Europa has a diameter of about 1,500 miles, a little less than half the size of the moon. While it’s very far away from the sun, the gravitational flexing caused by its close proximity to Jupiter could result in heat generation that keeps a subsurface ocean liquid. We have seen what appear to be cracks in the ice sheet on Europa, as well as plumes of water erupting from the surface.
NASA is talking about the specifics of the mission now, along with what it hopes to accomplish when it launches in the mid 2020s. Previous plans for a Europa mission called for a spacecraft that would orbit the moon, but there is a great deal of radiation that close to Jupiter that would fry a probe before long. The new mission plan calls for the probe to enter a highly eccentric orbit around Jupiter that would allow it to make multiple close flybys of Europa. The probe would basically dive quickly through the harshest radiation, but then retreat to a safe distance in preparation for the next flyby.
The plan currently includes 45 flybys of Europa over several years. During each encounter, the spacecraft could analyze the composition of the moon’s reddish streaks, measure the effects of Jupiter’s gravity, and search for water eruptions. If geysers are present, it might even be possible to fly the probe through one to collect samples from inside the moon.
If the mission does detect hints of life on Europa, the next step would be to land on the surface and begin the difficult task of breaking through the ice to take a closer look. Finding alien life would, of course, have huge implications for our place in the universe. If life can exist in a subsurface ocean orbiting a distant gas giant, it probably exists many, many other places too.