Exposure animation for asteroid 2020 HS7 as observed by the Tautenburg Observatory on 28 April 2020, a few hours after the Pan-STARRS project reported the discovery.
Credit: ESA / Tautenburg Observatory, S. Melnikov, C. Hoegner, B. Stecklum

On the evening of 27 April (European time), the NASA-funded Pan-STARRS project in Hawaii reported observations of a new asteroid, temporarily labelled P20ZIf8, collected with their second survey telescope over a timespan of less than an hour during the previous observing night. These observations immediately showed that the object was extremely close to our planet, and had a ~10% probability of being on a collision course, heading for a possible impact the following day.

This circumstance attracted the attention of many observers worldwide. Less than 50 minutes after the initial report by Pan-STARRS, the Xingming Observatory in China obtained the first follow-up astrometry, followed about an hour later by one of our collaborators, the Tautenburg observatory in Germany, which we had alerted asking for immediate observations. With this data, it became clear that the object was not going to collide with the Earth, but it was heading towards a very close fly-by the following day, roughly at the distance of Earth's geostationary orbit (although ~10° below it). The object was just a few metres in size, and therefore it would not have caused any significant threat even if it had been on a collision course. It had nevertheless been an interesting exercise to test the discovery and rapid follow-up capabilities of worldwide observers. Subsequent astrometry, obtained by additional collaborators of our Centre and by many other observatories worldwide, is sufficient to determine that the flyby happened on 28 April at about 18:49:40 UTC, and at a distance of about 42 745 km from the Earth centre. These numbers can be determined with a precision of just a few kilometres and a few seconds, showing that the trajectory of a nearby object can be established with exquisite accuracy even with just a day of data, if good observational coverage can be obtained.

The fly-by of this asteroid, now named 2020 HS7, ranks among the 50 closest ever recorded. Interestingly, the fly-by happened only 15 hours before the closest approach of (52768) 1998 OR2, a much larger kilometre-sized object that attracted the attention of the worldwide media. However, this latter object only approached our planet 16 times farther than the Moon (more than 6 million kilometres away), while 2020 HS7 came significantly closer to us and likely represented a more significant event for the astronomical community.

For further information please consult this ESA news.

Plots representing the orbit of asteroid 2020 HS7 in its close Earth fly-by on the 28 April 2020: on the left, the orbit as seen from the Ecliptic North Pole, on the right as it seen on the Ecliptic plane, crossing it with an angle of ~10 degrees (see Orbit Visualizer).