Image: 123rf/ Bjoern Wylezich
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Are we being invaded by aliens?

This was one of many questions asked after CCTV footage showed what appeared to be a meteor streak across the sky in North West at the weekend.

TimesLIVE spoke to Professor Roger Gibson from the School of Geosciences at the University of the Witwatersrand about the incident.

He said the meteor could have been the same one that appeared over China approximately one hour before reaching South Africa and Botswana.

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He said although eyewitnesses may have thought they saw it exploding as it hit the ground‚ it may not have landed.

“It was probably still 15-20 km up in the atmosphere and the explosion‚ called an airburst‚ was triggered as it descended into denser atmosphere closer to Earth’s surface‚” he said.

“After the explosion‚ fragments of the meteorite may have fallen to Earth over a wide area‚ possibly further along its path. We will need to see if eyewitnesses report finding fragments in the next few days‚ although this is not necessarily very easy‚” Gibson said.

How often do meteors appear in the atmosphere?

About 200 tons of rocky (asteroid) materials enter the Earth’s atmosphere every day. Most burn out without being noticed. If you look at the night sky outside the city you should see shooting stars (meteors) every few minutes.

These are dust-size particles entering the upper atmosphere at speeds of over 30‚000 km/h‚ so fast that the frictional resistance of the atmosphere causes them to start burning up.

“They are so hot that they glow. The reason why they disappear after only a short time is because they are burning away. Our satellites are bombarded by these micrometeorites all the time‚ so much so that astronauts often have to repair them‚” Gibson said.

Why they are useful to geoscientists?

Geoscientists refer to meteorites as “the poor man’s space probe”. “Meteorites are very interesting as they tell us something about the planets they are from. They come to us. They are space rocks (asteroids) that happened to cross Earth’s orbit and were pulled by gravitational force into the Earth. If we find the fragments on the Earth’s surface we then call them meteorites‚” Gibson said.

What’s the worst they can do?

If you want to take out risk insurance‚ you will find that you have far more of a chance of being hit by a car than a meteorite. A meteorite strike is likely‚ depending on its size‚ to cause fatalities. A giant meteorite impact caused the extinction of the dinosaurs 65 million years ago‚ mostly because of changes in the atmosphere that led to destruction to habitats.

Experts estimate that latest meteor may have been the size of a small car when it entered the atmosphere. “We expect on average one fragment of this size to enter the atmosphere per year‚” Gibson said.

He said that a meteor did not actually have to hit the ground to be destructive. An “airburst” explosion seemed to have happened in the Tunguska River area of Siberia‚ Russia‚ in 1908 which flattened over 2‚000 square kilometres of forest.

More than 1‚000 people were injured in Chelyabinsk (Russia) when the blast wave from an early morning airburst on February 15‚ 2013 shattered windows in six cities.

Many pieces of the Chelyabinsk meteorite were subsequently found.

Although less destructive‚ an airburst over the Thuate area of Lesotho on July 21‚ 2002 also produced thousands of meteorite fragments.


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