The sky — astronomy

Comets: Icy Travelers

A comet is a small body of ice, dust and rock that orbits the Sun. As it nears the Sun, its ice turns to gas, forming a glowing coma around the nucleus, and sunlight and the solar wind push material into a tail that always points away from the Sun.

What is a comet?

A comet is a small body made of frozen gases, dust and rock. People often call it a “dirty snowball”. The phrase is a simplification, but it captures the idea: the nucleus is a mixture of ice and dark dust. Far from the Sun, a comet is cold, dark and nearly invisible. As it approaches, it wakes up and can become one of the most beautiful sights in the sky.

Nucleus, coma and tail

A comet has three main parts.

Nucleus. This is the solid core. It is usually somewhere between hundreds of meters and a few tens of kilometers across, tiny compared with planets. Its surface is very dark and reflects only a small share of the light that reaches it.

Coma. When a comet nears the Sun, heat turns some of its ice straight into gas. This process is called sublimation (a change from solid to gas without passing through a liquid). The gas carries dust with it, forming a huge, fuzzy cloud around the nucleus. The coma can be far larger than the nucleus itself, sometimes larger than a planet.

Tail. A comet actually has two tails, and they often look different.

• The dust tail is broad, gently curved and yellowish or white, because dust reflects sunlight. Light pressure pushes the particles away, and they tend to trail along the comet’s orbit.
• The ion (gas) tail is thin, straight and bluish. It forms when solar radiation ionizes the gas (strips electrons from it) and the solar wind, a stream of charged particles from the Sun, sweeps it away.

An important fact: a tail does not show the direction in which the comet is moving. It always points away from the Sun. So after rounding the Sun, a comet travels with its tail ahead of it. Tails can stretch for millions of kilometers, yet they contain extremely thin material.

Where comets come from

According to current models, comets are leftovers from the early Solar System, formed about 4.6 billion years ago. They preserve material from that era, which is why scientists study them so eagerly.

Two reservoirs are usually distinguished:

• The Kuiper Belt is a region of icy bodies beyond the orbit of Neptune, where Pluto also lives. Many short-period comets, which orbit the Sun in under 200 years, come from there. The gravity of the giant planets, especially Jupiter, can nudge them toward the inner Solar System, so the story of many comets is tied to them.
• The Oort Cloud is a hypothetical, very distant spherical shell of icy bodies thought to surround the Solar System. Long-period comets, whose orbits take anywhere from hundreds to thousands or even millions of years, are believed to come from it.

The best-known comet is Halley’s Comet. It returns about every 76 years. It last passed its closest point to the Sun (perihelion) in 1986, and its next return is expected in 2061. It is an example of a periodic comet whose comeback can be predicted.

Comets are usually named after their discoverers or after the survey project that found them, so the name keeps a record of how each one was spotted.

A science highlight: the Rosetta mission

The European Space Agency (ESA) sent the Rosetta mission to comet 67P/Churyumov–Gerasimenko. The spacecraft arrived in 2014 and orbited the nucleus for more than two years, while its lander Philae made the first soft landing on a comet. The data revealed a complex shape, often described as two lobes joined together, and a surprisingly dark and varied surface. Measurements of water in the coma also showed a different hydrogen isotope ratio from that of Earth’s oceans, an important contribution to the debate about where Earth’s water came from.

Comets and meteor showers

As a comet travels, it leaves a trail of tiny grains and pebbles behind. If Earth’s orbit crosses such a trail, the particles hit our atmosphere at high speed and burn up as meteors. This is how meteor showers arise.

Examples:

• The Perseids in August come from dust left by comet Swift–Tuttle.
• The Orionids in October and the Eta Aquariids in May are linked to dust from Halley’s Comet.

There are exceptions. The Geminids are linked to the object Phaethon, which is classified as an asteroid rather than a classic comet. The line between asteroids and comets is not always sharp.

How to observe a comet

Most comets are faint and visible only through binoculars or a telescope. Comets bright enough for the naked eye are rare and often hard to predict, because a comet’s brightness depends on its makeup and on how it behaves near the Sun. Brightness forecasts are therefore approximate.

Some useful tips:

1. Look up current information on the comet from observatories and astronomy sites, and check where and when it will be in the sky.
2. Get away from city lights. Light pollution greatly reduces the visibility of fuzzy objects.
3. Allow about 20 minutes for your eyes to adapt to the dark.
4. Use binoculars if the comet is faint. See how to choose between binoculars and a telescope.
5. Comets usually drift slowly against the stars, so compare their position from night to night. Depending on its orbit, a comet may move near the ecliptic or far from it.

For general skywatching advice, see how to watch the sky. Never look at the Sun through binoculars or a telescope without a proper filter, even when a comet is close to it.

In short

A comet is an icy body whose nucleus forms a coma and tails when it nears the Sun. Comets are relics of the earliest history of the Solar System, and the dust they leave behind appears to us as shooting stars. Each one is a small time capsule.

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