Editing Precovery (section)

{{Short description|Process of finding the image of an object in images or photographic plates predating its discovery}}
[[File:Valetudo CFHT precovery 2003-02-28 annotated.gif|thumb|The Jupiter moon [[Valetudo (moon)|Valetudo]] was first discovered in 2017, but a number of precovery images have been identified since, including this one taken on 28 February 2003 by the [[Canada–France–Hawaii Telescope]], in which Valetudo's position is marked by the two orange bars.]]

In [[astronomy]], '''precovery''' (short for '''pre-discovery recovery''')<ref name="McNaught" /><ref name="AANEAS" /> is the process of finding the image of a [[celestial 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 planet]]s, but sometimes a [[comet]], a [[dwarf planet]], a [[natural satellite]], or a [[star]] is found in old archived images; even [[extrasolar planet|exoplanet]] precovery observations have been obtained.<ref name="HST-exoplanet" /> "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 [[asteroid]]s with a chance of [[impact event|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 [[galaxy|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 catalog]]s containing up to a [[1000000000 (number)|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 many minor planets.