When NASA was deciding where they were going to focus their efforts after landing Curiosity on Mars, they ultimately went with more Mars missions — the Maven orbiter in 2013, the InSight lander in 2016 and now another rover in 2020. However, although the price tag was a little too high, one other idea stood out to them: a mission to Jupiter's moon, Europa.
Although Jupiter has a total of 67 confirmed moons, the four largest ones stand out from the others. They are called the 'Galilean Moons' — named after their discoverer Galileo Galilei — and Europa is the smallest of these, and is the second of them out from the planet (Io is the closest of the four).
Europa's surface — at least the one that's visible to us — is a thick layer of water ice and planetary scientists have speculated for years that this layer of ice may be shielding a global ocean underneath it, which is kept warm due to tidal forces between Europa and Jupiter. The potential existence of this ocean and the possibility that it is in direct contact with Europa's rocky mantle, as well as the energy sources available to that ocean (not only from the tidal forces, but also radiation from Jupiter's magnetosphere), make this moon one of the best candidates in the solar system for finding life beyond the Earth.
The original proposal for a journey to the icy moon — called the Jupiter Europa Orbiter (JEO) — was supposed to be part of a joint mission with the European Space Agency (ESA). However, when NASA cancelled the JEO due to budget constraints, the team working on the plan went back and scaled-down not only the design of the probe, but also the scope of the mission. The ESA has continued with its own plans, though, and is scheduled to launch the Jupiter Icy Moon Explorer (JUICE) mission in 2022.
Now, instead of an orbiter that would have needed extensive shielding to withstand the punishment of being inside Jupiter's magnetosphere (which extends far out from Jupiter's surface) for the duration of the mission, the former JEO team is looking to send a small 'clipper' probe that would buzz by the moon several times, taking readings of its icy surface and tenuous oxygen atmosphere, and possibly confirming the presence of its subsurface ocean.
NASA already has one probe on its way to Jupiter, but its focus will be on the immense planet itself. The Juno spacecraft was launched last year and will arrive at its destination in 2016. It is programmed to enter a highly-eccentric polar orbit around the planet, similar to how many comets orbit the sun — approaching very close and then swinging far out before coming in for another close approach. This orbit will allow the spacecraft to avoid being fried in the intense radiation belt that surrounds Jupiter while it makes a detailed investigation of the planet's atmosphere, inner structure, gravity and magnetic field.