Comets are icy visitors of our solar system, appearing as bright, fuzzy streaks with long, glowing tails. These small, ancient objects are made of dust and ice and follow long paths around the sun. They are leftovers from the formation of the solar system nearly 4.6 billion years ago. For most of their lives, comets are quiet, frozen lumps, but when they near the sun, they transform into spectacular sights in the night sky.
The Cosmic Snowball: What Comets Are Made Of
The solid heart of a comet is called the nucleus. This nucleus is a mixture of rock, dust, and frozen gases like water, carbon dioxide, and methane. Because of this combination, scientists often call comets “dirty snowballs.” The nucleus is quite small, often measuring less than 10 miles across, about the size of a small town.
When this icy core travels closer to the sun, the heat causes the frozen materials to turn directly into gas, a process called outgassing. This released gas and dust forms a huge, fuzzy cloud around the nucleus called the coma. This glowing atmosphere can swell to be enormous, sometimes becoming up to 15 times wider than the Earth. The coma signals that the comet is becoming active, transforming it into a bright celestial object.
The Amazing Comet Tail
The most distinctive feature of a comet is its long, streaming tail, which only appears when the comet gets close enough to the sun. As the sun’s heat causes the ice to vaporize and release dust, radiation pressure and the solar wind push this material away. The solar wind is a constant stream of charged particles flowing outward from the sun.
The force of the solar wind and sunlight causes the tail to always point away from the sun, regardless of the comet’s direction of travel. This means the tail can sometimes appear to lead the comet as it moves away from the sun. Comets often develop two separate tails, each with a distinct appearance.
The dust tail is made of tiny, solid particles and tends to be white or yellowish. This tail usually follows a broad, gently curved path behind the comet’s orbit. The second tail is the ion or gas tail, made of electrically charged gas molecules that appear narrow and bluish. Because the solar wind affects this charged gas more strongly, the ion tail points almost perfectly straight away from the sun.
Where Comets Live and How They Travel
Comets spend most of their existence in the coldest parts of the solar system, far beyond the planets. They come from two main areas. The first is the Kuiper Belt, a doughnut-shaped region just past the orbit of Neptune. Comets from this region are called short-period comets because they take less than 200 years to complete one orbit around the sun.
The second, much more distant home is the Oort Cloud, a spherical shell surrounding the entire solar system. Comets from the Oort Cloud are long-period comets, and their orbits can take thousands of years to complete.
The gravity of planets or passing stars can sometimes nudge these frozen objects, sending them on a long, oval-shaped path toward the sun. As the comet travels in this highly elliptical orbit, it heats up and puts on its display when it nears the inner solar system. After swinging around the sun, the comet speeds back out into the cold darkness, returning again on its scheduled journey.