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SOURCE:MARK
E.
BAILEY,
ILLUSTRATION;
PRESTON
MORRIGHAN/SCIENCE
eyond orbit the most distant
planets, Oort cloud
known
as source
comets that reach system.
distribution dynamically
evolved, ``captured'' comets from
cloud been difficult explain.
ticular,
is
a severe
number
of comets observed
in short­peri­
od orbits that comet Halley.
page 2212 issue, Levison
et
investigate this so­called ``fading
problem'' combining models:
a
tailed model dynamical evolution
of
comets from
to intermedi­
short­period orbits,
a statisti­
model describes likelihood
current astronomical surveys would
discover these comets their inert ``aster­
oidal'' remnants. refined model
predicts orders magnitude more
comets short­period ``Halley­type''
(periods revolution
< years)
observed.
modern picture
origin comets first
developed
in detail Oort
proposed
planetary system
is surround­
ed enormous swarm
comets, randomly dis­
tributed extend
halfway
to nearest
Oort's model spheri­
symmetrical, center, and con­
tained about
â
10 comets
diameters greater
kilometers. model,
cometary space density
varied roughly
r
, where
r distance from
More recent models
(3--5) indicate
a greater
central condensation,
of mass
hypothetical dense inner core
at
3
4 astronomical
(AU), where
1 AU
is
distance Earth from
picture home comets
stood test time remarkably
However, inability
to predict
curately the observed nearly
isotropic intermediate and short­period
comets shows much cometary
world remains obscure.
It
is timely
fading problem (6--13) should
assessed light observations
theoretical understanding.
is first time
derstanding comets proved incom­
plete. years,
it was known that
tribution perihelion directions
of
period comets was nonuniform (14--17)
that Oort cloud
be affected
differential tidal gravitational
Galaxy (18).
early 1980s that Oort's picture, which
random stellar perturbations feed new
long­period comets into solar
system, overturned. The order
replaced
in which perturba­
tions Galactic were dominant
(19--21)
, stellar occasional
molecular cloud perturbations producing
occasional enhancements comet
flux---so­called comet showers---on time
scales
to million years.
Similarly, Jupiter­family comets (those
low­inclination ``direct'' orbits
< years) were long believed
to origi­
primarily result action
Jupiter
on long­period comets. belief
persisted even though many
Jupiter­family comets observed rela­
predictions 23). problem
resolved introducing into
models sources Jupiter­family
comets: either dense inner
Oort cloud 25), necessary any
to explain the outer cloud's dy­
namical survival the the solar
P
R
P
E
V
S
:
L AN
E TA
C
E NC
E
Where Have All
the Comets Gone?
E. Bailey
author
is
at Armagh Obser­
vatory, College Armagh BT61
Northern Ireland, E­mail:
meb@arm.ac.uk
Positions June 2002
50
Number
of
comets
1
)
1
)
0.000 0.001 0.002
Earth
Jupiter
Halley
Hale­Bopp
Saturn
fading problem. frequency distribution semimajor long­period and comets
superposed inner solar system orbits
of comets Halley Hale­Bopp provided
J. Asher. distribution
is shown scales (left, width
â
10
; right, width
--3
). The
whole histogram right
of
=
0 contained positive
of second plot.
similar
of shows fading problem
is solely issue dynamically new
comets. Without fading, from Oort cloud spike, distribution would
be Note exis­
tence
of
a puzzling cases comets originally slightly hyperbolic orbits;
are usually explained
result
of outgassing determination.
www.sciencemag.org SCIENCE 296 JUNE 2002 2151

www.sciencemag.org SCIENCE 296 JUNE 2002 2153
system, newly discovered Edge­
worth­Kuiper belt (26--30).
fading problem, however, has
a persistent thorn flesh the
theory. The cloud explains
long­period comets have such extremely
periods, with orbits stretching
in
more halfway
(see figure). But close exami­
nation shows frequency distribu­
values (where
a
is half
length elliptical orbit)
comets
at Astronomers
therefore introduced arbitrary ``fading''
function physical
namical evolution cometary nuclei.
It
been suggested comets
initially favorably perturbed,
so
as
to return inner planetary system,
(11--13).
Comets expected decay
because
of observed mass loss),
is direct evidence new comets
Oort cloud disappear
numbers. contrary, repeated
comet Halley suggests that kilome­
ter­sized cometary nuclei survive
hundreds perhaps thousands revolu­
before finally disintegrating.
Herein problem: comets
they must largely
of
probably large heliocentric
tances. comets decay, however,
there should many more interme­
diate­period comets observed,
Halley­type comets. Previous
mates (31--36) have now been indepen­
dently confirmed new, comprehen­
computer­based model
of Levison
et
argue the majority comets
physically disrupt.
important result raises
Where does mass An alterna­
possibility, favored
by Levison
et
long­period comets become
is, lose capacity under­
go outgassing) hence evolve into
albedo objects resembling asteroids, virtu­
invisible against blackness
of
space. These potentially hazardous
jects---the astronomical equivalent
of
coal cellars---have important
implications programs aiming
to
kilometer­size impactor
identifies
Another alternative cloud
comets may easily break essential­
ly unobservable smaller bodies
a
perihelion passages. Sun­grazing
Kreutz family
be
a prototype. These
comets have orbital periods hundreds
of
years, they appear have undergone
a
hierarchy fragmentation events during
at least millennia. There
at
dozen Sun­grazing families,
potentially containing
of thou­
sands comets subcometary frag­
ments with diameters
Astronomers using data from Solar
Heliospheric Observatory (SOHO)
satellite have detected nearly 500 such
comets less than
7 years, ob­
reported
on almost daily basis.
With recognition Kreutz
group unique observation
other examples comets,
disappearing before eyes, im­
portant cometary end­state may have been
identified. This, however, also raises the
specter
a cometary civilization,
as
a result encounters with possibly
dense cometary meteoroid streams con­
taining fragments
a that could pro­
catastrophic events Earth.
A possibility cloud
comets decay nothing substan­
than dust. disintegration
a kilometer­sized cometary nucleus into
component dust grains per­
a seriously counterintuitive result,
especially
as comets run noth­
harder wind. However,
comets have observed disinte­
grate apparent reason. comets
decay rapidly
to dense me­
teoroid trails should detectable---for
example, meteor showers, through im­
pacts spacecraft, possibly via ther­
infrared emission analogous the
discovered with Infrared
Astronomical Satellite (IRAS)
Perhaps, with previous questions
concerning origin comets, as­
tronomers are collectively missing
a trick.
A fundamental, though essential, tenet
of Oort theory observed
cometary flux
is
in
a steady the
numbers short­period other comets
in balance with observed near­parabol­
ic Oort cloud other
reservoirs. Could steady­state assump­
mistaken? This would
us
in the
uncomfortable position living spe­
epoch, perhaps within
a few million
years
of start
a comet shower
all sorts attendant repercussions;
while such models promise,
completely resolves fading prob­
(33,
If comets indeed disrupt, whether
unobservable bodies dust, Lev­
et make interesting point
perhaps behavior truly distin­
guishes nearly isotropic Halley­type
short­period comets from those the
Jupiter family, which their model origi­
from Edgeworth­Kuiper belt
rather Oort cloud. the
different fading behavior
of class­
es comet would provide first hard
evidence physical difference
tween ``inner'' ``outer'' comets, per­
haps result different origins and
thermal collisional histories.
present, comets remain puzzle:
They
to
be strong weak, and
there seems
to
be
a substantial missing
mass. Does provide
a clue ori­
cometary material?
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