Precovery (short for "pre-discovery recovery") is a term used in astronomy that describes 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. The name is based on pre-discovery recovery; recovery being the process of making new observations of a previously observed object after a period of non-observation. This happens most often with minor planets, but sometimes a comet, a dwarf planet, or a natural satellite is found in old archived images; even exoplanet precovery observations have been obtained.
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.
Note: The term "Precovery" refers to a pre-discovery image. "Recovery" refers to imaging of an object which was lost to our view (as behind the sun), but is now visible again.
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 (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 even been named, but subsequently lost. Consequently, its old name "Hermes" has been applied to it. Centaur 2060 Chiron was discovered in 1977, and precovery images from 1895 have been located.
Another extreme case of precovery concerned 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 other known planets of that time, Galileo mistook Neptune for a fixed star, leaving Neptune 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. In 1690, John Flamsteed did the same with Uranus, even cataloging it as "34 Tauri".
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