Asteroid 2017 BH30 zipped past the Earth just 32,200 miles away at its closest approach. By comparison, the Moon is never closer than 225,000 miles from Earth. BH30 was estimated to be 19 feet across, according to NASA's Jet Propulsion Laboratory.
The week prior, on January 25, 2017 AG13, an even larger asteroid, made a pass across the sky. While it did not come as close as BH30, at a little over 110,000 miles away, it was much larger, at some 50-110 feet across. That would make it at least as big as the Chelyabinsk meteor that caused over $30 million of damage when it exploded in midair over Russia in 2013.
On January 20, asteroid 2017 BX, between 10-40 feet across, blew by just 16,252 miles from Earth. That makes three Near Earth Object (NEO) close calls in just ten days, all noticed only days before they made their closest pass.
The next close pass identified by astronomers is 2012 TC4, a known asteroid 33-100 feet across, which will come within 13,200 miles of Earth during its expected October flyby.
"The larger these things are, the easier they are to spot. It's the little ones that we tend to not really find. And when we do, it's a little too late," said Antonio Paris, astronomy expert with Tampa's Museum of Science and Industry.
While humanity's space agencies are able to detect most large asteroids before they arrive, the B612 Foundation, a NEO-detecting nonprofit, says that there are "1 million smaller asteroids that might only wipe out a city or perhaps collapse the world economy," that have yet to be identified.
Humanity in the modern era has been lucky regarding asteroid impacts. Besides Chelyabinsk, there has not been a destructive meteor impact over a population center in centuries.
The 1908 Tunguska meteor flattened 770-square miles of remote Siberian forest, but did not cause any known fatalities. Had it hit a city, the death toll would have been in the millions, according to scientists. Medieval sources tell of the 1490 Ch'ing-yang event, which apparently killed at least 10,000 people in Ming China, the deadliest meteor on record.
In late 2016, NASA senior scientist Joseph Nuth said that if a large meteor were to be found on a collision course with Earth, "there's not a hell of a lot we can do about it." In response to his comments, former President Barack Obama commissioned a White House report to improve international preparedness for such an event.