The majority of asteroids and comets orbit the Sun at distances far from the Earth. However, a small fraction of them, the so-called Near-Earth Objects (usually abbreviated as NEOs), have orbits that can bring them closer to the Sun than the planet Mars, and approach the Earth. Their sizes range from 30km for the largest object (Eros) down to roughly 5m. Astronomers are interested in NEOs for two reasons. First, as they come close to the Earth, they are easier to study than other comets or asteroids. Secondly, NEOs can occasionally collide with the Earth, with the effect being dependent on the size.
The smallest NEOs will burn up and/or fragment at high altitudes in the Earth's atmosphere, as a fireball. Indeed, many of these events are seen each year by amateur astronomers and by satellites. Larger NEOs (between roughly 50m and 200m across) can survive down to the lower atmosphere, where their disruption can cause significant ground damage, over hundreds or even thousands of square kilometres. The last confirmed event like this occurred on 1908 June 30th over Tunguska, in Siberia. Even larger NEOs will generally impact the Earth itself, and cause a large explosion. It is now widely believed that the extinction of the dinosaurs 65 million years ago was associated with a 10km-wide NEO impacting in the Gulf of Mexico, causing tidal waves, firestorms and global climate change.
Tunguska-sized objects enter the Earth's atmosphere once every few hundred years. 1-km NEOs only hit the Earth every 500,000 years or so. However, NEOs arrive randomly (a bit like buses!), so the only way to be sure that we are not going to be hit is by finding them and calculating their orbits, thereby determining that none are coming our way.
There are several search programmes operating to find NEOs, primarily in the United States of America. Over 2,500 NEOs have been discovered, and a further 400 or so new NEOs are found every year. Once discovered, these NEOs are tracked by professional and amateur astronomers around the world. Teams of astronomers in America and Europe then calculate their orbits and assess if there is any risk of an impact in the foreseeable future.
Frequently asked questions:
Web sites for further NEO information: