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NASA has picked two concepts for a solar system mission planned to launch in the mid-2020s -- a comet sample return mission and a drone-like rotorcraft that would explore potential landing sites on Saturn's largest moon, Titan.
Which of these two mission will finally make it will be known only in 2019.
Both missions will receive funding through the end of 2018 to further develop and mature their concepts.
"NASA plans to select one of these investigations in the spring of 2019 to continue on to subsequent mission phases," the US space agency said on Wednesday.
"These are tantalising investigations that seek to answer some of the biggest questions in our solar system today," said Thomas Zurbuchen, Associate Administrator for NASA's Science Mission Directorate in Washington.
The Comet Astrobiology Exploration Sample Return (CAESAR) mission seeks to return a sample from 67P/Churyumov-Gerasimenko, a comet that was successfully explored by the European Space Agency's Rosetta spacecraft, to determine its origin and history.
The other selected mission, Dragonfly, is a drone-like rotorcraft that would explore the prebiotic chemistry and habitability of dozens of sites on Saturn's moon Titan, an ocean world in our solar system.
NASA announced the concepts following an extensive and competitive peer review process.
The concepts were chosen from 12 proposals submitted in April under a New Frontiers programme announcement of opportunity.
The selected mission will be the fourth in NASA's New Frontiers portfolio, a series of principal investigator-led planetary science investigations that fall under a development cost cap of approximately $850 million, NASA said.
Its predecessors are the New Horizons mission to Pluto and a Kuiper Belt object known as 2014 MU69, the Juno mission to Jupiter, and OSIRIS-REx, which will rendezvous with and return a sample of the asteroid Bennu.
The concepts selected for technology development are - Enceladus Life Signatures and Habitability (ELSAH) and Venus In situ Composition Investigations (VICI)
The VICI mission concept will further develop the Venus Element and Mineralogy Camera to operate under the harsh conditions on Venus.