Head of comet halley.
Most comets also develop tails as they approach the Sun. A comet’s tail is an extension of its atmosphere, consisting of the same gas and dust that make up its head. As early as the sixteenth century, observers realized that comet tails always point away from the Sun ( [link] ), not back along the comet’s orbit. Newton proposed that comet tails are formed by a repulsive force of sunlight driving particles away from the head—an idea close to our modern view.
Comet orbit and tail.
The two different components that make up the tail (the dust and gas) act somewhat differently. The brightest part of the tail is called the dust tail , to differentiate it from a fainter, straight tail made of ionized gas, called the ion tail. The ion tail is carried outward by streams of ions (charged particles) emitted by the Sun. As you can see in [link] , the smoother dust tail curves a bit, as individual dust particles spread out along the comet’s orbit, whereas the straight ion is tail pushed more directly outward from the Sun by our star’s wind of charged particles
Comet tails.
These days, comets close to the Sun can be found with spacecraft designed to observe our star. For example, in early July, 2011, astronomers at the ESA/NASA’s Solar and Heliospheric Observatory (SOHO) witnessed a comet streaking toward the Sun, one of almost 3000 such sightings. You can also watch a brief video by NASA entitled “Why Are We Seeing So Many Sungrazing Comets?”
The rosetta comet mission
In the 1990s, European scientists decided to design a much more ambitious mission that would match orbits with an incoming comet and follow it as it approached the Sun. They also proposed that a smaller spacecraft would actually try to land on the comet. The 2-ton main spacecraft was named Rosetta , carrying a dozen scientific instruments, and its 100-kilogram lander with nine more instruments was named Philae .