Damocloid asteroid
Encyclopedia
Damocloids are minor planet
s such as 5335 Damocles
and 1996 PW that have Halley family or long-period highly eccentric orbits typical of periodic comet
s such as Comet Halley
, but without showing a cometary coma
or tail.
Damocloids are believed to be nuclei
of Halley-type comets that have lost all their volatile materials due to outgassing
and become dormant
. Such comets are believed to 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 designations) subsequently showed a coma
and were confirmed to be comets: C/2001 OG108 (LONEOS)
, C/2002 CE10 (LINEAR), C/2002 VQ94 (LINEAR), C/2004 HV60 (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 beyond 90 degrees up to 270 degrees are in a retrograde orbit and orbit in the opposite direction of other objects.) David Jewitt defines a damocloid as an object with a Tisserand's parameter
relative to Jupiter TJ ≤ 2. This can also loosely be defined as (q
< 5.2 AU
, a
> 8.0 AU, and e
> 0.75) or i > 90 deg, but this definition that does not focus on Jupiter excludes objects such as , , and .
Using the Tisserand parameter (T-Jupiter < 2) there are currently 77 damocloid candidates, and about half of them still have poorly known orbits. Of these objects, 41 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.
The near-Earth object
has the highest relative velocity to the Earth (282,900 km/h) of objects that come within 0.5 AU
of the Earth.
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...
s such as 5335 Damocles
5335 Damocles
5335 Damocles is the archetype of the Damocloids, asteroids that are inactive nuclei of the Halley Family and long-period comets. It was discovered in 1991 and named after Damocles, a figure of Greek mythology....
and 1996 PW that have Halley family or long-period highly eccentric orbits typical of periodic comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s such as Comet Halley
Comet Halley
Halley's Comet or Comet Halley is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years. Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and thus the only naked-eye comet that might appear twice in a human lifetime...
, but without showing a cometary coma
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
or tail.
Damocloids are believed to be nuclei
Comet nucleus
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma...
of Halley-type comets that have lost all their volatile materials due to outgassing
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
and become dormant
Extinct comet
Extinct comets are comets that have expelled most of their volatile ice and have little left to form a tail or coma. The volatile material contained in the comet nucleus evaporates away, and all that remains is inert rock or rubble that can resemble an asteroid. Comets may go through a transition...
. Such comets are believed to originate from the Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
. This hypothesis is strengthened by the fact that a number of objects thought to be Damocloids (and assigned minor-planet provisional designations) subsequently showed a coma
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
and were confirmed to be comets: C/2001 OG108 (LONEOS)
C/2001 OG108 (LONEOS)
C/2001 OG108 is a Halley-type comet with an orbital period of 48.51 years. It was discovered on 28 July 2001 by the LONEOS telescope at Lowell Observatory....
, C/2002 CE10 (LINEAR), C/2002 VQ94 (LINEAR), C/2004 HV60 (Spacewatch) and possibly others. Another strong indication of cometary origin is the fact that some Damocloids have retrograde orbits
Retrograde motion
Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such...
, unlike any other minor planets. (Objects with an inclination beyond 90 degrees up to 270 degrees are in a retrograde orbit and orbit in the opposite direction of other objects.) David Jewitt defines a damocloid as an object with a Tisserand's parameter
Tisserand's parameter
Tisserand's parameter is a combination of orbital elements used in a restricted three-body problem.-Definition:...
relative to Jupiter TJ ≤ 2. This can also loosely be defined as (q
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...
< 5.2 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, a
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
> 8.0 AU, and e
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
> 0.75) or i > 90 deg, but this definition that does not focus on Jupiter excludes objects such as , , and .
Using the Tisserand parameter (T-Jupiter < 2) there are currently 77 damocloid candidates, and about half of them still have poorly known orbits. Of these objects, 41 have orbital observation arcs
Arc (geometry)
In geometry, an arc is a closed segment of a differentiable curve in the two-dimensional plane; for example, a circular arc is a segment of the circumference of a circle...
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
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
objects or centaurs.
The near-Earth object
Near-Earth object
A near-Earth object is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU. They include a few thousand near-Earth asteroids , near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to...
has the highest relative velocity to the Earth (282,900 km/h) of objects that come within 0.5 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
of the Earth.
External links
- David Jewitt, A first look at the Damocloids