By David Bryant
Every year in mid-August astronomers all over the world eagerly anticipate the Perseid Meteor Shower: some years the sky can be full of ‘shooting stars’, on others hardly a handful are seen.
This year was pretty good: I personally observed over sixty meteors in around two hours and even managed to photograph one as it passed close to the familiar ‘W’ shaped constellation, Cassiopeia:
Just occasionally a larger object will make it to ground level: it is then called a meteorite.
The most abundant types of meteorite are:
• Stony meteorites. These are left over from the formation of the Solar System or come from the surface of other planets.
A couple of hundred meteorites (known as achondrites) are known to have been blasted from the surface of the Moon or Mars by meteorite or cometary impacts: about the same number originated in the same way on asteroids such as 4-Vesta.
•Stony irons. Pallasites and mesosiderites are thought to derive from the core-mantle layer of shattered planets, or from collisions between stony and nickel-iron bodies.
• Iron meteorites These may have either condensed directly within the Solar Nebula or be the cores of disrupted, differentiated planetissimals.
Some meteor showers, however, are associated with the return of debris from periodic comets to our region of the Solar System.Beyond the furthest reaches of the Solar System, at a distance between 5,000 to 100,000 AU, (1 Astronomical Unit = the distance from the Earth to the Sun: around 150 million kilometres) lies an encompassing shell of trillions of cometary nuclei: the Oort Cloud. Since their formation from the solar nebula or capture by the Sun, these objects have collected a regolith of carbonaceous material and dust particles.
For reasons not yet fully understood (but possibly associated with the gravitational influence of passing dwarf stars) the orbits of these bodies can be perturbed, causing them to tumble inwards towards the Sun. Should this occur, by the time an Oort Cloud object reached the inner Solar System it would be travelling at a velocity of one hundred thousand kilometres an hour and will have acquired an outer shell of rocky material.
Encountering the electromagnetic radiation and streams of particles from the Sun, the icy nucleus may develop a coma and tail, becoming visible from the Earth as a typical comet.
In common with the great majority of these objects, it may swing around the Sun and pass harmlessly back into deep space. Possibly, however, it will be captured and enter an elliptical orbit, becoming a periodic comet that makes regular returns to the inner Solar System. Eventually it will lose most of its mass through sublimation, leaving a cloud of stony debris that might give rise to a new meteor shower, should its orbit cross that of the Earth.
The recent investigation of Comet 67P/Churyumov–Gerasimenko by the twin probes Rosetta / Philae demonstrated what many meteoricists had long suspected: although comets are primarily composed of water-ice, their surface is a regolith composed of whatever debris they have passed through during their adventures in space: it is this material that is responsible for periodic meteor showers like the Perseids in August (associated with Comet Swift-Tuttle), the Orionids in October (associated with Comet Halley) and the Leonids in November (associated with Comet 55P/Tempel-Tuttle)
Whenever a ‘good’ meteor shower (like those above) is due, the event is highlighted in the press and on TV. In the past there have been some incredible ‘meteor blizzards, like that in the old print below. Sadly, most years, non-astronomers are disappointed by the reality: even the one or two a minute I saw this August could not really be described as a spectacle.
Perhaps it is the publicity given to some showers or perhaps it is the antics of a couple of my American colleagues on TV making people more meteorite-aware, but after every Perseid Shower for the past four or five years I have received numbers of e-mails from people who think they have found one!
Here’s a recent example:
‘Hi! Is this a meteorite? I was watching out for meteors the other night when suddenly I noticed a black rock on the lawn: I’m certain it wasn’t there before!
I just went through the check list on your site: it is strongly magnetic, has a fusion crust, thumbprints, and there seems to be a widdsmanstatten pattern inside .I also clipped a little of edge and it is bright. I have added some pictures for you to look at. If it is a meteorite, I would be happy to offer it to you at the right price.’
Sad to say, I have no recollection of a single meteorite having been conclusively proven to have fallen during a periodic meteor shower. I should imagine the reasons are quite straight-forward:
• The size of particle that the Earth encounters when it passes through a field of cometary debris is quite small: probably the size of a grain of rice – far too small to reach the ground.
• The majority of periodic comets have visited the inner Solar System sufficiently often that most of the Earth-crossing debris has already been ‘hoovered up’.
• The trail of debris left by a comet is spread very thinly along its entire orbit around the Sun: the tiny region of the orbit of Halley’s Comet that the Earth crosses each year must statistically contain very few large chunks.
• The chances of anyone being fortunate enough to see one of these hit the ground and locate it are about the same as winning the lottery every week for a year!
In conclusion, the vast majority of meteoric material that arrives on Earth (about 300 tonnes a day!) is not of cometary origin: any that is will generally be far too small to reach the surface or be discovered.
If you’re interested in finding out more about comets, meteorites and their occasional impacts on our planet, my third book ‘Danger from the Skies’ is now available on Amazon or from my sales page:
http://www.david-bryant.co.uk/
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