Comets are insubstantial balls of frozen gas mixed with smaller amounts of dust & rock, and most are less than a few kilometres across. They are usually named after their discoverer, much like Halley's comet, which was depicted in Bayeux Tapestry and is visible to the naked eye every 76 years.
How do comets differ to meteors?
Comets are not the same as meteors, which streak through our atmosphere in a second or two, and can be much brighter than a comet. Comets are not in our atmosphere and they move very slowly with respect to the background stars. They are made up of leftovers of material that formed the solar system about 4.6 billion years ago. Their orbits are elliptical, meaning they go very close to the Sun and then far away again, with a complete orbit taking a few million years.
When it is far from the Sun, a comet shines by reflecting the sunlight, but as it approaches the Sun, it warms up and the ice surrounding the comet begins to sublimate (transforms from ice to gas). The Sun causes the gases of the comet to fluoresce, which increases the brightness of the comet.
The main body of a comet is called the nucleus, which can contain water, methane, nitrogen and ice, and is the only solid part. Not all comets have tails, but many do. Comet tails always point away from the Sun, with the dust tail looking yellow because of the reflection of sunlight. They fan out behind the comet, contrasting with the straight gas tail. Some comets appear to have a tail pointing towards the Sun, however this anti-tail is due to the angle of viewing of the dust tail.
Comets are classified based on the durations of their orbits around the Sun.
Short-period comets
• need more than 200 years to complete one orbit
• originate from Kuiper Belt: disk-shaped band of icy objects beyond Neptune's orbit
Long-period comets
• take more than 200 years to complete one orbit
• are thought to come from the Oort cloud - a cloud of icy planetesimals believed to surround the Sun at a distance of up to a 100 000 AU (astronomical unit - distance between the Sun and Earth)
Single apparition comets
• are not bound to the Sun but are on orbits that take them out of the solar system
• some comets smash into the Sun or get so close then break up & evaporate and these are called Sun-grazers.
Over 5000 known comets. Roughly one per year is visible to the naked eye.
Comments