P/2020 MK4 (PanSTARRS)

Lore & Background

P/2020 MK4 was discovered on 24 June 2020 by the Pan-STARRS survey at Haleakala Observatory in Hawaii. It was found in an outburst state and by late 2020 had returned to its regular brightness. The comet was recovered by the Lowell Discovery Telescope at an extremely faint apparent magnitude of 24.5 in September 2022, and was officially recognized as a comet by the Minor Planet Center on 20 November 2022, receiving the periodic comet designation P/2020 MK4.

Physical characteristics include a lower limit for the absolute magnitude of the nucleus of Hg = 11.30±0.03, which for an albedo in the range 0.1–0.04 gives an upper limit for its size in the interval 23–37 km. However, a follow-up study in 2022 showed the nucleus size is much smaller, at only 2.2 km in diameter. Its color indices indicate a neutral or gray nucleus.

Orbital evolution is highly chaotic due to strong interactions with giant planets, and numerical simulations suggest it may be ejected from the Solar System within the next 200,000 years. The simulations also indicate it may have experienced relatively close flybys with comet 29P/Schwassmann–Wachmann, during which one or both objects were transient Jovian satellites, and P/2020 MK4 may have crossed the coma of 29P when in outburst.

Reader's Guide

P/2020 MK4 (PanSTARRS) is significant as a Chiron-type comet or active centaur, a class of objects that bridge the gap between asteroids and comets in the outer Solar System. Its discovery in an outburst state and subsequent fading to regular brightness highlights the dynamic activity of such bodies. The comet's size remains uncertain, with an initial upper limit of 23–37 km based on albedo assumptions, but a 2022 study revised the nucleus diameter to just 2.2 km, illustrating the challenges in characterizing distant, faint objects. Its neutral color indices suggest a primitive surface composition. The chaotic orbital evolution, with potential ejection from the Solar System within 200,000 years, underscores the short dynamical lifetimes of centaurs due to giant planet interactions. The numerical simulations of close flybys with comet 29P/Schwassmann–Wachmann, including possible transient satellite captures and coma crossings, provide insight into the complex gravitational and physical interactions among these outer Solar System bodies. The comet's official recognition in November 2022 and its periodic comet designation mark it as a member of the population of active centaurs that are regularly monitored.

Did You Know?

• P/2020 MK4 was discovered in an outburst state on 24 June 2020 by the Pan-STARRS survey.
• A 2022 study revised its nucleus size downward to only 2.2 km in diameter.

Discovery and Designation

The designation P/2020 MK4 encodes several layers of information at a glance. The leading letter P identifies the object as a periodic comet, setting it apart from non-periodic comets carrying a C prefix, lost or disintegrated comets marked D, objects with unreliable orbits flagged X, interstellar visitors tagged I, or misidentified minor planets labeled A. The year 2020 together with the code MK4 pinpoints the epoch of discovery, while the name PanSTARRS credits the system responsible for the find. In comet nomenclature the discoverer's name is appended in nearly every case, and PanSTARRS follows that convention. Because the P prefix signals a periodic orbit, this object either completes a full circuit in under two hundred years or has already been witnessed crossing perihelion more than once. That classification carries a practical advantage: the next perihelion passage of a periodic comet can be forecast with a high degree of accuracy, a luxury not available for long-period comets whose trajectories are far more difficult to track.

Orbital Classification

Within the broader taxonomy of comets, P/2020 MK4 is catalogued among the unnumbered Jupiter-family comets discovered during the 2020s. Jupiter-family comets are officially defined by the condition that the ratio of their orbital period to Jupiter's falls between two and three. In looser usage the label extends to any comet whose period is under twenty years, whose orbital inclination is relatively low, and whose path loosely mirrors Jupiter's own circuit of the Sun. These comets share a gravitational neighborhood with the giant planet, and their dynamics are shaped by repeated close encounters. This classification distinguishes them from Halley-type comets, long-period comets originating in the Oort Cloud, and Kreutz sungrazers, each of which occupies a distinct orbital regime with its own set of observational and physical characteristics. The Jupiter-family grouping thus represents a specific dynamical family rather than a mere convenience category.

The Path to a Permanent Number

One of the most revealing features of P/2020 MK4's current designation is the absence of a numerical prefix. Under the International Astronomical Union's naming system, a periodic comet typically earns a permanent number—transforming from a provisional label like P/2020 MK4 into a form such as 123P/Name—only after its second confirmed perihelion passage. Until that milestone is reached, the object remains in a pool of unnumbered periodic comets, a group that also includes bodies such as P/2005 T5 (Broughton). If a comet fails to reappear after several perihelion passages or is presumed to have disintegrated, it is reclassified with a D designation; if it had already received a number, that number is retained with a D suffix, as in 3D/Biela or 5D/Brorsen. For P/2020 MK4, the route to a permanent identity therefore depends on sustained observation and confirmation of its return to perihelion.

Observational Challenges and Visibility

Jupiter-family comets such as P/2020 MK4 present persistent difficulties for observers tracking them across the sky. Their orbits are frequently perturbed by gravitational interactions with Jupiter, which can shift them away from their predicted positions and render them temporarily lost. Compounding this, these comets tend to possess low albedos and spend more time in close proximity to the Sun than their Oort Cloud counterparts. The repeated heating cycles cause them to deplete their volatile ices more rapidly, leaving them comparatively dimmer than comets with longer orbital periods. The release of gas and other material that forms the coma and tail also introduces non-gravitational effects that further complicate orbital calculations. Even for numbered periodic comets, whose next perihelion can be predicted with high accuracy, these accumulated errors from planetary perturbations and outgassing mean that locating a comet at the right moment and the right place in the sky remains a genuinely nontrivial task.