Above: Discovery images of ‘2019 UN13’. The object is faint and streaked, indicating its relative proximity to near-Earth space. Courstesy Catalina Sky Survey/NASA.
When Catalina Sky Survey (CSS) astronomer Teddy Pruyne submitted a new near-Earth asteroid candidate to the Minor Planet Center in the earliest hours of October 31, 2019, he had little idea the object’s orbit would soon bring it within a cosmic whisker of striking Earth’s atmosphere. Using CSS’s 28-inch (0.7-m) Schmidt telescope on Mt Bigelow in southern Arizona, Pruyne discovered the asteroid, now designated ‘2019 UN13’ while ‘blinking’ through four images taken within the constellation Aries. For Pruyne, 2019 UN13 appeared as four blips of light tracking across the image against the distant background stars.
Upon detection, he immediately reported the object as a new near-Earth Object (NEO) candidate to the Minor Planet Center (MPC) at Harvard University, which is the world’s clearinghouse and data bank for all such astronomical measurements. The new object’s positions and preliminary orbit were routinely shared with the NEO observing community and further observations were strongly requested. Additional observations from CSS’s follow-up telescope atop Mt Lemmon, Arizona, plus further observations from New Mexico Tech’s Magdalena Ridge Observatory and the Remote Astronomical Society’s New Mexico Observatory soon indicated that the object would indeed have a close Earth approach only about 7 hours after discovery. The additional observations also confirmed that the object was not an impact threat to Earth during this passage. Objects the size of 2019 UN13 impact Earth a couple or more times per year and typically burn up harmlessly in the upper atmosphere, occasionally dropping small fragments of meteorites to the surface.
At its closest approach about mid-day on Halloween, 2019 UN13, estimated to be about 3-7 feet across (1-2m), was 3900 miles (6200 km) above Earth’s surface, and only about ten times higher than the top of the atmosphere. Only one other near-Earth object, ‘2011 CQ1’, also discovered by CSS had a slightly closer approach without impacting Earth.