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STORY: Over the next five and a half years a NASA spacecraft, Europa Clipper, will make the 1.8-billion mile journey to one of Jupiter's moons.Its mission: to find out if life could exist in underground oceans of an ice-encased world.:: Why Europa?The moon is considered one of our solar system's most promising spots to search for life beyond Earth.Scientists have a keen interest in a vast salty liquid water ocean that previous observations have indicated resides under Europa's surface.That's beneath an icy shell believed to be 10-15 miles thick.NASA planetary scientist Bonnie Buratti said there is "very strong evidence that the ingredients for life exist on Europa".:: The ingredients for life Buratti pointed out that there are three main requirements for life to form.These are liquid water, an energy source, and certain chemistry - specifically organic compounds that could serve as food for any primitive organisms.Europa is considered to be an "ocean world."Though it is just a quarter of Earth's diameter, its deep subsurface ocean may contain twice the water in Earth's oceans.Because of its distance from the Sun, Europa receives only about 4% of the solar radiation that Earth gets.However, Buratti describes how the moon flexes as its orbit comes nearer and farther from Jupiter thanks to the gas giant's gravitational pull.That process produces heat, an energy source, on the moon.Scientists also speculate that there may be thermal vents at the bottom of the ocean where heat releases chemical energy.:: What will Europa Clipper do?Europa Clipper has three main scientific objectives.They are: gauging the thickness of Europa's outer layer of ice and its interactions with the subsurface below, figuring out the moon's composition, and determining its geology.It is the biggest spacecraft NASA has ever built for a planetary mission - larger than a basketball court.That's due to its sizeable solar arrays, used to power scientific instruments, electronics and its other subsystems.One of the nine instruments on board is MASPEX.It will sample gases to study Europa's ocean, surface and atmospheric chemistries.It will, Buratti said, look for "sophisticated organic molecules that could provide food."But there is a long wait for answers. Europa is expected to enter Jupiter's orbit in 2030 - having used the gravities of Earth and Mars to slingshot its way through the solar system.But Buratti says exploratory missions like this one always uncover something "that we could not have imagined."She said there will be something there, something "unknown," and that is going to be, quote, "so wonderful that we can't conceive of it right now. That's the thing that excites me most."