Not since 1910 have we been treated to so fine a year for
seeing comets. Don’t miss the chance to
see them yourself. Space.com has shown a
lovely photograph of two comets low in the western evening sky that should
inspire anyone to make the effort.
Sadly, evening cloudiness over Puget Sound has denied me the opportunity—it’s
not quite as nice for astronomy on the Washington coast as it was in Tucson!
Where do comets come from?
The simple answer, which the media pass on to us, is that they come from
the Oort Cloud, a vast swarm of dirty snowballs that orbit in random directions
around the Sun far outside the orbits of Neptune and Pluto. This explanation has the advantage that it is
sort or right—and the disadvantage that it is pretty inadequate.
Comets are usually divided into two families. First we have the long-period comets, which
typically take a million years to complete an orbit and spend most of their
time 10,000 AU from the Sun. These are
the Oort cloud comets. Their orbits are
quite close to random: about half are traveling the “wrong way” around the Sun.
which allows head-on collisions with planets at enormous closing speeds. Only those that approach to well within
Jupiter’s orbit ever get warm enough for wholesale evaporation of their ices,
which blows off vast streams of gases and dust that give comets their “hairy”
appearance, and hence the name “comet”, which comes from the Greek word for
hair. The overwhelming majority of the
Oort Cloud comets have never (“what, never?
Well, hardly ever”) approached close enough to the Sun to light up, and
hence to be discovered. At best, such a
comet has been observed only once.
Occasionally a long-period comet will pass close enough to
Jupiter or Saturn to have its orbit strongly affected by the planet’s gravity. These comets are diverted into relatively
tame low-inclination orbits that cross the orbits of several of the terrestrial
planets, often with orbital periods of 3 to 7 years and with aphelia close to
the orbit of the planet that kicked it. These are the short-period comets, which may
be observed through dozens to hundreds of trips around the Sun. They pass by repeatedly on regular schedules
with well-known orbital periods, and for that reason are often called “periodic
comets”.
There are several other fates possible for an Oort Cloud
comet that ventures into our planetary system besides becoming periodic comets. Some, after a traumatic close encounter with
a giant planet, will be hurled outward at a speed well above the escape
velocity of the Sun and become lonely wanderers in interstellar space. The chances of such a body ever entering
another planetary system and getting close enough to its star to light up as a
bright comet are extremely remote. Space
is big, and stars are small. No comet
interloper from another planetary system has ever been observed.
But there are other fates in store for the long-period
comets. Some may fly by one of the giant
planets and be diverted into orbits that have low inclination and cross the
orbits of several of the giant planets.
These bodies cannot avoid collisions or violent gravitational
interactions with these planets, and therefore have a short expected
lifetime. These bodies are called the Centaurs. They and a vast dynamically related group
called Trans-Neptunian Objects (TNOs), which, as their name suggests, orbit
near and beyond Neptune, can be both former and future comets. Pluto is one of the TNOs which happen to
belong to a subfamily of bodies that have reached an orbital accommodation with
Neptune, with a 3:2 orbital period resonance that prohibits them from ever
approaching Neptune closely or colliding with it. Bodies kicked into that neighborhood that were
not lucky enough to enter a safe resonant orbit would soon collide with
Neptune, be expelled from the Solar System, or become a Centaur.
In addition, the outer satellites of the giant planets,
those in retrograde orbits, are only very weakly bound to their planets. It is clear that these bodies may be captured
or lost into heliocentric orbits quite easily.
Such a lost satellite may become a Centaur; a newly captured satellite
probably was a Centaur.
Periodic comets may make hundreds of perihelion passages
before the supply of volatile ices near their surfaces is exhausted. The body ceases to emit gases and dust,
cometary activity fizzles out, and we are left with an icy comet core that is
covered with a layer of fine, extremely black dust that not only blocks solar
heating of the interior, but also has a very low thermal conductivity. Once a dust layer a few meters thick has
developed, all cometary activity ceases and the body has the appearance of an
extremely dark (D-class) asteroid. Many
near-Earth asteroids (NEAs) not only follow orbits similar to those of the
periodic comets, but some have even been observed to make the transition from
comet to asteroid. If a small impact
event opens a hole in the dust blanket, solar heating can again reach the
buried ice and a “jet” of gas and dust can erupt. Many short-period comets are active thanks
solely to one or a few such local jets.
And of course such a collision on a D asteroid may cause it to resume
cometary activity. Many NEAs that may be
dust-mantled icy cores of “extinct” comets can be recognized by their orbits
and their D-type reflection spectra. All
of these could again become comets.
The semantic distinctions between planetary retrograde
satellites, Centaurs, TNOs, long-period comets, periodic comets, and dark NEAs give
us useful ways of describing what and where a body is today, but they do not do
justice to the complex histories these bodies may have had before fitting
neatly into one of these convenient pigeonholes.
A Centaur may from time to time be perturbed into an Earth-crossing
orbit by one of the giant planets whose orbits they cross. Such a body, lighting up as it approached the
Sun, would then be termed a giant comet.
The Centaur 10199 Chariklo is about
260 km in diameter, compared to 6 km for a typical large comet nucleus such as
the body whose impact ended the Cretaceous Era and extinguished the last of the
dinosaurs. An impact of Chariklo with
Earth would deliver about 100,000 times as much energy as that global
extinction event, equivalent to about 4000 tons of TNT for each person on Earth. That would be about 2000 times as severe as
an all-out nuclear World War III.
Mankind would be extinguished and life on Earth would be set back to the
pre-Cambrian Era.
Unlike the dinosaurs, we have technologies that allow us to
find, track, predict, and even intercept potential impactors. It would be criminally negligent to ignore
the impact threat.
