Coma (cometary) in the context of Astronomical unit

An astronomical object, celestial object, stellar object or heavenly object is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms object and body are often used interchangeably. However, an astronomical body, celestial body or heavenly body is a single, tightly bound, contiguous physical object, while an astronomical or celestial object admits a more complex, less cohesively bound structure, which may consist of multiple bodies or even other objects with substructures.

Examples of astronomical objects include planetary systems, star clusters, nebulae, and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both a body and an object: It is a body when referring to the frozen nucleus of ice and dust, and an object when describing the entire comet with its diffuse coma and tail.

A comet is an icy, small Solar System body or interstellar object that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

The nucleus is the solid, central part of a comet, formerly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.

An astronomical object, celestial object, stellar object or heavenly object is a naturally occurring physical entity, association, or structure that exists within the universe. In astronomy, the terms object and body are often used interchangeably. However, an astronomical body, celestial body or heavenly body is a single, tightly bound, contiguous physical object, while an astronomical or celestial object admits a more complex, less cohesively bound structure, which may consist of multiple bodies or even other objects with substructures.

Examples of astronomical objects include planetary systems, star clusters, nebulae, and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both a body and an object: It is a body when referring to the frozen nucleus of ice and dust, and an object when describing the entire comet with its diffuse coma and tail.

These Solar System minor planets are the furthest from the Sun as of January 2026. The objects have been categorized by their approximate distance from the Sun on that date, and not by the calculated aphelion of their orbit. The list changes over time because the objects are moving in their orbits. Some objects are inbound and some are outbound. It would be difficult to detect long-distance comets if it were not for their comas, which become visible when heated by the Sun. Distances are measured in astronomical units (AU, Sun–Earth distances). The distances are not the minimum (perihelion) or the maximum (aphelion) that may be achieved by these objects in the future.

This list does not include near-parabolic comets of which many are known to be currently more than 100 AU (15 billion km) from the Sun, but are currently too far away to be observed by telescope. Trans-Neptunian objects are typically announced publicly months or years after their discovery, so as to make sure the orbit is correct before announcing it. Due to their greater distance from the Sun and slow movement across the sky, trans-Neptunian objects with observation arcs less than several years often have poorly constrained orbits. Particularly distant objects take several years of observations to establish a crude orbit solution before being announced. For instance, the most distant known trans-Neptunian object 2018 AG37 was discovered by Scott Sheppard in January 2018 but was announced three years later in February 2021.

In optics (especially telescopes), the coma (/ˈkoʊmə/), or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is also a form of chromatic aberration.

Stardust was a 385-kilogram robotic space probe launched by NASA on 7 February 1999. Its primary mission was to collect dust samples from the coma of comet Wild 2, as well as samples of cosmic dust, and return them to Earth for analysis. It was the first sample return mission of its kind. En route to Comet Wild 2, it also flew by and studied the asteroid 5535 Annefrank. The primary mission was successfully completed on 15 January 2006 when the sample return capsule returned to Earth.

A mission extension, codenamed NExT, culminated in February 2011 with Stardust intercepting Comet Tempel 1, a small Solar System body previously visited by Deep Impact in 2005. Stardust ceased operations in March 2011.

Damocloids are a class of minor planets such as 5335 Damocles and 1996 PW that have Halley-type or long-period highly eccentric orbits typical of periodic comets such as Halley's Comet, but without showing a cometary coma or tail. David C. Jewitt defines a damocloid as an object with a Jupiter Tisserand invariant (T_J) of 2 or less, while Akimasa Nakamura defines this group with the following orbital elements:

• q < 5.2 AU, a > 8.0 AU, and e > 0.75,
• or alternatively, i > 90°

However, this definition that does not focus on Jupiter excludes objects such as (127546) 2002 XU93, 2003 WG₁₆₆, and (661238) 2004 DA₆₂.