26 August 2014 sky & telescope
Death-defying Comet Catchdrives the coma and tail.
That advantage is also the diffi culty of the mission
— to design a spacecraft and instruments that can live
so long in a dirty environment of water, gas, dust, and
even possibly snowballs and pebbles. Not only does the
hardware need to survive, but the onslaught also makes
it diffi cult to fl y. The comet’s gravity is so weak that the
outward push of the gas and dust on the spacecraft, par-
ticularly against the large surface area of the solar panels,
quickly will make it impossible to continue orbiting the
comet. Depending on C-G’s activity level, Rosetta might
only have a few months in which to orbit the comet at
distances less than 50 km. The rest of the mission can be
better described as Rosetta “escorting” the comet, fl ying
back and forth at distances farther than 70 km with occa-
sional close, dive-bombing fl ybys down to maybe 10 km.
A comet’s coma can extend out to hundreds of thousands
of kilometers, so the spacecraft should still be inside the
coma even when keeping the nucleus at arm’s length. But
as in any dance, Rosetta will have to continuously adapt
to its partner in order to have a successful, many-months
tango with C-G.What Can We Learn from One Comet?
Comet C-G is a Jupiter-family comet, meaning that its
orbit is “controlled” mostly by Jupiter. It has an aphelion
of 5.7 a.u. and a perihelion of 1.2 a.u. The incident solar
energy changes by a factor of 23 during an orbit. That
change in solar heating is what drives the activity.
C-G has a short orbital period of about 6.5 years and
a rotation period of about 12.4 hours. From Earth-based
observations of C-G’s light curve, we have an estimated
model of the nucleus’s shape, which is somewhat irregu-
lar with an eff ective diameter of about 4 to 5 km.Comets interest space scientists in many fi elds because
they are relics, the leftover fl otsam and jetsam from the
solar system’s birth. They may represent some of the most
unprocessed material from 4.5 billion years ago, when
the planets formed. Having said that, comets are not
completely unprocessed. Any comet that has made several
passes by the Sun — and that includes these short-period,
Jupiter-family comets — will already have experienced
some modifi cation of its structure and physical makeup
by solar heating during previous passes. So the trick is
to disentangle the relative chemical abundances that are
due to the original composition from those that are due
to changes from past heating episodes. That’s why it is
necessary to have many instruments, so that we can study
the interior, surface, gas, and dust and parse the clues of
the comet’s origin from its evolution.
C-G is just one example, but we’ll be able to compare
what we learn from it with what we have seen during the
other, briefer visits we have made to comets. The Rosetta
mission will enable us to “translate” comets — much as
scholars used its namesake, the Rosetta Stone, to decipher
ancient Egyptian hieroglyphics. What we learn from C-G
will help us understand the connection between what
is happening on the small scale of the nucleus and the
large scale of the coma and tail, the latter two of which are
all we can observe from Earth. This knowledge will be a
powerful key in unlocking Earth-based observations of
other comets past, present, and future. ✦Joel Parker is a director at the Southwest Research Institute
and the host and producer of the radio science show “How
on Earth.” He has been a project manager and science team
member on several space missions, including LRO and New
Horizons, and is the deputy PI on the Rosetta-Alice project.This illustration depicts the Philae lander at work on Comet 67P/Churyumov-Gerasimenko. Immediately after touchdown in
November 2014, the lander will fi re a harpoon to anchor itself and prevent it from escaping the comet’s extremely weak grav-
ity. The minimum targeted mission time for Philae is one week, but surface operations might continue for many months.
ESA / AOES MEDIALAB