Short-period comets are generally defined as 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. 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.
At the shorter extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 20–30 degrees or less) are called Jupiter-family comets. Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets. As of 2012, only 64 Halley-type comets have been observed, compared with nearly 450 identified Jupiter-family comets.
Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.
Since their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets display a tendency for their aphelia to coincide with a gas giant's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. 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, in addition to being the swiftest of the giant planets. These perturbations can deflect long-period comets into shorter orbital periods, with Halley's Comet being a possible example of this.
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 hypothesised its existence). Vast swarms of comet-like bodies are believed 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 towards 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.
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Famous quotes containing the words short period, period and/or short:
“It is while we are young that the habit of industry is formed. If not then, it never is afterwards. The fortune of our lives therefore depends on employing well the short period of our youth.”
—Thomas Jefferson (17431826)
“Of all the barbarous middle ages, that
Which is most barbarous is the middle age
Of man! it isI really scarce know what;
But when we hover between fool and sage,
And dont know justly what we would be at
A period something like a printed page,
Black letter upon foolscap, while our hair
Grows grizzled, and we are not what we were.”
—George Gordon Noel Byron (17881824)
“Love requited is a short circuit.”
—Samuel Beckett (19061989)