Mother of the Perseids
Comet Swift–Tuttle
Every year, in mid-August, Earth crosses a trail of dust and the sky fills with shooting stars: the Perseids. They are left by comet Swift–Tuttle, which returns to the Sun every 134 years. With a nucleus 26 km across, it is the largest object that repeatedly passes close to Earth.
Perihelion passage of 12 December 1992, from the 3D simulation
- Nucleus
- 26 km
- Orbital period
- 134 years
- Perihelion
- 0.959 AU
- Aphelion
- 51.3 AU
- Last perihelion
- 12 December 1992
- Next perihelion
- 12 July 2126
Nucleus and tail
The nucleus of Swift–Tuttle is about 26 km across: more than twice the size of the asteroid that contributed to the extinction of the dinosaurs 66 million years ago. It has never been seen up close, and in the model on this page it is a sphere of that diameter. It rotates slowly, in about 2.8 days.
At each pass near the Sun the comet sheds gas and dust, which spread out along its whole orbit. It is these grains, the size of a grain of sand, that become Perseids: they enter the atmosphere at 59 km per second, among the highest speeds of any shower, and burn up about a hundred kilometres above the ground.
Orbit
Swift–Tuttle takes 134 years to travel a very elongated ellipse: at perihelion it passes 0.96 AU from the Sun, just inside Earth's orbit, and at aphelion it reaches 51 AU, beyond Pluto. Its orbit is tilted 113°: the comet travels in the opposite direction to the planets, which is why its dust hits Earth at such high speed.
Its next perihelion is expected on 12 July 2126; a few weeks later, on 5 August, the comet will pass about 23 million km from Earth and will be easily visible to the naked eye. In the 1990s a provisional calculation raised fears of an impact on that very occasion, but ancient apparitions, found in Chinese chronicles of 69 BC and AD 188, made it possible to reconstruct the orbit precisely: there is no risk for at least two thousand years.
Discovery and exploration
It was discovered, three days apart, by two American astronomers: Lewis Swift on 16 July 1862 and Horace Tuttle on the 19th. Four years later the Italian Giovanni Schiaparelli, in Milan, showed that the comet's orbit coincided with that of the Perseids: the first proof that meteor showers are born from comets.
Its return was expected around 1981, but the comet failed to appear: the calculations were off by eleven years. It was recovered only on 26 September 1992 by the Japanese amateur astronomer Tsuruhiko Kiuchi, using large binoculars. No spacecraft has ever visited it.
Frequently asked questions
How is Swift–Tuttle linked to the Perseids?
The Perseids are dust left by the comet along its orbit. Every year, between late July and the third week of August, Earth crosses that trail and the grains burn up in the atmosphere; the peak falls around 12 August. The link was demonstrated by Giovanni Schiaparelli in 1866.
Is Swift–Tuttle dangerous?
Not for at least two thousand years. Its orbit passes close to Earth's, and given its size and speed an impact would be catastrophic, but its passes are known precisely and none of them, over that span, poses a risk. In 2126 it will stay 23 million km away.
When will Swift–Tuttle return?
Its next perihelion is expected on 12 July 2126. It will be a favourable return: in early August the comet will be just 23 million km from Earth and easy to see with the naked eye. The last pass, in December 1992, was by contrast unremarkable.
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