Sky.and.Telescope_

22 August 2014 sky & telescope

Death-defying Comet Catch

Unlike previous cometary missions, which were all relatively

brief encounters, ESA’s Rosetta mission will have a long-

term relationship with its dirty snowball. Rosetta will

rendezvous with Comet 67P/Churyumov-Gerasimenko

(we call it “C-G” to avoid endless tongue-twisting conver-

sations) in August 2014. It will then escort the comet for

more than a year as it fl ies past the Sun.

During that time, we will watch — up close and per-

sonal — the life of the comet. We will watch as increasing

solar heating drives the vaporization of ices (such as water

and CO 2 ) from the nucleus as it approaches our star. We

will also see how those escaping gases may form ghostly,

geyser-like jets with entrained dust, watch the comet grow

a tail and coma and how these interact with the nucleus,

and fi nally observe the activity quiet down again after

C-G passes the Sun and heads back out to the deep-freeze

portion of its orbit.

As if those aren’t enough “fi rsts” for a mission, Rosetta

also carries a smaller spacecraft that will land on the

comet and do in situ studies of the surface. This lander

will work for several days or perhaps several weeks before

things get too hot or the solar panels become too dust-

covered to operate anymore.

The long hibernation and the upcoming adventure of

operating a spacecraft in the dangerous and dirty envi-

ronment of an active comet are just some of the mission’s

risks. But taking risks is often necessary when doing

cutting-edge research: if it were easy and safe, it would

have been done already. Rosetta will take those risks in

order to see in real time how a comet behaves, on a level

of detail that can’t be seen from ground-based telescopes

or even space telescopes such as Hubble.

Two L ab o rato rie s

To do this detailed level of study, you need an advanced

laboratory. Since we can’t bring a comet to the laboratory,

we are sending the laboratory to the comet. In fact, we are

sending two laboratories to the comet: there are 11 instru-

ments on the orbiter and 10 more on the lander (see side-

bars on pages 23 and 24). These include instruments that

study the dust particles, that take in and directly sample

the gas, and that study the plasma (ions and electrons)

around the comet and its interaction with the solar wind.

There are instruments that take images and instruments

that take spectra. We also have instruments that observe

over a wide range of wavelengths — from ultraviolet to

visible to infrared to microwave and out to radio. I like to

call this wide range of investigations “CSI Rosetta,” since

we have to look at all the clues to fi gure out what hap-

pened, as well as when, where, and how it happened. Each

After a tense wait, mission controllers celebrate the arrival of

Rosetta’s fi rst signal after the spacecraft woke up from hiberna-

tion on January 20th.

ESA / JÜRGEN MAI