Copernicus Journal for Young Scientists

Meteorites shoot across the sky every day. Every now and then a really big one strikes the Earth. How dangerous are they? Can they destroy a city, kill us?
You bet! 65 million years ago a 10-km-wide rock came down in Mexico. The effect was felt worldwide and was the end of the dinosaurs.
Where do these rocks come from? How strong is the impact of a big meteorite? What is the chance to be hit by one? Is Armageddon near?

On a clear summer evening, lights shoot across the sky. If you are lucky, there will be more, like a real shower of fire. If you go out at night in the summer and autumn, you will have a good chance to see these shooting stars or meteorites.

They are pieces of cosmic debris that plunge into the atmosphere with immense speeds of maybe 20,000 kilometres per hour. They leave behind glowing trails of air heated by the friction. Most of them are just tiny dust particles that completely vaporize on their way across the atmosphere.

The cosmic dust comes from the tails of comets. Like the Earth and other planets, comets orbit around the Sun, but in much more stretched elliptic paths. They are big blocks of ice, rocks and dust (‘dirty snowballs’) with tails, thousands of kilometres long, facing away from the sun. Those tails contain numerous dust particles.

Every now and then, the Earth crosses the path of a comet and sweeps up its tail of debris. Thousands of dust particles enter the atmosphere and rain down in meteorite swarms or showers. These swarms appear to come from the same part of the sky. One of those, the Leonids, is named after the constellation Leo from where they seem to shoot. It occurs every year in mid November, when the Earth crosses the orbit of Comet Tempel-Tuttle. The Perseid meteor shower is another one, peaking on 12 August. The meteorites spread out from the constellation Perseids and are parts of Comet Swift-Tuttle.

Most meteorites are not big enough to reach the surface of the Earth. Even then, most of them are no bigger than a sand grain. The really big guys usually are asteroids, planet-like objects drifting between the orbits of Mars and Jupiter. Very rarely, a collision with another object knocks an asteroid out of its orbit, sending it to the Earth.

Read what happens when a big rock strikes the Earth:

Over the Earth’s 4.6 billion years long history, many asteroids and meteorites collided with the Earth. This constant rain of cosmic debris is what made the Earth. In its early days, our planet grew steadily by collecting large and small particles.

Even now, 1 to 10 million kg of cosmic dust rains down each day from space. Why then do we find so few traces of impacts on Earth?

Meteor Crater, Arizona, USA

If a meteorite hits the surface of the Earth it makes a big hole, an impact crater. Remarkably, only 171 impact craters have been recognized on Earth, ranging in diameter from 15 metres to a few hundred km.

The larger impact craters usually are the oldest. The biggest one, the Vredefort Crater in South Africa (300 km) was made more than 2 billion years ago.

Only 20 years ago, the third largest impact was found in Mexico, buried beneath 100’s of metres of sediment. It is the Chicxulub crater, measuring 170 km across. It formed when an asteroid, maybe 10 km wide, hit the coast of Yucatan 65 million years ago. It was the asteroid that did in the dinosaurs. We will investigate this event in an article in the next issue of Copernicus.

The biggest impact crater in Europe is Nördlinger Ries in Germany, measuring 24 km across. That one is 15 million years old. There may be an impact crater near you . Find out where and pay a visit.

How dangerous are impacts? Read more:

Impact craters are like bomb craters. Indeed, we can compare the energy of an impact with that of nuclear bombs. Even relatively small impacts are as strong as a nuclear bomb. The Chicxulub impact that killed the dinosaurs was more than 1000 times stronger than the power of all nuclear bombs together!

Scientists have calculated that asteroids of that size fall on the Earth every 50 to 100 million years. If that would happen today, it would certainly kill most people on Earth.
But even a 500-m-wide rock would cause worldwide changes in climate. If it plunged into ocean, the giant waves (up to 60 metres high) would devastate coastal regions worldwide. Such meteorites strike the Earth more often: every few 100 thousand years or so.

Read about impacts hitting humans:
Tungusta, Siberia, in 1908

In 1908, a big fireball exploded above Tungusta, in Siberia. It blew over all the trees for 50 kilometres around and changed the world climate for several years. The Tunguska event was due to the explosion of a relatively small (less than 50 metres) rock 10 km above the ground.

There was no loss of human life due to the very sparse population. But imagine what would happen if a fireball like that hit a city like Paris, London or Berlin! Events such as Tunguska may occur every few 100 years. (See this Fact Sheet .)
October 2004, news got out that 2200 years ago, a 1-km-wide comet hit the area of Chiemsee in Bavaria, Germany (link ). Over an area of 27 by 58 kilometres scientists have found more than 80 smaller and larger impact craters. The whole area is littered with little bits of meteorite and molten rock.

Archeologists have found remains of Celtic settlements that were destroyed by the fireballs raining from the sky. This event certainly killed many people.

You see, asteroids and meteorites are a serious threat. But how great is that risk really?

Time for your own research:

experiment

Investigate what happens when a really big meteorite hits the Earth.
You will drop small projectiles (model meteorites) on different surfaces (flour, sand, plaster of Paris). You will observe shapes and sizes of your impact craters and compare them with big craters on the Earth and planets.

Some good questions for your research are:

There are small craters and big craters. Why are some craters bigger than others?
Scientists say the Earth and other planets formed by collisions of asteroids and meteorites. If the Earth is 4.6 billion years old, why don’t we find impact craters older than 2 billion years?
Why do the Moon and some planets show more impact craters than the Earth?
How much energy does an impact produce, e.g. compared with a nuclear bomb?
What are the chances that I will be hit by a meteorite, or my home town destroyed by an asteroid?

sources

Near Earth Object Program. NASA http://neo.jpl.nasa.gov/index.html

Walter S. Kiefer, 2003, Impact Craters in the Solar System. Space Science Reference Guide, Second Edition, Lunar and Planetary Institute.

Relic of ancient asteroid found. Rebecca Morelle, BBC News science reporter.
A large fragment of an asteroid that punched 160km-wide (100 miles) hole in the Earth's surface has been found.The beachball-sized fossil meteorite was dug out of the 145-million-year-old Morokweng crater in South Africa. http://news.bbc.co.uk/2/hi/science/nature/4757545.stm

Europe Under Attack

Dick van der Wateren

experiment

sources