Outer space is rather large. Some say it's even infinite. So when you hear that a comet that is 3km wide may be on a collision course with Earth, you may be thinking to yourself, "Pffft! Nobody knows how big a km is, and it's only three of them anyways. What's the big deal?" Well, it is a big deal. 

The Rosetta spacecraft is expected to eventually land on the comet 67P/Churyumov-Gerasimenko. I'm going to call it comet CG for short, I'm sure you understand. Now, comet CG never crosses Earth's orbit, so we don't have to worry about that. Rosetta recently made a rendezvous with comet CG and snapped a cool picture. Here is a scale illustration of what comet CG would look like next to Los Angeles. It's frickin' huge.