Let’s start with the scariest part about asteroid impacts. It isn’t a question of will a big one will impact? It’s a question of when. There are over 200 known impact craters on the Earth and likely millions more have been hidden by erosion and tectonic movement. And roughly every year a 20 kiloton explosion happens when a 5-10m asteroid impacts our atmosphere.
Over its lifetime the Earth has been positively peppered by impacts, and its still the final cosmic bullseye for a number of asteroids out there.
Will 2005 YU55 be the “big one”? In short, no. Despite it being the closest approach of a large (several hundred metre diameter) asteroid since 1976, it’s still going to be over 3/4 of the Earth-Moon distance away from us. That’s over 300,000 km away, and as it whizzes by at almost 50,000 km per hour it will hardly feel the Earth’s gravity. Don’t worry about it being “sucked in” by the Earth’s gravitational field, it’s just moving too fast. That’s the good.
So what’s the bad? Well, you’d hope that something coming this close to the Earth would put on a good show for skywatchers. But unfortunately YU55 is not even going to be visible to the naked eye (in fact the best radar observations are still not that impressive, see right). You’ll need a telescope with at least a 6 inch diameter to see it, and finding it will be made even harder by the full moon plus the speed at which the asteroid moves across the sky. Wait and hour and it will be in a completely different place. In fact, over a period of 10 hours it is going to move eastward across the sky by about 70 degrees. As night falls this evening it will start in Aquila (around 5 pm) and by 1 am it will be in the Great Square of Pegasus.
Since we started with scary, let’s end with two other scary things too. Asteroid 99942 Apophis (named for the Egyptian god of darkness) is going to get real close to the Earth on Friday the 13th April 2029. Based upon the latest orbital projections, it will come within 30,000 km of the Earth, so under 1/10th of the Earth-Moon distance. By the length scale of the solar system, that is about as close a shave as you can get. That’s even inside the orbital distance of geostationary satellites. And because it’s coming this close we can’t actually predict very well what will happen after the encounter. There’s a possibility it will loop back for another close encounter in 2036. We’ll only find out once we get more orbital data.
Lastly, over the next few years, massive new telescopes such as the Large Synoptic Survey Telescope will start looking at almost all the sky every three days. It’s going to revolutionize our knowledge of just what potentially hazardous (100m or so) asteroids are really out there. Right now, we just don’t know much about what’s out there. Remember we mentioned the 1976 flyby? Nobody knew about that when it happened. We only figured it out after the event.