Jupiter's moon Europa has been a priority destination for NASA's planetary program since the mid-1990s. With a deep ocean trapped beneath an icy shell on top and the rocky surface below, Europa is believed to have the chemicals and energy needed to host life. Over the course of almost two decades, I've seen plans for a better, really cheaper, faster mission that just needed a lot of new technology to be developed. As if to balance that plan out, there was a plan for the planetary equivalent of a Battlestar Galactica mission that was both unaffordable and also required technology that still doesn't exist. I thought we were close with the Jupiter Europa Orbiter (JEO, circa 2010) until new cost estimates showed that it, too, was unaffordable.
Now we have a proposed mission, the Europa Clipper, that doesn't require substantial technology development and that has a cost estimate (~$2B) that puts it well within the cost range of NASA's larger science missions. However, in today's era of declining US federal budgets, the Clipper's price tag is deemed unaffordable.
In a conversation with scientists on a NASA advisory panel, the head of the space agency's Science program, John Grunsfeld discussed whether NASA should look at a Europa mission for half that of the Clipper mission. If it could be done, then a Europa mission could fit in the established New Frontiers program of planetary missions. (I want to emphasize that Grunsfeld's conversation was informal and wasn't announcing a policy decision.)
Grunsfeld's comments made me curious. Estimates for the last two serious Europa proposals have come in at $4.7B (JEO) and ~$2B (Clipper). Was a mission for ~$1B (the recommended cost cap for future New Frontiers missions) credible? In my post today, I report on the results of my thought experiment .
To give you my conclusion first, yes, a Europa New Frontiers mission seems to be a credible idea to examine. However, I come away even more impressed with the Europa Clipper proposal and that's the mission I want to see fly.
The analysis below is somewhat wonkish as I document the assumptions and rational behind my thought experiment. Based on emails I receive from readers, getting any mission to Europa is a desire of many. I want to be clear on the analogies I'm drawing and assumptions I'm making. And remember that this is a thought experiment by an interested layman. It's all just fun speculation until a team of planetary scientists and engineers does the real work to evaluate the feasibility and science return.
A mission that orbits Jupiter faces a number of technical challenges. The spacecraft must be powered (and sunlight is dim at Jupiter). Jupiter possesses electronics-frying radiation (and Europa sits within the high radiation belt). That is in addition to the normal challenges of any planetary mission to operate a suite of instruments, store their data, and return the results to Earth.
We have proof that a capable Jupiter spacecraft can be built within a New Frontiers budget because NASA did so with the Juno spacecraft that is en route to Jupiter now. NASA qualified solar panels to work at Jupiter and developed shielding strategies to protect electronics from the worst of the radiation. The Juno spacecraft also carries a highly sophisticated instrument package to study the giant planet. (I was surprised to find a technical report on the mission that lists the total mass of the instruments at a large 155 kilograms. This, however, might have been a preliminary figure.)
In some ways, Juno is a simpler spacecraft than one that would study Europa. Juno spins like a top to provide stability rather than having to provide the more expensive rigid and precise 3-D pointing that would be needed for the cameras and other instruments to study a moon's surface. Juno's instruments also produce relatively small amounts of data compared to the instruments required to study Europa. That additional data for a Europa spacecraft requires expensive data storage and a more capable communications system and more power for the spacecraft.
In 2010, however, NASA completed studies of two New Frontiers-class missions to study Jovian moons as part of a planning process known as the Decadal Survey. I looked at those reports for clues about the capabilities a Europa New Frontiers mission might have in a credible design that meets the technical and environmental challenges of operating in the Jovian system. One of the reports described a multi-flyby spacecraft to study the volcanic moon Io (the Io Observer) and the other described a small orbiter for the icy moon Ganymede.
6 to 10
Time between flybys
7 to 21 days
Telemetry data rates
Like the Europa Clipper mission, the Io Observer would perform multiple flybys of its target moon to study its features. The cheaper Io Observer mission, however, would carry far few instruments, return much less data, and encounter its moon many fewer times than the Europa Clipper mission.
The two reports also showed that the New Frontiers missions would carry just four instruments each. (Technically, the magnetometer and plasma instrument for each mission are distinct, but they support each other's measurements and because they are low mass, I've combined them in the following table.)
Ganymede minimal orbiter
High resolution imager
Moderate resolution imager
Total mass (kg)
The science team for the Europa Clipper study has grouped the science goals into four categories and identified nine instruments to meet them. I looked at subsets of the proposed Clipper instruments that would have a similar mass to the instruments proposed for the Io Observer and fulfill one or more science goals. (A real mission's instruments also need to fit within the spacecraft's electrical power and data limits. However, I couldn't find a combination of instruments that fit within the mass limits but exceeded the power and data limits, so I'm showing only instrument masses.)
One option for a Europa New Frontiers mission would study the icy shell that covers the ocean and that has been shaped by the movement of ice blocks and plumes of ice and water within the shell. Understanding the icy shell would allow scientists to understand the forces that shaped the shell and that likely brought water from the oceans below to the surface. The topographic imager would be a camera that would image much of the surface in multiple colors at 25 to 200 m resolution.
The ice penetrating radar would study the structure of the ice and any “bubbles” of water within the ice. The radar proposed for the clipper mission would be particularly capable. Operating in its shallow mode, it would penetrate just 3 kilometers into the shell but would have a vertical resolution of structures of 10 m. Operating in its deep mode, it would penetrate 30 km but with a vertical resolution of 100 m. Less capable, and lighter, radar instruments are possible. The European JUICE mission to the Jovian system will carry a radar instrument that is just 12 kg but that can penetrate just 9 km with a vertical resolution of 30 to 90 m.
Icy shell studies
Ice penetrating radar
A second instrument option would not include the heavy (and data and power hungry) radar instrument, but would instead focus on studying the composition of the surface with an infrared spectrometer and a mass spectrometer. (The latter would “taste” molecules blasted from the surface by Jupiter's radiation or expelled from beneath the surface by possible plumes of gases.) A topographic imager again would study the surface geology which would also provide data on the forces that structure the icy shell. The magnetometer and plasma instrument would study the interaction of Jupiter's powerful magnetic field with the salty water in the ocean to study the extent and salinity (important to understand the composition) of the ocean beneath the icy shell.
Recently, there's been one reported observation of a plume of water being expelled by Europa. If this is confirmed, and the plumes are shown to be persistent and reliable, then a Europa mission might focus on studying those plumes. By doing so, it would study the composition of water either from reservoirs trapped within the ice or from the ocean below. Either way, this would be a unique opportunity to perform the kind of exciting science that the Cassini spacecraft has been doing with the plumes of Saturn's moon Enceladus.
For this instrument list, I have duplicated the minimum list of instruments recommended by a Decadal Survey report on potential missions to Enceladus. For this list, I show the mass of a more capable mass spectrometer than is currently planned for the Clipper mission. (For the technically inclined, the current Clipper mass spectrometer would measure only neutral particles and not ions. The alternative, and several times more massive mass spectrometer, would measure both and have capabilities similar to the Rosetta mission's ROSINA mass spectrometer.)
Before examining the question of whether a Europa New Frontiers mission would be a good investment, I want to emphasize that the instrument lists given above are to illustrate possible capabilities to show that good science likely could be done within the limits of a New Frontiers mission. A professional science and engineering team studying such a mission would almost certainly come up with a better alternative than any of these.
So, would a Europa New Frontiers mission be a good investment at ~$1B if ~$2B couldn't be found to do the Europa Clipper mission? The answer would have to come from a study conducted by planetary scientists and engineers. I'll suggest some of the questions they may ask to get to the answer.
One way to ask the question is whether we would end up knowing far more about Europa than we do today following the Galileo mission of the 1990s. The members of the Decadal Survey answered 'yes' to this question for missions of similar capability to study Io and Ganymede. (The former is on the list of candidate missions approved by the Survey. The latter was left off that list based on the hope that a European mission called JUICE would be approved (which it has been) to orbit Ganymede.) I don't see how a similar capability mission to Europa would be less valuable.
However, there is now an approved European mission, JUICE, which will arrive at Jupiter in the late 2020s. While its focus will be on Jupiter itself and Ganymede, it will make at least two close flybys of Europa. Right now, almost a decade before launch, the JUICE team is committing to just those two encounters. They know, though, of the importance of studying Europa, and I wouldn't be surprised if they don't eventually do a handful of flybys. (The limitation on the number of flybys will be the high radiation exposure each Europa encounter brings. Designing in additional radiation hardening is expensive.)
The JUICE spacecraft will carry a more capable instrument suite than a New Frontiers spacecraft would carry. So would a New Frontiers spacecraft with six to ten flybys provide enough additional science over what JUICE would do with two flybys? Or maybe five JUICE Europa flybys?
There are ways that a New Frontiers mission could compliment rather than compete with JUICE's measurements. Because of orbital mechanics, the JUICE encounters will occur at nearly the same point in Europa's orbit around Jupiter. This means that only one hemisphere of the moon will be sunlit for imaging and infrared spectroscopy. A New Frontiers mission could arrange its orbits to encounter Europa at a location where the other hemisphere is lit. (The Clipper mission proposes to vary its orbit to encounter Europa at two locations for near global mapping.)
If the reported Europa plumes are real and persistent, there's a hint that they operate only (or most strongly) at the point in Europa's orbit where it is furthest from Europa. Unless the gods smile upon us, this is unlikely to be the same point where JUICE will encounter Europa. The New Frontiers craft could tweak its orbit to encounter the plumes where they are most active for repeated passes through them.
It's also possible for the two spacecraft to carry complimentary instruments. JUICE will have good, but heavy, instruments that would allow it to search for and study any Europan plumes from a distance and over time. A New Frontiers spacecraft then could do the up close measurements going to plume locations spotted at a distance by JUICE. A New Frontiers craft could also carry a dust counter and thermal imager that won't be aboard JUICE to better characterize the plume structures and their sources.
The last measure of whether a New Frontiers mission would be good enough would likely be the toughest. Would the science done be good enough that NASA and other space agencies could avoid having to refly a similar mission later at another $1B+ to meet the science goals? This is a different question than asking whether a second spacecraft to make different measurements would be needed. If two New Frontiers missions at ~$1B each did the same science as a $2B Clipper, then doing two cheaper missions is just buying on the installment plan. But what if those cheaper missions couldn't do the right measurements or enough of them to answer the priority science questions?
Here the likely small number of encounters for a New Frontiers spacecraft may be the critical issue. The Clipper has the budget to design in the radiation hardening into the spacecraft and instruments to last for a planned 45 orbits. To keep costs low, a New Frontiers mission likely would not have the radiation hardening for that many orbits. The Io Observer study team thought that six to ten encounters was doable on a New Frontiers budget. (It's difficult for a layman to compare the radiation exposure for an Io flyby and a Europa flyby, so I don't know how to translate the Io limit for a Europa mission. My gut says it's probably many, many fewer encounters than is planned for the Clipper mission.)
I don't know the answer to this question, but suspect that the science definition team would wrestle with it.
So what's my personal takeaway from my thought experiment? A Europa New Frontiers mission seems like a credible idea to explore. However, it would take three New Frontiers-class spacecraft to fly all the instruments planned for the Europa Clipper (assuming ~40kg of instruments per spacecraft). The sum cost would be considerably higher than the cost for the Europa Clipper. In addition, the Clipper would have 45 flybys of Europa compared with just 6 to 10 flybys planned for the Io Observer (and by analogy for a Euorpa New Frontiers spacecraft).
I am deeply impressed with the capabilities planned for the Clipper mission, and if it flies it will be awesome, and the right science will be done to sufficient depth that we won't have to do it again. I want to see the Clipper mission fly.
I'm also not sure that it will be any easier getting $1B than $2B for a mission. NASA's current budget can't afford either one until early in the next decade. So doing either class of mission requires additional funds beyond what is currently planned. I view Uncle Sam like an ornery, miserly uncle who gives you hell whether you ask for $100 or $1000, so you might as well ask for the top figure. (Actually three ornery uncles: the President's budget office, and each house of Congress since they are controlled by different political parties.)
John Grunsfeld gets paid the big bucks to make the decision on whether to study and then propose a New Frontiers mission instead of the more expensive Europa Clipper.
I just want to get to Europa for an in-depth study of that world, and I hope Grunsfeld and his managers can pull either of those magic rabbits out of their hats.
Appendix: Approved JUICE instruments and proposed Europa Clipper instruments. Several of JUICE's instruments will be more focused on studying Jupiter and its magnetosphere than on the moons.
Ice penetrating radar
Visible-IR imaging spectrometer
Particle environment package/ mass spectrometer*
Ganymede laser altimeter
Sub-mm Wave instrument*
Radio & plasma wave investigation
Radio science experiment
*Package of 6 instruments on JUICE 2 instruments on Clipper
Some notes on New Frontiers mission costs: In the past, NASA selected New Frontiers missions that had a cost target of ~$750M for costs that would be managed by the missions' Principal Investigators (PIs): the spacecraft, instruments, mission operations, and data analysis. Additional costs borne by NASA included the launch, cost overruns (if any), increased costs due to factors such as delayed funding and schedule slips. (For example, NASA delayed the launch of Juno to pay the costs of other missions, which increased the time the Juno development team had to be paid.) The final quoted cost for the Juno mission is $1.1B with all these factors included.
A number of New Frontiers missions were studied for the Decadal Survey, and all had engineering team cost estimates, which included launch costs. Four New Frontiers missions had full cost reviews that included launch costs and reserves for were termed as “threats”: overruns, delayed funding, and the like. Including all these costs, these mission concepts were estimated to have total costs to NASA of $1.3B to $1.4B. Based on similarities in mission goals and broad design requirements, a simple multi-flyby Europa mission might have similar total mission costs.
Based on the results of the cost estimates, the members of the Decadal Survey recommended that future New Frontiers missions have Principal Investigator cost caps of ~$1B. NASA would also need to cover launch costs and any cost overruns from project delays or PI cost growth.
The commonly quoted cost estimate for the Europa Clipper, ~$2B, is an approximate mean of costs for several alternative implementations. Launch costs and additional costs from “threats” would be added costs to NASA. Given these, a New Frontiers mission might cost somewhat less than half the cost of a Clipper mission but would likely do much less than half the science (taking into account both a smaller instrument compliment, lower data rates, and fewer flybys).
My take is that on a dollar per science return basis, the Europa Clipper is likely a much better investment than a New Frontiers mission. If the latter is formally studied, I'll be interested to see if the professionals reach the same conclusion.