I know that tiny meteorites fall on the Earth all the time (something like 1000 tons of mass every day!), but when the 10th of August arrives I am more conscious of this and it has always been a special night for me.
In Italy (where I come from) there is a big tradition of stargazing for that night, la notte di San Lorenzo, and I have been looking up in the summer skies since I was a child. I have seen many shooting stars during those nights and I recall the sheer joy of the moment. Now that I live in the UK I am told there isn’t a similar well established and popular tradition for this event, and I am not quite ready to abandon my annual meeting with the stars.
First of all, why 10th of August? Well, that is the period of the year when the Earth passes through the cloud of the Swift-Tuttle comet’s debris, which orbits the sun once every 133 years. Some bits of ice and dust left by the comet dash into the atmosphere at more than 200,000 km/h, they burn up in the Earth's atmosphere and produce a trail of glowing vapour creating one of the best meteor showers of the year.
The shooting stars radiate from the direction of the constellation Perseus and this event is known as the Perseid meteor shower. They can be seen all over the sky, with the best viewing opportunities across the northern hemisphere.
When you are lucky enough to spot a meteor you see a very brief streak of light in the night sky, and it’s hard to imagine that the comet’s debris are just as small as a grain of sand or slightly bigger. They occur and disappear so quickly – just the right time to make a wish - that all you see is a white hot streaks of superheated air. But are you sure that it was a WHITE streak?
Shooting stars have different colours and we have different hues of light depending on the chemical composition of the debris. Entering the atmosphere the meteors vaporizes, and it emits a distinctive light spectrum based on its chemical makeup.
Another factor affecting the colour of the meteor is its speed. The air immediately in front of the meteor is compressed to higher pressures if it is faster and thus produces higher temperatures. A fast meteor will then appear blue, while a slow one will be red or orange.
We also have to consider the excitation of air molecules by the meteor’s passage. If the material is heated enough its molecules will be stripped of electrons and they become ionized. These charged particles will then interact with air molecules and cause them to glow (just as charged particles from the sun cause auroras). The colour of the light changes as layers of the meteoroid are stripped off and ionized.
Different elements are then responsible for the different colouring: sodium will produce an orange/yellow light, iron yellow, copper and magnesium blue/green, calcium and potassium violet.
The 2012 Perseid meteor shower will peak this weekend (11-12 August) and if you are patient (and lucky) enough you might be able to spot one or more shooting stars. The new moon was just a couple of days ago and the moonlight should not affect the viewing, so if the skies are dark enough and free of clouds you could see up to 10 shooting stars an hour!
I really hope to be able to continue my astronomical tradition, because even now that I have grown up and I know more about the chemistry of the shooting stars, they will always make 10th August a magical night.
Photo from NASA's Astronomy Picture of the Day
Credit: Katsuhiro Mouri & Shuji Kobayashi (Nagoya City Science Museum / Planetarium)