Falling rocks: meteors, meteorites and impacts

A shooting star is not a star. It is a tiny piece of rock or dust, often no bigger than a grain of sand, hitting the Earth’s air at enormous speed.

It is moving so fast that the air in front of it glows white hot. The grain burns away in a second or two, high above the ground. All you see is the streak.

Three words are easy to mix up. Out in space, the piece is a meteoroid. The streak of light is a meteor. If any of it lands, that piece is a meteorite.

On certain nights of the year there are many more meteors than usual. Those are meteor showers. They happen when the Earth passes through the dust left behind by a comet.

Speed is the reason. Anything arriving from space meets the Earth at between 11 and 72 km a second, tens of times faster than a rifle bullet. At that speed the air cannot get out of the way. It is squashed in front of the object and heated to thousands of degrees, and the object’s surface boils off. Most meteors shine between about 100 and 70 km up and never get lower.

About 44 tonnes of this material is estimated to fall on the Earth every day, nearly all of it as dust.

Showers. A comet sheds dust all along its orbit. If the Earth’s orbit crosses that trail, we run through it on the same date every year. The grains are travelling side by side, so their streaks all seem to spread out from one point in the sky, the way straight railway lines seem to meet in the distance. A shower is named after the constellation that point lies in.

Shower	Best around	Where the dust comes from
Perseids	12 August	Comet Swift-Tuttle
Leonids	17 November	Comet Tempel-Tuttle
Geminids	14 December	The asteroid Phaethon, unusually

Meteorites. A piece that starts out bigger than a fist or so may reach the ground. Most meteorites are stony, some are nearly pure iron and nickel, and almost all are chips off asteroids. A few have been knocked off the Moon and Mars by impacts there. They are the oldest things anyone can hold: the oldest grains inside them formed 4.567 billion years ago, and that is how the age of the Solar System is known. Two spacecraft have also gone and fetched samples that never had to survive the fall: Japan’s Hayabusa2 from the asteroid Ryugu in 2020, and NASA’s OSIRIS-REx from Bennu in 2023.

Bigger things. The larger the object, the rarer it is.

When	What happened
2013, Chelyabinsk, Russia	A rock about 20 m across burst roughly 30 km up. The blast broke windows across the city and about 1,500 people were hurt, mostly by flying glass.
1908, Tunguska, Siberia	An object perhaps 50 m across burst in the air and flattened about 2,000 square kilometres of forest.
About 50,000 years ago, Arizona	An iron meteorite about 50 m across dug Meteor Crater, 1.2 km wide.
66 million years ago, Mexico	An asteroid about 10 km across made the Chicxulub crater, some 180 km wide. It is the main reason the dinosaurs died out.

The Earth has been hit at least as often as the Moon, for its size. It shows fewer scars because rain, ice and moving continents wear craters away, while the Moon keeps every one.

Watching for the next one. Surveys have now found nearly all of the near-Earth asteroids more than a kilometre across, and none of them is on course to hit us in the next century or more. The search goes on for the smaller ones. In September 2022 NASA’s DART spacecraft was deliberately flown into Dimorphos, a small moon of an asteroid, and shortened its orbit by about half an hour. It was the first time people had changed the path of a body in space, and a first test of how an impact might one day be turned aside.