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Comets vs asteroids
Is it a comet?
Is it an asteroid?
Should astronomers emulate vets or zoo-keepers in seeking to understand the many and
various bodies that inhabit our Solar System? Iwan Williams speculates on issues discussed at the Royal
Astronomical Society Discussion Meeting on Comet–Asteroid Connections.
A
hot topic in Solar System studies, and
one that was given a good airing at the
RAS Discussion Meeting on 14 March
1997, is whether we live in a zoo or a safari
park. In a zoo, each species of animals lives
separately, in individual labelled enclosures. In
a safari park, in contrast, animals from a number of different species co-habit.
In the context of the Solar System, the question centres around asteroids and comets. With
comet Hale-Bopp shining brightly above us, it
is abundantly clear that there exists a family of
objects that outgas when close to the Sun and
posses a coma and a tail. Such objects have
been known since records began and can be
called “comets”. They are a cocktail of snow
and small dust grains, similar to the model first
proposed by Whipple in 1951. They probably
have a bulk density smaller than that of water,
have minimal strength as demonstrated by the
behaviour of comet Shoemaker-Levy 9 at
Jupiter, and are typically a few kilometres
across – comet Hale-Bopp at about 40 km
being one of the largest known.
The new and long period comets move on
highly elliptical orbits, with eccentricity typically greater than 0.9. The Jupiter family of
comets – those that have had their orbits
severely modified by the gravitational field of
Jupiter – have smaller eccentricities (mean
value about 0.5) and smaller semi-major axis
(mean about 3.5 AU).
Asteroids
In contrast, it is clear that there exists a family
of objects that we might call “asteroids”,
although the first member, Ceres, was discovered only 200 years ago, by Piazzi in Palermo
in 1801. These are much larger than comets.
An asteroid only 5 km across would be classified as small; Ceres, the largest, is 100 times
bigger than this. They have a mean density
twice that of water, considerable strength, and
a composition based on rock and metals. They
move on near-circular orbits (defined as having
an eccentricity less than 0.4).
The question under discussion is thus not
June/July 1997 Vol 38 Issue 3
Leonid meteor shower, 17 November 1966. (Kitt Peak/RAS Library.)
whether “asteroids” and “comets” are one and
the same thing, but rather what is the correct
classification or classifications, of the objects
that are currently being discovered in increasing numbers in various parts of the Solar System? And will any of these newer objects eventually evolve into, or have they evolved from,
standard comets and asteroids? In other
words, within our zoo or safari park, can we
correctly recognize the young animals or the
very old, of either species?
The first object of doubtful parentage, Apollo, was discovered in 1932 by Reinmuth at
Heidelberg, and is now recognized as one of a
large family of objects collectively known as
Near Earth Objects. As their name implies,
they have a perihelion distance at around 1 AU
and generally their aphelion is within the main
asteroid belt. Their orbits are thus less circular
than the main asteroids, but more circular than
the typical comet. They show no coma activity
and the reflectance spectrum is similar to that
of asteroids. They are bigger than standard
comets but smaller than a typical asteroid.
Their dynamical lifetime is much less than the
age of the Solar System and so they must form
within the system as we see it today.
Two standard explanations are offered. Near
Earth Objects could be expelled from the main
belt of asteroids as a result of gravitational perturbations from Jupiter, especially if their initial orbit was close to a mean motion resonance with Jupiter (where their orbital periods
are a ratio of small integer numbers, for example 3:2). This explanation has the added
advantage that it also explains the “Kirkwood
gaps” within the asteroid belt. Alternatively
they can be thought of as dead or dormant
comets. In this scenario, a dust layer builds up
on the surface of the cometary nucleus, gradually choking off the outgassing and leaving an
object which, on the surface, looks like an
asteroid. This explanation has the added
advantage of explaining where dead comets
actually go: they become Near Earth Objects.
Centaurs
Twenty years ago the first member of another
new group of objects, Chiron, was discovered
(by Kowal in 1977). This group still has only a
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Comets vs asteroids
handful of known members, collectively called
“Centaurs”. They are characterized by having
orbits lying always within the region of the
outer planets. The sizes of those discovered so
far is comparable to the larger asteroids and
hence they are much bigger than a typical
cometary nucleus. Of course, this may be an
artefact: detecting smaller members of the
group, even if they exist, would be very difficult.
Five years ago, the first member of what is
now known as the Edgworth–Kuiper Belt
(named after the two people who postulated its
existence), 1992QB1, was discovered by Jewitt
and Luu from Hawaii in 1992. These objects
orbit the Sun beyond Pluto on a near-circular
orbit, and appear to have sizes similar to or
slightly larger than the Centaurs. One year
later in La Palma, Williams, Fitzsimmons and
O’Ceillaigh discovered the first object in what
is either a subgroup, or arguably a different
species altogether, 1993SC. Observations
showed that this body moved on an orbit similar to that of Pluto, in other words moving on
a 3:2 mean motion resonance with Neptune.
Perihelion is close to Neptune’s orbit and aphelion in the main Edgworth–Kuiper Belt. The
size distribution within this subgroup is similar
to that in the main Edgworth–Kuiper belt.
A final family of objects, believed to exist,
but never actually seen, is the Oort cloud of
comets (proposed by Oort in 1951). This is
thought to be a large spherical cloud of comets,
beyond the orbit of Pluto, that acts as a reservoir from which new comets are released.
subsequently rediscovered when it appeared as
an asteroid. For the future, searches for comae
might provide an answer.
Meteor streams
An associated activity coupled to outgassing in
active comets is the ejection of meteoroids
leading to the formation of meteor streams.
One of the earliest associations identified was
between comet Temple–Tuttle and the Leonid
meteor shower, by Leverrier in 1867. The association of a meteor stream with an asteroid
could thus be taken as an indicator of past
cometary activity. The well known Geminid
meteoroid stream is associated with asteroid
3200 Phaethon, and some have taken this as
indicating that Phaethon was once a comet.
Given time, the veterinary surgeon’s
approach of investigating each object in turn
might work, but we are far from being able to
reach any definite conclusion through this
path. An alternative is to consult the zoo-keepers or park wardens (i.e. the dynamicists) and
Above: The
nucleus of
comet Halley,
seen by Giotto,
with irregular
outgassing
(ESA/NASA).
Left: Ida, a good
example of an
asteroid, with
craters and no
coma, but its
own moon
(NASA).
Ice traces
The above is just a catalogue of the animals
and their superficial characteristics, that we
find in our zoo or safari park, but listing them
does not by itself produce any clues as to their
parentage and offspring. Ideally, we would like
to do the equivalent of a DNA test, which
would answer everything. In our case the test
would be very simple, can we find significant
traces of ice or not? Ice would place an object
with the comets, no ice would mean it
belonged with asteroids. An auxiliary test that
would achieve the same end is determining the
bulk density – impossible without a mass
determination which is itself impossible without a very close fly-by.
Finding ice is also difficult, even in the framework outlined above. Any ice in the Near Earth
Objects is covered by dust in this scenario,
while even among the Centaurs, surfaces may
be heavily processed. The Edgworth–Kuiper
objects are just too far away for any direct
detection to be possible. One side effect of the
presence of ice could be outgassing leading to
the formation of a faint coma. Such a coma was
found around Chiron, while the reverse happened with comet Wilson–Harrington, which
was first observed to be outgassing, but has
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Objects. Some comets have also become dormant and can probably remain so for the short
span of time that we have observed Near Earth
Objects. The new comets certainly come directly from the Oort cloud, and some of them
undoubtedly evolve into short period comets.
Some short period comets also come from the
Edgworth–Kuiper belt, and are possibly what
we see in transit as Centaurs. Some of the
exhibits may also escape to appear at unexpected places, namely as satellites with Charon
ask them for their estimate of the probability
that a particular animal can escape from its
enclosure or migrate a specified distance within the park. They say that “asteroids” could
escape from the main belt and appear as Near
Earth Objects, though perhaps not in the numbers observed. They tell us that the Centaurs
are a wild group (i.e. chaotic) that could roam
more or less anywhere but, in contrast, the resonant Edgworth–Kuiper objects are very
happy where they are and show no signs of
going anywhere. The main Edgworth–Kuiper
belt members are also generally stable though
some can escape into the inner Solar System,
while the Oort cloud members can easily
migrate into the inner Solar System.
Hence my personal view, which is that if you
make the fences small enough you cannot tell
the difference between a zoo and a safari park.
In other words, some main-belt asteroids have
certainly escaped and are seen as Near Earth
(satellite of Pluto) and Triton (satellite of
Neptune), both being a good bet for Edgworth–Kuiper objects, while Phoebe (satellite
of Saturn) may be a captured Centaur. What is
clear is that much more research effort is called
for into both the dynamics and physical nature
of these objects. ●
Iwan Williams is professor of astronomy, with special interests in the study of asteroids, comets and
meteor streams, at the Astronomy Unit, Queen
Mary , University of London, London E1 4NS.
References
Edgworth K E 1943 J British Astron. Assoc. 53 181.
Jewitt D C and Luu J 1992 IAU Circular 5611.
Kowal C 1989 Icarus 77 118.
Kuiper G P 1951 in Astrophysics ed. J A Hynek (McGraw-Hill).
Oort J P 1950 Bull Astron. Inst. Netherlands 11 91.
Whipple F L 1951 Astroph. J. 111 375.
Williams I P, Fitzsimmons A, O’Ceallaigh D and Marsden B G
1995 Icarus 116 180.
June/July 1997 Vol 38 Issue 3