Lots of wild claims debunked about Comet C/2010 X1 (Elenin) at this great 2012 debunking site
Here is a quote:
Elenin will not collide with the earth
At its closest approach, Comet Elenin will be 34 million kilometers ( 21 million miles ) away from the Earth. It will also be 4 million kilometers ( 2.4 million miles ) above the orbit of the earth. See this image showing that Elenin will be well above Earth’s orbit, and this image, showing it will be well ahead of the earth int it’s oribt andthis video created by 2012hoax.org member obaeyensobaeyens using the orbital simulator AstroGrav.
“The comet never comes close to the Earth, but it is expected to be visible in binoculars during August and October. Part of the Internet chatter concerns its size. Comets are exceedingly small and enveloped in a tenuous cloud of gas and dust, so the only way to be sure of their actual dimensions is to visit with a spacecraft. This means its mass is less than one billionth the mass of the Earth. Needless to say, we will not be aware of the tiny gravitational pull from Elenin.”
The Earth will not pass through Elenin’s Tail
A common misconception of comets is that the tail streams out behind the comet. Even if that were the case, the images and video above show that we would not pass through the tail. However, the tail of a comet does not stream out behind the comet’s path, but rather points away from the sun. As such, the tail will never be in a position to cross the path of the earth.
Even if the earth were to pass through the tail of the comet, the only effect on the earth would be a nice show of meteors. Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.13