An asteroid is scheduled to pass an estimated 4 million miles from Earth today, its closest encounter in over 100 years.
Roughly the size of Mount Everest at 1.4 miles wide, asteroid 2011 UL21 was once predicted to have a one in 71 million chance of hitting Earth by the end of the decade—an estimate experts now put at zero. Later this week, a much smaller asteroid, roughly the size of a stadium and discovered days ago, is scheduled to pass within roughly 75% of the distance to the moon. Both rocks—remnants from the early solar system—are expected to be visible using a telescope. A planet-killing asteroid is not expected to threaten Earth for at least the next 1,000 years, though NASA recently conducted an exercise to gauge readiness (see more).
The asteroids come days before Asteroid Day on June 30, timed to commemorate the 1908 Tunguska event in Siberia. An explosion that day was believed to be the largest observed asteroid strike, roughly 1,000 times as powerful as the blast at Hiroshima.
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