Asteroids are pieces of rock left over from the formation of our solar system. About half a billion asteroids more than four meters in diameter orbit the Sun and travel through our solar system at speeds of up to 30 kilometers per second – roughly the same speed as Earth.

Asteroids certainly capture the public’s imagination well. It follows many Hollywood movies that imagine the devastation they could cause if a large one hit Earth.

Almost every week, we see headlines on the Internet about buses, trucks, vending machines, half-giraffes, and even asteroids the size of an entire giraffe. We also received headline warnings about “city killer”, “planet killer” and “Chaos God” asteroids.

Of course, the threats posed by asteroids are real. It is known that about 65 million years ago, life on Earth fell to its knees in a possibly massive asteroid impact that wiped out most of the dinosaurs. Even a four-metre object (for example, half a giraffe) moving at a relative speed of up to 60 kilometers per second will be powerful.

But beyond media labels, what are the risks associated with numbers? How many asteroids have hit Earth, and how many can we expect to fly?

What is the danger of a direct hit?

When it comes to asteroid impacts and impacts, the NASA chart below summarizes the overall risks. There are many more small asteroids than large ones, and small asteroids do much less damage than large ones.

As a result, Earth experiences frequent but minor collisions with small asteroids and rare but powerful collisions with large asteroids. In most cases, the smallest asteroids are greatly disintegrated when they enter Earth’s atmosphere and never even reach the surface.

When a small asteroid (or meteorite, an object smaller than an asteroid) hits Earth’s atmosphere, it creates a spectacular “fireball”, a very long-lived and bright version of a shooting star or meteorite. If any surviving part of the object hits the ground, they are called meteors. Most of the object burns in the atmosphere.

How many asteroids fly past Earth?

A very simple calculation gives you an idea of ​​how many asteroids you can expect to approach our planet. The numbers in the chart above estimate how many asteroids could hit Earth each year. Now let’s consider four-metre asteroids. On average, a four-metre asteroid crosses the Earth’s surface once a year.

If you double that space, you’ll get two per year. The radius of the earth is 6400 km. The radius of a sphere with twice its surface area is 9,000 km. Thus, about once a year, a four-meter asteroid approaches the Earth’s surface by 2,600 km – the difference between 9,000 km and 6,400 km.

Doubling the surface area again, you can expect two per year within 6400 km of the Earth’s surface. This aligns quite well with recent recordings of close encounters. A few thousand kilometers is quite a distance for objects several meters in size, but most of the asteroids reported in the media fly at much greater distances.

Astronomers consider anything that passes about 300,000 km closer to the Moon as a “close approach”. Being “close” to an astronomer does not usually mean that a member of the public says “close to”.

In 2022 there were 126 close approaches, and in 2023 we have 50.

Now think of really large asteroids that exceed a kilometer in diameter. The same very simplified logic as above can be applied. For every such civilization-threatening collision that occurs every half-million years, we can expect thousands of near misses (closer than the Moon) in the same time period. Such an event will occur in 2029, when the asteroid 153814 (2001 WN5) will fly 248,700 km from Earth.

How can we assess threats and what can we do about it?

It is estimated that about 95% of asteroids larger than one kilometer have been discovered, and the remaining 5% is constantly being sought in the sky. When a new one is found, astronomers make observations to assess any threat to Earth. The Turin scale classifies threats expected up to 100 years from now on a scale of 0 (no danger) to 10 (definite collision with a large object).

Currently, all known objects have a degree of zero. No object known today has a rating higher than 4 (a close encounter that deserves astronomers’ attention). So instead of hearing about giraffes, vending machines or trucks, what we really want to know from the media is the scale of the asteroid Turin.

Finally, technology has advanced enough that we have a chance to do something when we encounter a large number on the scale of Turin. Recently, the DART mission changed its trajectory by colliding the spacecraft with an asteroid. Such action in the future is likely to help protect Earth from collision for a sufficient period of time. Source