An archival precovery
During the first month of 2018 two objects reached a Torino Scale level of 1, and attracted the attention of observers with the goal of collecting additional observations necessary to remove the possible risk. One of them 2017 YZ1, was removed from the risk list within a few days thanks to new observations of various observers, including David Tholen from Hawaii, and our team working in collaboration with the OASI telescope in Brazil.
The other object, 2017 XO2, was harder to remove. By the time it reached Torino Scale 1 it was already quite faint, at magnitude 24, and difficult to observe for most telescopes. There was however another way to get the data needed to clarify the risk: we realised that the Pan-STARRS archive contained a significant amount of images obtained in November and December 2011 that should have covered the area where 2017 XO2 was located, and at that time the object was much brighter, around magnitude 20.
There was a problem however: the uncertainty of the asteroid's position in the sky was huge, many degrees long. Still, we decided to analyse all the dozens of images from one of the many detectors where the object could have been, and after many hours of careful searches we were able to locate the object in the November 5 image set, about 2 sigma away from the expected position.
These images then allowed us to find it in all the other sets from that year, for a total of nine good detections. None of them had been found by the automated Pan-STARRS system because there were only two detections for each night, while automated algorithms need at least three to safely identify a new moving object. Adding these observations to the orbit computation resulted in the immediate exclusion of all possible impact dates for the next century, and the object went directly from Torino Scale level 1 to a complete removal from our risk list.
Animation of two images of 2017 XO2 taken on 2011 November 25 by the Pan-STARRS telescope. The two images were exposed approximately 18 minutes apart, and the object is visible in the top half of the image. Credit: Pan-STARRS