DENVER — NASA has selected missions to return samples from a comet and to explore Titan with a drone as finalists for the next New Frontiers medium-class planetary science mission.

The two missions, selected from a pool of 12 proposals and announced by NASA Dec. 20, will receive funding for additional studies through 2018 before NASA picks one of them in the spring of 2019 for full development and launch in 2025.

One, called Comet Astrobiology Exploration SAmple Return, or CAESAR, would visit the comet 67P/Churyumov-Gerasimenko and collect a sample from its nucleus for return to Earth, the first such sample return mission. The mission would be managed by NASA’s Goddard Space Flight Center using a spacecraft built by Orbital ATK.

The other, Dragonfly, would send a spacecraft to land on Titan, Saturn’s largest moon and a world with a dense atmosphere. That spacecraft, similar to a quadcopter drone, would be able to fly to several locations on the surface, tens to hundreds of kilometers apart, to study its composition and habitability. The mission would be managed by the Applied Physics Laboratory (APL) of Johns Hopkins University, which would also build the spacecraft.

The announcement of two missions for further study surprised many in the planetary science community. NASA officials had previously suggested that the agency would select three, or possibly more, proposals for additional study.

Thomas Zurbuchen, NASA’s associate administrator for science, said at a media teleconference he had no set number of proposals in mind to select for further study. “I didn’t start with a number,” he said, focusing instead on the “outstanding and visionary science” the missions promised to carry out. “Basically, where I ended up with is the two that we’re currently talking about.”

Zurbuchen and others didn’t go into details about how CAESAR and Dragonfly beat out the other 10 proposals. “This is a very tough competition,” said Jim Green, director of NASA’s planetary science division. “The science in the two selected was just the top science.”

Both missions will go to objects already visited by earlier missions. In the case of CAESAR, comet 67P/Churyumov-Gerasimenko was studied by the European Space Agency’s Rosetta mission, which ended last year. That mission also deployed a lander, Philae, that operated briefly on the surface of the comet’s nucleus.

That previous mission will help the development of CAESAR. “The reason that we’re going back to Churyumov-Gerasimenko is that it provides us with an enormous amount of information about how to conduct our mission,” said Steve Squyres of Cornell University, the principal investigator of CAESAR. “We are able to design our mission, design our spacecraft, specifically for the conditions that we know to exist there.”

CAESAR is designed to collect both volatile ices and nonvolatile materials, which will be separated and placed in different containers in a return capsule. That return capsule will be provided by the Japanese space agency JAXA, based on its experience with the Hayabusa asteroid sample return missions.

NASA’s Cassini spacecraft studied Titan extensively in a 13-year mission that ended in September, while ESA’s Huygens probe landed on Titan’s surface in early 2005. “That is exceedingly useful for being able to characterize potential landing site and know where on Titan the best places to land are,” said Elizabeth “Zibi” Turtle of APL, the principal investigator for Dragonfly.

“Those missions left us with a lot of fundamental unknowns,” she added, such as the basic composition of Titan’s surface. “Dragonfly is designed to go back and build on what we learned from Cassini/Huygens and answers the fundamental unknowns that remain about Titan.”

Losing out on the competition were proposals that ranged from Venus orbiters and landers and a lunar sample return mission to missions to Saturn and its icy moon Enceladus.

However, NASA said it would provide technology development funding for two of those concepts, Enceladus Life Signatures and Habitability (ELSAH) and Venus In situ Composition Investigations (VICI). The exact amount of funding, and the scope of work, will be negotiated with those proposal teams.

“What are the investments in technology that would not only help a specific mission but also help a field?” Zurbuchen said of the decision to support technology for those two mission concepts. These would include life-detection instrument technology in the case of Enceladus and, for Venus, instruments that can operate in the high temperatures found on the planet’s surface.

The winning mission, CAESAR or Dragonfly, will be the fourth in NASA’s New Frontiers program of missions larger than Discovery-class spacecraft but less expensive than flagship-class missions. The first three missions are New Horizons, which flew by Pluto in 2015 and will fly by the Kuiper Belt object 2014 MU69 in just over a year; Juno, currently orbiting Jupiter; and OSIRIS-REx, a mission en route to the asteroid Bennu to collect samples for return to Earth.

Either CAESAR or Dragonfly, if selected, will operate well into the 2030s. Dragonfly would land on Titan in 2034, Turtle said, and operate there for several years, powered by a radioisotope thermoelectric generator. CAESAR, Squyres said, would return its samples to Earth on Nov. 20, 2038.

Proior to working on CAESAR, Squyres had led the Mars Exploration Rovers program, which landed the rovers Spirit and Opportunity on Mars in early 2004. Opportunity continues to operate, nearly 14 years after landing. “Knowing when the thing is going to end,” he said of CAESAR, “is a virtue.”