In 2013, an undetected asteroid exploded over Russia’s Chelyabinsk Oblast, injuring more than 1,000 people with shards of glass from blown-out windows.
As a result of this cosmic barrage, the US space agency is on constant alert for any rocky body headed our way.
NASA’s astronomers said: “On an average of every several hundred thousand years or so, asteroids larger than a kilometre could cause global disasters.
“In this case, the impact debris would spread throughout the Earth’s atmosphere so that plant life would suffer from acid rain, partial blocking of sunlight, and from the firestorms resulting from heated impact debris raining back down upon the Earth’s surface.
“Since their orbital paths often cross that of the Earth, collisions with near-Earth objects have occurred in the past and we should remain alert to the possibility of future close Earth approaches.
“It seems prudent to mount efforts to discover and study these objects, to characterise their sizes, compositions and structures and to keep an eye upon their future trajectories.”
Earth’s only surefire defence against minor asteroids is the planet’s atmosphere.
Every single day, hundreds of tons of space dust and small rocks pelt the atmosphere.
NASA estimates anything up to 82ft (25m) across will burn up before reaching the ground.
But incidents like the Chelyabinsk meteor prove even small asteroids can cause a lot of damage.
“Only once every few million years, an object large enough to threaten Earth’s civilisation comes along.
“Impact craters on Earth, the moon and other planetary bodies are evidence of these occurrences.”
Asteroids smaller than 0.6 miles or one kilometre across are big enough to cause “local damage to the impact care”.
Asteroids measuring between 0.6 miles and 1.24 miles (2km) are likely to have a global impact upon striking Earth.
NASA said: “At 5.4 kilometres in diameter, the largest known potentially hazardous asteroid is Toutatis.
“By comparison, asteroids that populate the main asteroid belt between Mars and Jupiter, and pose no threat to Earth, can be as big as 940 kilometres – about 583 miles – across.”
As of December 24, 2019, there are 889,214 known asteroids and 3,600 known comets in the solar system.
The vast majority of these space rocks race around the Sun within the asteroid belt.
Occasionally, the space rocks come close to enough to Earth for NASA to study.
Thankfully there is no known asteroid or comet currently headed directly towards Earth.