158P/Kowal–LINEAR, also known by its minor planet designation (118401) LINEAR, is a celestial object that holds the unique distinction of being classified as both a periodic comet and a numbered minor planet (asteroid). It is a notable example of an object exhibiting characteristics of both types of solar system bodies, sometimes referred to as a "main-belt comet" due to its asteroid-like orbit.
Discovery and Nomenclature
The object has a complex discovery history that led to its dual classification:
- Initial Asteroid Discovery: On September 7, 1999, the Lincoln Near-Earth Asteroid Research (LINEAR) program discovered an asteroid-like object, which was designated 1999 RE70. This object was later numbered as the minor planet (118401) LINEAR.
- Comet Discovery: Two months later, on November 7, 1999, astronomer Charles T. Kowal, observing from Palomar Observatory, discovered a faint cometary object with a visible coma and tail. This object was designated P/1999 VS38 (Kowal).
- Identification: Subsequent orbital calculations quickly revealed that 1999 RE70 and P/1999 VS38 (Kowal) were, in fact, the same object.
- Dual Designation: As a result of this identification, the object received a periodic comet designation, 158P/Kowal–LINEAR, acknowledging both discoverers and the distinct observations of its cometary activity. Its asteroid designation, (118401) LINEAR, remains valid.
Orbital Characteristics
158P/Kowal–LINEAR orbits the Sun in the outer regions of the asteroid belt, with characteristics that are somewhat intermediate between typical asteroids and Jupiter-family comets:
- Orbital Period: Approximately 14.9 years.
- Semi-major Axis: Around 5.9 astronomical units (AU).
- Perihelion: About 2.8 AU (within the asteroid belt).
- Aphelion: Approximately 9.0 AU (near the orbit of Saturn).
- Eccentricity: Roughly 0.5, indicating a moderately elliptical orbit.
- Inclination: Around 17 degrees to the ecliptic.
Its orbit suggests that it spends most of its time in the outer asteroid belt, and its cometary activity is observed as it approaches perihelion.
Physical Characteristics and Significance
As a periodic comet, 158P/Kowal–LINEAR periodically displays a coma and tail, indicative of volatile materials (like water ice) sublimating from its surface when heated by the Sun. However, its asteroid-like orbital characteristics mean it is part of a growing class of objects referred to as main-belt comets. These objects are thought to be icy bodies that formed in the colder outer solar system but have somehow migrated into the asteroid belt, or they represent primordial icy bodies that formed in situ within the asteroid belt and have retained their volatiles.
The study of 158P/Kowal–LINEAR and other main-belt comets provides valuable insights into:
- The distribution and evolution of water and other volatiles in the early solar system.
- The potential sources of Earth's water.
- The processes that can lead to cometary activity in objects previously thought to be inert asteroids.
See Also
- Main-belt comet
- Comet
- Asteroid
- Periodic comet
