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B
y studying comets,
astronomers hope to
shed new light on
various questions about the early
solar system, the formation of
Earth, and even the origins of life.
Earth is the only planet known
to have a surface ocean of liquid
water. The origin of this water is
one of the enduring mysteries of
Earth science. A leading theory is
that the hot, young planet sweated
out the water from its rocks,
releasing water vapor into the
atmosphere. Once the planet
had cooled sufficiently, this vapor
condensed and fell as a deluge of
rain that filled the oceans. Another
theory argues that at least some
of the water arrived from space,
specifically in the hundreds of
thousands of icy comets that rained
down on Earth during the first half
billion years of its existence and
vaporized on impact.
A flyby of Halley’s comet in
1986 by a flotilla of spacecraft led by
ESA’s Giotto got the first close-up
look at a comet’s core, or nucleus.
The Halley encounter provided
conclusive proof that comets are
largely made from water ice mixed
with organic dust and chunks ofUNDERSTANDING COMETS
rock. This revived the theory that
this is where Earth’s oceans came
from. One theory concerning the
origin of life was that the complex
chemical building blocks necessary
for life, such as amino acids and
nucleic acids, arrived on Earth
from space. Perhaps these organic
compounds were also delivered to
Earth by comets. The only way to
find out was to send a spacecraft
to meet up with a comet and land
on its surface. In 2004, the ESA-led
Rosetta mission blasted off on a
10-year journey to do just that.Fresh target
Rosetta’s intended target
was Comet 67P/Churyumov–
Gerasimenko, or 67P for short. In
1959, this comet had been captured
by the gravity of Jupiter, which had
pulled it into a shorter six-year orbit
of the sun. Before that, 67P had
been circling the sun much farther
away. This excited the Rosetta
scientists because the tail of a
comet—its most familiar feature—
is caused by solar radiation heating
the surface of the nucleus, whichIN CONTEXT
KEY DEVELOPMENT
ESA—Rosetta (2004)BEFORE
1986 The Halley Armada of
eight spacecraft, led by ESA’s
Giotto, make observations
of Halley’s comet.2005 The Deep Impact
mission fires a probe at comet
Tempel 1 to create a crater
in the surface, and analyzes
what is underneath.2006 The Stardust mission
collects a capsule of cometary
dust from the tail of comet
Wild 2 and returns to Earth.AFTER
2015 New Horizons flies
by Pluto and begins an
exploration of the Kuiper belt.2016 NASA’s OSIRIS-REx
spacecraft is launched with a
mission to collect and return
a sample from the asteroid
101955 Bennu.In 2005, the Deep Impact impactor
collided with comet Tempel 1,
releasing debris from the comet’s
interior. Analysis showed the comet
to be less icy than expected.Giotto ignited the planetary
science community in Europe.
Gerhard Schwehm
Giotto Project scientist