Comet Elst–Pizarro is a body that displays characteristics of both asteroids and comets, and is the prototype of main-belt comets. Its orbit keeps it within the asteroid belt, yet it displayed a dust tail like a comet while near perihelion in 1996, 2001, and 2007.
- As a comet it is formally designated 133P/Elst–Pizarro.
- As an asteroid it is designated 7968 Elst–Pizarro.
7968 Elst–Pizarro imaged at La Silla Observatory in August 1996. The narrow tail is visible.
|Discovered by||1979 OW7:|
M. R. S. Hawkins
R. H. McNaught, S. J. Bus 
Eric W. Elst, Guido Pizarro
|Discovery date||24 July 1979 (1979 OW7) |
14 July 1996 (1996 N2)
|Main-belt comet |
Main-belt asteroid 
Themis/ Beagle family 
|Epoch 13 January 2016 (JD 2457400.5)|
|Uncertainty parameter 0|
|Observation arc||13350 days (36.55 yr)|
|Aphelion||3.66751 AU (548.652 Gm) (Q)|
|Perihelion||2.6524 AU (396.79 Gm) (q)|
|3.1600 AU (472.73 Gm) (a)|
|5.62 yr (2051.7 d)|
Average orbital speed
|0° 10m 31.656s / day (n)|
|Jupiter MOID||1.51427 AU (226.532 Gm)|
|1.3(?) g/cm³ |
|3.471 h (0.1446 d)|
|Temperature||160 K |
|17.24 to 20.71|
15.3R (2004) 
15.49R (2010) 
Elst–Pizarro was reported in 1979 as minor planet 1979 OW7, with its image on a photographic plate being completely stellar in appearance. Its 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. The cometary nature of Elst–Pizarro was first discovered when a linear dust feature was observed with the ESO 1-metre Schmidt telescope at La Silla Observatory on 7 August 1996.
Subsequently, around the next perihelion in November 2001, the cometary activity appeared again, and persisted for 5 months. It again came to perihelion on 8 February 2013. The outgassing was found to happen only on small part of the surface measuring less than 600m in effective diameter, likely being the relatively recent (younger than 100 million years) impact crater.
At present, seven other objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 4015 Wilson–Harrington (107P/Wilson–Harrington), 60558 Echeclus (174P/Echeclus), 118401 LINEAR (176P/LINEAR), (323137) 2003 BM80 (282P/2003 BM80), (300163) 2006 VW139 (288P/2006 VW139), and (457175) 2008 GO98 (362P/2008 GO98). As a dual status object, astrometric observations of 7968 Elst–Pizarro should be reported under the minor planet designation.
Castalia is a proposed mission concept for a robotic spacecraft to explore Pizarro and make the first in situ measurements of water in the asteroid belt, and thus, help solve the mystery of the origin of Earth's water. The lead is Colin Snodgrass, from The Open University in the UK. Castalia was proposed in 2015 and 2016 to the European Space Agency within the Cosmic Vision programme missions M4 and M5, but it was not selected. The team continues to mature the mission concept and science objectives. Because of the construction time required and orbital dynamics, a launch date of October 2028 was proposed.
Castalia mission scientists are now considering instrument contributions for the Chinese ZhengHe comet exploration mission, which will conduct detailed remote sensing and in-situ measurements at Elst–Pizarro in 2032 following a proposed launch in 2024.
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- "IAUC 6457". IAU-Central Bureau for Astronomical Telegrams. 21 August 1996.
- Henry H. Hsieh (May 2010). "Main Belt Comets". Hawaii. Archived from the original on 2011-08-06. Retrieved 2010-12-15. (older 2010 site) Archived August 10, 2009, at the Wayback Machine
- David Jewitt. "Main Belt Comets". UCLA, Department of Earth and Space Sciences. Retrieved 2010-12-15.
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- Liang Liang Yu; Hsia, Chih-Hao; Ip, Wing-Huen (2019). "Low activity main belt comet 133P/Elst-Pizarro: New constraints on its Albedo, Temperature and Active Mechanism from a thermophysical perspective". arXiv:1912.06266 [astro-ph.EP].
- Hsieh, Henry H.; Jewitt, David C.; Fernández, Yanga R. (2004). "The Strange Case of 133P/Elst–Pizzarro: A Comet Among the Asteroids". The Astronomical Journal. 127 (5): 2997–3017. Bibcode:2004AJ....127.2997H. CiteSeerX 10.1.1.362.9950. doi:10.1086/383208.
- Hsieh, Henry H.; Jewitt, David C.; Lacerda, Pedro; Lowry, Stephen C.; Snodgrass, Colin (2010). "The return of activity in main-belt comet 133P/Elst–Pizarro". Monthly Notices of the Royal Astronomical Society. 403 (1): 363–377. arXiv:0911.5522. Bibcode:2010MNRAS.403..363H. doi:10.1111/j.1365-2966.2009.16120.x.
- "Main-Belt Comets May Have Been Source Of Earths Water". Space Daily. 23 March 2006.
- "Strange Comet Discovered at ESO". ESO. 16 September 1996.
- "Dual-Status Objects". Minor Planet Center. 2008-03-06. Retrieved 2018-02-13.
- Snodgrass, C.; Jones, G.H.; Boehnhardt, H.; Gibbings, A.; Homeister, M.; Andre, N.; Beck, P.; Bentley, M.S.; Bertini, I.; Bowles, N.; Capria, M.T.; Carr, C.; Ceriotti, M.; Coates, A.J.; Della Corte, V.; Donaldson Hanna, K.L.; Fitzsimmons, A.; Gutiérrez, P.J.; Hainaut, O.R.; Herique, A.; Hilchenbach, M.; Hsieh, H.H.; Jehin, E.; Karatekin, O.; Kofman, W.; Lara, L.M.; Laudan, K.; Licandro, J.; Lowry, S.C.; et al. (2018). "The Castalia mission to Main Belt Comet 133P/Elst-Pizarro". Advances in Space Research. 62 (8): 1947–1976. arXiv:1709.03405. Bibcode:2018AdSpR..62.1947S. doi:10.1016/j.asr.2017.09.011.
- Snodgrass, Colin (October 2018). "Exploring the next frontier: the Main Belt Comets – Chinese MBC mission" (PDF). University of Edinburgh. p. 20. Retrieved 20 June 2019.
- 7968 Elst–Pizarro at AstDyS-2, Asteroids—Dynamic Site
- 7968 Elst–Pizarro at the JPL Small-Body Database