After a meteor exploded over Russia today, injuring nearly 1,000 people, Dr Hugh Lewis, an astronautics expert from the University of Southampton, answers the key questions on the asteroid.
Where did the meteor come from?
No-one knew about the meteor before it was spotted streaking across the Russian sky and though detection systems are in place to spot larger meteors and asteroids, this was "very small" at around five metres in diameter.
It is not likely to be related to the asteroid 2012 DA14, which is passing Earth at a narrow distance tonight.
What was it made of?
"It is likely to be based on rock similar to what you would find on Earth," Dr Lewis said.
He added that it may have contained some metals, such as iron.
The meteor streaking across the Earth
How big was it?
The meteor was measured to weigh 10 tons. Dr Lewis said it would have probably measured around five metres in diameter, comparatively "very small".
How fast was it travelling?
The Russian Academy of Sciences said it was going at least 33,000 miles per hour and shattered about 18-32 miles above the ground, releasing several kilotons of energy.
Dr Lewis said meteors could often travel "much faster" than that.
What would have happened if it had hit Earth?
Objects of that size rarely penetrate the Earth's atmosphere.
But if it had more metal in its make-up, it may have got through and hit the Earth, he said.
That would leave a crater three times it size and a "quite substantial" blast which would emanate a large amount of heat transferred from the kinetic energy of the meteor.
"You would probably see considerable fatalities if it hit a populated area," he said.
The meteor hit a zinc factory
What can be done if we know a meteor or asteroid is going to hit Earth?
Scientists are constantly monitoring space for asteroids and meteors, and there are options to deal with them, including nuclear weapons, Dr Lewis said.
But if the object is between 0.6 and 1.2 miles in size, it would cause a global catastrophe.
"There are surveys going on to look for the larger stuff. It depends how soon you could pick them up," he said.
"If you detect them early then we can contemplate deflecting an asteroid.
"As a very, very last resort you can always call upon nuclear weapons.
"And if everything goes wrong you do have the option to evacuate people.
"But if you get something between 1km-2km (0.6-1.2 miles) in size then you are talking global catastrophe so evacuation probably wouldn't do any good."