‘City killers’ and half-giraffes: how many scary asteroids really go past Earth every year?

tech.hindustantimes.com:

Asteroids are chunks of rock left over from the formation of our Solar System. Approximately half a billion asteroids with sizes greater than four metres in diameter orbit the Sun, travelling through our Solar System at speeds up to about 30 kilometres per second – about the same speed as Earth.

Asteroids are certainly good at capturing the public imagination. This follows many Hollywood movies imagining the destruction they could cause if a big one hits Earth.

Almost every week we see online headlines describingasteroids the size of a “bus”, “truck”, “vending machine”, “half the size of a giraffe”, or indeed a whole giraffe. We have also had headlines warning of “city killer”, “planet killer” and “God of Chaos” asteroids.

Of course, the threats asteroids pose are real. Famously, about 65 million years ago, life on Earth was brought to its knees by what was likely the impact of a big asteroid, killing off most dinosaurs. Even a four-metre object (half a giraffe, say) travelling at a relative speed of up to 60 kilometres per second is going to pack a punch.

But beyond the media labels, what are the risks, by the numbers? How many asteroids hit Earth and how many can we expect to zip past us?

In terms of asteroids hitting Earth, and their impact, the graphic below from NASA summarises the general risks.

There are far more small asteroids than large asteroids, and small asteroids cause much less damage than large asteroids.

So, Earth experiences frequent but low-impact collisions with small asteroids, and rare but high-impact collisions with big asteroids. In most cases, the smallest asteroids largely break up when they hit Earth's atmosphere, and don't even make it down to the surface.

When a small asteroid (or meteoroid, an