By Duncan Steel 28/07/2020

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Four weeks ago I wrote about the last time a sizable asteroid hit our planet – the ‘Tunguska Event’ of 1908, when an object about 50 metres across exploded above the Siberian taiga – but smaller cosmic rocks shoot close by Earth fairly frequently. In the past 42 hours (as I write) a tiny asteroid less than five metres in size has been discovered by astronomers in Arizona, tracked from Mt John Observatory in New Zealand, and then additionally followed by observers in Croatia and France. As a result we know it will miss us, but will today fly by at a distance that is less than the altitude of geostationary satellites.

This asteroid is called 2020 OY4. The ‘2020’, obviously-enough, is the year of discovery. The ‘O’ denotes the half-month of discovery (the second half of July): we use 24 of the letters in the alphabet, leaving out I and Z. The ‘Y4’ is a label that signifies its order in discoveries reported to the International Astronomical Union’s Minor Planet Center. With automated spotting of asteroids with robotic telescopes there are thousands of ‘discoveries’ every night nowadays – most of which are repeat detections of objects that are already in our data banks. Many, though, are new: this asteroid 2020 OY4 could not have been found previously because it is too small and so very faint, beyond the grasp of our telescopes unless it happens to pass close by Earth. Which is what it is doing, right now.

The first humankind was aware of its presence was late on Sunday (NZST) when a telescope at Mt Lemmon near Tucson, Arizona, picked it up. The University of Arizona team then obtained further observations with another telescope at the same site.

Within four hours of its discovery, and with daylight approaching in the western United States, observatories further west (into nighttime) were needed to take over the tracking. Step forward the redoubtable team of Pam Kilmartin and Alan Gilmore of Mt John Observatory (part of the University of Canterbury), at Lake Tekapo. Starting shortly before midnight Sunday and stretching through to almost 06:00 on Monday morning they acquired eleven accurate positions for this little asteroid, measuring its location compared to background stars as it zipped across the southern sky, beyond the reach (at that time) of most observatories in the northern hemisphere. As it moved north, and the Earth turned, the two telescopes in Europe were able to obtain a few more positions for the asteroid, enabling mathematicians who work on orbital dynamics to determine that it will miss our planet by a safe distance. It has passed to the north of NZ, across the centre of Australia, and at 17:31 (5:31 pm) NZST on Tuesday 28th July it will make its closest approach to Earth, over southern Africa. No further observations will be feasible because it has passed into the daytime sky.

How close will it come? As I wrote above, it will miss Earth by a bit less than the altitude of geostationary satellites. These fly above consistent points on the equator, at a height near 35,800 km. Asteroid 2020 OY4 will pass about 35,170 km above Earth’s surface, give or take 50 km. It seems quite remarkable that such a tiny rock – estimated to be between two and five metres in dimension – could be discovered and then within a day or so astronomers can say for sure that it will miss our planet, even giving the miss distance to within just 50 km, a minute length on any cosmic scale.

If you wish to know more about this particular asteroid, take a look here for the NASA-JPL Near-Earth Object web page about it, or go to this page concerning 2020 OY4 at the Near-Earth Objects Dynamics site at the University of Pisa in Italy. Below is the first side of the CAFS (Close Approach Fact Sheet) issued by the European Space Agency’s Near-Earth Object Coordination Centre.

The orbit diagram above shows how this little asteroid goes out to about the distance of Mars away from the Sun, then dips inwards to skim past the orbit of Venus, passing Earth on the way. For a slant view, see the graphic at the head of this blog post. Note that in both these orbit diagrams the automated plotting has overlaid the name of the asteroid (2002 OY4) across the label for the Earth: yes, it really is coming close on the scale of the solar system!

Tiny rocks like this one are not actually anything to worry about. Something this small would explode high in the atmosphere, although it would be something to behold: if this one had  been on a collision course with Earth, it would have entered the atmosphere at about 60,000 kilometres per hour, and released energy equivalent to about three kilotonnes of TNT (one-quarter the Hirishoma nuclear bomb), but apart from shaking people up a little such an explosion occurring far above the height that jumbo jets fly would have little effect on us far below. The last time that an asteroid caused real damage was in February 2013, when a bigger asteroid (about 15-20 metres in size) blew up over Chelnyabinsk in Russia, putting over a thousand people into hospital. And near-misses (near-hits?) by relatively small asteroids occur frequently; for example, take a look here for a list.

It’s bigger asteroids (and comets) that we need to take more seriously. Now, I don’t want to worry you (or, make you concerned about your grandchildren, as on this one I am looking 70 years into the future), but let me tell you about 2020 NK1… You can tell from that designation that it has just been discovered in the past few weeks (i.e. the ‘N’ tells you: between July 1-15). And this is a big one, estimated to be about half-a-kilometre wide. So far as we know, there are seven times between the years 2091 and 2101 that this humdinger will pass close by Earth, with a collision being possible. It’s the first asteroid in a while to have a non-zero assignation on the Torino Scale for asteroid impact risk. We would hope that as more observations of it are collected over the years (no need to panic!) it will become possible to be sure that 2020 NK1 will miss the Earth, just as we are now sure that (99942) Apophis (formerly 2004 MN4) will pass safely by our planet in April 2029.

But what if, to the great misfortune of humankind, it turns out that 2020 NK1 is on a collision course with Earth? Before I tell you that, let me write that the current best-estimate of the likelihood of this happening is only about one-in-30,000. The astronomical figures, though, are daunting. We know that the impact speed would be near 32 kilometres per second (over 100,000 kph). Our estimate of its mass is about 180 million tonnes. The energy released would be about 23 gigatonnes of TNT equivalent: about 460 times the energy of the most-powerful hydrogen bomb ever tested (which was around 50 megatonnes).

Watch this space? Watch this rock, methinks. 2020 NK1 is the sort of natural cosmic projectile which we need to know about, and plan for the future. If it is due to wreak havoc in 70-80 years’ time, we have the notice period to do something about it.

Addenda, 2020 July 28, 17:00 NZST:
On 2020 OY4, the actual observations (i.e. measured positions) are available freely on this webpage at the Minor Planet Center. You will see that of the 25 positions that were available to determine the trajectory of the asteroid, and therefore the fact that it would miss the Earth, eleven came from Alan Gilmore and Pam Kilmartin at Mt John/Lake Tekapo. Alan tells me that they obtained additional observations last night (the 27th), as this little rock was zooming across the night sky: the closer an object is, the faster the apparent speed.

On 2020 NK1, the asteroid that might strike Earth sometime between 2091 and 2101 (peculiar thing is, we can calculate the precise possible times of impact, there being seven such opportunities, though whether any will actually occur we cannot yet say)… This webpage lists the 19 initial positions measured, leading to the realisation that this is a dangerous asteroid, and of those 19 Alan and Pam obtained seven. When such optical measurements of position are made, there is always some uncertainty in the precise locations relative to background stars, and only locations in the sky plane (that is, x and y positions) are feasible. On the other hand, if we can get radar echoes from an asteroid, we can determine its distance from us (the z dimension) and also its line-of-sight speed (z-dot) with considerable precision, and if such radar sounding is possible then we can quickly improve our knowledge of the asteroid’s orbit around the Sun, and whether it might be on a timetable to collide with the Earth. The trouble is, there are only two radars that are capable of performing such observations, both operated by the US, at Goldstone in California and Arecibo in Puerto Rico. As chance would have it, there is a radar opportunity later this week (that is, the asteroid will be close enough, and within the sector of the sky that the radars can reach), and so my expectation is that with radar measurements we will be able to say for sure that 2020 NK1 will pass safely by Earth late this century. If I am wrong in that anticipated outcome, then our great-grandchildren may have a problem; though maybe I should just term it a challenge.

Update, 2020 July 29, 15:00 NZST: The tiny rock 2020 OY4 passed Earth by as expected. An article on the web site is linked here.

Another addendum, 2020 August 01, 17:40 NZST: Alan Gilmore has kindly informed me today that at Arecibo Observatory they were able to get radar astrometry on 2020 NK1 on Friday 31st (after a frustrating Thursday 30th, when observations were impossible due to a tropical storm). In consequence, our knowledge of the orbit of that asteroid has been improved and – in line with my expectation expressed above – it has been possible to demonstrate that this half-kilometre object will not impact Earth between 2091-2101, that having been a possibility prior to the new data being obtained. Sighs of relief all round those who knew about the risk.

And more, 2020 August 13: The Arecibo Observatory mentioned above has been damaged when a cable snapped on August 10th, resulting in a 30-metre section of the dish being smashed. At present it is out of action, so it is fortuitous that 2020 NK1 was tracked just in time, else we might have needed to wait a long time before we could be sure that the asteroid posed no danger to us.