Surprise! NASA just issued a last minute asteroid notice: Today, a 100-foot (30 meter) asteroid called 2014 DX110 is going to fly by Earth closer than the Moon. The closest point will be 217,000 miles (about 350,000 kilometers) at around 4PM Eastern Standard Time.
I love how they rush to downplay these close encounters of the apocalyptic kind to calm the population: "As happens about 20 times a year with current detection capabilities, a known asteroid will safely pass Earth Wednesday closer than the distance from Earth to the moon." When they say "current detection capabilities" they are referring to the fact that other asteroids fly by without us noticing. Like the asteroid that exploded over Russia last year, without any alert whatsoever.
NASA detects, tracks and characterizes asteroids and comets using both ground- and space-based telescopes. The Near-Earth Object Observations Program, commonly called "Spaceguard," discovers these objects, characterizes a subset of them and identifies their close approaches to determine if any could be potentially hazardous to our planet.