NASA is in various stages of planning two multi-billion dollar missions to Jupiter's intriguing, ice-covered moon of Europa. One, a flyby mission known as the Europa Clipper, will makes dozens of passes of the moon down to an altitude of about 25km as it assesses the nature of the ice, the ocean below, and looks for clues of habitability. A second, even more ambitious mission, would seek to actually land on Europa, sample its ice, and look for signs of life.
Both missions, but especially the lander, would be among the most complex, daring, and costly planetary science missions that NASA has have attempted. However, both the Clipper and lander are not equally likely to occur. The Clipper is more established. It has been progressing through NASA's multi-tiered review process and has a launch date of 2022. In the President's budget request for fiscal year 2019, it also received $265 million in funding.
The lander mission has always seemed more tenuous, partly because it represents such a breathtaking challenge to land on an icy moon so far away—a nightmare glacier that is irradiated by nearby Jupiter, and where the creaky surface rises and falls. In terms of complexity, the Europa Clipper spacecraft has a mass of about 6 tons, and the lander spacecraft will probably end up with a mass of about 16 tons.