Comet Tails

Why does comets usually have two tails?
- Mahesh (age 19)
India

The two tails are one with dust and with gas respectively. Comets are originally made of ice with dust embedded and gas. As they approach the solar system, the ice evaporates because of sunlight. Pressure from solar radiation and solar wind (stream of charged particles from the sun) cause the dust and gas particles to be sweeped from the nucleus of the comet.

Dust is solid (a dust tail is visible because sunlight is reflected on the solid particles). Since the solar radiation pressure is relatively weak, a dust tail becomes curved as the end of the tail is left behind the orbit. 

On the other hand, a gas tail is formed by ionization; sunlight takes electrons from the gas atoms and makes them into ions. Ionization causes the gas tail to glow, making it visible. The trajectory of the gas tail (or ion tail) follows the direction of solar wind, or more specifically the magnetic field lines of solar wind plasma. So the gas tail looks perpendicular to the orbit of the comet. 

Therefore, comets get to have two tails pointing in different directions. You may find the following webpages helpful. 

http://hubblesite.org/reference_desk/faq/answer.php.cat=solarsystem&id=19

https://en.wikipedia.org/wiki/Comet_tail

Hope this helps!

SHC

(published on 01/04/2016)