Comets and Meteor Showers

A comet is a block of ice, dust, and rock with a nucleus a few kilometers across that orbits the Sun in a very elongated path. Far from the Sun it is dark and frozen. As it approaches, heat turns part of the ice into gas, which carries dust away and forms the coma, the cloud around the nucleus, and the tail.

Many comets have two tails: a dust tail, broader and curved, and a gas tail, thinner and bluish. Both always point away from the Sun, pushed by sunlight and the solar wind, and can stretch for millions of kilometers.

Halley is the most famous: it returns every 76 years on average and is the only short-period comet that typically becomes bright enough to see with the naked eye. It passed in 1986 and is expected to reach perihelion on July 28, 2061, in a position more favorable for viewing from Earth than last time.

Encke has the shortest return among well-known comets: it approaches the Sun every 3.3 years and, on the most favorable passages, can reach binocular visibility under a dark sky.

Other long-return comets mark generations: Swift-Tuttle, which passed in 1992, returns in 2126, and Tempel-Tuttle returns every 33 years. Even when distant, they leave dust that forms meteor showers.

As a comet passes, it leaves a trail of dust along its path. When Earth crosses that trail, the grains enter the atmosphere at tens of kilometers per second and burn up, streaking the sky: those are shooting stars. Because Earth crosses the same trail at the same time each year, the showers recur on the same dates annually.

Halley, for example, produces two showers: the Eta Aquariids in May and the Orionids in October. The Perseids in August come from Swift-Tuttle, and the Leonids in November from Tempel-Tuttle. The Geminids in December are among the strongest of the year and originate from an asteroid, Phaethon.

The meteors in a shower appear to radiate from a single point in the sky, the radiant, which gives the shower its name: the Perseids seem to come from the constellation Perseus. They usually appear more often after midnight, when the radiant is high and there is no bright Moon.

The key is a dark sky: away from city lights, with no bright Moon and clear weather. Dry climates, like the Atacama Desert and the Elqui Valley in Chile, have many clear nights per year; in Chapada dos Veadeiros in Brazil, the sky tends to be clearer during the dry season, from May to September.

You don’t need binoculars or a telescope to see meteors: lie back and look at the sky, let your eyes adapt to the dark for about 20 minutes, and be patient. For faint comets, binoculars help a lot, and at observatories open to visitors guides usually show where the comet is.

None of this is guaranteed: clouds, a full Moon and city lights greatly reduce what you see, and each meteor shower's rate is for a perfect sky. A comet's brightness is also only confirmed when it gets close to the Sun.