An anonymous reader writes "NASA's Juno mission to Jupiter will perform a close 'fly-by' of the Earth in a few hours. To assist with its radio and plasma wave experiment, the mission is asking amateur radio operators to send a 'Morse Code' message to the probe as it passes."
The page has all the info you need: "The activity will begin at 18:00 UTC on October 9, 2013 and continue until 20:40 UTC. This page will clearly indicate when you should key up or key down to transmit 'HI' to Juno in Morse Code (see examples below). The Morse code pattern below can also act as a guide. The 'HI' message will be repeated every 10 minutes, beginning at 18:00, 18:10, 18:20, etc. "