There are hundreds of millions of asteroids in our solar system, which means new asteroids are being discovered fairly frequently. It also means that close encounters between asteroids and Earth are fairly common. Some of these close encounters end with the asteroid impacting Earth, occasionally with severe consequences.
A recently discovered asteroid named 2023 BU has made headlines for passing very close to Earth today.
2023 BU was discovered in Crimea on Jan 21 by amateur astronomer Gennadiy Borisov and passed six days later on Jan 27 just about 3,600 km from Earth’s surface (near the southern tip of South America).
This distance is only slightly greater than the distance between Perth and Sydney and is only about 1 percent of the distance between Earth and our moon.
The asteroid also passed through the region of space containing a significant portion of the man-made satellites orbiting the Earth.
All of this makes 2023 BU the fourth closest known asteroid encounter with Earth, ignoring those that struck the planet or our atmosphere.
How is 2023 BU classified as an asteroid and threat? 2023 BU is unremarkable, apart from passing so close to Earth. The asteroid’s diameter is estimated to be only 4–8 m, which is on the small end of the asteroid size range.
There are probably hundreds of millions of such objects in our solar system, and it is possible that 2023 BU has come close to Earth many times before over the millennia. Until now we were not aware of the fact.
On average, an asteroid with a diameter of 4 meters will hit Earth every year and an asteroid with a diameter of 8 meters about every five years. Asteroids of this size pose little risk to life on Earth upon impact, as they largely break up the atmosphere. They create spectacular fireballs, and some of the asteroids could make it to Earth as meteorites.
Now that 2023 BU has been discovered, its orbit around the Sun can be estimated and future visits to Earth predicted. It is estimated that there is a 1 in 10,000 chance of 2023 BU hitting Earth sometime between 2077 and 2123.
So we have little to fear from 2023 BU or any of the many millions of similar objects in the solar system.
Asteroids must be more than 25 m in diameter to pose a significant risk to life if they collide with Earth. To challenge the existence of civilization, they would have to be at least a kilometer in diameter.
It is estimated that there are fewer than 1,000 such asteroids in the solar system that could hit Earth every 5,00,000 years. We know more than 95 percent of these objects.
Will there be more nearby asteroid passes? 2023 BU was the fourth closest asteroid flyby ever recorded. The three narrower passes were caused 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 in the solar system’s nearly five billion years of existence, and this situation will continue in the future.
What has changed in recent years is our ability to detect asteroids of this size, allowing all threats to be characterized. That an object about 5 m in size can be detected many thousands of kilometers away by a very dedicated amateur astronomer shows that the technology for making meaningful astronomical discoveries is within the reach of the general public. That is very exciting.
Amateurs and professionals can continue to work together to discover and categorize objects so that threat analysis can be performed. Another very exciting recent development came last year with the Double Asteroid Redirection Test (DART) mission, in which a spacecraft successfully collided with an asteroid and changed direction.
DART makes plausible the concept of deflecting an asteroid off a collision course with Earth when a threat analysis identifies a serious risk with adequate warning.
