Halley's orbital period has varied between 74–79 years since 240 BC.
Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a circle and 1 being a parabolic trajectory).
The perihelion, the point in the comet's orbit when it is nearest the Sun, is just 0.6 AU.
This is between the orbits of Mercury and Venus.
Its aphelion, or farthest distance from the Sun, is 35 AU (roughly the distance of Pluto).
Unusual for an object in the Solar System, Halley's orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or, clockwise from above the Sun's north pole.
The orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic. (
Because it is retrograde, the true inclination is 162°.)
Owing to the retrograde orbit, it has one of the highest velocities relative to the Earth of any object in the Solar System.
The 1910 passage was at a relative velocity of 70.56 km/s (157,838 mph or 254,016 km/h).
Because its orbit comes close to Earth's in two places, Halley is associated with two meteor showers: the Eta Aquariids in early May, and the Orionids in late October.
Halley is the parent body to the Orionids.
Observations conducted around the time of Halley's appearance in 1986 suggested that the comet could additionally perturb the Eta Aquariids meteor shower, although it might not be the parent of that shower.

Halley is classified as a periodic or short-period comet; one with an orbit lasting 200 years or less.
This contrasts it with long-period comets, whose orbits last for thousands of years.
Periodic comets have an average inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, so Halley's orbit is atypical.
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 resembling 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 2015, only 75 Halley-type comets have been observed, compared with 511 identified Jupiter-family comets.

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System.
If Halley was once a long-period comet, it is likely to have originated in the Oort cloud, a sphere of cometary bodies that has an inner edge of 20,000–50,000 AU.
Conversely the Jupiter-family comets are generally believed to originate in the Kuiper belt, a flat disc of icy debris between 30 AU (Neptune's orbit) and 50 AU from the Sun (in the scattered disc).
Another point of origin for the Halley-type comets was proposed in 2008, when a trans-Neptunian object with a retrograde orbit similar to Halley's was discovered, 2008 KV42, whose orbit takes it from just outside that of Uranus to twice the distance of Pluto.
It may be a member of a new population of small Solar System bodies that serves as the source of Halley-type comets.

Halley has probably been in its current orbit for 16,000–200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations.
The non-gravitational effects can be crucial; as Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path.
These orbital changes cause delays in its perihelion of four days, average.

In 1989, Boris Chirikov and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations.
These studies showed that its dynamics were chaotic and unpredictable on long timescales.
Halley's projected lifetime could be as long as 10 million years.
These studies also showed that many physical properties of Halley's Comet dynamics can be approximately described by a simple symplectic map, known as t
Halley's orbital period has varied between 74–79 years since 240 BC. Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a circle and 1 being a parabolic trajectory). The perihelion, the point in the comet's orbit when it is nearest the Sun, is just 0.6 AU. This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is 35 AU (roughly the distance of Pluto). Unusual for an object in the Solar System, Halley's orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or, clockwise from above the Sun's north pole. The orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic. (Because it is retrograde, the true inclination is 162°.) Owing to the retrograde orbit, it has one of the highest velocities relative to the Earth of any object in the Solar System. The 1910 passage was at a relative velocity of 70.56 km/s (157,838 mph or 254,016 km/h). Because its orbit comes close to Earth's in two places, Halley is associated with two meteor showers: the Eta Aquariids in early May, and the Orionids in late October. Halley is the parent body to the Orionids. Observations conducted around the time of Halley's appearance in 1986 suggested that the comet could additionally perturb the Eta Aquariids meteor shower, although it might not be the parent of that shower.
Halley is classified as a periodic or short-period comet; one with an orbit lasting 200 years or less. This contrasts it with long-period comets, whose orbits last for thousands of years. Periodic comets have an average inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, so Halley's orbit is atypical. 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 resembling 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 2015, only 75 Halley-type comets have been observed, compared with 511 identified Jupiter-family comets.
The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System. If Halley was once a long-period comet, it is likely to have originated in the Oort cloud, a sphere of cometary bodies that has an inner edge of 20,000–50,000 AU. Conversely the Jupiter-family comets are generally believed to originate in the Kuiper belt, a flat disc of icy debris between 30 AU (Neptune's orbit) and 50 AU from the Sun (in the scattered disc). Another point of origin for the Halley-type comets was proposed in 2008, when a trans-Neptunian object with a retrograde orbit similar to Halley's was discovered, 2008 KV42, whose orbit takes it from just outside that of Uranus to twice the distance of Pluto. It may be a member of a new population of small Solar System bodies that serves as the source of Halley-type comets.
Halley has probably been in its current orbit for 16,000–200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations. The non-gravitational effects can be crucial; as Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path. These orbital changes cause delays in its perihelion of four days, average.
In 1989, Boris Chirikov and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations. These studies showed that its dynamics were chaotic and unpredictable on long timescales. Halley's projected lifetime could be as long as 10 million years. These studies also showed that many physical properties of Halley's Comet dynamics can be approximately described by a simple symplectic map, known as t
Halley's Comet