# What comet causes the Perseids?

109P/Swift-Tuttle, a comet with a 26 km nucleus on a 133-year orbit, now about 43 au away. The August meteors are its debris. See it in a 3D model.

## The answer

The Perseid meteor shower is debris from comet 109P/Swift-Tuttle. Every August, peaking mid-month, Earth crosses the stream of particles the comet has shed along its orbit, and each streak in the sky is one of those grains burning up in the atmosphere. NASA puts the nucleus at 26 kilometres across, which makes it one of the largest objects that regularly crosses Earth's orbit.

## The comet is nowhere near us

This is the part that surprises people: the comet responsible for August's meteors is currently about 43 astronomical units from the Sun, further out than Pluto usually sits. It last passed the Sun in 1992 and takes 133 years to go round, so it will not return until 2125. The shower happens every year anyway because the debris stream stays behind, spread along the whole orbit, and Earth crosses that stream on schedule whether the comet is near or not.

## Seeing the orbit makes it obvious

A sentence cannot really show how a comet 6.4 billion kilometres away delivers meteors to your back garden, but an orbit can. The model draws Swift-Tuttle's full ellipse through the solar system from JPL's recorded elements, and where that ellipse threads past Earth's orbit is exactly where the Perseids come from. The distance on this page was computed for the date given; the model recomputes it for any date you set.

See it in the model: https://astro.observer/orrery/?body=109p-swift-tuttle

Sources:
- NASA, 109P/Swift-Tuttle: https://science.nasa.gov/solar-system/comets/109p-swift-tuttle/
- NASA, the Perseids: https://science.nasa.gov/solar-system/meteors-meteorites/perseids/
- JPL small-body database, the orbit this model propagates: https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html

Gallery: https://astro.observer/