Halley's comet is probably the best known comet by non-astronomers. It flies by earth once every 75-76 years and is visible without a telescope. It last appeared in 1986, so many readers may have seen it. I, unfortunately was not born yet, so I have to wait until 2061 to see it. Records exists dating back to roughly 240 BC referencing sight of the comet.
The comet is peculiar in that it revolves around the sun in the opposite direction of the planets. Looking down from the "north" all the planets orbit counterclockwise, while Halley's comet orbits clockwise. It has a bright head full of rocky material with shocked gas in front of it. The tail is due to the solar wind stripping material from the comet, illuminating it, and pushing it away from the head.
There's been some talk in the non-scientific literature that Halley's comet will someday crash into and destroy Earth…. Well to comfort your nerves it's probably not going to happen. It's really difficult to calculate the path of Halley's comet because it is heavily affected by the gravity of the sun and other planets. Computer models show that in 2134, Halley will reach it's closest point to earth: 8,077,826 miles away. This is still 36 times farther away than the moon. So unless the sun or Jupiter influences Halley's orbit enough to send it hurdling towards Earth, we should be fine.
Thanks to Nick Ruffle for this post idea!
Next Topic: Cat's Eye Nebula
