Asteroid sentry takes up post on web Nasa has developed an automatic look-out for asteroids which could hit the Earth. The Sentry system constantly updates the orbits of known asteroids as new information comes in. Objects which pose any risk are posted on the system's website and withdrawn once it becomes clear they are safe. Sentry was designed by Nasa's Near-Earth Object Programme over a period of two years. Objects normally appear on the system's so-called 'risks page' because their orbits can bring them close to the Earth's orbit but it isn't always immediately clear how close. A spokesman says...