The flypast - the closest ever - will allow scientists to measure deep inside the 17 mile wide, potato-shaped moon to see just what it is made of. Phobos, thought to be an asteroid captured by Mars's gravity, has an unusual low orbit that is taking it spiralling slowly towards an impact. That led to US President Dwight Eisenhower being briefed in 1960 that it could be a space station launched by an advanced Martian civilisation.