1. Home
  2. Astronomy
  3. Comets and asteroids: debris of
  4. The “long-haired” comets

13.3 The “long-haired” comets  (Page 5/9)

<< Chapter < Page
  Astronomy     Page 5 / 9
Chapter >> Page >

Head of comet halley.

The head of Comet Halley. In this photograph the bright head, or coma, is seen at left, with the tail trailing away toward the right.
Here we see the cloud of gas and dust that make up the head, or coma, of Comet Halley in 1986. On this scale, the nucleus (hidden inside the cloud) would be a dot too small to see. (credit: modification of work by NASA/W. Liller)

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.

Comet Orbit and Tail. The Sun is drawn at the left-hand focus of a blue ellipse representing the orbit of a comet. The comet is drawn at six positions along the ellipse, and at each position the tail of the comet points away from the Sun. Beginning at upper right the comet has a very short tail. Moving counter clockwise, the comet’s tail gets longer as it nears perihelion (closest approach to the Sun, at lower left) and gets shorter as it recedes toward the right.
The orientation of a typical comet tail changes as the comet passes perihelion. Approaching the Sun, the tail is behind the incoming comet head, but on the way out, the tail precedes the head.

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.

Comet Hale-Bopp’s Tails. Panel (a), at left, is an image of Hale-Bopp the nucleus is at lower left, with the white dust tail (labeled) extending to center-right, and the blue ion tail (labeled) extending to top-center. A white line is drawn across the nucleus toward the left indicating the direction of motion of the comet. An arrow points to the direction of the Sun at lower left. Panel (b) shows two B+W images of Comet Mrkos at different times with long dust and ion tails.
(a) As a comet nears the Sun, its features become more visible. In this illustration from NASA showing Comet Hale-Bopp , you can see a comet’s two tails: the more easily visible dust tail, which can be up to 10 million kilometers long, and the fainter gas tail (or ion tail), which is up to hundreds of millions of kilometers long. The grains that make up the dust tail are the size of smoke particles. (b) Comet Mrkos was photographed in 1957 with a wide-field telescope at Palomar Observatory and also shows a clear distinction between the straight gas tail and the curving dust tail. (credit a: modification of work by ESO/E. Slawik; credit b: modification of work by Charles Kearns, George O. Abell, and Byron Hill)

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 .
<< Chapter < Page Page > Chapter >>
Practice Key Terms 3

Read also:

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Astronomy' conversation and receive update notifications?

Ask