The two proposals were selected from a group of 12 submitted to the New Frontiers program, which supports mid-level planetary science missions.
The first, called Dragonfly, would be an unprecedented project to send a flying robot to an alien moon. Equipped with instruments capable of identifying large organic molecules, the quadcopter would be able to fly to multiple locations hundreds of miles apart to study the landscape on Titan.
This large, cold moon of Saturn features a thick atmosphere and lakes and rivers of liquid methane, and scientists believe that a watery ocean may lurk beneath its frozen crust.
It's "an environment that we know has the ingredients for life," said lead investigator Elizabeth Turtle, a researcher at Johns Hopkins University's Applied Physics Laboratory. With Dragonfly, "we can evaluate how far prebiotic chemistry has progressed."
The Comet Astrobiology Exploration SAmple Return, or CAESAR, mission would circle back to the comet 67P/Churyumov-Gerasimenko, which was visited by the European Space Agency's Rosetta spacecraft from 2014 to 2016.
After rendezvousing with the Mount Fuji-size space rock, CAESAR would suck up a sample from its surface and send it back to Earth, where it would arrive in November 2038 (mark your calendars!).
NASA has sampled a comet before; the Stardust mission collected dust from a comet's gassy outer envelope, called its "coma". But this would be the first mission to return material from a comet's icy surface.
"Comets are among the most scientifically important objects in the solar system, but they're also among the most poorly understood," said Cornell University researcher Steve Squyres, the lead investigator for the mission.
Researchers believe that comets delivered water and organic molecules to early Earth, potentially contributing to the origins of life.
Surface samples from 67P/Churyumov-Gerasimenko will include precious "volatile" molecules that easily turn to gas but are important for understanding the body's origin and history.
The selection of the two concepts was announced in a news conference Wednesday. The missions now enter a concept study phase, when the scientists involved can further develop their proposals.
A final selection will be made in July 2019, and whichever spacecraft is chosen will launch sometime in 2025.
Two of those proposals were also selected for further technological development: the Enceladus Life Finder, which would look for markers of biological activity in the geyser plumes shooting out of Saturn's moon, and the Venus In situ Composition Investigations, which would be the first NASA spacecraft to conduct in-depth exploration of Venus in almost 30 years.
NASA has three New Frontiers missions already in flight: New Horizons, which flew past Pluto in 2015; and Juno, which is orbiting Jupiter; as well as OSIRIS REx, a spacecraft en route to the asteroid Bennu that will send back a sample from the rock's surface in September 2023.
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