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.