Destination: Europa

     Take a rocket to the Outer Solar System, hang a sharp right when you hit Jupiter (trust me you'll see it). Go on for about 600,000 kilometers and you'll arrive at one of the most interesting moons in all of Sol: Europa.
     The moon was first discovered in 1610 by Galileo Galilei and is one of the four original Galilean Moons. It was, at the time, thought to be the second farthest moon from Jupiter, though later observations by the Voyager probe showed four more inner satellites, making Europa Jupiter's sixth moon. The moon is about one fourth the size of Earth, having a radius of 1560 km, and orbits its host planet every three and a half days at an average range of 670,000 km.
     So why should you care about tiny little Europa? Two words will answer your question: Liquid Water. Evidence suggests that under a few kilometers of hard icy crust lies a massive planet-wide ocean. This ocean could very easily harbor all sorts of simple and complex lifeforms, protected from radiation via the ice above, instead of a typical atmosphere. Creatures that evolved here would likely rely on nutrients from thermal vents, similar to the ones found in the deeper parts of the Earth's oceans, and would have evolved completely different sensory organs then what we're used to. Creatures never exposed to visible light might "see" in electromagnetic fields, much like a shark's sense of electroreception (click to learn more). The search for extraterrestrial life might well lead us into our own backyard.

     Though no probe has ever specifically been sent to Europa, many have studied it as they passed by, including Pioneer 10 and 11, Voyager 1 and 2, Galileo, and New Horizons. Several missions to the moon have been planned over the years, but were cancelled for various reasons. In 2012, the ESA selected the Jupiter Icy Moon Explorer (JUICE) as a planned mission, with a launch date in 2022. JUICE is an orbiter, meaning while it will be able to collect a large amount of data on the moon, it won't be able to directly probe for life. For that we need something a little more invasive; like a nuclear-powered "melter probe". A probe of this type would melt/drill its way to the subsurface ocean, and then release one or more autonomous probes to scan for life and collect samples. The picture is an artist's concept of this method.