Asteroid Misses Earth Narrowly, by Cosmic Standards
“We’re safe. From this one.”
That was a quote from NASA’s planetary defense officer, Lindley Johnson, after a large asteroid soared narrowly—by cosmic standards—past the Earth on Wednesday morning, the closest approach by such an object in more than a decade, according to scientists at NASA.
The asteroid, discovered three years ago and known by astronomers as 2014 JO25, is approximately 2,000 feet end to end, larger than the Willis Tower in Chicago, though its unusual shape made its size difficult to assess before it passed. It was about 1.1 million miles away from Earth—about 4.6 times the distance between Earth and the Moon—when it passed by at about 7:30 a.m. CST time.
JO25—which some enthusiasts have nicknamed “The Rock” after the actor Dwayne Johnson—approached from the direction of the Sun. It will be visible through telescopes for several nights.
The last time a similarly sized asteroid passed within such a close distance was September 2004, when Toutatis, a 3.1 mile-long asteroid, came within four lunar distances of Earth.
It is the first time in four centuries that JO25 has been so close to the planet, and the asteroid will not come within a similar distance for another 500 years, NASA said in a news release.
Congress gave NASA the task of tracking all large objects that come within any reasonable range of Earth. The agency’s Near-Earth Object Observations Program tracks any object that is 140 meters (about 460 feet) or larger that comes within five million miles of the Earth. NASA, using statistical analysis, projects that there are somewhere between 25,000 and 26,000 asteroids in that category, though it has identified only about 7,700 thus far.
“That’s the current program objective, although we know that asteroids smaller than that, were they to impact, could do significant damage to a citywide area,” said Mr. Johnson, of NASA. He added that the criteria would keep the agency focused on asteroids that could wipe out a region the size of a state.
Scientists at NASA have said that such large asteroids have no significant chance of hitting Earth within the next hundred years. But the agency still takes the time to run drills preparing for such a contingency.
Mr. Johnson, who has been working on the problem for NASA since 2003 and became its first planetary defense officer about 15 months ago, said that NASA has proposed a mission concept for how such asteroids would be deflected while still in space, called a kinetic impact technique. Two spacecraft would be called upon to nudge an asteroid called Didymos, changing the speed of its orbit as a test of the way a threatening asteroid might be dealt with in the future.
“You only need to change the velocity of the asteroid by a hair, just a centimeter per second,” Mr. Johnson said. “If you do that a few years in advance of impact time, it will make it miss instead of hit the Earth.”
That test would take place in 2022.
 Jonah Engel Bromwich, New York Times (April 19, 2017). A version of this article appears in print on April 20, 2017, on Page A17 of the New York edition with the headline: “It Was a Near Miss for Earth, By the Universe’s Standards”.