Comets

What a comet is

A comet’s nucleus is a lump of ice, rock and dust, usually a few kilometers across. For most of its orbit it is frozen and invisible. As it approaches the Sun, sunlight warms the ice, which turns straight to gas and carries dust off the surface. That forms a fuzzy cloud around the nucleus, the coma, and often two tails: a curved, yellowish dust tail and a straight, faintly blue gas tail. Both point away from the Sun, so a comet moving outward travels tail first.

Short-period comets, which return within about 200 years, mostly come from the Kuiper Belt beyond Neptune. Long-period comets fall in from the far more distant Oort Cloud and may not have visited the inner Solar System for thousands of years, if ever.

Seeing one yourself

Bright comets are rare and notoriously hard to forecast. A comet’s brightness depends on how much gas and dust it releases, and that can change suddenly; some fade or break apart, others brighten unexpectedly. A listed magnitude is also misleading, because the light is spread over the coma: a comet of magnitude 6 looks far fainter than a star of magnitude 6.

Binoculars are the best instrument. Comets are often brightest near the Sun, which puts them low in the west after sunset or low in the east before dawn, so a clear, low horizon matters. Get away from city lights, let your eyes adapt, and look for a small, soft glow that stays fuzzy when you focus.

In the app, SkyMate’s Comets & Asteroids data pack covers about 15,000 orbits from the IAU Minor Planet Center and computes where each one is for your place and time. On the web we don’t list individual comets yet, because a page with a stale position or brightness would mislead you.

Comets you can meet every year

When Earth passes through the trail of dust a comet has shed, the grains burn up in the atmosphere as meteors. These showers in SkyMate’s catalog come from comets: