Observing the Sky: Other Celestial Bodies - Comets

Comets are balls of dust and ice that orbit the Sun in long, elliptical orbits. When far from the Sun, a comet looks like a dirty snowball. Closer to the Sun, the comet is warmed and its frozen water and carbon dioxide turn from solid ice to gas in a process called sublimation. A tail of ionized gas flows from the comet, blown away by the solar wind, a stream of particles that comes from the Sun. The tail doesn't flow behind the comet the way long hair would if you were running, instead the solar wind keeps the tail always pointing away from the Sun. Pressure from the photons of light from the Sun cause the comet's dust tail to flow in a slightly different direction. With each pass by the Sun, a comet loses more material and eventually, it will fade away completely.

Comets with orbits of 200 years or less are called short-period comets. They come from a region beyond the orbits of Pluto and Neptune, called the Kuiper (rhymes with "wiper") Belt. Kuiper Belt Objects orbit the Sun at a distance of about 30 to 50 astronomical units (that's 30 to 50 times the Earth's distance from the Sun!), and they range in size from .062 miles (0.1 km) to 744 miles (1,200 km). Long-period comets can take from 200 to over 100,000 years to orbit the Sun. Astronomers believe they come from the Oort Cloud, a region of space 50 to 100 astronomical units (AU) from the Sun.

Comet Halley is probably the most famous comet, and it is the only one that has an orbit small enough to bring it near the Sun (and into Earth's view) more than once in a person's lifetime—every 76 years. The comet's official name is P/Halley. The 'P' is added to the names of all comets with periodic orbits that keep them circling the Sun. Comets with hyperbolic orbits pass the Sun only once.

Learn how the Navy helped make some new discoveries about comets.