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Not so deep impact

We saw it coming, predicted where it was going to strike and then we went to pick up the bits! For the first time ever an asteroid has been spotted in space and we followed as it smashed into the Earth's atmosphere and exploded over a North African dessert.

This was not a civilisation destroyer like the one that many scientists think wiped out the dinosaurs, but a small fragment of rock about the size of a London Taxi. It collided with the Earth on Tuesday, October 7th 2008.

The asteroid, called 2008TC3, was first sighted on the morning of October the 6th by astronomers using the 1.5 meter Mt. Lemmon telescope in Arizona, USA. The asteroid first appeared as a tiny dot on computer screen thousands of times too faint to be seen with the naked eye. In fact, it looked just like any one of the millions of faint stars seen every night.


What gives it away as being an asteroid is its movement. This is checked by taking two pictures a few hours apart. If any of the stars appear to move then this proves that they are not stars but objects within the solar system orbiting the sun. By looking at how fast they move it is easy to tell how far away they are. As objects get further away from the sun its gravity gets weaker, and so they move more slowly. This is why Mercury, which is close to sun and experiences a really strong gravitational pull, has to move so quickly to stay in orbit.

An orbit is really a balancing act between gravity and speed. Pluto, which is about a hundred times further away from the sun, experiences a much weaker pull, and so moves much more slowly.

When the speed of the asteroid was calculated it was found to be moving at about the same speed as the Earth: about 30Km per second. This meant it was really close. When other observatories around the world picked it up with their giant telescopes the precise orbit could be calculated. Bingo! It was on a collision course with Earth! The picture shows the trail at dawn where the asteroid had entered the atmosphere only two hours earlier.

Asteriod Entry

Asteroids are very common: the belt between Mars and Jupiter is thought to contain about 1.5 million rocks with a diameter of more than 1Km (0.62 miles). Some of them are really huge; Ceres is about 450Km in diameter (about 280 miles across).

The asteroid belt is a massive collection of bits left over from the birth of the solar system. They should have formed another large planet but the gravity from nearby Jupiter is so strong that it keeps on pulling the planet apart leaving us with the asteroid belt. Nearly all the asteroids are confined within this region, but sometimes there is collision between asteroids in the belt that throws them into the Earth's orbit. These are known as Near Earth Objects (NEOs).

Asteriod Belt

At the moment over 6000 asteroids are known to exist and more are being discovered all the time. They can be very dangerous. In 1909, a small piece of old comet hit the atmosphere above the Tunguska River in Siberia. The blast knocked pine trees over for a radius of about 40Km (about 25 miles across). It produced a blast equivalent to a very large atomic bomb, about 10 to 15 million tonnes of TNT. The heat from the blast set the trees on fire and fried countless reindeer! If it had hit London it would have killed over six million people, but strangely it left no crater!

This is why the discovery and tracking of 2008TC3 is so important. If we could just detect a big one a few days in advance and predict where it would land we could at least evacuate a city if we need to.

Why are asteroids so dangerous?

Very good question, but it basically comes down to speed. These things move fast, really fast. The faster something goes the more kinetic energy it carries. Almost anything travelling at 30Km per second (about 19 miles per second) is carrying a lot of energy. Our taxi-sized rock carries about as much energy as 1000 tonnes of TNT, and this is a really small one by asteroid standards!

This is what an asteroid about the size a large bus can do. The crater is about 1.2Km in diameter and 200 meters deep. The heat from the impact would have caused fires for about 100Km (about 63 miles) in any direction. It happened about 49,000 years ago in Arizona. It would have been quite a sight....from a distance though because you wouldn't want to be too close!

Flatenned Trees

No crater in Tunguska but a whacking great hole in Arizona. What's up?

Another very good question. Expeditions were sent out for years after Tunguska looking for a crater. It should have been huge. The impact lit up the sky for hundreds of miles around, but they didn't find one. This is because there are many different types of asteroids. The one that hit above Tunguska may not even have been a true asteroid. It may have been a comet.

Comets are made of lots of frozen water, carbon dioxide and small bits of rock and dust. When this hit the top of the atmosphere travelling at 30Km per second (about 18 miles per second) it simply heated up due to the immense friction with the atmosphere, and then it vaporised in a flash of heat. Nothing actually reached the ground but only the heat and the shock wave. The shock wave flattened the trees and the heat caused them to burst into flame.

The crater in Arizona was caused by a large lump of iron. This did not vaporise when it hit the atmosphere, but carried straight on. It did vaporise when it hit the ground though causing a massive crater. Very few large lumps of iron have never been found around the crater, but if you scramble around in the dust at the bottom of the crater with a magnet you will pick loads of iron filings!

Earth Crater

The Earth is subject to impacts on a regular basis. If you want to see how many things have hit the Earth over its history just have a look at the moon with its many craters. The only difference is that the weather and vegetation on Earth has wiped its surface clean.

We have been hit many times in the past and will be hit many times in the future. It may be worth getting to know the enemy.

The first thing you need to know is where they are, and this is where Spaceguard comes in. Spaceguard is an organisation that co-ordinates observations from a group of observatories working all over the world. Very few telescopes are actually dedicated to looking for NEOs so the chances of spotting even quite a big one are low. Once the object has been spotted, its orbit calculated and an impact confirmed we still can't be sure what it will do.

There are, in fact, many different types of asteroid. Some are old dead comets like the Tunguska object. Some are 'rubble piles', these are loosely assembled collections of large rocks that will break up and scatter over a large area. Some are single rocks made largely of carbon-rich minerals and some are the metal monsters capable of holding together as they come through the atmosphere and slamming into the ground. While they are in space it is very difficult to work out which is which.

Could we stop the 'big one'?

Possibly! An asteroid only 1000 meters in diameter (that's about 10 football pitches!) is capable of destroying human civilisation. If we discovered one of these early enough it would definitely be worth a go!

How could we destroy an asteroid before it hit Earth?

There are a number of possibilities:

Nuclear weapons

Nuclear Asteriod Defense

We could use nuclear weapons to try and split the object up into smaller fragments. This would be a very dangerous game as it could result in a shower of lethal fragments that would be impossible to track.


A spacecraft could be collided with the asteroid at high velocity in the hope of nudging it off course. This could work if it was spotted early enough. The closer the asteroid is to the Earth the bigger the push would have to be.

We could soft land a spacecraft on the surface of the object and use its rocket motor to gently push it off course. Again, this is reliant on spotting the asteroid early!
There are many other possibilities, but all are really dependent on getting to the object early. All a bit unlikely at the moment though.

The most important thing would seem to be getting a better network of telescopes capable of spotting potential threats early. This, however, costs a lot of money. It probably won't be until after the first big impact over a populated area that the world will get the message.


Perform an experiment to produce an impact crater as perfect as meteor crater in Arizona. Your impactor should be a marble or ball bearing. The difficult part is choosing a target that behaves in a way that scales to the way Rocks behave when hit by an object travelling at 20 or 30 times the speed of sound. Suggested substances to try would include sand, flour (very messy!), wax, custard etc!

The big questions:

How big is the threat from asteroid collision?

How many asteroids exist?

How many are a threat to the Earth?

Is it worth spending money guarding against an impact from space?