At approximately 09:30 UT on 14 April 2018 the Catalina Sky Survey, in Arizona, discovered a bright magnitude 15 object moving at a sky speed of 10" per minute, in the constellation Libra.

Follow-up observations obtained over the next few hours, by Catalina's own facilities and by other observers in the US Southwest, quickly showed that this was an unknown asteroid, which was about to have a close fly-by with our planet less than a day later. The asteroid was subsequently designated 2018 GE3.
It is not very unusual for small asteroids to be discovered just a few hours before close approach. However, what made this discovery quite unique is that this object was much bigger, having an estimated diameter between 50 m and 100 m.

The fly-by of 2018 GE3 happened around 06:40 UT on 15 April, at half the distance of the Moon. It reached magnitude 12 just before the closest approach, and then went towards the direction of the Sun, becoming unobservable from the ground.
The fly-by was one of the closest ever recorded for an object of this size. The only other object of comparable (or slightly larger) size that was seen coming closer was 2002 MN: it flew-by at just 0.3 lunar distances on 14 June 2002. It was discovered only three days before by the then-active LINEAR survey.

&imagePreview=1

How is it possible that an object of this size, coming from the dark side of the sky, could be missed by all asteroid surveys until less than a day before the close passage? The reasons can be found in the "geometry" of the approach trajectory that was quite unfortunate for a variety of reasons. 

- The asteroid became brighter than magnitude 22.5 (the typical limit of large surveys like Pan-STARRS1) on 31 March, only two weeks before its fly-by. This time interval, unusually short for object of this size, is due to its higher-than-average incoming radial speed of ~30 km/s. Objects of the same size, on a more typical and slower NEO orbit, can become visible a couple of weeks earlier. However, they may not necessarily move in a way that makes them recognizable as NEOs that early on, so the actual warning time may be shorter.

Sky Coverage plot prepared with the Minor Planet Center's tool (https://minorplanetcenter.net/iau/SkyCoverage.html) for the timespan between 2 and 13 April 2018, and including only the deeper surveys. The approximate path of 2018 GE3 during the same time period is marked by the red arrow. The Pan-STARRS1 survey (dark blue) came close to the right position on 12 April, but stopped too far South. The Catalina Sky Survey's largest telescope, the 1.5-meter at Mt. Lemmon (purple), was observing mostly North of the equator for the same two weeks. Credits: Minor Planet Center

- More importantly, at that time the object was only 30° away from the fully illuminated moon, making it effectively unobservable. It got even closer to the moon for the next two days, and reached a sufficient distance from it only around 7 April.

- Also, one of the main asteroid discovery telescopes, the Pan-STARRS1 survey, was not operational due to poor weather until 12 April, and therefore did not cover the area of the sky where 2018 GE3 was located.

The currently existing surveys, especially those capable of reaching the faint limiting magnitude needed to detect 2018 GE3 early on, have the capability to cover the entire observable night sky only every few weeks. It is therefore possible for an object like this to "slip" though their monitoring pattern and come close to our planet unannounced.

This underlines the importance and also the potential of the Fly-Eye Telescope which is currently built in Italy under ESA contract. It will have a very large field-of-view of 6.7 x 6.7 square degrees, which allows to scan one complete hemisphere in 48 hours. Putting one fly-eye telescope in the Northern and one in Southern hemisphere will provide a global coverage. Asteroids like 2018 GE3 should then be detected with at least one week of warning time, sufficient to make a reliable estimate of a possible impact area and to initiate adequate precaution measures.