133P/Elst-Pizarro

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7968 Elst-Pizarro
(133P/Elst-Pizarro)
Discovery ^A

Discoverer	M. R. S. Hawkins and R. H. McNaught<ref name="MPEC1996-R07">MPEC 1996-R07</ref> and/or S. J. Bus <ref name="IAUC6473">IAUC 6473</ref> (as 1979 OW₇) as well as Eric W. Elst and Guido Pizarro (as 1996 N₂)
Discovery date	24 July, 1979 (as 1979 OW₇) <ref name="IAUC6457">IAUC 6457</ref> and 14 July, 1996 (as 1996 N₂)
Alternate designations ^B	1996 N₂, 1979 OW₇
Category	Main-belt comet
Orbital elements ^C
Epoch 22 September, 2006 (JD 2454000.5)
Eccentricity (e)	0.1644
Semi-major axis (a)	472.811 G m (3.161 AU)
Perihelion (q)	395.073 Gm (2.641 AU)
Aphelion (Q)	550.548 Gm (3.680 AU)
Orbital period (P)	2052.262 d (5.62 a)
Mean orbital speed	16.64 km/s
Inclination (i)	1.386°
Longitude of the ascending node (Ω)	160.220°
Argument of perihelion (ω)	132.138°
Mean anomaly (M)	310.762°
Physical characteristics ^D
Dimensions
Mass
Density
Surface gravity
Escape velocity
Rotation period	0.1446 d (3.471 h) <ref>Planetary Data System (PDS) lightcurve data</ref>
Spectral class
Absolute magnitude	14
Albedo (geometric)
Mean surface temperature

Comet Elst-Pizarro is a remarkable body in that it displays characteristics of both asteroids and comets <ref name="spacedaily">Main-Belt Comets May Have Been Source Of Earths Water, Space Daily, Mar 23, (2006).</ref>, and is the prototype of main-belt comets. Its orbit keeps it within the asteroid main belt, while it displays a dust tail like a comet while near perihelion.

As a comet it is formally designated 133P/Elst-Pizarro.
As an asteroid it is designated 7968 Elst-Pizarro.

Elst-Pizarro was reported in 1979 as minor planet 1979 OW₇, with its image on a photographic plate being completely stellar in appearance. The orbit remains entirely within the orbits of Mars and Jupiter, with eccentricity 0.165, typical of a minor planet in the asteroid belt. However, the images taken by Eric W. Elst and Guido Pizarro in 1996, when it was near perihelion, clearly show a cometary tail. Since this is not normal behaviour for asteroids, it is suspected that Elst-Pizarro has a different, probably icy, composition.

Subsequently, around the next perihelion in late 2002, the cometary activity appeared again, and persisted for several months <ref name="HHsieh">Main Belt Comets page by Henry Hsieh</ref>.

At present, there are only four other objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), Comet 107P/Wilson-Harrington (4015 Wilson-Harrington), 60558 Echeclus (174P/Echeclus), and 118401 LINEAR (176P/LINEAR (LINEAR 52)). 3200 Phaethon could be a member of this group ^{[citation needed]}.

[edit] References

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