Fireball of 03 March 2012

Last night, 03 March 2012 at around 2145GMT, my Twitter stream became flooded with reports from people saying they’d seen a giant meteor streaking across the sky.

The first I heard about it was a tweet from @VirtualAstro:

ALERT! Reports coming in of sightings of fireballs (large meteors/ Shooting stars) in the North and South of England

It turns out that these were probably all reporting the same sighting. For a brief spell the hashtag #ukcomet started to gain prominence, but it wasn’t a comet at all, rather a large chunk of space rock burning up in the Earth’s atmosphere.

On any clear dark night you can see a few meteors – which are also known as shooting stars – as the Earth hurtles round the Sun hoovering up all the bits and pieces of debris floating about in space.

Most meteors are the size of a small pebble and as they get hoovered up by the Earth they pass through the atmosphere. This generates frictional heating as the space-rock rubs past air molecules, and eventually the rock will burn up completely. This happens in part of the atmosphere called the mesosphere, which is about 75km (45 miles) above the Earth’s surface. During the brief period of frictional heating not all the energy produced is converted into heat, some of it gets converted into light, which is why we see them streaking across the sky. Usually a meteor will be moving so fast, and burn up so quickly, that is appears as a very quick flash of light, of less then a second in duration.

But there are bigger bits of rock out there too, and when something bigger than around 10cm enters the Earth’s atmosphere we might get a far more spectacular display, something called a fireball, or bolide meteor.

This is what happened last night. Rather than friction heating up the rock (there was probably a bit of that going on too) the energy seen in fireballs is generated by ram pressure. This is as a result of the large rock crashing into the atmosphere and causing all of the air in front of it to rapidly compress, forming a shock wave. The air in this shock wave heats up (did you know that compressed air heats up? Feel the tube on a bicycle pump next time you’re blowing up a tire) and flows around the rock, causing it in turn to heat up. This process starts the rock glowing, and when it’s bright enough we see it as a fireball.

Fireballs are much brighter than standard meteors – in fact the IAU defines a fireball as any meteor brighter than magnitude -4 – and last longer in the sky, and so they’re much easier to spot. Therefore even though they’re much rarer than your common or garden meteors they tend to get spotted by lots more people, and are even visible in big cities, hence the flurry of reports late last night.

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