Damocloids are a
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differently ...
of
minor planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s such as
5335 Damocles and
1996 PW that have
Halley-type or long-period highly
eccentric orbit
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
s typical of periodic
comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...
s such as
Halley's Comet, but without showing a
cometary coma or
tail.
David Jewitt
David Clifford Jewitt (born 1958) is a British-American astronomer who studies the Solar System, especially its minor bodies. He is based at the University of California, Los Angeles, where he is a Member of the Institute for Geophysics and Pl ...
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 , , and .
Using the Tisserand's parameter with respect to Jupiter of 2 or less, there are currently 220 damocloid candidates .
Of these objects, 189 have orbital
observation arc
In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...
s greater than 30 days providing reasonably decent orbits.
Their average radius is eight kilometers assuming an
albedo
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
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
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as bein ...
. Other damocloids include: , , , , and
20461 Dioretsa.
Retrograde objects such as Halley's Comet and damocloid 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 (and assigned minor-planet provisional designation
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calc ...
s) subsequently showed a coma and were confirmed to be comets: C/2001 OG108 (LONEOS), (LINEAR), (LINEAR), (Spacewatch) and possibly others. Another strong indication of cometary origin is the fact that some damocloids have retrograde orbits, unlike any other minor planets. (Objects with an inclination
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.
For a satellite orbiting the Ea ...
beyond 90 degrees up to 180 degrees are in a retrograde orbit and orbit in the opposite direction of other objects.)
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 ''(also see )''. Tisserand's parameter
Tisserand's parameter (or Tisserand's invariant) is a value calculated from several orbital elements ( semi-major axis, orbital eccentricity and inclination) of a relatively small object and a larger " perturbing body". It is used to distinguish d ...
with respect to Jupiter (TJupiter) is also given. Akimasa Nakamura's criteria and a TJupiter of less than 2 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.ʻOumuamua
Oumuamua is the first known interstellar object detected passing through the Solar System. Formally designated 1I/2017 U1, it was discovered by Robert Weryk using the Pan-STARRS telescope at Haleakalā Observatory, Hawaii, on 19 Octob ...
and 2I/Borisov, two interstellar objects.
Notes
See also
* Extinct comet
* List of centaurs (small Solar System bodies)
The following is a list of centaurs, a group of non-resonant small Solar System bodies whose orbit around the Sun lie typically between the orbits of Jupiter and Neptune (5 to 30 AU). Centaurs are minor planets with characteristics of comets, an ...
References
External links
A first look at the Damocloids
David Jewitt, January 2005
David J. Darling
David Darling (born 29 July 1953 in Glossop, Derbyshire) is an English astronomer
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observ ...
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Distant minor planet groups and families