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Europa Clipper illustration
The proposed Europa Clipper mission would go into orbit around Jupiter, instead of Europa itself, making several dozen close flybys of the icy moon. At an estimated cost of $2.1 billion, it is less than half the cost of previous Europa orbiter concepts, but NASA is looking for ways to further reduce costs. (credit: NASA/JPL)

Europa on the cheap

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For decades, scientists have dreamed of sending a mission to Europa, one of Jupiter’s four large Galilean moons. The fleeting flybys of the moon by Voyagers 1 and 2 in 1979, and the more frequent flybys by Galileo in the 1990s, revealed a world with a icy crust, crisscrossed by fractures and other complex features. Those and other observations led scientists to conclude that Europa has, below that icy crust, a subterranean ocean of liquid water that, combined with the energy from tidal heating that keeps that ocean liquid and organic compounds, provide the basic requirements for life.

Instead of going into orbit around Europa, the Europa Clipper spacecraft would instead go into orbit around Jupiter and make repeated close flybys of the moon.

As compelling as the scientific case is for Europa, NASA has yet to send a dedicated mission there. Part of that is due to the technical challenges involved in launching a spacecraft to Jupiter and operating in the high radiation environment created by Jupiter’s powerful magnetic field. Those technical obstacles drive up the costs of those proposed missions, and reduce the chances they can be funded. NASA has studied Europa orbiter mission proposals since the late 1990s, but those efforts typically foundered on costs and shifting priorities.

In 2011, the latest planetary science decadal survey—the once-per-decade report outlining priorities for planetary science missions based on science and other factors—identified a Europa orbiter as the second-highest priority large, or “flagship” mission, behind a rover to collects samples on Mars for later return to Earth. That ranking, though, came with a caveat: an independent study of the mission concluded it would cost $4.7 billion, too high a price tag. For a Europa mission to remain highly ranked, NASA needed to bring down the cost significantly. (See “Tough decisions ahead for planetary exploration”, The Space Review, April 4, 2011).

NASA has been working on reducing a cost of a Europa orbiter mission, in large part by no longer making it an orbiter. The concept now favored within the space agency is something called “Europa Clipper.” Instead of going into orbit around Europa, the spacecraft would instead go into orbit around Jupiter and make repeated close flybys of the moon. The most recent mission designs involve 45 flybys of Europa over three and a half years, with the vast majority of them coming within 100 kilometers of the moon’s surface. That approach minimizes the spacecraft’s exposure to radiation as well as propellant needed for entering orbit around Europa.

Europa Clipper is still in its earliest phases, with a mission concept review planned for September. A notional design for the mission, presented at a meeting of NASA’s Outer Planets Assessment Group (OPAG) in January, showed a spacecraft equipped with nine instruments, including cameras, spectrometers, and an ice-penetrating radar, whose booms extended from either side of the spacecraft’s bus like parts of an old-fashioned television antenna. Launched on an Atlas V 551 in November 2021, the spacecraft would arrive at Jupiter in April 2028 after gravity assist flybys of Venus and the Earth. That design has a cost estimate of about $2.1 billion, less than half the independent cost estimate for the earlier Europa orbiter mission in the decadal survey report.

That work on Europa Clipper has largely been done at the insistence of Congress, rather than by NASA’s own initiative. In the final fiscal year 2013 budget, Congress set aside $75 million of NASA’s planetary sciences budget specifically for Europa mission studies, and followed that up with $80 million in the fiscal year 2014 budget, even though NASA requested no money in either year’s budget request for Europa. The text of the final 2014 appropriations bill noted that the $80 million “shall be for pre-formulation and/or formulation activities for a mission that meets the science goals outlined for the Jupiter Europa mission in the most recent planetary science decadal survey.”

When NASA released its fiscal year 2015 budget proposal last month, it contained, for the first time, a line item for Europa mission studies. NASA requested $15 million for fiscal year 2015, but was short on details in the document on how the money would be used. “Following the analysis of options for a lower-cost Europa mission conducted in FY 2014, NASA will continue developing the architecture for a potential Europa mission,” the budget justification document stated.

“My desire, to be quite honest, would be to target a Europa mission that we could fly for a billion dollars or less,” Bolden said.

At the formal rollout of the budget March 4, NASA administrator Charles Bolden and chief financial officer Beth Robinson were vague on what the scope of the Europa mission they had in mind. “We’re frankly just not sure at this point” how large and expensive a Europa mission would be, Robinson said. “It will take us years. It is a very challenging mission.”

However, NASA is looking for ways to make it a less expensive mission. Speaking at the American Astronautical Society’s Goddard Memorial Symposium outside Washington the day after the budget rollout, Bolden said that NASA was “committed” to flying a mission to Europa in the 2020s, but would seek to bring the costs down from even the Europa Clipper’s $2.1-billion estimate.

“What I’ve asked people to do is that I want the science community to come together with industry, academia, and international partners, and my desire, to be quite honest, would be to target a Europa mission that we could fly for a billion dollars or less,” he said. “That may or may not be possible, because the one thing we don’t want to do is to fly a mission of a certain amount of money that has no valuable scientific return.”

Later at the Goddard Symposium, NASA associate administrator for science John Grunsfeld said that NASA would put out a request for information (RFI) “post haste” for ideas of a Europa mission that would fit within the cost cap of NASA’s New Frontiers program of mid-sized planetary science missions. “That’s part of formulating the cost bogey for an eventual Europa mission,” he said. “The exciting thing is that the administration is talking about doing a Europa mission, rather than just studying indefinitely the possibility of doing a Europa mission.”

The idea of such a low-cost Europa mission has generated considerable skepticism in the scientific community. It would require cutting the cost of the Europa Clipper concept by half, which was already more than 50 percent cheaper than the previous Europa orbiter mission designs. While it might be feasible to launch a Europa mission within that billion-dollar cost cap, some scientists wonder if the mission would be able to do enough science to justify the expense.

“Is that possible?” Bolden asked about a billion-dollar Europa mission. “I don’t know. Almost every single one of my advisors says it can’t be done.”

At the “NASA Night” session of last month’s Lunar and Planetary Science Conference (LPSC) outside Houston, Jim Green, director of NASA’s planetary sciences division, defended the move to at least study the concept of doing a low-cost Europa mission. “That’s a phase space that perhaps we haven’t significantly looked at, and we owe it to ourselves to be able to determine if there are any viable missions at a billion dollars or less,” he said.

Yet, Green sounded perhaps a little skeptical that such a low-cost mission could meet most of the scientific goals laid out in the decadal survey. “We owe it to the administration to do that last check,” he said of the RFI, citing advances in technology. “Maybe, just maybe, there’s a way to do the preponderance of the planetary decadal science objectives for less than a billion dollars. That’s important to check, and after we see what the responses are and evaluate those, we will then chart a course to execute the program accordingly.”

Any skepticism that Green voices in his LPSC comments may be shared by others in NASA. “If we can do a Europa mission for a billion dollars or less, we’re going to do it,” Bolden said in comments Thursday to a joint meeting of the Space Studies Board and Aeronautics and Space Engineering Board in Washington. “Is that possible? I don’t know. Almost every single one of my advisors says it can’t be done.” The exception, he added, was NASA chief scientist Ellen Stofan, who “hasn’t said it can be done, but she said we ought to look at it.”

That RFI for a low-cost Europa mission, he said, was part of a broader effort to find ways to lower costs of NASA programs in general. “We want to challenge the conventional wisdom on the cost of missions of scientific significance and import,” he said, noting that NASA centers and contractors alike had gotten use to missions having certain high costs. “I don’t like it that way. I want to get the most bang for the buck.”

SLS illustration
NASA’s Space Launch System, fitted with a payload fairing adapted from existing launch vehicle designs, could launch a Europa mission on a direct trajectory, cutting travel times by several years. The cost of such a launch, though, could be an obstacle to its use. (credit: NASA)

As NASA examines low-cost options for a Europa mission and continues studies of the larger Europa Clipper concept, there are several other factors that could alter planning for a Europa mission.

One alternative approach could speed up the Europa Clipper mission—or some alternative to it—by replacing the Atlas V currently baselined for the mission with the Space Launch System (SLS) heavy-lift rocket. While Europa Clipper spends more than six years en route to Jupiter, making one gravity assist flyby of Venus and two of Earth, the same spacecraft launched on the SLS could go directly from Earth to Europa in less than half the time: one scenario has the spacecraft launching in June of 2022 and arriving in March of 2025.

In recent months, NASA officials from Bolden on down have discussed the possibility of using SLS for launching a Europa mission. At January’s OPAG meeting, Steve Creech, SLS assistant program manager for strategy and partnerships, said that with even the large design margins in the early phases of SLS’s development, the rocket an easily launch in excess of five tons directly to Jupiter. “There’s a nice opening on the [SLS] manifest for Europa” in the early 2020s, he added.

One issue, though, with using the SLS is its cost, which is likely to be significantly more than the most powerful version of the Atlas V. Science missions are at least somewhat shielded from launch costs, as the cost caps for programs like New Frontiers and the smaller Discovery line of planetary missions exclude launch services. Yet, as one OPAG attendee said in a question to Creech about the use of SLS, “I kind of doubt we can afford to pay for it.”

Creech said that while discussions were underway within NASA on how to account for SLS costs charged to science mission, the goal was to make the SLS “cost neutral” for those missions; that is, subsidized so that the cost of a launch to a mission would be no more than alternative vehicles like the Atlas V 551 currently baselined.

“At the most basic level, the SLS guys need a reason to launch. They need people who want to use the rocket and, frankly, we need big rockets,” said Curt Niebur, outer planets program scientist at NASA headquarters, during the OPAG meeting.

Scientific discoveries could also have an effect on any Europa mission. In December, scientists announced they had detected plumes of water vapor erupting from the south polar regions of Europa in ultraviolet observations by the Hubble Space Telescope. Such plumes had been seen erupting from another icy world, Saturn’s moon Enceladus, but never before from Europa.

“If those plumes are connected with the subsurface water ocean we are confident exists under Europa’s crust, then this means that future investigations can directly investigate the chemical makeup of Europa’s potentially habitable environment without drilling through layers of ice,” said Lorenz Roth of Southwest Research Institute, lead author of the study, in December’s announcement.

How the existence of plumes will affect the science mission is still being studied, but the prospect of directly sampling the interior of the moon using these plumes excites some scientists. “Wouldn’t you love to have a highly capable mass spectrometer flying through those plumes and making those measurements?” Stofan asked during a talk at a meeting of the Universities Space Research Association in Washington last month.

While Europa Clipper spends more than six years en route to Jupiter, making one gravity assist flyby of Venus and two of Earth, the same spacecraft launched on the SLS could go directly from Earth to Europa in less than half the time.

Europa, though, may be getting more competition for astrobiological studies from Enceladus. Last week, scientists reported findings confirming the presence of a subsurface ocean in Enceladus. That, coupled with the large plumes already detected, makes that world a more promising candidate for a mission focused on astrobiology than Europa, at least in the eyes of some scientists.

In a paper published in the journal Astrobiology last week, a group of scientists led Chris McKay of NASA Ames argued that Enceladus is the only known world beyond Earth that has all the known requirements for life: water, energy, carbon, and nitrogen. Moreover, any life detected there would likely have developed independently of the Earth, unlike Mars or even Europa, where there is the possibility of the exchange of materials via meteorites. “If life is detected on Enceladus, it is more likely that we will have found a second genesis of life in our Solar System,” they wrote.

In the paper, McKay and others argue for an Enceladus sample return mission: a spacecraft that would sample material in that moon’s plumes and return them to Earth for analysis. “[T]he plume of Enceladus currently represents the best astrobiology target in the Solar System and that missions to the plume must go beyond demonstrating habitability to the search for evidence of life and would best involve both in situ analyses and sample return,” they write.

Europa, though, still has the interest of the agency as well as its patrons in Congress that have funded mission studies even when NASA didn’t request them. But as technical capabilities and scientific priorities shift, even NASA’s newfound interest in a Europa mission is no guarantee it will fly any time soon.