Main-belt comet
Encyclopedia
Main-belt comets are bodies orbiting within the main asteroid belt
which have shown cometary activity during part of their orbit. The Jet Propulsion Laboratory
defines a main-belt asteroid as an asteroid with a semi-major axis
(average distance from the Sun) of more than 2 AU
but less than 3.2 AU, and a perihelion (closest approach distance to the Sun) of no less than 1.6 AU.
that are undistinguishable from the orbits of many standard asteroid
s. While quite a few short-period comets have semimajor axes well within Jupiter's orbit, main-belt comets differ in having small eccentricities
and inclination
s similar to main-belt asteroids. The first three identified main-belt comets all orbit within the outer part of the asteroid belt.
It is not known how an outer Solar System body like the other comets could have made its way into a low-eccentricity orbit typical of the asteroid belt, which is only weakly perturbed by the planets. Hence it is assumed that unlike other comets, the main-belt comets are simply icy asteroids, which formed in an inner Solar System orbit close to their present positions, and that many outer asteroids may be icy.
. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia. The activity persists for a month or several out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile
material helping to expose them to solar radiation.
When discovered in January 2010, P/2010 A2
(LINEAR) was initially given a cometary designation and considered a member of this group, but P/2010 A2 is now believed to the remnant of an asteroid-on-asteroid impact.. Observations of 596 Scheila
indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter.
Identified members of this morphology class include:
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
which have shown cometary activity during part of their orbit. The Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
defines a main-belt asteroid as an asteroid with a semi-major axis
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...
(average distance from the Sun) of more than 2 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
but less than 3.2 AU, and a perihelion (closest approach distance to the Sun) of no less than 1.6 AU.
Orbits
Unlike comets, which spend most of their orbit at Jupiter-like or greater distances from the Sun, main-belt comets follow near-circular orbits within the asteroid beltAsteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
that are undistinguishable from the orbits of many standard asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s. While quite a few short-period comets have semimajor axes well within Jupiter's orbit, main-belt comets differ in having small eccentricities
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...
and inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
s similar to main-belt asteroids. The first three identified main-belt comets all orbit within the outer part of the asteroid belt.
It is not known how an outer Solar System body like the other comets could have made its way into a low-eccentricity orbit typical of the asteroid belt, which is only weakly perturbed by the planets. Hence it is assumed that unlike other comets, the main-belt comets are simply icy asteroids, which formed in an inner Solar System orbit close to their present positions, and that many outer asteroids may be icy.
Activity
Some main-belt comets display a cometary dust tail only for a part of their orbit near perihelionApsis
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...
. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia. The activity persists for a month or several out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
material helping to expose them to solar radiation.
When discovered in January 2010, P/2010 A2
P/2010 A2
P/2010 A2 is a small Solar System body that displayed characteristics of both an asteroid and a comet, and thus, was initially given a cometary designation. Since it has the orbit of a main-belt asteroid and showed the tail of a comet, it was listed as a main-belt comet...
(LINEAR) was initially given a cometary designation and considered a member of this group, but P/2010 A2 is now believed to the remnant of an asteroid-on-asteroid impact.. Observations of 596 Scheila
596 Scheila
596 Scheila is a main-belt asteroid and main-belt cometorbiting the Sun. It was discovered on 21 February 1906 by August Kopff from Heidelberg...
indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter.
Composition
It has been hypothesized that main-belt comets may have been the source of Earth's water, since the deuterium-hydrogen ratio of the Earth's oceans is too low for classical comets to have been the principal source.Members
The term 'main-belt comet' is a classification based on orbit and the presence of an extended morphology. It does not imply that these objects are comets or that the material surrounding their nuclei was ejected by the sublimation of volatiles, as on comets.Identified members of this morphology class include:
| Full Name |
Hsieh |
Jewitt David C. Jewitt David C. Jewitt is a professor of astronomy formerly at the University of Hawaii Institute for Astronomy, now at UCLA. He was born in 1958 in England, and is a 1979 graduate of the University of London. Jewitt received an M.Sc. and a Ph.D. in astronomy at the California Institute of Technology in... |
Semi-major axis 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... (a) |
Perihelion (q) |
Perihelion date |
|---|---|---|---|---|---|
| 133P/Elst–Pizarro | Y | Y | 3.15 | 2.64 | 2013-Feb-08 |
| 176P/LINEAR 118401 LINEAR 118401 LINEAR is an asteroid and main-belt comet which was discovered by the Lincoln Near-Earth Asteroid Research 1-metre telescopes in Socorro, New Mexico on September 7, 1999. LINEAR was discovered to be cometary on November 26, 2005, by Henry H. Hsieh and David C... |
Y | Y | 3.19 | 2.57 | 2011-Jul-02 |
| 238P/Read 238P/Read 238P/Read is a Main-belt comet discovered on 2005 October 24. It has an orbit within the main asteroid belt and has displayed the coma of a traditional comet.... (P/2005 U1) |
Y | Y | 3.16 | 2.36 | 2011-Mar-10 |
| P/2008 R1 (Garradd) | Y | Y | 2.72 | 1.79 | 2013-Jan-25 |
| P/2010 A2 P/2010 A2 P/2010 A2 is a small Solar System body that displayed characteristics of both an asteroid and a comet, and thus, was initially given a cometary designation. Since it has the orbit of a main-belt asteroid and showed the tail of a comet, it was listed as a main-belt comet... (LINEAR) |
Y | 2.29 | 2.00 | 2009-Dec-04 | |
| P/2010 R2 (La Sagra) | Y | 3.09 | 2.62 | 2010-Jun-25 | |
| 596 Scheila 596 Scheila 596 Scheila is a main-belt asteroid and main-belt cometorbiting the Sun. It was discovered on 21 February 1906 by August Kopff from Heidelberg... |
Y | Y | 2.92 | 2.44 | 2012-May-19 |
External links
- Henry Hsieh's Main-Belt Comets page has extensive details on Main-belt comets
- David Jewitt. Main Belt Comets
- Planetary Society article on MBCs
- Discussion of possible differences in characteristics of the water in MBCs and other comets
- YouTube Interview with David Jewitt (discussion on main-belt comets starts around 9 minutes into video)
- Impact trigger mechanism diagram by David JewittDavid C. JewittDavid C. Jewitt is a professor of astronomy formerly at the University of Hawaii Institute for Astronomy, now at UCLA. He was born in 1958 in England, and is a 1979 graduate of the University of London. Jewitt received an M.Sc. and a Ph.D. in astronomy at the California Institute of Technology in...
- Comet-like appearance of (596) Scheila
- Project T3: Finding Comets in the Asteroid Population
