Elst–Pizarro, the first main-belt comet
An asteroid in the main belt that grows a dusty tail, showing that ice survives among the asteroids meteorites come from.
Key facts
- Designation
- 133P/Elst–Pizarro, also asteroid (7968)
- Type
- Main-belt comet (an active asteroid)
- Orbital period
- about 5.6 years
- Closest to the Sun
- about 2.6 AU (about 390 million km)
- Farthest from the Sun
- about 3.7 AU
- Tilt of orbit
- about 1.4°
- Nucleus
- about 3.8 km across
- Discovered
- seen as a comet by Eric Elst and Guido Pizarro at La Silla, Chile, on 7 August 1996; first seen as an asteroid in 1979
- Returns
- active near each perihelion
Orbit
Elst–Pizarro orbits in the outer main asteroid belt between Mars and Jupiter, on a nearly circular path close to the planets' plane. That is an ordinary asteroid orbit, not a comet's.
Size and nature
It is about 3.8 km across and spins quickly, about every three and a half hours. Near the closest part of its orbit to the Sun, it grows a thin, straight dust tail, and the tail returns at each pass. That repeated behaviour points to ice warming and turning to gas, carrying dust with it, rather than a single collision.
History
Eric Elst and Guido Pizarro photographed it in August 1996 and saw a tail. It was then found to be the same as asteroid 1979 OW7, so it has both a comet name and an asteroid number. In 2006 astronomers found two more objects behaving the same way, and named the new class main-belt comets.
Why it matters for meteorites
Many meteorites, such as the carbonaceous chondrites, show that their parent asteroids once held liquid water. Main-belt comets show that some ice still survives today in the asteroid belt itself. The line between asteroids and comets is less sharp than it seemed, and the parents of some meteorites may be closer in nature to comets than once thought.
Key dates
| Date | What happened |
|---|---|
| 7 August 1996 | Eric Elst and Guido Pizarro saw a tail on an asteroid, revealing the first main-belt comet. |
Did you know?
About two dozen main-belt comets and other active asteroids are now known.
Elst–Pizarro has both an asteroid number and a comet number.
A mission to a main-belt comet has been studied but not yet launched.