We didn’t die from 2023 BU, but why did we only find out about the asteroid on Saturday?

If you are still reading this it’s because the asteroid didn’t destroy Earth…

And yet, another day, another asteroid getting a little too close to Earth for comfort. Asteroid 2023 BU has just grazed Earth, skimming in-between ‘low Earth orbit’ which is 2,000km and geostationary orbit which is 36,000km. This is close enough to nab an incredibly unlucky satellite.

So, as asteroid 2023 BU begins its sail back into the distance, it’s a good time to ask questions about why it’s so difficult to track small asteroids until they’re only a few days away. And as more and more satellites are inserted into orbit, it’s also worth understanding if a rogue asteroid could be a problem for one of these satellites.

Asteroid 2023 BU has had a similar story to many small asteroids. A few days before it hits or misses us, someone discovers it. The media goes wild, nothing happens, and we all move on with our lives.

In this case, 2023 BU was discovered on Saturday the 21 January and flew past us at its closest approach (around 3,900 km above Earth) earlier this morning. The asteroid was discovered by amateur astronomer Gennadiy Borisov, who also discovered the first interstellar comet 2I/Borisov in 2019.

Excitingly, 2023 BU is the fourth-closest approach we’ve ever been able to track that didn’t end up hitting Earth. The other three were discovered in 2020 and 2021.

2023 BU is about the size of a delivery van at 4-8 metres long, and scientists were not worried about it at all. Even if it had hit our atmosphere, an asteroid of that size would have burnt up completely 30 kilometres before it hit the ground.

“In this instance I think we’ll live through it,” jokes Dr Jason Held, CEO of Saber Astronautics. “And look on the bright side, if it does hit, you won’t feel a thing!”

Saber Astronautics’ Mission Control Centre is based in Adelaide, and has worked in ‘space traffic management’ for satellites. Luckily though even the satellites probably don’t need to worry too much.

“I think most people in the industry have a big sky, little bullet kind of philosophy … with asteroids hitting satellites,” he said.

Much more likely is a satellite hitting another satellite, he told Cosmos. But he didn’t completely rule it out.

“But I don’t do Vegas. I am not known for being lucky.”

NASA’s test last year to change the path of an asteroid named Dimorphos was incredibly successful. This means that if an Armageddon asteroid was coming for us all, with enough notice NASA could send up a little spacecraft to nudge it out of our way.

But that only works if we know if the asteroids are coming. NASA knows where pretty much all of the near-Earth asteroids that are larger than one kilometre. An asteroid that big would spell global catastrophe if it hit Earth so it’s a good idea to keep an eye on them.

We’re also finding more and more asteroids larger than 140 metres in size – if an asteroid this size fell into a city it could cause mass casualties. NASA estimates they’ve located about 40% of those.

The smallest asteroids – like 2023 BU – which are unlikely to cause too much damage if they hit, are still mostly untracked. NASA suggests they’ve found only 0.4%.

But that’s not to say that these smaller asteroids can’t cause problems. In 2013 an asteroid about 20 metres long exploded over Chelyabinsk Oblast in Russia. No one had any idea it was coming until it entered the atmosphere and exploded. Around 1,500 people sought medical attention, and thousands of buildings were damaged.

So why can’t we track these smaller asteroids too?

“The sensors that track all this stuff, the best ones, are still run out of the US military. There is also all these commercial sensors that are just beginning. There’s a few in Australia, there’s a few in the United States,” says Held.

“But asteroids don’t have money to pay for sensor time.”

However, we are getting better at tracking these smaller asteroids. The Centre for Near Earth Object Studies has already found over 18,000 asteroids smaller than 100 metres.

Projects and telescopes like NEOWISE and ATLAS have been helping to track those asteroids around us. NEOWISE keeps an eye on asteroids before they get to close, and ATLAS is a last-ditch effort to allow people to evacuate an area before potential impact.

But, in the meantime, amateur astronomers like Borisov might be the best we’ve got. You can rewatch 2023 BU as it hit its closest point to Earth as part of the Virtual Telescope Project.