Things that hit the Earth: how it works

How often, and how bad. These are NASA’s rough figures. They are averages over a very long time, not a timetable.

Size of the object	Arrives about once every	What it does
10 metres	10 years	A very bright fireball and a bang that can break windows
50 metres	1,000 years	Flattens the area round it
140 metres	20,000 years	Digs a crater 1 to 2 km wide
1 kilometre	700,000 years	Digs a crater about 10 km wide
10 kilometres	100 million years	Leaves a crater about 100 km wide and changes the whole world

Four that we know about.

When and where	What happened
About 66 million years ago, Mexico	The Chicxulub crater, about 180 km wide. Linked to the loss of more than 70 per cent of species
About 50,000 years ago, Arizona	Meteor Crater, 1.2 km wide and 180 m deep, from an object 30 to 50 m across
1908, Tunguska, Siberia	An object less than 100 metres across probably burst in the air. Trees were knocked flat over hundreds of square miles
15 February 2013, Chelyabinsk, Russia	A house-sized asteroid burst in the air with the force of about 440,000 tonnes of TNT. Hundreds were hurt, mostly by broken glass

Why Chicxulub is hard to see. The crater is buried under later limestone, and its centre is just off the coast. What shows on the map in this lesson is a shallow curved trough only 3 to 5 metres deep and about 5 km wide, where the ground has sunk along the crater’s buried edge. Somebody walking across it would probably not notice. It was picked out by radar from the Space Shuttle Endeavour in February 2000.

The watch. A near-Earth object is an asteroid or comet whose path brings it within about 1.3 times the Earth’s distance from the Sun. On 11 November 2026 NASA’s database listed 43,238 of them. An asteroid is called potentially hazardous if it is at least 140 metres across and its path comes within 7.5 million km of the Earth’s, and 2,551 were on that list. The name is a label for the ones worth watching closely. It does not mean any of them is going to hit.

A grey surface completely covered in sharp-edged boulders and rubble, lit from one side, filling the whole picture. — **A photograph.** The last complete picture DART sent, two seconds before it hit, from about 12 km away. The patch of ground shown is 31 metres across. The asteroid is a loose heap of boulders. Credit: NASA/Johns Hopkins APL.

What DART did. DART weighed about half a tonne and hit at roughly 22,500 km an hour. NASA had said in advance that changing the orbit by 73 seconds would count as success. The change was 32 minutes, give or take 2, which is more than 25 times as much. A good part of the push came from the rubble that was blasted off the far side, which acted like the exhaust of a rocket.

A bright blue point of light with a long straight blue tail running to the upper right, and a scatter of faint blue dots round it. — **A photograph.** Dimorphos on 19 December 2022, nearly four months after the crash, by the Hubble Space Telescope. The tail is dust knocked off by DART. The faint dots round the asteroid are 37 boulders, from 1 to 7 metres across, drifting away at less than 1 km an hour. The blue is the colour chosen for the picture. Credit: NASA, ESA and David Jewitt (UCLA); image processing by Alyssa Pagan (STScI).

What comes next. The European Space Agency’s Hera spacecraft was launched on 7 October 2024 to go and look at the result. It is due to reach the two asteroids in late 2026, to measure the mass of Dimorphos and see what DART did to it.

A push like DART’s is small. It works only if the asteroid is found many years before it would arrive, so that a tiny change in its speed has time to grow into a miss. That is why the watching matters as much as the hitting.

How it works

How it works

Where this comes from