What Is a Meteor Shower? Comet Dust Hitting the Sky on Schedule

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.

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, named after a constellation that no longer exists and coming from an asteroid-like body
• •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

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.

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.

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.

Counting by eye is only one way meteors are studied. Because the trail of a meteor is a column of ionised gas, it reflects radio waves, and radar stations and even amateur radio listeners can detect meteors in daylight and through cloud. This is how astronomers found that some of the strongest showers of the year are invisible to the eye: the Arietids, in early summer, arrive from a radiant close to the Sun, so they fall during the day and were only discovered once radar was pointed at the sky in the years after the Second World War.

Networks of automated cameras spread across a country add the other half of the picture. When two or more stations film the same meteor, the trail can be triangulated to find its height, speed and direction, and from that its orbit around the Sun can be calculated backwards. That is how a stream is linked to its parent comet with confidence rather than guesswork, and when a rare larger object survives to the ground, the same geometry narrows the search area so that fresh meteorites can be found within days, before rain and weather have altered them.

The rates quoted for meteor showers are not counts that anyone actually made but standardised figures called zenithal hourly rates, or ZHR. A ZHR is the number of meteors a single observer would see in an hour under an ideal sky, dark enough to show stars of magnitude 6.5, with the radiant directly overhead. Real observers are never in that position, so organisations such as the International Meteor Organization ask volunteers to record their counts together with the faintest star they can see and the height of the radiant, and then correct each count upwards to the ideal.

Here is how the correction works. Suppose you count 30 Perseids in one hour while the radiant is 30 degrees above the horizon. The number of meteors scales roughly with the sine of the radiant's altitude, which for 30 degrees is 0.5, so you divide by 0.5 and get 60. If your faintest visible star is magnitude 5.5 rather than 6.5, you also multiply by a factor for the faint meteors you missed, about 2.2 for the Perseids, giving roughly 130. Run the sum backwards and a ZHR of 100 shrinks to a couple of dozen meteors an hour in a suburban garden.

Meteor showers are made of tiny grains that burn up completely, but larger rocks occasionally survive to reach the ground as meteorites. On 28 February 2021 a bright fireball crossed the sky over England, recorded by doorbell cameras and dedicated meteor cameras. From the footage, scientists calculated where any fragments might have landed.

The next morning a family in the town of Winchcombe in Gloucestershire found black fragments on their driveway. It turned out to be a rare type of carbonaceous meteorite, rich in carbon and water-bearing minerals, recovered so quickly that it was barely contaminated by Earth. Researchers studying it found minerals and compounds that help explain how water may have reached the early Earth.

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.