Approximately ten days later, the NASA spacecraft would return to Europa and receive the data collected by the penetrator's instruments.

While the penetrator concept is exciting, the devils are in the details. Planetary penetrator missions have been studied for many potential missions. Russia launched two on a mission intended for Mars but which never left Earth orbit. NASA delivered two tiny penetrators to Mars, but they were never heard from after they were released by their carrier spacecraft. Japan spent years developing penetrators for the moon but eventually cancelled the project because of development problems.

A key problem with penetrators is that they need relatively flat landing sites for successful landings. Europa's surface is covered in slopes and rough terrain. Also, penetrators are built to tolerate high vertical velocities, but any lateral velocity can destroy the payload inside. (Put another way, engineers can design for high G's in one direction, but it's hard to design for all directions.) This means that the retro rocket must successfully kill all but the smallest lateral movement so the penetrator moves only vertically during its descent.

Another problem with penetrators is that the space inside is small and any instruments must be built to withstand high impact forces. As a result, there's usually significant development required. The penetrator concept report states that the instruments to study the chemistry of the ice are at a low state of development for use in a penetrator.

Issues such as these have kept penetrators as a great idea that has never been matured enough to become a reliable tool for planetary exploration.

My take on the report describing the penetrator concept is that delivering a penetrator for Europa appears to be a high-risk possibility both for completing the development in time for a launch and for actual delivery. Another significant problem is that the concept craft would have a mass greater than 300 kg, well above the 250 kg NASA is offering.

The other concept studied by ESA's engineers would be a daughter spacecraft that would be a straightforward use of existing technologies. The original idea was for a small spacecraft that could fly through plumes erupting from Europa. This idea seems to have lost its appeal. First, diligent searches have failed to confirm the original observation of a possible plume (which was made at the limits of detectability). Second, NASA's Europa spacecraft is highly capable with cutting edge instruments, and it could fly through any plumes itself.

Between the Juno, JUICE, and Europa missions, almost all of the Jupiter system is already targeted for detailed study. An exception, though, is the extremely volcanic moon Io that sits deep within Jupiter's radiation field. That moon became the target for the second study.

In this concept, NASA's craft would release the European orbiter shortly after the two jointly enter Jovian orbit. The ESA craft then would fire its own engine to lower the perijove of its orbit to encounter Io at least twice.