OK, our next caller is… from Mars! ESA

It is midnight on 1 January 2004 and you want to send a greeting on your mobile phone to a friend. Sorry, the line is too busy, try again later. If you think you are alone with this problem, you are wrong. Space agencies have had to work out ingenious solutions to prevent similar ‘engaged, call later’ tones from happening on Mars. For the first time, there will be seven spacecraft on the Red Planet at the same time. Will they all be able to ‘phone home’?