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.)
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.
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.
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.
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.