NASA recently began laying out the groundwork for the technology it will need to fly an unmanned mission to Jupiter's intriguing moon Europa.

Scientists say Europa - which orbits the planet Jupiter about 778 million km (484 million miles) from the Sun - could support life because it might have an ocean of liquid water under its miles-thick frozen crust. NASA said in December the Hubble Space Telescope observed water vapor above the frigid south polar region of Jupiter's moon Europa, providing the first strong evidence of water plumes erupting off the moon's surface.

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In the 2011 National Research Council's decadal survey which guides NASA future mission planning, scientists said a mission to explore Europa and its subsurface ocean offer "one of the most promising environments in the solar system for supporting life -- should be the second priority for NASA's large-scale planetary science missions.

"This moon, with its probable vast subsurface ocean sandwiched between a potentially active silicate interior and a highly dynamic surface ice shell, offers one of the most promising extraterrestrial habitable environments in our solar system and a plausible model for habitable environments outside it. The Jupiter system in which Europa resides hosts an astonishing diversity of phenomena, illuminating fundamental planetary processes. While Voyager and Galileo taught us much about Europa and the Jupiter system, the relatively primitive instrumentation of those missions, and the low data volumes returned, left many questions unanswered. Major discoveries surely remain to be made," the report states.

However, the NRC's study said NASA should fly the Jupiter Europa Orbiter (JEO) only if NASA's budget for planetary science is increased, the report says, and JEO's mission scope is made more affordable. The independent estimate put the overall price tag at a Europa mission at $4.7 billion. The White House 2015 NASA budget request includes $15 million to begin to develop a mission to Europa, so there are no guarantees even that will make is through the congressional financial ringer.

Other "cheaper" Europa ideas are on the board too and are outlined nicely in this Space Review story here.

Beyond the potential budgetary battles, NASA's Science Mission Directorate said it will soon issue three components to begin making up a Europa trip.

The first announcement will be a Request For Information soliciting brief descriptions of mission concepts that address Decadal Survey science objectives for Europa via missions costing less than $1 billion. Upon review of the RFI responses, NASA may solicit and award funds for further study of credible concepts that are judged to be technically feasible, to fit within the 1B cost cap, and to provide adequate science return. This RFI is targeted for release by the end of April.

The second announcement, forecasted here for the end of April 2014, will be a preview Community Announcement describing the planned key parameters of the mission plan or Program Element Appendix. The Community Announcement is intended to convey similar information of interest to potential proposers.

The third announcement will be the Europa science instrument investigations itself, currently planned for release by July 2014, to solicit proposals for instrument investigations conducted by a Principal Investigator as part of a robotic mission to explore Europa.

NASA said that at the conclusion of this formulation phase, it is planned that a subset of these investigations may be selected to continue into Phase B and subsequent mission phases.

NASA also wrote about some key facts about Europa and why it is such a potentially interesting system for further exploration

If the sun were as tall as a typical front door, Earth would be the size of a nickel and Europa would be the size of the dome on the back of the nickel.

One day on Europa (the time it takes for Europa to rotate or spin once) takes about 3.5 Earth days. The length of Europa's day is the same as the amount of time it takes Europa to orbit Jupiter. Jupiter makes a complete orbit around the sun (one Jupiter year) in about 12 Earth years (4,333 Earth days).

Like many other moons (including Earth's moon), Europa is locked by gravity to its planet so that the same side always faces toward Jupiter.

Europa's surface is mostly solid water ice. It is extremely smooth and crisscrossed by fractures.

Europa has an extremely thin oxygen atmosphere -- far too thin to breathe.

Europa and Jupiter have been visited by eight spacecraft, which have performed flybys. Galileo is the only mission to make repeated visits to Europa.

With abundant liquid water, and energy and chemistry provided by tidal heating, Europa could be the best place in the solar system to look for present day life beyond Earth.

If Europa's ocean is proven to exist, it would possess more than twice as much water as Earth.