NASA's Juno Spacecraft Sets Solar Power Distance Record

January 14, 2016 | by Jonathan O'Callaghan

Photo credit: Juno will arrive at Jupiter on July 4 this year. NASA/JPL-Caltech

Break out the history books, because NASA’s Jupiter-bound Juno spacecraft has just set an impressive record. It is now the furthest spacecraft from the Sun powered only by solar power.

Juno achieved the record on Wednesday, January 13, when it reached 793 million kilometers (493 million miles) from the Sun. This eclipses the previous record of 792 million kilometers (492 million miles), set by ESA’s Rosetta spacecraft in October 2012, which is currently in orbit around Comet 67P/Churyumov-Gerasimenko.

"Jupiter is five times farther from the Sun than Earth, and the sunlight that reaches that far out packs 25 times less punch," said Rick Nybakken, Juno's project manager from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, in a statement. "While our massive solar arrays will be generating only 500 watts when we are at Jupiter, Juno is very efficiently designed, and it will be more than enough to get the job done."

Until now, all spacecraft that have traveled this far or beyond have used nuclear power, or specifically the decay of plutonium-238 in a radioisotope thermonuclear generator. This includes spacecraft like Cassini and the Voyager probes.

But for Juno a decision was made to go with solar power, in part due to a shortage of plutonium-238 at the time the mission was devised (although production has now restarted) and also to test out the feasibilities of using solar power at this distance. Future missions, including NASA’s planned Europa Multiple-Flyby Mission to Jupiter’s icy moon in the 2020s, may also use solar power if Juno is a success.

An infographic of solar-powered explorers. A higher resolution version is available here. NASA/JPL-Caltech

“I don’t want to discount the fact that we effectively have a canary on the way to the Jupiter coal mines for us,” Barry Goldstein, project manager for the Europa mission at JPL, told Scientific American last year. “But we have been tracking the performance of Juno’s arrays against our modeling and our tests at lower-intensity radiation doses, and they’re tracking right on.” 

Juno is scheduled to arrive at Jupiter on July 4 this year, when it will begin year-long observations of the gas giant, probing its clouds and aurorae. It will perform multiple flybys of the planet, orbiting the world 33 times and dipping to within 5,000 kilometers (3,100 miles) above the cloud tops every 14 days. It is the first mission to Jupiter since the Galileo mission ended in 2003.

Powering Juno are three solar arrays, each 9 meters (30 feet) long, with a total of 18,698 solar cells. At its furthest, Juno will be 832 million kilometers (517 million miles) from Earth, and this record is unlikely to be broken any time soon, perhaps only slightly eclipsed by the upcoming Europa mission.

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