Credit: TIM PUCKETT

Every 33 years, or thereabouts, the Leonid meteor shower is spectacular — so many meteors fall that it is called a storm. The last time was in 1966, when up to 40 meteors a second were seen; and back in 1833, watchers even heard sounds. That could be telling us about processes occurring on comet 55P/Tempel-Tuttle (shown here), the creator of the Leonids.

As comets are heated by sunlight, their ices sublime. The breeze of this sublimation lifts debris off the comet nucleus, and some of these pieces eventually get swept up by the Earth. We see them as meteors — usually just a streak of light, created by a body a few millimetres in diameter. Larger bodies produce brighter fireballs.

Sounds are much rarer, but in 1833 many observers heard hissing, crackling and popping noises. These were probably ‘electrophonic sounds’, created by very-low-frequency radio waves, in turn generated by turbulent plasma in the wake of a disintegrating meteoroid. According to Martin Beech (Astron. J. 116, 499-502; 1998), the falling bodies must have been well over a metre across to produce such sounds.

That is much too big to have been lifted off the nucleus of Tempel-Tuttle by sublimation pressure. So how did these great lumps of dirt escape the comet? Perhaps there are cavities in the nucleus that are only occasionally lit by the Sun, producing violent outbursts that can shoot out large bodies.

We may be in for another big show from the Leonids this year, around 17 November. Then, it may even be possible for astronomers to see these large meteoroids directly, or to see them hitting the Moon. And if you're watching, listen out.