What Is a Comet Made Of?

The solid part of a comet is the nucleus, typically a few kilometres across. It contains water ice, frozen carbon dioxide, carbon monoxide, ammonia and methane, mixed with dust and rocky material.

The European Rosetta mission, which orbited comet 67P from 2014, refined the picture considerably. The nucleus turned out to be extremely dark, more porous than expected, and much less dense than solid ice, so a loosely packed rubble pile is closer than a snowball.

Far from the Sun a comet is inert. As it approaches, solar heating turns the ices directly from solid to gas, a process called sublimation, releasing dust trapped inside.

This creates a large fuzzy cloud called the coma around the nucleus, which can be far bigger than the solid body. Radiation pressure and the solar wind then push material away from the Sun, forming the tail.

Comets usually have two distinct tails pointing in slightly different directions:

• •A dust tail, yellowish white, curved, made of grains pushed gently by sunlight
• •An ion tail, bluish and straight, made of charged gas swept by the solar wind

Short period comets, which return within about two hundred years, come mostly from the Kuiper Belt and scattered disc, though Halley-type comets probably started further out beyond Neptune. Halley's Comet, returning roughly every seventy six years, is the famous example.

Long period comets come from the Oort Cloud, a vast spherical shell of icy bodies thought to extend a substantial fraction of the way to the nearest star. These can take thousands or millions of years to complete one orbit.

Comets are leftovers from the formation of the solar system, largely unaltered since. Studying them is close to sampling the original material that planets were built from.

They may also have delivered water and organic molecules to the early Earth, though the isotope ratios measured at comet 67P did not match Earth's oceans well, which pushed attention towards asteroids as a more likely source. The debate is unresolved and genuinely interesting.

Comets leave debris along their orbits, and when Earth passes through one of these trails the particles burn up in the atmosphere, producing a meteor shower.

This is why showers happen on predictable dates each year. The Perseids in August come from comet Swift-Tuttle, and the Leonids in November from comet Tempel-Tuttle. The particles are mostly the size of a grain of sand, and the streaks they produce are the result of them entering the atmosphere at tens of kilometres per second rather than of any great size.

Most of what scientists know about comets comes from telescopes and passing spacecraft, but one mission actually brought comet material home. NASA's Stardust spacecraft flew through the coma of comet Wild 2 in 2004 and caught tiny dust grains in a very light, porous material called aerogel. The grains slowed gradually as they burrowed in, leaving carrot-shaped tracks rather than being destroyed by the impact. A capsule carrying the samples parachuted into the Utah desert in 2006, and laboratories around the world were able to study the particles directly.

The results were surprising. Alongside the expected material from cold regions, the dust contained minerals such as olivine that can only form at very high temperatures, close to the young Sun. Somehow these grains had travelled out to the cold outer solar system and been frozen into a comet. This suggested that material in the early solar system was mixed far more widely than many models had assumed. It is a good example of how a direct sample can overturn a tidy picture that had seemed reasonable from a distance.

For most of history, comets appeared without warning and were often read as omens. The comet that appeared in 1066, before the Norman conquest of England, is shown in the Bayeux Tapestry with figures pointing up at it. Later, astronomers showed that comets follow the same laws of gravity as planets. Edmond Halley used Isaac Newton's work to argue that comets seen in 1531, 1607 and 1682 were one object on a long orbit, and he predicted it would return around 1758.

Halley died before that date, but the comet was spotted on Christmas night in 1758, and it now bears his name. Its return was a powerful confirmation of Newton's laws and showed that at least some comets are regular members of the solar system. Comets can also collide with planets. In 1994, fragments of comet Shoemaker-Levy 9, which had been pulled apart by Jupiter's gravity, crashed into the giant planet one after another, leaving dark scars in its atmosphere that were visible from Earth through telescopes.

The first close look at a comet's nucleus came in March 1986, when the European spacecraft Giotto flew within about 600 kilometres of Halley's Comet. Its images showed an elongated, potato-shaped body roughly 15 kilometres long, darker than coal, with bright jets of gas and dust bursting from a few active patches on the side facing the Sun rather than from the whole surface. Giotto was battered by dust, and its camera stopped working shortly before closest approach. The jets matter, because they act like small rocket engines, nudging a comet's orbit so that its return can differ slightly from predictions.

In 2005 NASA tried a more direct experiment. Its Deep Impact mission released a copper-rich impactor of about 370 kilograms into the path of comet Tempel 1, and the two met at more than 10 kilometres per second on 4 July. The collision threw out a huge, bright plume of extremely fine dust, showing that the surface was covered in powdery material and that the comet was weak and porous inside. The plume also contained water ice and carbon-based compounds. Years later another spacecraft flew past Tempel 1 and photographed the scar left behind, a crater roughly 150 metres across.

Rosetta carried a small lander called Philae, and on 12 November 2014 it became the first spacecraft to land on a comet's nucleus. The landing did not go to plan. Its harpoons, meant to anchor it in the weak gravity, failed to fire, and Philae bounced twice, coming to rest about a kilometre from its target in the shadow of a cliff. Running on its main battery, it still worked for around sixty hours, sending back data and images before falling silent. Rosetta photographed it wedged in a crack in 2016, shortly before the mission ended.

Rosetta itself spent two years alongside comet 67P, and the results reshaped ideas about comets. The nucleus has two lobes joined by a narrow neck, giving it the shape of a rubber duck, which most researchers think formed when two smaller bodies came together very gently. Instruments sniffing the gas around the comet detected glycine, the simplest amino acid, along with phosphorus, both ingredients relevant to life. They also found surprisingly abundant oxygen molecules, which suggests the comet has preserved very old ices. In September 2016 the mission ended with Rosetta itself making a slow, controlled descent onto the comet's surface.

A comet is a porous mix of ice, dust and rock that only becomes visible when solar heat turns its ices to gas, producing a coma and two separate tails, and it preserves material largely unchanged since the solar system formed.