TORONTO — Designers of a proposed NASA mission to study the icy and potentially habitable Jupiter moon of Europa have decided to use solar panels rather than a nuclear power source for the spacecraft, while keeping spacecraft’s launch options open.

In an Oct. 3 presentation at the 65th International Astronautical Congress here, Europa Clipper Deputy Project Manager Thomas Magner of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said that using large solar panels for the mission was both technically viable and less expensive than a radioisotope thermoelectric generator (RTG).

“For the last two years, we did extensive risk reduction work looking at whether solar is feasible,” Magner said. “We found that solar works fine, so our final decision about two months ago was to go to solar.”

Those tests included subjecting solar cells to a range of temperatures and radiation conditions the spacecraft would experience during the mission. Engineers also examined whether the large solar panels, with an area of 50 square meters, would cause jitter that would adversely affect the spacecraft’s scientific instruments.

Europa Clipper would not be the first solar-powered Jupiter mission: NASA’s Juno spacecraft, launched in 2011 and scheduled to go into orbit around Jupiter in July 2016, also uses solar panels. Magner said they tested solar cells used for Juno, exposing them to a wider range of temperatures to reflect the different trajectory that Europa Clipper would take to reach Jupiter.

A solar-powered Europa Clipper would be less expensive than one that used an RTG, Magner said, although he did not quantify the difference in costs. It would eliminate the need for an environmental impact statement that is required for nuclear-powered spacecraft and launch approval from the director of the Office of Science and Technology Policy. It would also free up constrained supplies of plutonium-238, the isotope used in RTGs, for other potential missions.

Magner said after his presentation that using solar power would increase the spacecraft’s mass in order to accommodate the panels and structures, although it remained well within current design margins. A few other tweaks would be needed to the spacecraft’s design to incorporate the solar panels, he said.

While NASA has not yet decided to develop Europa Clipper or any other mission to Europa, Magner said the project passed a NASA-sponsored mission concept review in September “with flying colors” and predicted the agency would move ahead with it.

“We expect later this year for NASA to select us as the mission they want to go forward with to Europa, and we’ll get a new start early next year,” he said.

If NASA does choose to proceed with Europa Clipper, it will defer a choice of launch vehicle for several years. One mission design could use an existing rocket, such as the workhorse Atlas 5. In that scenario, Europa Clipper would launch in mid-2022 and, after a series of gravity assist flybys of Venus and Earth, arrive at Jupiter in early 2030.

An alternative approach would use NASA’s still-in-development Space Launch System heavy-lift rocket to send Europa Clipper on a direct trajectory to Jupiter. A mid-2022 launch would allow the spacecraft to reach Jupiter in fewer than three years, without the need of any flybys.

Launching on SLS, Magner said, would reduce the costs of operating Europa Clipper given the much shorter travel time to Jupiter, and also simplify the thermal design since the spacecraft would not have to perform a Venus flyby in order to reach Jupiter. However, it was unclear how much more Europa Clipper would have to pay for an SLS launch.

Steve Creech, NASA’s SLS assistant program manager for strategy and partnerships, said here that the agency was still evaluating how much of SLS’s fixed costs a science mission like Europa Clipper would have to pay for a launch. “That’s a decision above our pay grade,” he said.

Magner said he anticipated that NASA would hold off on making a launch vehicle choice until the program completes its preliminary design review (PDR), currently scheduled for 2018. “Our intention is to maintain dual launch capability through PDR,” he said, “and then, based on [NASA] headquarters direction, proceed with just one launch vehicle.”

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