Large objects have hit the Earth many times over its 4,500 million year history. We know this because these Near Earth Objects (NEOs) left clues in the Earth's rocks. When a large rocky object hits another, it leaves an impact crater. NEOs which make craters are travelling at thousands of miles per hour when they hit the ground. The Earth has more than 160 large impact craters on its surface.
Meteor Crater in Arizona, USA was the first impact crater discovered on Earth. The NEO which made Meteor Crater (also called Barringer Crater) was just 40-50 m wide. When it crashed, its energy made an explosion which left a huge crater in the ground. Meteor Crater is 1,200 m wide, which is as long as 11 football pitches!
Some craters are harder to find, like the Chicxulub Crater. This is off the coast of Mexico and as been covered by the sea floor. We think the same event which made Chicxulub crater led to the extinction of the dinosaurs! The largest crater on Earth is the Vredefort crater in South Africa. It is 300 km wide and 2,000 million years old.
In 1908, a meteor or comet broke up in the air above Tunguska, Siberia. Scientists have worked out it was under 50 m in size when it exploded. This is large for a meteor but tiny for an asteroid! It made a shock wave which flattened 80 million trees! In 2013, a similar, but much smaller event took place in Chelyabinsk, Russia. Remember, events like Tunguska or Chelyabinsk do not happen often.
What are the chances of a large NEO hitting Earth?
The risks from a NEO increase with its size. The bigger the NEO, the more energy it has when it hits the Earth. NEOs wider than 1 km are the biggest danger to Earth. Impacts from these large objects could damage a large area. They could even affect the whole world.
Large NEOs are not likely to pass close to our planet often. We know they will hit the Earth a few times each million years on average. These statistics cannot tell us what year a serious collision will occur. This is why it is important to track the orbits of NEOs. If we know the path a NEO will take, we can plan a way to prevent a collision. We could use rockets or explosives to nudge the NEO out of harms way!