By Duncan Steel 24/09/2020


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Sorry for the late notice on this one, but I only just heard myself, in common with most of the human race.

A small asteroid, somewhere between the size of a truck and the size of a house in dimensions, will hurtle past the Earth tonight, dipping closer to our planet’s surface than the altitude at which TV transmission and weather satellites orbit high above the equator. Not only that, but it seems that it will pass across the sky above New Zealand and Australia, meaning that astronomers here could get the best view during its closest approach, at about 11:18pm NZST.

At that time the asteroid would be visible in the constellation Pisces, fairly close to Mars in the sky (the bright reddish object that will rise in the east at about 8:15pm), but actually very close to the Earth: only 22,000 km away from us, about half the distance around the equator. (Satellites in geostationary orbit remain at an altitude just below 36,000 km, so this space rock is coming closer than them.)

To be able to see this asteroid you would need a telescope larger than those many astronomy enthusiasts possess, an instrument with an aperture of at least 15-20 centimetres being necessary. A variety of websites are available which indicate where and how to look, if you do have access to such a telescope. The following website – EarthSky.org – not only tells you how to view the asteroid directly, but also has a web feed through which you can see the asteroid’s passage near Earth over the internet through feeds from various telescopes.

A problem for observing from NZ is that the asteroid will be quite low in the sky, at most around 20 degrees above the horizon. The indefatigable team of Pam Kilmartin and Alan Gilmore at the University of Canterbury’s Mount John Observatory near Lake Tekapo will try to get measurements on its position before the time of close approach using the 1-metre telescope there, but they report that observing conditions do not look good. It is also fairly difficult to track on such an object at closest approach, because it will be travelling at almost 8 km per second (near 28,000 kph) relative to the Earth. Therefore, while it will dip closer to us than geostationary satellites, it will not be there for long!

I apologised above for the late notice, but in fact the asteroid was only discovered on September 18th by a near-Earth object search team at the University of Arizona. The current designation indicates that: 2020 SW. The year is obvious, the ‘S’ shows it was found in the second half of September, and the ‘W’ is an ordering starting with ‘A’ for the first asteroid discovered in the half-month (though ‘I’ is not used because of the possible confusion with the numeral one).

Over the past several days (or nights) further observations have enabled astronomers to determine this rock’s orbit around the Sun, and be sure that while it will pass close by Earth, it will not hit us. At least this time around… If you look at the orbital diagram at the head of this blog post you will see that 2020 SW makes two close approaches to Earth’s orbit each time it loops around the Sun, taking 373 days to complete such a trip. It also looks like it might perhaps hit Venus at some time in the future, though truth be told the ways in which asteroid orbits change under the gravitational perturbations imposed during close approaches to a planet make any long-term predictions infeasible. The good news is that we know that this particular asteroid will not collide with Earth at any time within the next fifty years. Indeed this is really our last chance to track it until the next close approaches in 2040 and 2041.

But what if it were indeed heading for us? The bad news is that it would release energy equivalent to about 10 kilotonnes of TNT, much the same as the Hiroshima nuclear bomb at the end of WW2. The good news is that most of that energy would be dissipated in the upper atmosphere, above an altitude of around 20 km, twice as high as jetliners fly, with little real danger to the world below apart from a phenomenal flash in the sky. As I wrote, though, we know it will miss us not only tonight, but also for at least the next half-century.

For a NASA press release telling you more about this near-miss, see here. For the NASA Jet Propulsion Laboratory webpage giving the nitty-gritty details about this small asteroid, see here. Information about this asteroid is also available from the European Space Agency at this webpage.