Comet

Hale-Bopp comet seen from Earth, 1997.

A comet is an astronomical object, with an orbit that takes it from relatively far from its star, to a close approach to its star. If it survives its close approach, it will then recede.

Astronomers had long recognized that, theoretically, a comet might approach Sol, our sun, from outside the Solar system, but it is only in very recent times that comets of interstellar origin have been discovered.

Lessons learned from Halley's Comet

Astronomer Edmund Halley was the first person to propose that comets could be periodic. In particular, he suggested that there was a bright comet, subsequently named after him, that entered the inner Solar system approximately every 89 years. He asked his friend, Isaac Newton, to calculate where and when they should look for its next appearance. Halley's query to Newton was his trigger to develop calculus.

As comets like Halley's Comet approach the inner Solar system, they shed enough dust and gas that the reflected light makes them appear very bright. They come from the outer Solar system, where objects are cold. As they come closer to Sol the light intensity increases enough to heat up the comet's surface. Icy veins, of water ice, frozen Carbon Dioxide, Methane, and other compounds with a low melting point, sublimate. When those icy veins contain dust, sand, pebbles and boulders, they too are released, and reflect light.

The period of Halley's comet was not as predictable as a clockwork, because it shed mass every time a vein of compounds with a low metling point sublimated, and because, when those veins sublimated the material it shed could squirt out like a tiny rocket motor, directly changing its orbit, by a tiny increment.

Halley found that earlier observers had recognized great comets, coming from the same part of the sky, many times in the past, without realizing that, instead of being multiple comets, it was the same comet returning on a regular basis. One of the previous instances is recorded in the Bayeux tapestry.

The life of a comet

Astronomers have witnessed the destruction of comets. They have seen comets as they approached Sol. All comets are not observable when they approach Sol. There has to be a period when they are unobservable because they are on the other side of Sol, or because they are too close, and Sol drowns them out. Astronomers have seen some comets emerge, from around Sol, in two separate pieces, because losing material as they heated up destroyed much of their structural integrity. Other comets are not visible at all, and are presumed to have lost all structural integrity, and completely broken apart.

Jupiter sweeps up comets. Astronomers have long expected this. In the early 1990s Astronomers saw Comet Shoemaker-Levy's capture by Jupiter. As it orbited Jupiter, its gravity caused the comet to fracture into approximately two dozen fragments. Astronomers recorded the dramatic collision of those fragments, into Jupiter.

If close approaches to Sol don't weaken a comet's structural integrity, so it breaks apart, eventually a comet that was once riddled with pocket of frozen volatile compounds, will lose all those volatile compounds, so it is transformed into a cinder, that no longer gets bright, when it approaches the sun, as there is nothing left to sublimate.

And, if that cinder keeps orbiting Sol, long enough, perturbations from the other planets, particularly Jupiter, will circularize its orbit. Once its orbit is closer to circular, it won't heat up and cool down. A Halley's Comet year is 89 Earth years. Uranus's year is 84 Earth years. If Jupiter succeeded in circularizing Halley's Comet's year, its orbit would be very close to that of Uranus.