In astronomy, precovery (short for pre-discovery recovery)[1][2] is the process of finding the image of an object in images or photographic plates predating its discovery, typically for the purpose of calculating a more accurate orbit. This happens most often with minor planets, but sometimes a comet, a dwarf planet, a natural satellite, or a star is found in old archived images; even exoplanet precovery observations have been obtained.[3] "Precovery" refers to a pre-discovery image; "recovery" refers to imaging of a body which was lost to our view (as behind the Sun), but is now visible again (also see lost minor planet and lost comet).

Orbit determination requires measuring an object's position on multiple occasions. The longer the interval between observations, the more accurately the orbit can be calculated; however, for a newly discovered object, only a few days' or weeks' worth of measured positions may be available, sufficient only for a preliminary (imprecise) orbit calculation.

When an object is of particular interest (such as asteroids with a chance of impacting Earth), researchers begin a search for precovery images. Using the preliminary orbit calculation to predict where the object might appear on old archival images, those images (sometimes decades old) are searched to see if it had in fact already been photographed. If so, a far longer observation arc can allow a far more precise orbital calculation.

Until fast computers were widely available, it was impractical to analyze and measure images for possible minor planet discoveries because this required much human labor. Usually, such images were made years or decades earlier for other purposes (studies of galaxies, etc.), and it was not worth the time it took to look for precovery images of ordinary asteroids. Today, computers can easily analyze digital astronomical images and compare them to star catalogs containing up to a billion or so star positions to see if one of the "stars" is actually a precovery image of the newly discovered object. This technique has been used since the mid-1990s to determine the orbits of a great many minor planets.


In an extreme case of precovery, an object was discovered on December 31, 2000, designated 2000 YK66, and a near-Earth orbit was calculated. Precovery revealed that it had previously been discovered on February 23, 1950 and given the provisional designation 1950 DA, and then been lost for half a century. The exceptionally long observation period allowed an unusually precise orbit calculation, and the asteroid was determined to have a small chance of colliding with the Earth. After an asteroid's orbit is calculated with sufficient precision, it can be assigned a number prefix (in this case, (29075) 1950 DA).

The asteroid 69230 Hermes was found in 2003 and numbered, but was found to be a discovery from 1937 which had been named "Hermes", but subsequently lost; its old name was reinstated. Centaur 2060 Chiron was discovered in 1977, and precovery images from 1895 have been located.[4]

Another extreme case of precovery concerns Neptune. Galileo observed Neptune on both December 28, 1612 and January 27, 1613, when it was in a portion of its orbit where it was nearly directly behind Jupiter as seen from Earth. Because Neptune moves very slowly and is very faint relative to the known planets of that time, Galileo mistook it for a fixed star, leaving the planet undiscovered until 1846. He did note that the "star" Neptune did seem to move, noting that between his two observations its apparent distance from another star had changed. However, unlike photographic images, drawings such as those Galileo made are usually not precise enough to be of use in refining an object's orbit. In 1795, Lalande also mistook Neptune for a star.[5] In 1690, John Flamsteed did the same with Uranus, even cataloging it as "34 Tauri".

One of the most extreme cases postulated is of the discovery of Ganymede. This again involved Galileo, who is usually stated to have discovered it in 1610. It has been postulated by Xi Zezong that Ganymede was discovered by the Chinese astronomer Gan De in 365 B.C., when he catalogued it as a small red star next to Jupiter during naked eye observation.[6]

Dwarf planets

Artistic comparison of Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus, Salacia, 2002 MS4, and Earth along with the Moon.

Discovery and precovery dates for well-known dwarf planets, minor planets and probable dwarf planets:

19521Chaos19981991 5.0
787992002 XW9320021989 5.5
556372002 UX2520021991 3.87
3072612002 MS420021954[13]3.7
555652002 AW19720021997 3.5
2002 XV9320021990 5.42
174567Varda20031980 3.1
849222003 VS220031991 4.1
2089962003 AZ8420031996 3.54
4555022003 UZ41320031954 4.38
4440302004 NT3320041982 4.4
2309652004 XA19220041989 4.1
905682004 GV920041954 4.25
1751132004 PF11520041992 4.54
120347Salacia20041982 4.36
1203482004 TY36420041983 4.52
1454512005 RM4320051976 4.4
1454522005 RN4320051954 3.89
2024212005 UQ51320051990 3.4
4703082007 JH4320071984 4.49
229762Gǃkúnǁʼhòmdímà20071982 3.69
5236712013 FZ2720132001[20]4.1
4722712014 UM3320142003 5.2
5237942015 RR24520152004[21]3.6

See also


  1. McNaught, R. H.; Steel, D. I.; Russell, K. S.; Williams, G. V. (March 7–11, 1994). "Near-Earth Asteroids on Archival Schmidt Plates". In Jessica Chapman; Russell Cannon; Sandra Harrison; Bambang Hidayat (eds.). Proceedings, The future utilisation of Schmidt telescopes. IAU Colloquium 148. 84. Bandung, Indonesia: Astronomical Society of the Pacific. p. 170. Bibcode:1995ASPC...84..170M.
  2. AANEAS: A Valedictory Report Archived 2012-07-28 at the Wayback Machine by D.I. Steel, R.H. McNaught, G.J. Garradd, D.J. Asher and K.S. Russell, 25 March 1997
  3. Villard, Ray; Lafreniere, David (April 1, 2009). "Hubble Finds Hidden Exoplanet in Archival Data". HubbleSite NewsCenter. NASA. Archived from the original on April 5, 2009. Retrieved 2009-04-03.
  4. "JPL Small-Body Database Browser: 2060 Chiron (1977 UB)" (2009-09-17 last obs). Archived from the original on 2011-06-09. Retrieved 2010-02-08.
  5. Fred William Price (2000). The planet observer's handbook. Cambridge University Press. p. 352. ISBN 9780521789813. Retrieved 2009-09-11.
  6. Galilean Moons– Gan De. Available at!/topic/1989270/ Archived 2017-12-01 at the Wayback Machine Accessed 27th November 2017.
  7. "Charles Messier, premier observateur de l'astéroïde Pallas". Archived from the original on 16 March 2016. Retrieved 7 May 2018.
  8. Wild, W. J.; Buchwald, G.; Dimario, M. J.; Standish, E. M. (December 1998). "Serendipitous Discovery of the Oldest Known Photographic Plates with Images of Pluto". American Astronomical Society. 30: 1449. Bibcode:1998DPS....30.5514W.CS1 maint: uses authors parameter (link)
  9. Chamberlin, Alan. "JPL Small-Body Database Browser". Archived from the original on 7 May 2018. Retrieved 7 May 2018.
  10. "JPL Small-Body Database Browser: 38628 Huya (2000 EB173)" (2009-06-13 last obs). Archived from the original on 2018-05-07. Retrieved 2010-02-09.
  11. "JPL Small-Body Database Browser: 28978 Ixion (2001 KX76)" (2009-05-21 last obs). Archived from the original on 2015-11-05. Retrieved 2010-02-08.
  12. "JPL Small-Body Database Browser: 50000 Quaoar (2002 LM60)" (2009-09-12 last obs). Archived from the original on 2011-06-11. Retrieved 2010-02-08.
  13. "JPL Small-Body Database Browser: (2002 MS4)". 2011-12-12. Archived from the original on 2012-04-15. Retrieved 2015-01-28.
  14. "JPL Small-Body Database Browser: 90377 Sedna (2003 VB12)" (2010-01-05 last obs). Archived from the original on 2016-03-25. Retrieved 2010-02-08.
  15. "JPL Small-Body Database Browser: 90482 Orcus (2004 DW)" (2009-04-28 last obs). Archived from the original on 2012-02-08. Retrieved 2010-02-08.
  16. "JPL Small-Body Database Browser: 136108 Haumea (2003 EL61)" (2010-01-26 last obs). Archived from the original on 2011-06-09. Retrieved 2010-02-08.
  17. "JPL Small-Body Database Browser: 136199 Eris (2003 UB313)" (2009-11-20 last obs). Archived from the original on 2011-05-12. Retrieved 2010-02-08.
  18. "JPL Small-Body Database Browser: 136472 Makemake (2005 FY9)" (2010-01-26 last obs). Archived from the original on 2011-08-30. Retrieved 2010-02-08.
  19. "JPL Small-Body Database Browser: 225088 (2007 OR10)" (2009-10-19 last obs). Archived from the original on 2012-07-01. Retrieved 2010-02-08.
  20. "JPL Small-Body Database Browser: 2013 FZ27)" (2014-03-26 last obs). Retrieved 2015-04-13.
  21. "JPL Small-Body Database Browser: 2015 RR245)" (2016-06-08 last obs). Archived from the original on 2016-12-27. Retrieved 2016-12-26.
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