Comets
Warning
This section is incomplete.
Comets are small, icy bodies that orbit the Sun. As they approach the Sun, they warm and begin releasing gases, often from melting ice. This produces an extended cloud of gas surrounding the small body. The gas forms into a tail that points away from the Sun, driven by the solar wind, which emanates radially from the Sun in all directions.
Comet C/2020 F3 NEOWISE as seen in the sky. This is enhanced slightly through photography, but the comet and its tail were clearly visible to the unaided eye. This photo approximates how a prominent comet will look in the sky—most comets are not this bright. Credit: Joshua Wachuta, CC BY-SA 4.0
Nucleus
The nucleus of a comet is characterized by the “dirty snowball” model. Rock, water, ice, astrophysical dust, frozen carbon dioxide, methane, and other compounds and molecules. The nucleus can be up to about 60 kilometers (35 miles) in diameter to under one kilometer in diameter.
The elongated nucleus of Comet 103P/Hartley. Captured by NASA’s EPOXI mission in 2010 onboard the Deep Impact spacecraft, the nucleus has been measured to be just over one kilometer (three-quarters of a mile) in diameter. Credit: NASA/JPL-Caltech/UMD
A comet can only encounter the Sun so many times before it disintegrates. Each pass results in mass loss to the comet and adds stresses to the integrity of the nucleus. Comets that are drawn close to the Sun in their orbit—into the inner Solar System—have a limited lifetime. Comet Hartley, pictured above, is estimated to have about 100 more passes by the Sun before it breaks apart.
Tail
The tail of a comet only appears when materials inside the nucleus begin to vaporize as it approaches the Sun. The tail has two main streams: a dust tail and a gas tail. The dust tail, made of material from the comet, spews off in a curved fashion like the steam from a locomotive when it’s moving down the tracks. The gas tail, composed of gases ionized by the Sun’s ultraviolet light, always points directly away from the Sun.
Structure of a comet, showing the nucleus, coma, and tails. Credit: ESA, CC BY-SA IGO 3.0
Short-period Comets
Short-period comets are informally defined to have an orbital period of less than 200 years. These comets, like Halley’s, tend to be in elliptical orbits that reach the outer planets when they’re farthest from the Sun. Halley’s comet, which has an orbital period of about 75 years, travels a little beyond Neptune’s orbit before starting its long return toward the Sun.
These comets are believed to originate from the Kuiper Belt, a disk of objects around and beyond Neptune. Often, their aphelia (their farthest distance from the Sun) is proximate to the outer planets. Halley’s Comet’s aphelion, for example, is near the orbit of Neptune.
Long-period Comments
Long-period comets can have orbital periods from 200 years to million of years. They have highly eccentric orbits that take them far beyond the outer planets. These comets originate from the Oort Cloud, where primordial planetesimals reside and are occasionally perturbed into an orbit that takes them into the realm of the planets and close to the Sun.
Comets are prone to orbital change as they pass through the Solar System. The outer planets, particularly Jupiter, can alter the orbit of an incoming comet. Jupiter has over two times the mass of all the other planets combined, so it is a force, mostly for good, keeping comets out of the Solar System or changing their orbits. SOme comets collide with Jupiter or the Sun, and some are sent out of the Solar System altogether.
Extinction and Replenishment
Because comets are not long-lasting, on cosmic timescales, the well of comets must be replenished. Long-period comets typically do not survive 50 orbits, whereas Jupiter-family comets (those near the orbit of Jupiter) can survive over 1,000 orbits.
Eventually, the volatile materials inside the comet evaporates—a little each passage—and the remaining chunk of rock might more resemble an asteroid. Alternatively, comets can plunge into a planet or the Sun, or break apart into tiny pieces. Others can be ejected from the Solar System to become interstellar travelers.
The replenishment comes from the Oort Cloud, the well of icy planetesimals. This material formed closer to the Sun, but was driven out once the planets in the Solar System formed. But, every now and then, as the Sun and accompanying Solar System travel around the Milky Way Galaxy, a passing cloud of gas, or a nearby star encounter can dislodge comets from their orbits in the Oort Cloud and send them hurtling into the inner Solar System. Their orbits are affected by this initial perturbation, but also the gravitational interactions of the planets as they approach the Sun.