Precovery

When an object is of particular interest (such as asteroids with a chance of impacting Earth), researchers begin a search for precovery images.

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.

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.

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.

As modern survey archives reach fainter magnitudes and are more comprehensive, significant precovery images have become easier to locate.

The Jupiter 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.