A large asteroid whizzed past the Earth last week in the first event of its kind for 35 years, enthralling sky-watchers across the world. We find out how many more are out there and what would happen if one of them hit us.
âGetting closer. Ready to die? Just kidding.â Luckily for us, it was. If its mock Twitter page is anything to go by, @AsteroidYU55 was âhuge, sarcastic, bit scary, lonely and looking forward to striking Earth in November 2011â.
Thankfully, the 400metre-wide asteroid only came within 325,000km (202,000 miles) of our planet.
In the end, 2005 YU55, to give it its full name, hurtled past at 48,000kph (30,000mph), coming slightly closer than the Moon.
Nasa keeps a running list of recent NEO âclose approachesâ to Earth âYU55 was just one of four to pass near Earth on November 8 alone.
It also has a ârisk tableâ of Near-Earth Asteroids (NEAs) that could cause âpotential future Earth impact eventsâ in the next 100 years. The current tally lists 374 asteroids. The man who discovered the asteroid six years ago says there was never any chance of it hitting us.
Dr Robert McMillan, head of the Spacewatch project at the University of Arizona said: âThe prediction we had was very solid because we had hundreds of observations of this object. The orbit was very well determined so there wasnât any question about the trajectory.â
The last time an asteroid of a similar size came this close to Earth was in 1976, when asteroid 2010 XC15 came within 190,000 miles. Our next encounter is expected with asteroid 2001 WN5 in 17 years.
Dr McMillan estimates it could be three times the size of the last weekâs passing asteroid. If an asteroid that big hit Earth, it would be catastrophic.
âThe calculations suggest that if a 1km diameter asteroid were to hit the Earth, the aerosols thrown into the upper atmosphere would stay up there for a couple of years.
âIt would make the Sun so dim that it would shut down agriculture for two years. Thatâs what we call a civilisation-ending asteroid,â he said.
But donât panic just yet: âSomething a kilometre in diameter probably doesnât hit the Earth more often than an average of half a million years,â said Dr McMillan.
He discovered YU55 by using telescopes to take three images of the night sky at 25-minute intervals to pick out fast-moving objects.
The next step was to pass the information on to the Minor Planet Center in Cambridge, Massachusetts, which determined YU55 to be a Near-Earth Object (NEO). Prof Iwan Williams, from the school of physics and astronomy at Queen Mary, University of London, compares the Earth to a car going around the M25 on its own and an asteroid is a pedestrian who might step out.
Prof Williams believes it is smaller asteroids that may be of more concern in future.
âThey will do more damage and are more frequent. These big ones that are meant to cause the end of the worldâ¦ never going to happen.â