An unexpected very close approach
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.
An even closer unexpected approach
Less than a week after the discovery of 2020 HS7 by University of Hawaii’s Pan-STARRS survey, on 4 May 2020 another telescope funded by NASA’s Planetary Defense Program, University of Arizona’s Catalina Sky Survey, discovered a new object that was already very close to Earth, and possibly in an impacting trajectory.
The events that unfolded over just a few hours showed how quickly the NEO community can react to a possible impactor, and gather the observations needed to accurately determine the trajectory of an incoming object.
The new asteroid, temporarily designated C2QQFV2 by the Catalina team, was detected in a set of four observations exposed by the Mt. Lemmon telescope between 05:53 and 06:16 UTC. Once the object was detected by their pipeline, and confirmed real by their observers, it was quickly reported to the MPC, which published the discovery observations at 06:38 UTC.
Within a few minutes, the various services monitoring the MPC’s NEO Confirmation Page detected the new object, and run an analysis from which it became evident that the object was already very close, and had a ~5% chance of being on a collision course, for an impact happening less than 6 hours later.
These automatic alerts triggered various observers, including Catalina itself and the University of Hawaii’s ATLAS survey, to gather additional follow-up observations of the new object. At 07:12 UTC, less than an hour after the original discovery was achieved, the first follow-up observations started coming in, and within two hours, three other telescopes had collected additional observations from other locations.
With this new dataset, it immediately became evident that we had another near miss: the object would fly-by at about 12:03 UTC, at just 13 400 km from the Earth’s centre.
The object, calculated to be between 3 and 6 metres in diameter, is now designated 2020 JJ, and it is already quickly receding from Earth. Had it been in a collision course, it would not have caused a significant threat, given its small size.
What is even more impressive is that the final dataset, collected over just two hours, is sufficient to determine the fly-by circumstances to a precision of about 10 seconds in time, and 5 kilometres in distance. Had the object been in a collision course, we would have been able to predict the point of entrance into the Earth’s atmosphere with a comparable precision, showing how effective immediate optical follow-up can be at localising a possible imminent impact threat.
