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By Alan Boyle

It's likely to be a decade or more before NASA begins an up-close study of Europa, an ice-covered moon of Jupiter that's thought to harbor a habitable ocean — but the space agency has already picked out the scientific instruments for the job.

"We're excited about the potential of this new mission and these instruments to unravel the mysteries of Europa in our quest to find evidence of life beyond Earth," John Grunsfeld, associate administrator for NASA's Science Mission Directorate, said Tuesday in an announcement laying out the lineup.

The instruments will fly aboard a spacecraft that's due to make 45 or more flybys of Europa over the course of three years, Europa program scientist Curt Niebur said. The closest flyby would pass 16 miles (26 miles) above the icy surface. NASA expects the mission to launch sometime in the 2020s, but many of the details — including the precise cost and the launch vehicle — still have to be worked out.

During a Tuesday news conference, Niebur said the spacecraft won't carry a "life detector," because scientists don't yet know enough about Europa's interior to answer the big question. But the probe's instruments should be able to determine whether the watery ocean that scientists assume lies beneath Europa's ice is habitable.

"They can actually, from orbit, teach us about the chemistry happening at the bottom of Europa's ocean, without actually having to dive into that ocean," Niebur said.

Nine instruments were selected out of 33 proposals that were put forward by teams of researchers in response to NASA's solicitation:

  • Plasma Instrument for Magnetic Sounding: PIMS can use magnetic readings to determine the thickness of Europa's icy shell, as well as the depth and saltiness of the ocean below.
  • Interior Characterization of Europa Using Magnetometry: ICEMAG will measure Europa's magnetic field and work in conjunction with PIMS to take electromagnetic soundings of the subsurface ocean.
  • Mapping Imaging Spectrometer for Europa: MISE can take spectral readings to map the distribution of organic materials, salts and other chemicals on the moon's surface.
  • Europa Imaging System: Niebur said EIS will be able to map 90 percent of Europa's surface with a resolution of 50 meters (164 feet) per pixel. For some areas, the resolution could be up to 100 times better.
  • Radar for Europa Assessment and Sounding: Ocean to Near-Surface: REASON is a dual-frequency ice-penetrating radar system that's expected to reveal the hidden structure of Europa's ice shell.
  • Europa Thermal Emission Imaging System: E-THEMIS is a "heat detector" that should be able to spot vents where plumes of water are erupting through the ice.
  • Mass Spectrometer for Planetary Exploration/Europa: MASPEX will analyze Europa's extremely thin atmosphere and any surface material that's ejected into space.
  • Ultraviolet Spectrograph/Europa: UVS will watch for water plumes like the ones that the Hubble Space Telescope detected several years ago. (Those plumes have since faded away.)
  • Surface Dust Mass Analyzer: SUDA will measure the composition of small, solid particles ejected from Europa — in effect, sampling the surface from orbit.

Another proposed instrument — the Space Environmental and Composition Investigation near the Europan Surface, or SPECIES — will be developed for future mission opportunities, NASA said.

The last probe to study the moon up close was NASA's Galileo spacecraft, which orbited Jupiter between 1995 and 2003 and made 11 flybys of Europa. Galileo's data led scientists to the conclusion that Europa had a watery ocean beneath its icy shell, boosting the interest in follow-up missions.

Niebur said NASA was allocating $110 million over the next three years for work on the Europa spacecraft's instruments, but it's up to Congress to appropriate the money for the full mission. The estimated cost is on the order of $2 billion, excluding the launch vehicle.

As for the launch date, Jim Green, director of NASA's Planetary Science Division, would say only that NASA is planning for liftoff sometime in the 2020s. Mission planners say getting to Jupiter would require a six-year cruise if an existing launch vehicle was used — but half that time or less if NASA went with its heavy-lift Space Launch Vehicle, which is currently under development.

The Europa spacecraft wouldn't be the only probe in the neighborhood: NASA's Juno spacecraft is due to go into orbit around Jupiter next year — but will focus on the giant planet itself rather than any of its 60-plus moons. The European Space Agency's JUICE mission is set for launch in 2022 to study Jupiter's moons — but will spend most of its time on Ganymede and Callisto, with only two flybys set aside for Europa.

Green and Niebur said the Europa flyby mission, which has been referred to as the Europa Clipper, would complement those other studies. It also could set the stage for a potential landing on Europa and even more ambitious goals.

"It would be great to think that the results from this particular mission would lead in the next decade to some new and exciting concepts about potentiall getting underneath the icy shell ... but that's indeed in the distant future," Green said.