2020 QG, the closest close approach
On the late morning (European time) of 16 August 2020 the Zwicky Transient Facility, a multi-purpose astronomical survey using the 1.22 m Schmidt telescope at Mt. Palomar, discovered a new moving object. Six astrometric observations, covering a timespan of a bit less than an hour, already showed that the object was likely small but very close to us.
As often happens with these very close objects, obtaining follow-up observations may not be easy, since the rapidly increasing ephemeris uncertainty makes them hard to locate even just a few hours after discovery.
Fortunately, as soon as the sun set over Europe, two facilities supported by ESA's Planetary Defence Office were ready to observe, and actually targeted this asteroid. The Tautenburg observatory in Germany got it first, using their 1.34 m Schmidt telescope, the largest instrument of this type in the world. Less than an hour later the object became observable from Tenerife, where our team observed it with ESA's own OGS telescope.
With these observations it became immediately evident that the object was indeed a very close approacher: it had passed by the Earth earlier that morning, missing our planet by less than 3000 km. It was the closest asteroid ever observed to pass by our planet without hitting it, and the fifth closest overall, if we include the four small asteroids that actually impacted.
Thanks to the observations from Tautenburg and Tenerife, the asteroid became easy to observe by other observatories, both in Europe and in the US. The following day it received its official designation by the Minor Planet Center: 2020 QG.
We now know it was a very small object, with a size of only a few meters, comparable with the other four impactors discovered so far. Had it hit the Earth, it would not have caused any significant damage on the ground.
Left image: it shows the 1.3x1.3 square degree field with 2020 QG in the zoomed cutout. The circle marks the position uncertainty. Credit: TLS Tautenburg. Right image: close approach visualisation for asteroid 2020 QG. In white the Moon and its orbit. Behind it, the orbit of the asteroid and its direction. In yellow, the position vector of the Sun relative to the centre of the Earth: Credit: ESA / NEOCC.