An asteroid the size of an aircraft carrier made a close but harmless swing by Earth on Tuesday evening.
The space rock known as 2005 YU55 flew past the Earth around 6:30 p.m. ET, closer than the moon's orbit to our planet.
Scientists at NASA's Near-Earth Object Program, which tracks asteroids and comets, had ruled out any chance of impact in advance. But they used the close encounter to learn more about asteroids.
The last time a cosmic interloper this size came this close to Earth was in 1976 and it won't happen again until 2028.
Since late last week, antennas at the space agency's Deep Space Network in California have been monitoring 400-metre-wide asteroid as it approached from the direction of the sun.
The Arecibo Observatory in Puerto Rico planned a viewing Tuesday when the asteroid was scheduled to make its closest pass at a distance of 325,072 kilometres at 6:28 p.m. ET, after sunset in Eastern Canada.
Researchers will now analyze radar data, which can generate bright, 3D images of objects that look dark to human eyes, to glean details about the asteroid's surface features and shape.
On Monday afternoon, NASA released new, more detailed radar images of the asteroid as it moved closer to Earth.
Since its discovery six years ago by a University of Arizona astronomer, scientists have learned a great deal about 2005 YU55.
Its surface is coal black, and it spins slowly through space.
Amateur skygazers who wanted a glimpse needed two things: a good sky chart and a 15-centimetre telescope or larger since the asteroid is very dark-coloured and too faint to detect with the naked eye. Even with a telescope, sighting is not guaranteed. The glare from the moon, which will be close to full on Tuesday night, may make the asteroid difficult to spot.
Astronomy writer David Dickinson charted the expected path of the asteroid on his blog Astroguyz, showing that it will appear high in the southwest to observers in eastern North America, passing through the constellations Aquila, Delphinus, and Pegasus as it heads westward. It is expected to pass within a degree of Altair, the brightest star in the constellation Aquila, at 6:07 p.m. ET.