From time to time we see comets in the sky. Like asteroids, they are large, dusty, icy rocks several hundred meters long and a few kilometers in diameter. There are billions of them. They formed at the same time as the solar system but they stayed away from the Sun.
There are two large reservoirs of comets: the Kuiper Belt beyond Neptune and the huge cloud of Oort that surrounds the entire solar system and is far behind Pluto.
Most of the “pebbles” making up these clouds are so small and located far from the Earth that they are invisible. In time some of these small icy bodies collide or are deviated from their path. When this happens, some of these rocks may come close to the Sun. When the ice that composes them begins to melt under the heat of the Sun, we observe behind them a long bright tail: the “pebbles” becomes visible, a comet is born!
Comets always have two tails: a white or slightly yellowish one consisting of dust which is often thick and slightly curved; a so-called ionic blue tail which is always straight, consisting of gases and particles (plasma).
The tail of a comet can measure several hundred million kilometers; the record is 2 times the distance from Earth to the Sun, 300 million km! This tail is always directed in the opposite direction to the Sun. It can be narrow, multiple, or fan-shaped depending on its position relative to the Earth. Once the comet has passed the Earth and the Sun, it returns where it came from, often beyond Pluto. It starts to freeze again, it no longer emits gas and dust, it loses its shiny tail and often becomes invisible again.
Since comets revolve around the Sun, some come back periodically, like Encke’s Comet who visits us every 3.5 years. The famous comet Halley visits us every 75 years for more than 4000 years. She will return in 2061.
How do comets die? Yes, the effect comets die too … In general, by dint of losing material, they eventually fall apart and completely melt in a few thousand years. Sometimes comets run straight on the Sun and totally melt in its burning atmosphere. More rarely, some can hit a planet.
Thus, in July 1994, Comet Shoemaker-Levy 9 passed too close to Jupiter. The giant planet captured it and the comet crashed into it. For a week the debris of the comet penetrated the clouds of Jupiter’s atmosphere and created huge explosions larger than the Earth, forming a cloud of 3300 km and the temperature was approaching 17000 degrees! The explosion of the main debris was so violent that it was visible from Earth.
Fortunately nowadays such a collision is very very rare. But it is likely that it is a similar collision that killed all living dinosaurs on Earth 66 million years ago.