List of Solar System objects by greatest aphelion

Motion of the Solar System's barycenter relative to the Sun

As many of the objects listed below have some of the most extreme orbits of any objects in the Solar System, describing their orbit precisely can be particularly difficult and sensitive to the time the orbit is defined at. For most objects in the Solar System, a heliocentric reference frame (relative to the gravitational center of the Sun) is sufficient to explain their orbits. However, as the orbits of objects become closer to the Solar System's escape velocity, with long orbital periods on the order of hundreds or thousands of years, a different reference frame is required to describe their orbit: a barycentric reference frame. A barycentric reference frame measures the asteroid's orbit relative to the gravitational center of the entire Solar System, rather than just the Sun. Mostly due to the influence of the outer gas giants, the Solar System barycenter varies by up to twice the radius of the Sun.

This difference in position can lead to significant changes in the orbits of long-period comets and distant asteroids. Many comets have hyperbolic (unbound) orbits in a heliocentric reference frame, but in a barycentric reference frame have much more firmly bound orbits, with only a small handful remaining truly hyperbolic.

The orbital parameter used to describe how non-circular an object's orbit is, is eccentricity (e). An object with an e of 0 has a perfectly circular orbit, with its perihelion distance being just as close to the Sun as its aphelion distance. An object with an e of between 0 and 1 will have an elliptical orbit, with, for instance, an object with an e of 0.5 having a perihelion twice as close to the Sun as its aphelion. As an object's e approaches 1, its orbit will be more and more elongated before, and at e=1, the object's orbit will be parabolic and unbound to the Solar System (i.e. not returning for another orbit). An e greater than 1 will be hyperbolic and still be unbound to the Solar System.

Although it describes how "unbound" an object's orbit is, eccentricity does not necessarily reflect how high an incoming velocity said object had before entering the Solar System (a parameter known as V_infinity, or V_inf). A clear example of this is the eccentricities of the two known Interstellar objects as of October 2019, 1I/'Oumuamua. and 2I/Borisov. 'Oumuamua had an incoming V_inf of 26.5 kilometres per second (59,000 mph), but due to its low perihelion distance of only 0.255 au, it had an eccentricity of 1.200. However, Borisov's V_inf was only slightly higher, at 32.3 km/s (72,000 mph), but due to its higher perihelion distance of ~2.003 au, its eccentricity was a comparably higher 3.340. In practice, no object originating from the Solar System should have an incoming heliocentric eccentricity much higher than 1, and should rarely have an incoming barycentric eccentricity of above 1, as that would imply that the object had originated from an indefinitely far distance from the Sun.

Due to having the most eccentric orbits of any Solar System body, a comet's orbit typically intersects one or more of the planets in the Solar System. As a result, the orbit of a comet is frequently perturbed significantly, even over the course of a single pass through the inner Solar System. Due to the changing orbit, it's necessary to provide a calculation of the orbit of the comet (or similarly orbiting body) both before and after entering the inner Solar System. For example, Comet ISON was ~312 au from the Sun in 1600, and its remnants will be ~431 au from the Sun in 2400, both well outside of any significant gravitational influence from the planets.

Comet West in 1976

Examples of comets with a more well-determined orbit. Comets are extremely small relative to other bodies and hard to observe once they stop outgassing (see Coma (cometary)). Because they are typically discovered close to the Sun, it will take some time even thousands of years for them to actually travel out to great distances. The Whipple proposal might be able to detect Oort cloud objects at great distances, but probably not a particular object.

The following group of bodies have orbits with an aphelion above 400 AU, with 1-sigma uncertainties given to two significant digits. As of October 2022, there are 66 such bodies.[16]

Orbits of three known sednoids: Sedna, 2012 VP113, and Leleākūhonua

The following asteroids have an incoming barycentric aphelion of at least 1000 AU.