Giant asteroid passes near Earth
An asteroid that is 400m (1,300ft) wide has passed by Earth, much to the delight of astronomers.
Although invisible to the naked eye, scientists said they spotted strange structures on its surface as it spun past at 30,000mph (48 280.32 km/h).
Asteroid 2005 YU55's was the closest an asteroid has been to Earth in 200 years, according to Nasa.
It is also the largest space rock fly-by Earth has seen since 1976; the next visit by a large asteroid will be 2028.
The aircraft-carrier-sized asteroid was darkly coloured in visible wavelengths and nearly spherical, lazily spinning about once every 20 hours as it raced through our neighbourhood of the Solar System.
Ron Dantowitz, the director of the Clay Centre Observatory in Massachusetts, followed the asteroid through a telescope.
"We're tracking the asteroid itself, so the stars are moving by in the background and the asteroid is actually streaking by at about 30,000mph," he said.
"As we track it, it looks like the stars are moving in the background and the asteroid is locked on in the centre view.
"It's not so much that we can see it tumbling like a rock in space, we're examining it for the brightness and colour."
Nasa said it had been no closer than 201,700 miles (324,600km), as measured from the centre of the Earth. The rock reached its closest point to Earth at 23:28 GMT on Tuesday.
It will now trace a path across the whole sky through to Thursday.
The asteroid often travels in the vicinity of Earth, Mars and Venus, but Nasa said this fly-by had been the closest the asteroid had come to Earth in at least 200 years.
"This is the closest approach by an asteroid that large that we've ever known about in advance," said Lance Benner of Nasa's Jet Propulsion Laboratory.
But he stressed that there had been no chance that the pass would be anything other than a close encounter.
"2005 YU55 cannot hit Earth, at least over the interval that we can compute the motion reliably - which extends for several hundred years," he said.
Instead, the pass gave astronomers a rare opportunity to study the asteroid in detail.
In particular, two radio telescopes - the Goldstone Observatory in California, US and the Arecibo Observatory in Puerto Rico, US - tracked radio echoes off it in a bid to understand better what it is made of and how it is shaped.
The precise details of the asteroid's path will also help scientists to predict where it will go much further into the future.
Earth has several regular visitors like 2005 YU55 - most famously the Apophis asteroid. Apophis has in the past been claimed as a possible future impactor when it returns to our neighbourhood in 2029 and again in 2036.
There is, according to the latest calculations, no danger from Apophis either. However, it will pass much closer to Earth on 13 April 2029 - at a distance of 18,300 miles (29,500km).