Precoveries

In astronomy, precovery (short for pre-discovery recovery) is the process of finding the image of an object in old archived images or photographic plates 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. While the term "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 planets and Lost comets).

Calculating the orbit of an astronomical object involves measuring its position on multiple occasions. The more widely separated these are in time, 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, which is only sufficient 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 been in fact photographed already. If so, a far more precise orbital calculation can then be made.

Until fast computers were widely available, it was impractical to analyze and measure images for possible minor planet discoveries because this involved a considerable amount of manual 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 an enormous number of minor planets.

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Wikipedia