ASTROGEOLOGY 6 (ADVANCED LEVEL)

(i)                 Comets

            A comet is an icy ball of rock that grows a tail when it travels close to the Sun. It grows a tail because as comets heat up, gas and dust are forced from the body and trail behind it. The Sun illuminates this trail, which causes it to glow. The glow is visible in the night sky, and what we on Earth see is a streak.

The way a comet orbits - and the length of its orbit - varies greatly from one comet to another. They can have a short period and orbit every few years, or a very long period and orbit every 100,000 years.

-          Most of the shorter-period comets come from the Kuiper belt, a massive region of small objects (including Pluto) out beyond Neptune.

-          The longer-orbit comets come from the Oort cloud, a massive cloud of icy planetesimals very far out in the outer part of the solar system.

Every year, Earth sees a few comets. Some of these are comets we expect because they regularly orbit the sun. The others are new comets that have never been seen before.

The best-known comet is Halley's Comet. It last appeared in 1986 and will appear again in 2061. Halley's Comet has been seen every 76 years since 240 BC.

(web source: www.study.com ) 

 

Figure 4: A comet