An asteroid as big as a city block shot relatively close by the Earth on Saturday, the latest in a series of visiting celestial objects including an asteroid the size of a bus that exploded over Russia in February, injuring 1,500.
Discovered just six days ago, the 460-foot long (140-meter) Asteroid 2013 ET passed about 600,000 miles (950,000 km) from Earth at 3.30pm EST (2030 GMT). That's about 2-1/2 times as far as the moon, fairly close on a cosmic yardstick.
"The scary part of this one is that it's something we didn't even know about," Patrick Paolucci, president of Slooh Space Camera, said during a webcast featuring live images of the asteroid from a telescope in the Canary Islands.
Moving at a speed of about 26,000 mph, (41,843 kph), the asteroid could have wiped out a large city if it had impacted the Earth, added Slooh telescope engineer Paul Cox.
Asteroid 2013 ET is nearly eight times larger than the bus-sized asteroid that exploded over Chelyabinsk, Russia, on February 15. The force of the explosion, equivalent to about 440 kilotons of dynamite, created a shock wave that shattered windows and damaged buildings, injuring more than 1,5000 people.
Later that day, another small asteroid, known as DA14, passed about 17,200 miles (27,680 km) from Earth, closer than the orbiting networks of communications and weather satellites.
"One of the reasons why we're finding more of these objects is that there are more people looking," Cox said.
Two other small asteroids, both about the size of the Russian meteor, will also be in Earth's neighborhood this weekend. Asteroid 2013 EC 20 passed just 93,000 miles (150,000 km) away on Saturday - "a stone's thrown," said Cox.
On Sunday, Asteroid 2013 EN 20 will fly about 279,000 miles (449,007 km) from Earth. Both were discovered just three days ago. "We know that the solar system is a busy place," said Cox.
"We're not sitting here on our pale, blue dot on our own in nice safety ... This should be a wakeup call to governments."
Nasa has been tasked by the US Congress to find and track all near-Earth objects 0.62 miles (1 km) or larger in diameter, and estimates about 95% have been identified.
However, only about 10% of smaller asteroids have been discovered, Nasa scientists have said.
The effort is intended to give scientists and engineers as much time as possible to learn if an asteroid or comet is on a collision course with Earth, in hopes of sending up a spacecraft or taking other measures to avert catastrophe.
About 100 tons of material from space hit Earth every day. Astronomers currently expect an object about the size of what hit Russia to strike the planet about every 100 years.
Meteorite explodes over Russia, more than 1,000 injured
A meteorite streaked across the sky and exploded over central Russia on February 15, 2013, raining fireballs over a vast area and causing a shock wave that smashed windows, damaged buildings and injured more than 1,000 people.
People heading to work in Chelyabinsk heard what sounded like an explosion, saw a bright light and then felt the shock wave, according to a Reuters correspondent in the industrial city 1,500 km (950 miles) east of Moscow.
The fireball, travelling at a speed of 30 km (19 miles) per second according to Russian space agency Roscosmos, had blazed across the horizon, leaving a long white trail that could be seen as far as 200 km (125 miles) away.
Car alarms went off, thousands of windows shattered and mobile phone networks were disrupted. The interior ministry said the meteorite explosion, a very rare spectacle, also unleashed a sonic boom.
No deaths were reported but the emergencies ministry said 20,000 rescue and clean-up workers were sent to the region after President Vladimir Putin told emergencies minister Vladimir Puchkov to ease the disruption and help the victims.
The interior ministry said about people had been injured, at least 200 of them children, and most from shards of glass.
Experts drew comparisons with an incident in 1908, when a meteorite is thought to have devastated an area of more than 2,000 sq km (1,250 miles) in Siberia, breaking windows as far as 200 km (125 miles) from the point of impact.
Simon Goodwin, an astrophysics expert from Britain's University of Sheffield, said that roughly 1,000 to 10,000 metric tons of material rained down from space towards the earth every day, but most burned up in the atmosphere.
"While events this big are rare, an impact that could cause damage and death could happen every century or so. Unfortunately there is absolutely nothing we can do to stop impacts."