L10. Asteroids, Comets, and Meteors

Unit 2 · The Solar System

R-report

L10. Asteroids, Comets, and Meteors

On a clear night you can sometimes see a streak of light flash across the sky in under a second. Most of those "shooting stars" are grains of dust smaller than a pea — and they are leftovers from the birth of the solar system.

Leftovers from the beginning

About 4.6 billion years ago, the solar system formed from a collapsing cloud of gas and dust. Small grains stuck together into larger clumps, and clumps collided to build the planets. Not everything got used up, and the leftovers are still out there. - Asteroids are rocky and metallic bodies, most of them in the asteroid belt between Mars and Jupiter. They range from tiny boulders to Ceres, which is nearly 1,000 kilometres across. - Comets are mixtures of ice, dust, and rock — often described as dirty snowballs. They come from the cold outer solar system, far beyond Neptune. Despite the way films show it, the asteroid belt is mostly empty. Spacecraft fly through it routinely without any danger of collision, because the average gap between objects is hundreds of thousands of kilometres. These leftovers are scientifically precious. They never melted and re-formed like planets did, so their chemistry is a nearly untouched sample of what the young solar system was made of.

Why comets grow tails

A comet spends most of its long, stretched orbit far from the Sun as a dark lump of ice a few kilometres across — the nucleus. Nothing much happens out there. As it swings inward, sunlight heats the ice, which turns straight from solid to gas. That gas and the dust carried with it form a glowing cloud around the nucleus called the coma, which can be larger than a planet. Then the comet grows tails, and there are two of them. - The dust tail is pushed gently by sunlight and curves along the comet’s path. - The ion tail is made of charged gas swept straight back by the stream of particles flowing out from the Sun. Here is the detail people find surprising: both tails point away from the Sun, not backwards along the direction of travel. On its way out of the inner solar system, a comet travels tail-first. Each pass costs a comet some material, so comets do not last forever. Halley’s Comet returns every 76 years and was last seen in 1986.

Meteors, meteorites, and impacts

Three similar words describe three different things, and keeping them straight is worth the effort. - Meteoroid: a small piece of rock or dust travelling through space. - Meteor: the streak of light it makes when it enters Earth’s atmosphere and burns up from friction, typically 75 to 100 kilometres above the ground. - Meteorite: the piece that survives and reaches the ground. Earth sweeps up tonnes of this material daily, almost all of it dust. Meteor showers happen when Earth crosses the dusty trail left by a comet, so they recur on the same dates each year — the Perseids in August, the Geminids in December. Large impacts are rare but real. About 66 million years ago an object roughly 10 kilometres across struck what is now Mexico and contributed to the extinction of the dinosaurs. Astronomers now run survey programmes to find and track near-Earth objects, and in 2022 NASA’s DART mission deliberately crashed a spacecraft into a small asteroid to test whether its orbit could be nudged. It worked.

Putting the investigation together

Asteroids and comets are the material left over from building the planets — rock and metal in the inner solar system, ice and dust in the outer. When Earth runs into some of that debris we see meteors, and occasionally a piece survives to become a meteorite.

These small bodies punch above their weight scientifically. They preserve the original chemistry of the solar system, they may have delivered water and carbon-rich molecules to the early Earth, and a large one can reshape the history of life. Studying them is part planetary history and part practical risk assessment.