Precovery
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Precovery is a term used in astronomy that describes the process of finding the image of an object (usually a minor planet, but sometimes a comet or even a natural satellite) in old archived images or photographic plates, for the purpose of calculating a more accurate orbit. The name is based on pre-recovery; recovery being the process of making new observations of a previously-observed object after a period of non-observation.
To accurately calculate an orbit for any solar system object, it is necessary to measure its angular 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 are available, which is only sufficient for a preliminary (not very precise) orbit calculation.
When an object is of particular interest, for instance a near-Earth asteroid with a chance of impacting the Earth, a precovery effort is made. Using the preliminary orbit calculation to predict where the object might appear on old archival images (sometimes from decades ago), those images are searched to see if it had been in fact photographed at the time. If so, a far more precise orbital calculation can then be made.
Before modern inexpensive and fast computers, 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 ago for other purposes (studies of galaxies, etc.) and it was not worth anyone's while to look for run-of-the-mill minor planets. Today, computers can easily analyze digital astronomical images and compare them to online star catalogues containing up to a billion or so star positions, and the identification of minor planet candidates is automated to a much greater extent. Such modern techniques (since about the mid 1990s) have enormously increased the rate of minor planet discoveries.
[edit] Examples
An extreme case of precovery is the following. On December 31, 2000, a discovery was made, 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).
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, 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.
