There are hundreds of millions of asteroids in our solar system, which means that new asteroids are discovered frequently. It also means that close encounters between asteroids and Earth are fairly common. Some of these close encounters end with the asteroid hitting Earth, sometimes with dire consequences.
A recently discovered asteroid, named 2023 BU, has made news as it flew very close to Earth today.
Discovered by amateur astronomer Gennady Borisov in Crimea on 21 January, 2023 BU passed about 3,600 km above Earth’s surface (near the southern tip of South America) six days later on 27 January.
That distance is only a fraction of the distance between Perth and Sydney and only 1 percent of the distance between Earth and our Moon.
The asteroid also passed through the region of space that contains a significant proportion of man-made satellites orbiting Earth.
All of this makes 2023 BU the fourth-closest known asteroid encounter with Earth, ignoring those that have impacted the planet or our atmosphere.
How does 2023 BU rate as an asteroid and a threat? 2023 BU is not remarkable except that it passed so close to Earth. The asteroid is estimated to be only 4–8 m in diameter, which is at the small end of the asteroid size range.
There are likely hundreds of millions of such objects in our Solar System, and it is possible that 2023 BU may have come close to Earth several times before millennia. Until now, we have been oblivious to this fact.
For context, on average a 4-metre-diameter asteroid will impact Earth every year and an 8-metre-diameter asteroid every five years or so. Asteroids of this size pose little risk to life on Earth, as they are large. Environments break down in scale. They make spectacular fireballs, and some asteroids may make it to the ground as meteorites.
Now that 2023 BU has been discovered, its orbit around the Sun can be estimated and future visits to Earth can be predicted. It is estimated that there is a 1 in 10,000 chance that 2023 BU will impact Earth sometime between 2077 and 2123.
Therefore, we have nothing to fear from 2023 BU or any of the millions of similar objects in the Solar System.
Asteroids must exceed 25 meters in diameter to pose any significant risk to life in a collision with Earth; To challenge the existence of civilization, they would have to be at least one kilometer in diameter.
It is estimated that there are fewer than 1,000 such asteroids in the Solar System and one may impact Earth every 500,000 years. We know about more than 95 percent of these objects.
Will there be more close asteroid passes? 2023 BU was the fourth closest pass by an asteroid ever recorded. Three close passes were by very small asteroids discovered in 2020 and 2021 (2021 UA, 2020 QG and 2020 VT).
Asteroid 2023 BU and countless other asteroids have passed very close to Earth during the roughly five billion years of the Solar System’s existence, and this situation will continue into the future.
What has changed in recent years is our ability to detect asteroids of this size, so that any threats can be identified. That an object about 5 meters in size can be detected thousands of kilometers away by a very dedicated amateur astronomer shows that the technology to make important astronomical discoveries is within reach of the general public. It is very exciting.
Amateurs and professionals can continue to search and classify objects together, so threat analysis can be done. Another very exciting recent development came last year by the Double Asteroid Redirection Test (DART) mission, which successfully rammed a spacecraft into an asteroid and changed its direction.
DART makes the concept of redirecting an asteroid from a collision course with Earth plausible if the hazard analysis identifies a serious risk with sufficient warning.