The Long-Haired Stars
The word comet comes from a Greek word meaning wearing long hair, and it fits. A comet is a small, icy body that warms up as it nears the Sun and starts releasing gas, a process called outgassing. That gas and dust form a fuzzy cloud around it, and sometimes a long tail.
The sizes are wild. The solid core is usually a few hundred meters to tens of kilometers across. Yet the cloud around it can grow up to 15 times wider than Earth, and the tail can stretch beyond one astronomical unit. A bright comet can stretch across 30 degrees of the sky, the width of 60 Moons.
As of November 2021, astronomers knew of 4,584 comets. That is a tiny fraction of the total, because the distant reservoir of comet-like bodies is thought to hold about one trillion. Roughly one comet a year is visible to the naked eye, and about once a decade one becomes bright enough to be called a great comet.
Dirty Snowball or Deep Fried Ice Cream?
The core of a comet, its nucleus, is a loose mix of rock, dust, water ice and frozen gases such as carbon dioxide, carbon monoxide, methane and ammonia. In 1950 Fred Whipple described comets as icy objects with some dust and rock, and the name dirty snowballs stuck. After NASA's Deep Impact probe blasted a crater in Comet Tempel 1 in 2005, some said icy dirtballs fit better. A 2014 study compared them to deep fried ice cream: a hard crust of crystal ice over colder, less dense ice inside.
Comets are also shockingly dark. Their low albedo makes them some of the least reflective objects in the Solar System. Halley's Comet reflects about four percent of the light that hits it, and Comet Borrelly less than three percent. Asphalt reflects seven.
They carry interesting chemistry too. In 2009 the amino acid glycine was confirmed in comet dust brought back by NASA's Stardust mission. In 2014 the Philae lander became the first spacecraft ever to land on a comet.
Tails That Point Away From the Sun
As a comet heats up, it grows a huge, extremely thin atmosphere called the coma. The solid core is usually less than 60 kilometers across, but the coma can be millions of kilometers wide. About a month after an outburst in October 2007, comet 17P/Holmes briefly had a thin dust cloud larger than the Sun.
A comet may have two tails, one, or none. The dust tail is made of grains about the size of human hairs, pushed by sunlight into a gentle curve. The ion tail is made of charged molecules and points straight away from the Sun. On rare occasions, such as when Earth passes through the plane of a comet's orbit, an antitail pointing the other way can be seen.
Comets even give off X-rays, which greatly surprised astronomers in 1996. Highly charged particles from the Sun fly through the coma and steal electrons from it, a process called charge exchange, and that releases X-rays. Uneven heating can also blast gas out of weak spots like a geyser, making the nucleus spin or even split apart.
Visitors From the Deep Freeze
Comets spend most of their lives far away in cold storage. Short-period comets come from the Kuiper belt and the scattered disc beyond Neptune. Long-period comets are thought to come from the Oort cloud, a huge sphere of icy bodies that reaches halfway to the nearest star. Out there it is about 10 kelvin. A passing star or the pull of the galaxy can nudge one inward toward the Sun.
The trip is often one way. About half of long-period comets are thrown out of the Solar System on their first pass, by the gravity of the planets. Comet C/1980 E1 once took about 7.1 million years to orbit the Sun, until a 1980 close pass by Jupiter flung it onto an escape path.
Some come from outside entirely. ʻOumuamua, seen in 2017, is probably a comet. 2I/Borisov, named after the astronomer who spotted it in 2019, is considered the first detected interstellar comet, and 3I/ATLAS also has a coma. Others end with a bang: in July 1994, pieces of Comet Shoemaker-Levy 9 crashed into Jupiter.
The Comet That Came Back on Time
For centuries people argued about what comets even were. Aristotle believed they happened in the air, not out in space. In the 16th century, Tycho Brahe and Michael Maestlin measured the parallax of the Great Comet of 1577 and showed it had to be at least four times farther away than the Moon.
Then came the big prediction. In 1705 Edmond Halley used Isaac Newton's method on 23 comet sightings. He noticed that the comets of 1531, 1607 and 1682 had very similar orbits and decided they were the same object. He predicted it would return in 1758 or 1759. Three French mathematicians, including Nicole-Reine Lepaute, then pinned down its closest approach to the Sun to within a month. When it came back as predicted, it became known as Halley's Comet.
Comets also leave crumbs. When Earth passes through their dusty trails, we get meteor showers, like the Perseids every August from Comet Swift-Tuttle. In 1910, Earth passed through Halley's tail, and news of the toxic gas cyanogen in it led some people to buy gas masks and anti-comet pills.
