(from The ARRL Letter on Sept. 12, 2013)
NASA's Juno spacecraft will fly past Earth Oct. 9 to receive a gravity assist, putting it on course for Jupiter. To celebrate, the Juno mission is inviting amateur radio operators around the world to say "HI" to Juno in a coordinated Morse code message. If enough operators participate, Juno's "Waves" radio and plasma wave experiment should be able to detect the message. The Say "HI" to Juno web page (http://www.jpl.nasa.gov/hijuno/ ) will be updated with additional information as the event approaches. All transmissions will take place on 10 meters, with the precise (suggested) frequency determined by the last letter of your call sign (see Juno Flyby Frequency Table below).
Sponsors say Juno will have a better chance of detecting signals from many operators if the transmissions are spread out across the spectrum. The Juno Waves instrument is a broadband receiver, and the detector being used for this event has a passband 1 MHz wide.
While the Waves instrument is sensitive to radio signals in all amateur bands below 40 MHz, sponsors chose 10 meters because experience with the University of Iowa instruments on the Galileo and Cassini Earth flybys showed significant ionospheric shielding at lower frequencies. Sponsors actually are hoping for poor band conditions on Oct. 9 so that an appreciable fraction of the radiated energy can escape the ionosphere into space.
Indicators on the Say "HI" to Juno web page will instruct participants when to transmit and when to stop transmitting. Each will have a timer to indicate how long until you switch from one mode to the other. Stations should transmit a legal station identification as the FCC or non-U.S. regulators require. Participants should consider their stations to be operating as attended beacon stations and should avoid transmitting on top of ongoing communications.
Stations with directional antennas should check the web site for information on what headings to use during the event. Visit the Say "HI" to Juno web page (http://www.jpl.nasa.gov/hijuno/ ) for full details and to obtain the latest information. The activity begins Oct. 9 at about 18:01 UTC and continues until about 20:41 UTC. Operators taking part should make sure their computer clocks are synchronized to network time prior to the event. The web page will indicate when you start or stop transmitting (key down/key up).
Participants can receive a QSL card for contacting Juno. Email email@example.com  with your call sign and mailing address. Additional information is available at NASA's Juno (http://www.nasa.gov/juno ) website and the Mission Juno (http://missionjuno.swri.edu/ ) website.
-- NASA Jet Propulsion Laboratory; thanks to John Andrews, AC0XY