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The power of precoveries
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Asteroid 2017 US imaged by the Catalina Sky Survey on 13 October 2017, three days before their own discovery of the object. The detections are extremely faint, too faint to be detected automatically by the pipeline, but visible to the trained human eye.

Credit: Catalina Sky Survey / University of Arizona / NASA


If you check our current risk list, you will notice that a significant number of objects in the top positions are extremely “old”, discovered in the first decade of the century.

Some of them actually only have observations taken around the time of discovery, and have not been seen ever since. The orbits of these objects are now very uncertain, and it is therefore difficult to obtain new observations and revise the impact threat assessment, unless they are re-observed by chance.

There is however a way to get some additional data. It is called "precovery search", and it consists of a systematic search of existing image archives in order to locate additional detections of the object, not recognised at the time the images were obtained.

In the past, we successfully found precovery observations of a significant number of objects in our risk list. Some of them were actually sufficient to fully exclude the threat posed by the object.

In order to continue this systematic effort, we started a very promising collaboration with the Catalina Sky Survey, a project of NASA’s Planetary Defense Program, to search for precovery detections in their extremely large and complete image archive, collected over two decades of survey work. Last week we reported the first results of this search to the Minor Planet Center: precovery detections of three objects in the top-20 positions of our risk list (2008 JL3, 2008 UB7 and 2017 US).

An example of one such detection is presented in this image: the object, 2017 US, is extremely faint, but it is visible to the human eye on the CCD images, and its position is measurable with accurate astrometric tools. The corresponding measurements resulted in a revised assessment of the impact threat, which was slightly lowered. The threat for the two other objects was also revised as a result of the precovery detections: one remained basically unchanged, the other was slightly increased. Overall, our knowledge of the orbits for all three objects was improved, in some cases quite significantly.

This search shows the power of historical image archives for threat assessment, and in particular the value of the extensive archives generated by Catalina and by the other asteroid surveys.