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Summary: www.sciencemag.org SCIENCE VOL 335 20 JANUARY 2012 297
PERSPECTIVES
together. The results from comet C/2011 N3
(SOHO) are thus pioneering a new method of
cometary study.
Understanding the physical construc-
tion of comets sheds important light on how
matter accreted from tiny, micrometer-sized
specks of dust and molecules of gas to build
kilometer-sized ice and rock-rich bodies, the
comets, in the first million years of the solar
system's existence (9). This is still a great
mystery--most studies of the aggregation
of protoplanetary gas and dust, using their
known physical parameters (bulk modulus,
porosity, surface cohesion, dielectric con-
stant, etc.) at the expected interaction speeds
of a few kilometers per second or more, show
that particles should build up to millimeter-
to centimeter-sized objects quite easily along
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