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Van Kane

Europa: How Less Can Be More

Posted by Van Kane

26-08-2014 18:55 CDT

Topics: Jupiter's moons, Europa, Mercury, MESSENGER, Europa Clipper, JUICE

Three factors make exploring Europa hard. First, we want to explore an entire complex world, and mapping its features requires acquiring vast amounts of data. Second, Europa lies far from the Earth, which necessitates capable communications and power systems (read, “expensive”) to return the data to Earth. Third, Europa lies well within the harsh radiation fields surrounding Jupiter, which both requires significant radiation hardening (again, read, “expensive”) and limits the life of any spacecraft that explores this world. These factors can make a mission concept that seems like less actually be more.

The limiting factor on science for most planetary orbiters is not the time the instruments can make observations. Rather it is the time available to return data to Earth because many instruments can gather data far faster than the communications system can transmit it to antennas on Earth. (There also are a limited number of antennas to listen to planetary spacecraft, so few missions receive continuous coverage, and spacecraft often cannot continuously transmit either because they must turn to observe the planet or the planet itself blocks communication.) 

To get a sense of the challenges, compare the problems of exploring Mercury and Europa. A mission to Mercury must deal with the intense heat coming from both the Sun and the planet surface. However, a spacecraft designed to overcome that challenge can continue to function until its fuel is exhausted. As a result of the luxury of spending years in orbit around Mercury and the fact that Earth is never more than 222 million miles away, NASA’s MESSENGER mission has been able to return terabytes of data to Earth. Between its orbital insertion in March 2011 and March 2012, the spacecraft generated 2.3 TB of data to be archived by NASA. The mission continues to operate today, so the total returned to date should be substantially more. The maximum data rate for this $446M (2008 dollars) mission is 104 kilobits per second.

By comparison, it’s cold at Jupiter, but it is the intense radiation around Europa that limits spacecraft life. Different mission studies have assumed lifetimes in orbit between one ($1.6B estimated cost, 2015 dollars) and nine months ($4.3B estimated cost), many times shorter than the approximately four Earth years MESSENGER will have at Mercury. While Jupiter is never closer than approximately 2.7 times as far from Earth as Mercury, more capable spacecraft systems would allow data rates of around 135 kilobits per second. With a lifetime of one month in orbit, the data return would be around 334 gigabits, and with a lifetime of nine months, around 4.5 TB. (Different mission designs made different assumptions about data return, so nine month mission data return isn’t a simple multiple of the one month mission data return.)

Comparison of mission data returns

Van R. Kane

Comparison of mission data returns
Comparison of data return for different Europa mission concepts with the data archived from NASA's Mercury MESSENGER orbiter from its first terrestrial year in orbit.

These challenges for exploring Europa have been well known since the Galileo Jupiter orbiter in the 1990s all but proved that Europa likely has a vast ocean that could harbor life that lies under a relatively thin icy shell. As mission planners and budget directors have wrestled with this problem, we’ve been through at least five distinct eras of Europa mission planning. (There have also been various proposals by independent teams for simpler and cheaper missions, which may or may not have been feasible for their proposed costs.)

Immediately following the Galileo spacecraft’s discoveries, JPL conducted preliminary mission studies that envisioned a capable spacecraft using conventional technology to orbit Europa.

In the late 1990s, NASA’s then Administrator redirected efforts to a mission concept that would use yet-to-be-developed technologies (the X-2000 project) to dramatically lower mission costs to the neighborhood of MESSENGER’s cost. By the time the program was canceled in 2002, mission estimates had shot from around $190M to $1.4B (early 2000’s dollars). 

Not to be outdone, the next NASA administrator proposed the Battlestar Galactica of missions, the Jupiter Icy Moons Orbiter (JIMO) that would orbit the moons Callisto and Ganymede in addition to Europa. This mission depended on the development of radically new capabilities such as space-rated nuclear fission reactors to power the spacecraft. This $16B concept died quietly when the administrator left NASA.

JUICE (JUpiter Icy moon Explorer)


JUICE (JUpiter Icy moon Explorer)

If the previous two efforts were perhaps fanciful, the next effort, the 2008 Jupiter Europa Orbiter (JEO) concept was based solidly on feasible technology. This highly capable spacecraft would have conducted extensive studies of the Jovian system before beginning nine months in orbit around Europa with a highly capable instrument suite. This was the mission any fan of Europa really wanted. Unfortunately, an estimated $4.3B price tag doomed the concept.

JEO data by instrument

Van R. Kane

JEO data by instrument
The bulk of the data that would have been returned by the 2008 Jupiter Europa Orbiter concept would have been produced by the ice penetrating radar, infrared spectrometer, and a trio of cameras.

Following the JEO studies, NASA conducted studies of three missions that each would have a firm cap of $2B: an orbiter, a multi-flyby spacecraft, and a lander. It was quickly realized that the latter would not be feasible until a previous mission had better studied Europa’s surface to find the best combinations of most scientifically while still safe landing sites.

That left the choice of an orbiter that would spend 30 days circling the moon and a multi-flyby spacecraft that would spend less than a cumulative 6 days close to Europa during 34 flybys. The scientists who reviewed the two missions solidly backed the multi-flyby concept (that has evolved into the current Europa Clipper concept).

So how can 6 days of science be better than 30? For the comparison that follows, I’ll use the assumptions of the 2012 studies. Since that time, the capabilities of the multi-flyby concept have been substantially enhanced into the Europa Clipper concept. Because the orbiter concept didn’t have the additional two years of fine-tuning of the multi-flyby craft, comparing their 2012 conceptions allows comparison of equally developed concepts.

Europa data volume by instrument

Van R. Kane

Europa data volume by instrument
Comparison of data that would be returned by instrument for the 2012 multi-flyby and orbiter mission concepts.

Between each of the flybys, the multi-flyby spacecraft would have seven to ten days to transmit data stored during each brief encounter back to Earth. That would let the multi-flyby craft have up to a year of time to transmit its data compared to just 30 days for the orbiter. The result would be almost three times as much data returned to Earth. (Differing assumptions about how much of the time antennas would listen to the spacecraft mean that the amount of data returned is not a multiple of time.)

The larger data return of the multi-flyby spacecraft would enable the spacecraft to carry two high priority instruments that generate large amounts of data. The more data hungry of these, the ice penetrating radar, would study the structure of the icy shell beneath the surface. This would allow scientists to study whether bodies of water are trapped within the ice between the surface and the ocean below and fracturing of the shell. The radar might penetrate through the shell to the top of the ocean to measure the total depth of the icy shell. These measurements will help scientists understand how material is transported between the ocean and the surface.

The second instrument, a short-wave infrared spectrometer, can identify materials exposed on Europa’s surface and map their distribution. Scientists believe that Europa’s surface exposes materials transported from the ocean below, where we can easily see it and eventually study it with a lander. The interaction of materials on the surface with Jupiter’s radiation field creates chemicals that may be transported to the ocean below to be available for use by any life. This spectrometer would map the presence and distribution of these materials across Europa’s globe.

Both the 2012 orbiter and the multi-flyby spacecraft would carry a third data-hungry instrument, a topographic imager that would map the surface.

Not all potential instruments require high data rates. The orbiter would have carried a trio of instruments that required measurements from around the globe: a laser altimeter to measure surface tides to enable estimates of the thickness of the icy shell and a magnetometer and plasma instruments that would have enabled estimates of the volume and salinity of the underlying ocean. Unfortunately, the measurements of these instruments are lower priority than those for a radar and shortwave infrared spectrometer. (In the 2012 study, the multi-flyby spacecraft also would have carried a heavy, power-intensive, but low data rate mass spectrometer that would directly sample material sputtered from the surface.)

The importance of the ice penetrating radar and mid-IR spectrometer tipped the weight of opinion in favor of the multi-flyby concept. Given a limited number of encounters that would fly over just a tiny fraction of Europa’s surface, they key was to distribute those flybys to fly over key locations. 

With two years of further study, the multi-flyby concept that evolved into the Europa Clipper concept has added an additional eleven flybys (for a total of 45) and several instruments compared to the 2012 concept. By balancing the placement and number of encounters with many months to return data, the Europa Clipper concept would enable a $2B mission that conducts the most crucial measurements of the $4.3B JEO concept. The $1.6B orbiter concept couldn’t match this feat.

However, the Europa Clipper is not NASA’s plan for a Europa mission. White House budget analysts and NASA’s senior management are looking for a $1B concept that wouldn’t do the job of the Europa Clipper but would still do significant science. Earlier this summer, they reportedly received six proposals that target this cost cap. NASA ’s managers are examining the proposals to ensure that they are both fiscally and technically feasible within the budget. In the meantime, they are not releasing any information about the types of missions proposed.

From what I understand, much of the scientific community and many NASA managers are skeptical that a meaningful mission can be done within a $1B budget. Sometime in the coming months we will learn whether NASA thinks any of the proposals have merit. If they do, then the broader scientific community will weigh in with its assessment. 

I’ve argued in a previous post that a $1B mission is likely technically possible, but I have doubts about whether it could address enough high priority science to be worth the expenditure. The coming months will see if I’m proved wrong or not.

In the meantime, NASA continues to refine the Europa Clipper concept, which so far has shown the best balance between doing more with less to perform the critical science for the next step in exploring this world.

Europa Clipper concept


Europa Clipper concept
Current concept for the Europa Clipper mission, which is an evolved version of the 2012 multi-flyby concept.


Jupiter Europa Orbiter Final Report

2012 Europa mission concept studies

Current concept for the Europa Clipper mission

See other posts from August 2014


Or read more blog entries about: Jupiter's moons, Europa, Mercury, MESSENGER, Europa Clipper, JUICE


Eriknz: 08/27/2014 02:57 CDT

Why can't NASA work internationally to explore Europa. That would enlarge the potential for mission design tremendously.

Arbitrary: 08/27/2014 03:59 CDT

A lander would have the benefit of radiation shielding by Europa itself. If it lands in a valley or crater it would only need to bring a shielding roof. The mass budget can be spent on instruments instead of on shielding. And a seismometer on the ground should be the highest priority. One reconnaissance flyby should be enough to find a good landing location. It should not be impossible for it to hop to a second location on that airless low gravity body. @Eriknz International cooperation means much more politics and special interests fighting for "employment opportunities" and more levels of incompatible bureaucracies. It costs much more and takes much longer time and has a high risk of suddenly being canceled for completely unrelated reasons. Fusion power research stalled when it got international. Europeans choose rocket design based on the influence of the politicians from where solid and liquid rocket engines are manufactured. Every election in every country changes the plans.

Enzo: 08/27/2014 05:23 CDT

"In the 2012 study, the multi-flyby spacecraft also would have carried a heavy, power-intensive, but low data rate mass spectrometer that would directly sample material sputtered from the surface." Maybe not such a bad idea if the plumes that seem to have been observed are real.

Stephen: 08/27/2014 08:47 CDT

"This was the mission any fan of Europa really wanted. Unfortunately, an estimated $4.3B price tag doomed the concept." Actually, it was the 2013 Decadal survey's own cost analysis which affixed a price tag of that order, and that was for $4.7 billion. (The JEO team itself estimated $3.4 billion.) But that merely begs a question. If those JPL studies of 2012 were so conclusively in favour of the Clipper that all the boffins sided with it rather than with the JEO why then did the Decadal Survey recommend the JEO as the Europa mission of choice? In fact why was there no Europa flyby mission in some form in that Survey at all? "That left the choice of an orbiter that would spend 30 days circling the moon and a multi-flyby spacecraft that would spend less than a cumulative 6 days close to Europa during 34 flybys." That comparison is misleading since it appears to suggest that the 30-day orbiter would have dropped straight into a Europan orbit. In reality,a JEO would have dropped into a Jovian orbit first and spent the better part of 3 years doing 24 flybys of various Galilean moons, including 6 of Europa, to pump the orbit down, before finally dropping into a Europan orbit as the final stage, just as MESSENGER did three flybys of Mercury before going into a Mercurian orbit.. Presumably, it would not have been impossible to have increased the number of Europan flybys by decreasing the number of Ganymede and Callisto ones. Either way, the JEO would have had the best of both worlds. A flyby component with its opportunities for lengthy data returns combined with an comparatively brief orbiter component at the end.

stone: 08/27/2014 09:16 CDT

@Eriknz and Arbitrary The last years provided the clear evidence that a international cooperation of NASA are killed not because of political or monetary reasons from abroad but only by mismanagement from US side. The Jupiter mission and the joint mars rover missions would have provide good chance to for international cooperation but instead of cooperation NASA only projects were started. The joint rover mission 2018 which would have used a Curiosity clone is now one slot later and now called 2020, no cooperation but a lot of very angry people at ESA. The Jupiter mission was not much different. NASA is not a good cooperation partner on misson level.

Van: 08/27/2014 01:49 CDT

The Decadal Survey did not request a Europa multi-flyby mission, but did state that the cost of a Europa mission had to be substantially reduced before it should be included in NASA's program. So far as I know, the 2012 study was the first to conduct an in-depth study of the science possible from a multi-flyby mission and then compare it to a similar cost (<$2B) orbiter. The 2012 orbiter concept envisioned a number of Callisto, Ganymede, and Europa flybys prior to Europa orbit insertion. However, the orbiter payload was optimized for measurements from orbit, and the science from flybys would have been limited. The report does not discuss flyby science.

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