an asteroid's path, and that could be vital. a 160—metre—wide rock like dimorphos could devastate populated areas, but smaller ones are a problem too. anything bigger than the 20 metre asteroid that broke up over russia in 2013 and injured hundreds of people are a concern. even the smaller objects can cause quite a lot of damage. a 25—metre asteroid, they will be really hard to spot with telescopes, so we are always pushing the technology and the science we can do and then we will try to detect where every single object is so we know what is coming in the future. the spacecraft will take nearly a year to travel the 7 million miles to its destination. no—one has ever tried anything like this before, but it could be the best chance of defending our planet if an asteroid is ever on a disastrous collision course. rebecca morrelle, bbc news. that launch is due to take place and pretty much four hours from now, vanderburgh air