5.5 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Comet | 4/10 | https://en.wikipedia.org/wiki/Comet | reference | science, encyclopedia | 2026-05-05T13:31:54.670973+00:00 | kb-cron |
Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. They have aphelion 4–35 AU. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits. Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods. Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesized its existence). Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable. When flung into the orbit of the sun, and being continuously dragged towards it, tons of matter are stripped from the comets which greatly influence their lifetime; the more stripped, the shorter they live and vice versa.
=== Intermediate period === Comets with periods greater than 30 years but less than 200 years have been called intermediate period comets, but other definitions have been used. They can be considered to have aphelion 35–1000 AU.
=== Long period ===
Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years. They have aphelion greater than 1000 AU. Less than a dozen of such comets are observed annually, but studying them is important as a way to learn more about the Oort cloud where they primarily originate. Jan Oort himself discovered the key facts about these comets including there nearly parabolic orbits oriented randomly around the Sun. Subsequent studies showed that galatic tides and the gravitational push of passing stars are sufficent to create a comet from objects in the Oort cloud. About half of these comets are ejected from the Solar System on their first pass by the Sun due to the gravity of the planets. As long-period comets approach the Sun their orbital eccentricity oscillates in sync with the orbit of Jupiter and falls as the comet passes the orbit of Uranus. As of 2025, three objects have been discovered with an eccentricity significantly greater than one: 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS, indicating an origin outside the Solar System. While ʻOumuamua, with an eccentricity of about 1.2, showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet. On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet. 3I/ATLAS has an eccentricity of about 6.1, and also has a coma, indicating that it is also a comet. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known solar comet with a reasonable observation arc. Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. Comets from interstellar space are moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). When such objects enter the Solar System, they have a positive specific orbital energy resulting in a positive velocity at infinity (
v
∞
{\displaystyle v_{\infty }\!}
) and have notably hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
=== Oort cloud and Hills cloud ===