the us space agency, nasa, has unveiled a system it hopes it might use to protect the earth from asteroids. the bbc�*s tim allman has the story. space is vast and full of wonder. countless stars, cou ntless pla nets and countless lumps of rocks streaking through the cosmos. earth has always been a potential target, exposed and vulnerable — until now. this is a computer simulation of dart, the double asteroid redirection test, which could be our first line of defence if one of those lumps of rock comes heading our way. nature has given us a set up where we have an asteroid, a nearby asteroid, approaching close to earth, so that we can observe from earth—based observatories, but this is a test. and this is how