The early hours of January 27th 2023 saw the closest approach to Earth of asteroid 2023 BU. The fact that this particular space rock was only discovered on January 21st, just a week earlier, combined with it passing just 3,600 km from the surface of the Earth (0.03x the distance between the Earth and the Moon) got the media rather excited. It’s trajectory brought it closer to the Earth than orbit of our geostationary satellites, but still well above the 200-300 km of things like the International Space Station located in low Earth orbit. Given how far apart geostationary satellites are, our communications infrastructure was not in any significant danger (this time).
This particular asteroid was estimated to have a diameter of 4-8 metres and was travelling at a speed of around 9.3 kilometres per second as it passed by. This might sound big, but it’s tiny by asteroid standards. If it had hit the atmosphere, it would have most likely burnt up entirely, leaving only tiny fragments reaching the ground, if at all. For comparison, the rock that disintegrated over Chelyabinsk in 2013 was estimated to be 20 metres in diameter – that one exploded in the atmosphere, showering small chunks of debris over the town. Assuming a similar density to the Chelyabinsk rock, asteroid 2023 BU likely had a mass of less than 1,000 tonnes.
The thing is with an asteroid passing this close to a much larger object, the encounter will change its future orbital trajectory. Prior to this encounter, observations show that this asteroid orbited the Sun every 359 days. Observations made after the encounter allowed experts to model its new orbit, finding that it now orbits the Sun every 425 days. It won’t be back at the Earth now until December 24th 2029 when it will be some 14 million km at closest approach. Nothing to worry about. In fact, they’ve modelled its position all the way to 2139. The closest it will pass to us in that time is 528 thousand km in January 26th 2066.
The thing is, this happens all the time. As of today, according to the IAU’s Minor Planet Center, there are 31,207 known near-Earth asteroids, 850 of which are larger than 1 kilometre in size, and 2,328 potentially hazardous asteroids. And we’re finding new ones all the time. Just this year (we’re still only in February) we’ve had at least eleven objects pass closer than the Moon, at least five of which were not discovered until after closest approach! Again, don’t panic, they’re all pretty small and would be highly unlikely to do any damage.
One of these actually impacted the atmosphere. Asteroid 2023 CX1 was discovered less than seven hours before impact! Again, don’t panic, it was tiny, about 1 metre in diameter, and burned up as an impressive fireball somewhere over the English Channel / Northern France (above). You can see reports of sightings on the IMO fireball report catalogue. This was only the seventh impacting asteroid to be discovered before it actually hit the atmosphere. It’s still pretty difficult to find these things in advance.
If you want to look at the population characteristics, JPL’s Center for Near Earth Object Studies has some data and charts you can play with – I’ve included a couple below showing the discovery rate of NEOs, colour-coded by survey, and the size distribution.
The above plot shows the increase in discovery of near-Earth objects. The surveys that have discovered the most objects are the Catalina Sky Survey and Pan-STARRS, although many are still discovered by amateur astronomers – including 2023 BU and 2023CX1! This is one of the science goals of the Vera C. Rubin telescope‘s Legacy Survey of Space and Time (LSST), to make an inventory of the solar system.
This one shows the size distribution of NEOs discovered so far. As you can see, there are not many in the 1000+ metres category – luckily! Those are the ones most likely to cause us damage, but they are also the easiest to spot. The thing with space rocks is that they are rocks. Rocks are usually pretty dull looking, they are often dark colours and don’t reflect much light. That is a problem when your trying to find them with an optical telescope – they don’t reflect much light, so are pretty faint and therefore difficult to detect.
If you’re a keen astronomical observer and are looking for a project, here’s the Minor Planet Center’s list of NEOs needing confirmation. More observations are always welcome, helping to pin down asteroid orbits, and you don’t need sophisticated equipment to contribute.
Keep watching the skies – there will be more of these! But they will get harder to spot as we launch more and more satellites, and install more and more lights.