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The Perseid Meteor Shower

The streak of light is not a rock burning. It is usually a grain of comet dust smaller than a pea, and what you see is air.

In plain English

Every August, Earth drives through a river of dust left behind by a comet, and the dust hits the top of our atmosphere at fifty-nine kilometres per second. That is London to Paris in under six seconds.

Almost none of it is big. A typical Perseid is a grain somewhere between a grain of sand and a pea. It never reaches the ground, never lands in a field, never becomes a rock you could pick up. It arrives, it glows for a second, and it is gone.

And the glow is the part everyone has slightly wrong. The light is not the particle burning up from friction. At that speed the grain compresses the air in front of it faster than the air can get out of the way, and the shock wave heats that air to thousands of degrees. The grain vaporises into it. What reaches your eye is mostly excited air and a smear of metal atoms, glowing at a height of eighty to a hundred and twenty kilometres: the edge of space itself, far above any aircraft, above the aurora's lower reaches, above almost all of the atmosphere.

You are not watching something fall. You are watching the sky get hit.

Five things to file under "wait, what?"

  • The comet behind it is the most dangerous large object we know about. Comet 109P/Swift–Tuttle has a nucleus about 26 kilometres across, around two and a half times the size of the asteroid that ended the dinosaurs, travelling roughly four times as fast. It is the largest object in the solar system, the Moon aside, that repeatedly comes close to Earth.

  • In 1992 it was briefly put on a collision course. When Swift–Tuttle returned that year, its orbit was uncertain enough that if the following perihelion in 2126 were off by about fifteen days, it would hit Earth on 14 August 2126. The calculation was refined within months and the miss is comfortable: 22.9 million kilometres. Every orbit for the next two thousand years has now been worked out, and Earth is fine. It is still, by some distance, the most carefully watched comet in the sky.

  • Nobody has ever known what a shooting star was until 1866. That year Giovanni Schiaparelli (the same astronomer who would later describe canali on Mars) noticed the Perseid stream's orbit matched Swift–Tuttle's almost exactly. It was the first time anyone linked a meteor shower to a comet, and the first time anyone could answer the question of what, physically, a shooting star is made of.

  • The point they radiate from is an optical illusion. Perseid meteors all travel on parallel paths. They appear to fan out from a single point in the constellation Perseus for the same reason railway tracks appear to converge at the horizon: perspective. The radiant is not a place. It is the direction Earth is heading.

  • The Church got there first. In the medieval calendar the Perseids were the Tears of Saint Lawrence, who was martyred on 10 August in the year 258, two or three days before the peak. Chinese records of the shower go back further still, to around 36 AD. People have been noticing this every August for the better part of two thousand years.

The full story

Where the dust comes from

A comet is a few kilometres of ice, dust and rock on a long orbit. Each time it swings close to the Sun, the ice sublimates, turning straight from solid to gas, and the escaping gas drags dust off the surface with it. That dust does not stay with the comet. It spreads out along the comet's orbital path until, over centuries, the whole orbit is a thin, enormous tube of debris.

Swift–Tuttle takes about 133 years to go round. It last passed the Sun in 1992 and will not be back until 2126, so no one alive has seen it and no one alive will. But its orbit crosses ours, and every July and August Earth ploughs through the tube. The specific grains that make tonight's meteors were shed on earlier passes, some of them centuries ago. You are looking at the exhaust of a visitor that left long before you were born.

Why it looks like burning

The common explanation, that friction with the atmosphere heats the rock until it burns, is close enough to be memorable and wrong enough to be worth correcting.

At fifty-nine kilometres per second, a Perseid grain is moving far faster than air molecules can move out of its way. The air piles up in front of it and compresses, and compressing a gas heats it. This is ram pressure, and it is the same reason a bicycle pump gets warm. The temperatures involved run to thousands of degrees, and the grain does not so much burn as ablate: its surface vaporises directly into the shock front, atom by atom.

Those liberated atoms (iron, magnesium, sodium, calcium) then collide with air molecules at enormous energy, knocking their electrons into higher states. When those electrons drop back down, they emit light. That is your meteor: a column of excited air and vaporised metal, ionised along a track a few kilometres long, glowing for something under a second.

It is also why meteors have colours. Sodium burns yellow-orange, magnesium blue-green, and the ionised nitrogen and oxygen of the air itself gives the reds. A bright Perseid seen properly is not white.

Why the Perseids specifically

Not all showers are equal, and the Perseids have three things going for them.

They are fast. At 59 km/s they are near the upper end of what is possible: the theoretical maximum for anything gravitationally bound to the Sun and hitting us head-on is around 72 km/s. Faster means brighter, and it means more of the long, dramatic streaks rather than brief flecks.

They are rich. Swift–Tuttle is a big comet that has been shedding for a long time, so the stream is dense. Under dark skies at the peak you might see fifty to a hundred an hour. Under a suburban sky, expect a fraction of that, and be pleased with it.

And they arrive in August, in the northern hemisphere, when standing outside at two in the morning is not an act of endurance. The Geminids in December are arguably a better shower. Far fewer people have ever seen them.

The night of 12–13 August 2026

This year is the best in nearly a decade, for a reason with a pleasing symmetry to it.

The Moon is new on 12 August 2026. A new moon is not in the night sky at all, which means no moonlight washing out the faint meteors, and the faint ones are most of them. The last time the Perseid peak lined up with a moonless sky this well was 2018, and the next comparable year is around 2029.

That same new moon is the one that passed directly in front of the Sun this morning, producing the total solar eclipse whose path crossed Greenland, Iceland and northern Spain. A new moon is precisely a moon between us and the Sun; occasionally it lines up exactly and we get an eclipse. The geometry that darkened the sky at midday is the same geometry that leaves it dark tonight.

Practical notes, briefly. Best after midnight and before dawn, with roughly two to four in the morning the strongest window: that is when your side of Earth has rotated to face into the debris stream, so you are on the windscreen rather than the rear window. Do not look at Perseus; meteors appear all over the sky and looking at the radiant gives you the shortest trails. Lie back, take in as much sky as you can, and give your eyes a full twenty minutes to adapt without checking your phone. No telescope. No binoculars. A telescope is the exact wrong instrument: it shows you a tiny piece of sky, and you need all of it.

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