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.