Rethinking Europa

Originally, the gap between large-scale outer planets missions, known as flagships, was about a decade. Twelve years passed between the launch of the Voyagers and Galileo, and eight years went by between Galileo and Cassini. Had NASA been able to keep up that cadence, a new mission should have flown around 2007, which roughly aligns with the 2003 Decadal Survey's recommendation of a comprehensive mission for Europa. Instead, Juno, a mid-price mission from NASA's New Frontiers program meant to answer specific questions about Jupiter's interior, launched in 2011.

The next decadal survey, which took effect in 2013, once again prioritized a Europa mission—this time just behind a Mars sample return. Prockter said the sample return was given a higher rating for pragmatic reasons: it could be split into multiple, less-expensive chunks.

The Europa mission finally became a reality in 2015, but only after scientists and engineers extensively modeled Jupiter's radiation fields based on data from Galileo, as well as flybys from probes like Voyager and Cassini.

"We went back to the drawing board, and we said okay, what if we don't actually go into orbit around Europa, but instead we do what Galileo did, which is we stay in orbit around Jupiter, but we just encounter Europa every now and then?" said Prockter. "We found that we could get almost as good science doing that, but we didn't need as much (radiation) shielding, we could live a lot longer, we didn't need as much mass, and we could do that for considerably lower cost. And so that's what we did."

The outer planets workforce

The end of a space mission can cause an exodus of talent, as the associated workforce scrambles to find other sources of employment.

Prockter was a graduate student during the Galileo mission. Her Ph.D. thesis was on Europa and Ganymede. After Galileo ended, one of her colleagues went to work for aerospace giant Lockheed Martin, while another moved to the oil and gas industry.

Prockter joined the Johns Hopkins Applied Physics Lab in Baltimore, Maryland. First, she worked on the NEAR mission, which sent a spacecraft to the asteroid Eros, and then, she transitioned to the MESSENGER Mercury mission. Though she enjoyed the work, neither mission had anything to do with her interest in icy satellites, she said.

Over the years, she maintained a small foothold in Europa geology through occasional grants, and estimates that at one point, less than 10 people were active in the field. By the time Europa Clipper arrives at Jupiter—2025 at the very earliest—she will have been waiting for new Europa data for more than two decades.

"That's assuming I don't have a heart attack and drop dead or something," she said. "And if that mission is delayed by any reason, who knows, I'll be thinking about retirement by the time we get there."

Like Galileo, the end of the long-lived Cassini mission will also have an acute impact on early career scientists.

Morgan Rehnberg was a Ph.D. student at the University of Colorado studying Saturn's rings using Cassini data. He completed his Ph.D. in May, and promptly left academia for public outreach. He's now the director of scientific presentation at the Fort Worth Museum of Science and History.

Rehnberg has always been a passionate science communicator, and told me that drove his decision to leave academia more than anything else. Nevertheless, his choice wasn't just between public outreach and more ring science, it was between public outreach and an entirely different field.

"Cassini is basically the only game in town when it comes to studying planetary rings up close," he said.