Just to show that we don't always know where the Earth-periling asteroids are, 2012 XE54, estimated to be 15 - 50m across (not the three miles across reported in some outlets more consistently prone to exaggeration), was first discovered on December 9th and had its closest approach, at 224,500km on December 11th. That's only 60% of the distance between the Earth and Moon.
For those that do follow the exaggerating press, a note: any asteroid that's 3 miles across would have been discovered and mapped years ago. We're left seeking out the tiny bodies, such as this one.
Furthermore an impact from an object of this size (and at its reported velocity of 13km/sec) would cause significant damage in a localised area, but unlikely to have significant global effects - some have compared its likely effect to the Tungaska event in 1908 in Siberia. Think of a rock the size of a house colliding with the Earth at 13km/sec.
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Of interest to astronomers was the fact that just prior to its closest approach, the asteroid was briefly eclipsed by the Earth, an event recorded by a number of observers.
Asteroid 2012 XE54 has already had its orbit accurately determined. The Jet Propulsion Laboratory has a 3D animation available for viewing here. The object is in a slightly elliptical orbit (8.3 degrees to the earth's orbit) reaching near to the orbit of Venus at its closest (0.85AU) and well beyond Mars (1.95AU) at its furthest point from the Sun. With an orbital period of 2.72 years, it is unlikely to re-encounter the Earth on any of its next few rotations.
