A tale of two fireballs

Meteor over the skys of Reno Nevada April 22nd 2012 (image: Lisa Warren/NASA)

On the evening of Saturday 3rd March, at about 21.40 GMT, there were widespread sightings of a bright fireball travelling from northern Scotland to southern England. Many people phoned the emergency services, but the possibility of an aircraft-related incident was soon ruled out. According to the BBC the Met Office tweeted “Hi All, for anyone seeing something in the night sky, we believe it was a meteorite.” In the following days the fireball incident was given extensive coverage in the media. These reports were essentially given over to eyewitness accounts of the fireball, with a little bit of expert comment providing a scientific perspective. Articles in the Mail Online and BBC News are fairly typical of the overall coverage. Reports in The Sun went a bit further than most in claiming that a lady in Redcar, Cleveland had found a meteorite sample from the fireball. The report has never been officially verified, with the material shown in the photo that accompanies the article looking suspiciously like the by-product from an industrial smelting process. In fact, according to Len Entwisle, the fireball co-ordinator of the British Astronomical Association’s meteor section, it is extremely unlikely that any material from the fireball would have fallen over the UK. In an email to me he says “This fireball would appear to have had a quite shallow angled entry into the Earth’s atmosphere….It would appear to have had a trail from the entry height of 900 to 1000 km to an end height at about 60 km. At that end height one would not expect a “fall”.

The following month, on Sunday 22nd April at about 07.51 Pacific time, the residents of California and Nevada witnessed an even more spectacular display. A bright daytime east-to-west moving fireball was seen to fragment towards the end of its trajectory accompanied by a loud sonic boom heard over a wide region in the area of Lake Tahoe. Residents of Coloma and Lotus townships, California reported hearing whistling sounds and a “welding” odour. Weather radar sweeps detected falling meteorites and a radar-defined strewn field was delineated, centred on the Sutter’s Mill Historic site. The first 5.5g sample of the meteorite was collected on 24th April, with the total recovered mass to date being 437g.

The Wikipedia entry for the meteorite, now officially known as Sutter’s Mill, records it as being a CM2. That would make it the same class as Murchison, an important meteorite that has been extensively studied on account of the complex inventory of organic molecules that it contains. However, intriguingly, the official Meteoritical Bulletin entry is less specific and merely classes it as “C” i.e. just a carbonaceous chondrite. The bulletin entry states “The meteorite is unusually hard compared to CM2 chondrites, and microprobe totals for the matrix are also high for CM2, suggesting possibly incomplete aqueous alteration or, alternatively, mild thermal metamorphism.” All of which suggests that Sutter’s Mill may turn out to be a very unusual meteorite.

Two fireballs then, the one over the UK was spectacular, but now a fast fading memory. On the other hand, the Sutter’s Mill fireball may just have been the first blast of a scientific adventure. We will have to wait and see.

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