Damocloid
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 of 2 or less, while Akimasa Nakamura defines this group with the following orbital elements:
However, this definition that does not focus on Jupiter excludes objects such as,, and.
Using the Tisserand's parameter with respect to Jupiter of 2 or less, there are currently 220 damocloid candidates as of 2022. Of these objects, 189 have orbital observation arcs greater than 30 days providing reasonably decent orbits. Their average radius is eight kilometers assuming an albedo of 0.04. The albedos of four damocloids have been measured, and they are among the darkest objects known in the Solar System. Damocloids are reddish in color, but not as red as many Kuiper-belt objects or centaurs. Other damocloids include:,,,, and 20461 Dioretsa.
Retrograde objects such as Halley's Comet and damocloid 343158 Marsyas can have relative velocities to Earth of.
Origin
Damocloids are thought to be nuclei of Halley-type comets that have lost all their volatile materials due to outgassing and become dormant. Such comets probably originate from the Oort cloud. This hypothesis is strengthened by the fact that a number of objects thought to be damocloids subsequently showed a coma and were confirmed to be comets: C/, , C/2002 VQ94|, and possibly others. Another strong indication of cometary origin is the fact that some damocloids have retrograde orbitsthey have an inclination beyond 90 degrees up to 180 degrees.List
this list from the JPL SBDB contains 20 numbered and 268 unnumbered bodies that meet Akimasa Nakamura's criteria for being classified as a damocloid, that is, either a retrograde orbit, or the following orbital elements:q < 5.2 AU, a > 8.0 AU, and e > 0.75 . Tisserand's parameter with respect to Jupiter is also given. Akimasa Nakamura's criteria and a TJupiter of less than two are largely equivalent as only a few listed bodies do not meet the defined TJupiter threshold. Most damocloids are also listed on MPC's list of other unusual minor planets. The orbital data is sourced from JPL-numbered and -unnumbered element files. The list includes A/-designated objects that were mistakenly identified as a comet, but are actually minor planets. However it excludes hyperbolic bodies such as A/2019 G4 as well as unbounded, interstellar objects such as 1I/ʻOumuamua and 2I/Borisov.