Titan Mare Explorer
Speaking of intriguing moons, NASA has long been interested in the Saturnian satellite Titan, which has a thick atmosphere and methane oceans. In 2011, the Titan Mare Explorer proposed launching a specialized boat to sail Titan's seas by 2023. It would have been the first time any extraterrestrial sea had been explored, but NASA chose to develop the InSight Mars mission instead. The Titan Saturn System Mission will replace it, and is scheduled for launch within 20 years.