Comet Swift–Tuttle
The 26-kilometre comet whose dust gives the Perseids every August, and the largest object that regularly passes close to Earth.
Key facts
- Designation
- 109P/Swift–Tuttle
- Type
- Halley-type comet
- Orbital period
- about 133 years
- Closest to the Sun
- 0.96 AU (about 143 million km), just inside Earth's orbit
- Farthest from the Sun
- about 51 AU, beyond Pluto's average distance
- Tilt of orbit
- 113° (retrograde)
- Nucleus
- about 26 km across
- Discovered
- July 1862, by Lewis Swift (16 July) and Horace Tuttle (19 July) in the United States
- Returns
- last at perihelion December 1992; next July 2126
Orbit
Swift–Tuttle takes about 133 years to go round the Sun. Its orbit dips just inside Earth's at one end and reaches far beyond Neptune at the other. Like Halley's Comet it moves backwards relative to the planets, so its dust meets Earth at about 59 km/s. Its path comes remarkably close to Earth's own orbit, which is why its shower is so rich.
Size and nature
At about 26 km across, the nucleus is more than twice the size of the asteroid thought to have ended the age of the dinosaurs. That makes it the largest known body that repeatedly sweeps close to Earth's orbit. Calculations show no risk of collision for thousands of years; its closest future pass, in the year 4479, is expected to be at a comfortable distance of a few million kilometres.
History
Lewis Swift and Horace Tuttle found it independently within three days of each other in July 1862. In 1866 the Italian astronomer Giovanni Schiaparelli showed that the Perseid meteors travel along the same orbit. It was the first time a meteor shower had been tied to a particular comet, and it proved that shooting stars are comet dust. Astronomers expected the comet back around 1981; it arrived 11 years late, in 1992, and was spotted again on 26 September of that year. Older Chinese records of comets in 69 BC and AD 188 are thought to be earlier passes.
Meteor showers
Earth crosses the comet's dust every August, producing the Perseids, the best-loved shower of the northern summer. The Perseids are rich in bright meteors and fireballs, because the comet has laid down a dense stream over thousands of years. Rates rose in the early 1990s, when fresh dust from the comet's 1992 return was close to Earth.
Its meteor showers
- The Perseids peak around 12–13 August; about 100 an hour at best; 59 km/s
Key dates
| Date | What happened |
|---|---|
| 16 July 1862 | Lewis Swift discovered the comet now called Swift–Tuttle. |
| 19 July 1862 | Horace Tuttle independently found Comet Swift–Tuttle. |
| 26 September 1992 | Comet Swift–Tuttle was recovered on its first return since 1862. |
Did you know?
Perseid meteoroids are mostly smaller than grains of sand, yet they hit the air with more energy per gram than TNT.
The Perseids have been seen for about 2,000 years; Chinese records mention them in AD 36.
The comet is next due at perihelion in July 2126.