Like the planets, asteroids orbit the Sun with some regularity and stability. They range in size from a few metres up to Ceres, the largest of the asteroids at just under 1000km. Their make up is very similar to that of the rocky inner planets of the Solar System rather than the gaseous giants and, on occasions, the larger ones can be seen in the night sky.
Comets on the other hand are a little more unpredictable. Orbiting the Sun in the cold depths of the Solar System, their composition is much more like that of a dirty snowball with rock and ice the main ingredient. Most cometary nuclei will lay dormant for millions of years but occasional collisions within the orbiting clouds will send one comet toward the inner solar system. They warm up as they fall closer to the Sun and as their temperature rises, the ice melts. Because of the conditions in space, the ice doesn’t turn to liquid, instead it turns straight to a gas in a process called sublimation. The ice turns into a vast coma measuring millions of km surrounding the tiny solid nucleus.
Pressure from the Sun, called the solar wind, blows against the coma producing the trademark tail of the comet. Its interesting to note that the tail of the comet always points away from the Sun, it has nothing to do with the comet’s direction of motion. With particularly bright and spectacular comets, its possible to see not just one, but two tails. Because comet’s are made up of rock and ice, they will often have a dust tail (from the rock getting dislodged from the nucleus) and a gas tail from the sublimated ice. Comet Hale Bopp which was seen in 1997 is a fine example of this.
Unfortunately comet’s are notoriously unpredictable and often new ones are seen just before they grace are night skies. Whilst there are a number of very well known comets that return after a number of years there are also big and bright ones that can suddenly appear and take us all by surprise.
As comet’s orbit the Sun, they shed material and when the Earth moves through this path of cometary debris we experience a meteor shower.
Mark’s Observing Tip
Asteroids are small and often dark so telescopes or binoculars are typically needed to spot them. Comets on the other hand, can become quite bright and even be visible to the unaided eye. For comet’s the naked eye or binoculars are often the best way to observe them.