On November 8th of this year, the 400-meter-wide asteroid 2005 YU55 will pass the Earth, missing us by the comfortable margin of 325,000 kilometers (200,000 miles).
While this is the largest asteroid (that we know of) to swing past us for the next 17 years or so, YU55 is not an immediate threat to Earth. Its orbit does bring it close enough to our planet that it's been deemed a potentially hazardous asteroid, but the orbit is well-enough known that we can rule out an impact for at least the next century. That's long enough for me personally to not be concerned.
I've seen some small amount of buzz on the usual conspiracy sites about this asteroid, and I do see some folks trying to play this up a bit (search on "YU55 doomsday" for example), but fear-mongering chatter is surprisingly low for this event. I expect that by this fall you'll be seeing breathless YouTube videos accusing NASA of covering up a imminent impact -- and I don't say this blithely; it's happened before. Remember asteroid 2007 TU24? No? That's because nothing happened, despite the claims of panic-promoters.
As you can see in this JPL animation below, in November YU55 will miss us by a cosmic mile as well (click to embiggen and get a clearer animation):
You can see the Earth at the center (the diagonal line if the Earth's orbit around the Sun), the Moon orbiting the Earth, and the path of YU55. The scale on the bottom is a million kilometers, about 620,000 miles. The Moon's orbit is roughly 770,000 km (475,000 miles) wide. The path of YU55 cuts a shallow chord across the Moon's orbit, well away from our planet.
Still, there's a chance for some real science on this rock. At that distance, it'll appear so small (1/4 arcseconds across, where the Moon is 1800 arcsec across for comparison) that it'll be too small even for Hubble to make much of it -- at best, in Hubble's cameras it will appear to be just two pixels across. And that's even if Hubble could track it, which it can't.
But the Deep Space Network of radio telescopes can actually get very high resolution imagery using sophisticated techniques, possibly getting images with a resolution of just 4 meters -- the size of an SUV -- on the asteroid. That means YU55 will be 100 pixels across, enough to see some details on the surface, including craters, boulders, and even possibly a moon if it has one. Pretty cool.
So anyway, just in case the icky underside of the internet tries to play this up later this year, shouting doom-and-gloom, let me be clear:
Image credit: NASA/Cornell/Arecibo. Animation: NASA/JPL-Caltech