What Is a Meteor Shower? Comet Dust Hitting the Sky on Schedule
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On the night of 12 November 1833 the sky over the eastern United States filled with falling stars at a rate of tens of thousands an hour, and people woke their neighbours believing the end of the world had come. It was the Leonid meteor shower, at the peak of its 33-year cycle, and the astronomers who studied it worked out for the first time that the streaks were not weather but grit from space, striking the atmosphere from a single direction along the Earth's orbit. Every year the planet runs through the same trails on the same dates, and the calendar of showers is one of the few astronomical events that can be watched from a garden with no equipment at all.
What a meteor is
A meteor is the streak of light, not the object. The object is a meteoroid, a fragment of rock, metal or comet dust, usually the size of a sand grain or a pea, and it hits the upper atmosphere at 11 to 72 kilometres a second, depending on whether it is catching the Earth up or meeting it head on. At those speeds the air in front of it is compressed and heated to thousands of degrees, the grain vaporises at an altitude of 80 to 120 kilometres, and the glowing trail of ionised air and vapour is what is seen, a hundred kilometres up and gone in under a second. A grain the size of a pea makes a bright meteor; a fireball brighter than Venus is a pebble; and only an object the size of a fist or larger survives to reach the ground as a meteorite, which shower meteoroids, being fragile comet dust, almost never do.
Where the dust comes from
Comets shed. Each time one passes near the Sun its ice boils off and carries dust with it, and the dust spreads out along the comet's orbit into a stream that persists for centuries. Where the Earth's orbit crosses such a stream, the planet sweeps through it on the same date each year, and for a night or a week meteors arrive in numbers. Because all the grains in a stream travel on parallel paths, the meteors appear to spread out from one point in the sky, the radiant, in the constellation that gives the shower its name; the Perseids radiate from Perseus, the Geminids from Gemini, though the radiant is a perspective effect and the meteors themselves appear anywhere. The parent comets are known for most major showers:
- •Perseids, peaking around 12 August: from comet Swift-Tuttle, which returns every 133 years; up to 100 an hour, the most watched shower because of the warm nights
- •Geminids, around 14 December: from the asteroid Phaethon, a burned-out comet; up to 150 an hour, the strongest of the year
- •Quadrantids, around 3 January: brief and intense, from a body named after a constellation that no longer exists
- •Leonids, around 17 November: from comet Tempel-Tuttle; usually modest, but a storm of thousands an hour every 33 years, as in 1833, 1966 and 1999
- •Eta Aquariids in May and Orionids in October: both from Halley's comet, the Earth crossing its stream twice a year
Why some years are storms
A stream is not uniform. Each return of the comet lays down a fresh, dense ribbon of dust that slowly spreads, and if the Earth happens to pass through a young ribbon rather than the diffuse old stream, the rate jumps a hundredfold. Astronomers can now calculate where the ribbons from each past return lie, and predicted the Leonid storms of 1999 to 2002 to within an hour, an achievement that let satellite operators turn their spacecraft edge-on to the stream. The Leonid rate in 1966 over the American southwest was estimated at 40 meteors a second, and the 1833 storm is the reason the phrase falling stars entered American folk memory.
The rest of the sky's grit
Outside the showers, sporadic meteors arrive at a few an hour on any dark night, the background of dust left by comets whose streams have dispersed and by collisions among the asteroids. The Earth sweeps up about 100 tonnes of it a day, most of which drifts down as microscopic dust and can be collected on rooftops with a magnet. Larger visitors are rarer: a fireball bright enough to cast shadows happens somewhere every day, a meteorite fall that is witnessed and recovered a few times a year, and an object the size of the Chelyabinsk one, twenty metres, once in a century or so. A meteor shower is the gentle end of a process whose other end made the craters on the Moon.
Watching one
No telescope is needed and would hinder, since meteors are fast and appear anywhere. The rules are to get away from streetlights, to choose a night when the Moon is down, since moonlight halves the count, to lie back and look at the darkest part of the sky rather than at the radiant, to allow half an hour for the eyes to adapt, and to watch after midnight, when the observer's side of the Earth faces forward into the stream and the meteors are more numerous and faster. The advertised hourly rate is for perfect conditions with the radiant overhead; a real observer sees perhaps a third of it. A good Perseid night from a dark field yields a meteor every minute or two, an occasional fireball, and the sense, which the astronomers of 1833 were the first to have, of the Earth moving through a cloud of dust that a comet left behind before there were people to see it.
The takeaway
A meteor shower happens when the Earth passes through the stream of dust a comet has shed along its orbit, so that grains the size of sand strike the atmosphere at tens of kilometres a second and vaporise a hundred kilometres up in streaks that appear to radiate from one point in the sky. The showers recur on fixed dates each year, become storms when the Earth crosses a fresh ribbon of dust, and are best watched from a dark place after midnight with no Moon and no telescope.