2018 October 19: The Close Approach of Small Asteroid 2018 UA.
This episode illustrates how well the asteroid observing community communicates and coordinates observations of potentially dangerous asteroids. 2018 UA was discovered and originally designated as ZU1CE58 by Gregory J. Leonard of the Catalina Sky Survey on 2018 Oct 19 at 03:59 UT with the 0.68-m Schmidt telescope and a 10Kx10K CCD (IAU/MPC site code 703). Their discovery was promptly posted on the Minor Planet Center's (MPC's) Near-Earth Object Confirmation Page. The NASA/JPL Center for NEO Studies also promptly computed initial very approximate possible trajectories that included potential impacts on Earth within hours and a size potentially as large as 200 meters. But that computed orbit at the time of discovery was necessarily so uncertain that it was not even clear whether it was geocentric or heliocentric.
Worsening weather at the Catalina Mountains prompted the discoverers' request for additional follow-up observations by other stations. The object was moving fast and could have been lost before a sufficiently accurate orbit could have been determined. Spacewatch was in operation at the time and responded with additional observations of the object's trajectory with both the 0.9-meter and 1.8-meter telescopes.
Examples of Spacewatch images of 2018 UA taken with the 1.8-meter telescope on 2018 Oct 19 at 08:27 UTC are illustrated here, here, and here. The exposure time of each image was 30 seconds. This zoomed-in field illustrated is 3.2 x 5 arcminutes. These three images out of a total of 9 are separated at intervals of 92 seconds. The trailed nature of the images of the asteroid and their separation illustrate the speed of the object across the sky, which was 18 arcseconds per minute at the time.
Because the MPC's initial predictions were necessarily approximate, the asteroid's rate deviated from them, accelerating faster than expected. Spacewatch was still able to recover the object because of the prompt timing of response to the request. The MPC used these observations to update their prediction of the path on the sky, allowing another observer, Masayuki Suzuki in Japan, to observe it again 2 hours later remotely with the Q62 0.5-meter iTelescope in Siding Spring, Australia .
As a result of these recoveries of 2018 UA, the MPC was able to issue Minor Planet Electronic Circular 2018-U19, designating the object as 2018 UA and providing approximate post-encounter orbital elements. That designation is assigned automatically according to a prearranged calendric sequence; it is only a coincidence that "UA" also is short for the University of Arizona.
The revised estimate of the object's size is now between 3 and 6 meters in diameter, depending on how reflective its surface is. A graphical representation of its encounter trajectory is available.
At the time of closest approach to Earth, the sky plane angular rate had increased to 3.3 degrees per minute due to its close distance of only 1.1 Earth radii from the surface of the Earth. Close approach occurred on Oct 19 at 14:52 UT over the south Pacific; that event was not observed. The object then receded off into the distance and into daylight, likely not to return to the vicinity of Earth for many years, if at all. In fact, it is not listed as a potential impactor within the next 100 years. Without the prompt communication and follow-up observations, 2018 UA may have remained on the list of potential impactors for the indefinite future.
Another treatment of this event is in the latest European Space Agency's Space Situational Awareness Newsletter.