What’s out there in space lurking close to the Earth and is it going to hit us?
Near-Earth Objects (NEOs) are asteroids, comets and extinct comets which have orbits passing near the Earth. They range in size from less than one to tens of kilometres across.
Observing them is difficult as their apparent brightness reaches a peak before they pass near the Earth. This means that they are generally detected as their brightness is diminishing and the time when they can be observed is limited to a few weeks or less.
The closest recorded encounter between a natural object and the Earth took place on 31 March 2004. Meteoroid 2004 FU162 passed just 6500 km above the ground. 2004 FU162 is only 10 m across so if it had collided with the Earth it would almost certainly have exploded harmlessly in the upper atmosphere.
Another close approach to the Earth occurred on 19 May 1996, when 1996 JA1 passed the Earth at a distance of 450,000 km or about as far away as the Moon. With a diameter of about 500 metres, this is probably the largest asteroidal object recorded so close to the Earth.
Only two more are likely to pose any threat in the foreseeable future. Firstly, in the year 2880 the 1.1 km wide asteroid 1950 DA will pass close to the Earth. At the moment the odds of a collision are at most 1 in 300. If 1950 DA really is set to strike the Earth, humanity has more than 800 years to work out a way to deflect it.
Secondly, the 390 m wide minor planet 2004 MN4 (Apophis) will pass close to the Earth in the year 2029. The gravity of our planet will alter the orbit of Apophis around the Sun and it will make two further close passes in the years 2035 and 2036. There is a very low risk of impact in the 2036 encounter.
The Tunguska Event
Some estimates though put the chances of a collision as high as once every 100 years. In the last century, one of the most dramatic collisions of an object with the Earth was the Tunguska event in Siberia in 1908 - an explosion with the power of a 20-megaton nuclear bomb. Trees in the forest were flattened up to 30 km from the explosion, which was heard up to 1000 km away.
Even a collision between a small object and a city could cause great damage. If an asteroid like 1996 JA1 hit the Earth at a typical velocity of 20 km per second, the energy released would be about the same as a 0.4-megaton nuclear weapon or 30 times the force of the bomb dropped on Hiroshima.