First look
The idea, in plain words
This hole in the Arizona desert is 1.2 kilometres wide and 180 metres deep. It was made about 50,000 years ago, in a few seconds, by a lump of rock and metal from space. The lump was only 30 to 50 metres across. What did the damage was its speed.
Bigger ones have come. About 66 million years ago, a far bigger one struck what is now the coast of Mexico. The crater it left is about 180 kilometres wide and is buried now, but its edge still shows as a faint curved dip in the ground. The impact is linked to the loss of more than seven in every ten kinds of living thing on the Earth, the dinosaurs among them.
The small ones are common and the big ones are very rare. A rock ten metres across arrives about once in ten years and makes a brilliant fireball and a bang. One big enough to dig a crater like Arizona’s comes about once in a thousand years. One ten kilometres across, big enough to change the whole world, comes about once in a hundred million.
The last one to hurt people came on 15 February 2013. A rock the size of a house came in over the city of Chelyabinsk in Russia and burst in the air with the force of about 440,000 tonnes of explosive. The blast broke windows over a wide area, and hundreds of people were hurt, mostly by flying glass.
So astronomers keep watch. Telescopes sweep the sky every clear night for anything whose path comes near the Earth’s, and by 11 November 2026 more than 43,000 such objects had been found. Once an orbit is known, it can be worked out many years ahead.
In 2022 came the next step: seeing whether one could be moved. NASA picked a small asteroid called Dimorphos, 160 metres long, which circles a larger one. It was no danger to anybody. On 26 September a spacecraft called DART was flown straight into it at about 22,500 kilometres an hour. This is one of the last pictures it sent, eleven seconds before it hit.
It worked. Before the crash, Dimorphos took 11 hours 55 minutes to go once round its companion. Afterwards it took 11 hours 23 minutes. For the first time, people had changed the path of something in the sky.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Asteroid
- A small body of rock or metal in orbit round the Sun.
- Near-Earth object
- An asteroid or comet whose orbit brings it close to the Earth's.
- Impact crater
- A bowl blasted out of a surface by something striking it at many kilometres per second.
- Meteor
- The streak of light made when a meteoroid burns up in the air.
- Meteorite
- A piece of a meteoroid or asteroid that has survived the fall and reached the ground.
- Orbital period
- The time taken to go once round an orbit.
- Comet
- A small body of ice, dust and rock that grows a cloud of gas and a tail when it comes near the Sun.
Where this comes from
- NASA, Asteroid facts (past impacts, how often each size arrives, near-Earth objects)
- NASA, Shaded relief with height as colour, Yucatán Peninsula (PIA03379)
- NASA, DART’s final images prior to impact
- NASA, DART mission impact changed asteroid’s motion in space
- NASA/JPL, Small-Body Database (count of near-Earth objects on 11 November 2026)
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.