26/08/2026
Our next feature is ‘C’ for ‘Comet’ ✨
Comet, shooting star, meteorite… What’s the difference? 🤔
A shooting star, or meteor, typically appears and fades in less than a second. It's the streak of heated atmosphere created by a small fragment of space rock (a meteoroid) ploughing into Earth’s atmosphere at high speed. A meteorite is a fragment of a large meteoroid or asteroid that has not burned up completely, and lands on Earth. Random meteors and meteorites may come from the population of meteoroids and asteroids orbiting the Sun, from asteroid impacts with planets and other asteroids, or from comets. Speaking of which...
A comet is something quite different. It’s an icy body rather than rocky or metallic, perhaps smaller than a kilometre across, though sizes of a few kilometres are common. When far from the Sun, beyond the outer planets, they are typically invisible, being small and dark (comet Halley is currently just beyond the orbit of Neptune, and you would need a long exposure with a very large telescope, to show even just a small fuzzy dot). As comets approach the Sun, the ice can melt*, releasing trapped dust and gas to form the characteristic tail**.
There are many comets in the inner Solar System each year, but very few reach naked-eye brightness. There have been no great comets seen in our skies since the 19th century, with obvious tails stretching across the sky. Comets Hale-Bopp and McNaught, in 1997 and 2007, were among the best we’ve seen in the last few decades.
Some comets have been seen in the inner Solar System more than once and are known as periodic comets. Their reappearances can be predicted with reasonable accuracy, but some comets appear for the first time, not to return for thousands or millions of years. Comet Halley’s next return will be in 2061, when it should be seen in our evening sky in July and August.
It is thought that perhaps more than a trillion comets are to be found in the Oort Cloud—the outer region of the Solar System, extending more than a quarter of the distance to the nearest star.
From July 2025, astronomers have been observing an interstellar comet, 3I/ATLAS, which passed through the inner Solar System a few months later. This is the third interstellar object that we have found and may indicate comets are regularly “shared” between nearby stars. Based on analysis of its chemistry, astronomers believe it could be more than twice as old as our Solar System.
* Strictly speaking, the ice “sublimes” or “sublimates”, passing straight from a solid to a gas.
** Comets can have both dust and gas tails, sometimes appearing separate, depending upon their motion relative to Earth and the Sun.
📸: Rob McNaught
