And it's happening again.
On September 9, 2018, the near-Earth asteroid 2018 RC will come within 136,000 miles from the Earth’s surface, or about half the distance to the moon. The asteroid is about the size of a 20 story building and will come barrelling past Earth just after noon UTC on Sunday, September 9 - just five days after it was discovered. That's scary. If it was going to wipe out all life on Earth we wouldn't have known it even existed until we were 5 days away from dying.
The near-Earth asteroid 2018 RC, discovered by the ATLAS survey in Hawaii on September 3, 2018, and announced the next day by the Minor Planet Center, has an estimated diameter in the range of 100-230 feet. Just before its flyby, it will be visible with small (4 inch or larger) telescopes, as a 12th magnitude dot of light. At that brightness, it won't be visible to the eye alone. It will be around 8:00 AM so I wouldn't be up to see it anyway.