207
See also: Halley’s comet 74–77 ■ Asteroids and meteorites 90–91 ■
Astrophotography 118–19 ■ The Kuiper belt 184T
he arrival of a comet can
be a spectacular event,
with the brightest of
them remaining visible even
during the day. Yet US astronomer
Fred Whipple showed that these
dazzling astral visitors are, in
fact, extremely dark objects.
In 1950, Whipple proposed that
comet nuclei—the “bodies” of these
early solar system remnants, as
opposed to the bright, gassy tails
visible from Earth—are a rough
mix of meteoric materials and
volatile ices. The ices are mainly
frozen water, along with frozen
gases such as carbon dioxide,
carbon monoxide, methane, and
ammonia. The rest is rock and
dust. A black crust of tarry organic
compounds, similar to crude oil,
coats the surface. Comet nuclei are
among the darkest objects in the
solar system. Only 4 percent of the
light that falls on them is reflected.
Fresh black asphalt, by comparison,
reflects nearly twice as much.
Whipple’s “icy conglomerates”
concept explained how comets could
repeatedly send out trails of vaporas they passed the sun. The idea
was accepted, albeit with the
catchier name “dirty snowballs”
(later modified to “icy dirtball” on
the discovery that comets contain
more dust than ice). However,
Whipple had to wait until 1986 for
his ideas to be confirmed. That year,
the Giotto spacecraft drew alongside
Halley’s comet, and took close-range
pictures of the dark nucleus usually
hidden by the bright coma. ■NEW WINDOWS ON THE UNIVERSE
COMETS
ARE DIRTY
SNOWBALLS
THE COMPOSITION OF COMETS
IN CONTEXT
KEY ASTRONOMER
Fred Whipple (1906–2004)
BEFORE
1680 German astronomer
Gottfried Kirch is the first to
find a comet using a telescope.
1705 Edmond Halley shows
that the comet of 1682 is the
same object as the comets
of 1531 and 1607.
AFTER
2003 A survey by The
Astrophysical Journal finds
that over 50 years Whipple’s
1950 and 1951 papers were the
most cited papers in astronomy.
2014 Rosetta completes a
rendezvous with comet 67P/
Churyumov–Gerasimenko and
successfully sends the Philae
lander onto its surface.
2015 New research suggests
comets are like “deep fried
ice cream,” with an icy crust,
a colder and more porous
inside, and a topping of
organic compounds.
Images from the Giotto spacecraft
revealed the nucleus of Halley’s comet
to be a dark peanut-shaped body,
ejecting two bright jets of material.