http://www.newsweek.com/everything-you-need-know-about-amazing-green-comet-21p-1101875 > The comet known as 21P/Giacobini-Zinner (21P for short) is currently flying towards the inner Solar System at speeds of more than 51,000 miles per hour. On September 10, it is set to make its closest approach in 72 years to both the Sun and our planet. > But what exactly is a comet and what do we know about 21P?