SPACE AR ZONE

There’s nothing more spectacular in the night sky than a really bright comet, so when can we expect the next one?

WORDS ANDREW MAY

It’s always exciting to see a comet in the sky, and they’re few and far between, so when is the next one due? The surprising answer is that comets aren’t really that rare at all. During July 2022, no fewer than 15 comets are potentially visible in the night sky in the Northern Hemisphere, and a similar figure in any other average month. However, while many of these comets can be viewed by an amateur astronomer using binoculars or a telescope, they are rarely bright enough to be seen without optical aids. And even with them, the typical comet is far from being the spectacular sight you might expect.

When someone asks when the next comet is coming, they’re usually talking about a very bright naked-eye object. But before we can answer that question, we need to look in more detail at just what is meant by a comet in the first place. Sometimes described as ‘dirty snowballs’, comets are small objects orbiting the Sun that are composed of rock and dust mixed with frozen water, carbon dioxide, ammonia and methane. As the comet’s orbit takes it closer to the Sun, the frozen materials start to warm up and vaporise, creating the long tail which is a comet’s most distinctive feature.

The natural home of comets is the outer Solar System beyond Neptune, but some of them travel on highly elongated orbits that periodically bring them through the inner Solar System, where they may potentially be visible from Earth. For example, the most famous comet of all, Halley’s Comet, makes an appearance in our skies every 75 years or so. Halley’s most recent visit to the inner Solar System occurred in 1986 – recently enough that we were able to study it from close range with an armada of space probes.