed objects do exist among small

bodies like asteroids and comets, and in fact three of the

five other comets imaged by flyby missions have this general

shape. But Comet 67P is a wonderfully extreme example.

There are two main possibilities: Either two objects slowly

collided and stuck together, or the neck region preferentially

eroded due to an abundance of volatile ices there that escaped

20 m

Rosetta’s

shadow

March–July 2015

Increasing comet activity February 24, 2015

Close encounter

November 12, 2014

Philae lands on nucleus

October 7, 2014

“Selfie” while orbiting close to comet

August 6, 2014

Orbiting close to comet

July 14, 2014

Shape of nucleus revealed