The NASA Juno spacecraft is headed for Jupiter. Launched August 2011, it is scheduled to arrive July 4, 2016. On October 9, 2013, Juno coasted by the Earth and used gravity assist to speed it up and redirect it slightly so it will coast the rest of the way toward Jupiter. It received a boost in speed of more than 8,800 mi/hr (about 7.3 km/s). It is currently right on course. You can read about that flyby in this news release from the mission website.
During the flyby of Earth, scientists turned on Juno’s Waves instrument. This instrument will eventually measure radio and plasma waves in Jupiter’s magnetosphere. The reason for turning it on during flyby, three years before reaching Jupiter, was to record amateur ham radio signals. Ham radio operators around the world were invited to take part in a public outreach project. They were invited to say “HI” to Juno during the flyby. They all broadcast the same low power Morse-coded “HI” message at the same time from their home antennas. Over 1400 operators from every continent, including Antarctica, participated.
Morse code characters use dots and dashes, or “dits and dahs.” To transmit the letter H, operators needed to send 4 dots, or dits, in a row. These were followed by a space in time. Then came 2 dots, or dits, for the letter I to complete HI. As Juno neared Earth, each operator viewed a web page run by mission scientists that told them when to make their dits and how long to hold them. Each dit lasted 30 seconds. The HI message was repeated every 10 minutes for a total of 16 times until Juno had passed Earth.
The green dots at top represent pieces of the repeated message that Juno was able to detect. Grey dots were transmissions, but not detected by the Waves instrument. The ham radio signals were processed to isolate them from the background noise. Scientists on the Waves team were not sure the signal could be detected. When the the recording was played back at high speed, they were thrilled to see and hear the signal in the data.
Bill Kurth, University of Iowa Physics and Astronomy, and lead investigator of the Waves Team, describes the project in this 4 minute documentary. You can hear for yourself how it sounded to Juno. All participants were happy with the success of the team effort.