I was rewatching Apollo 13 over the weekend - a fabulous movie, by the way, if you’ve never seen it - about how, in 1970, three astronauts returned safely to Earth following an explosion on their spacecraft, two days into a journey to the Moon. Among the many problems faced by the crew and mission control was one of navigation. In order to conserve electrical power, they had to shut down their computers, along with their navigation systems, until they powered them up again just before reentry. Of course, this made it far more difficult to plot their course – but it was also far more important that they do so – so that they would be in the right position when they approached the Earth.