The asteroid Eros, taken by the NEAR spacecraft. Image courtesy NASA/JPL-Caltech

The depressing answer to this question is a certain yes, unless we can find a way to deflect them. Large asteroids have hit the Earth at various points in its history, leaving evidence in the form of impact craters and deposits of rare materials such as iridium.

The larger the asteroid, the lower the chance of a collision. Smaller objects hit the Earth more frequently. About five hundred football sized rocks strike the Earth each year. In contrast, the asteroid that may have led to the extinction of the dinosaurs 65 million years ago was about 10 km across and an extremely rare event. Perhaps sixty objects greater than 5 km across have struck the Earth in the last 600 million years.

Current risk

At present there are no asteroids that are predicted to collide with the Earth. Astronomers are searching for a particular class called Near-Earth Objects (NEOs), which are asteroids that have orbits that cross or pass near the path of the Earth around the Sun. To date around 500 NEOs have been found with a diameter of greater than 1 km. If an object of this size hit the Earth it would cause destruction on a large scale.

Future risks

False colour image of the Potentially Hazardous Asteroid 1950 DA during its encounter with the Earth in March 2001, assembled using echoes from the Goldstone radar in California. Image: NASATwo objects present some risk in the future. Firstly, on 13 April 2029 asteroid 2004 MN4 (named 99942 Apophis) passed sufficiently close to the Earth that the gravity of our planet changed its path through space. As a result the asteroid will make two further close passages in 2035 and 2036. In both cases there is a very small chance that Apophis will collide with the Earth. Further observations are likely to indicate that this asteroid will pass at a safe distance.

Secondly, in the year 2880 asteroid 1950 DA will approach the Earth with at most a 0.3% chance of an impact. 1950 DA is 1.1 km across and so large enough to merit concern. Once again, further observations will probably rule out the possibility of a collision.

Find out more

• The Minor Planets (asteroids - fact file)
• Near-Earth Asteroids (fact file)
• Asteroids (from GCSE astronomy online course)

External links

• NASA impact risk table - lists potential future Earth impact events that the JPL Sentry System has detected based on currently available observations.
• Spaceguard project - focus for studies into the threat posed to the Earth by collisions with asteroids and comets.