“These are tremendously exciting missions,” Dr. Green said.
Each team now will get $4 million and about one year to flesh out its idea. NASA will decide in mid-2019 which one of the two to build. The selected mission is to launch by the end of 2025.
NASA flies two types of science missions. In the first kind, which includes the Mars Curiosity rover and an coming spacecraft that will explore Jupiter’s moon Europa, the agency decides where it wants to go and then builds and operates the mission itself.
But for the second type, NASA solicits suggestions from inside and outside the space agency. Previous New Frontiers missions include New Horizons, which zipped past Pluto two years ago; Juno, currently looping around Jupiter; and Osiris-Rex, currently en route to an asteroid.
For this round of New Frontiers, NASA will spend up to $850 million for the spacecraft, instruments and mission operations. Add in the cost of the rocket to get the spacecraft off the ground, and the total price tag will be about $1 billion.
Steven W. Squyres, a professor of physical sciences at Cornell University who leads the Caesar proposal, said his team chose Comet 67P because of the wealth of data collected by the Rosetta mission, which flew alongside the comet from 2014 to 2016.
“We are able to design our mission, design our spacecraft specifically for the conditions that we know to exist there,” Dr. Squyres said. “And what that does is just dramatically improve the chances for success for a very difficult activity, which is grabbing a piece of a comet.”
Comets are believed to contain primitive ingredients from the early solar system that went into the building of the planets.
Caesar would scoop up at least 100 grams from the comet, separating the volatiles — constituents that could evaporate — from the more solid substances. The spacecraft would then head back to Earth and drop off the sample in a capsule.
“The sample will arrive back on Earth on the 20th of November, 2038,” Dr. Squyres said. “So mark your calendars, and once it’s been delivered to laboratories worldwide, I think it’s going to produce groundbreaking science for decades to come.”
The Dragonfly mission would follow up on observations made by the Cassini-Huygens mission that concluded in September. The Huygens probe, built by the European Space Agency, landed on Titan in 2005. The Cassini orbiter made many flybys of Titan during its 13 years in orbit around Saturn.
Powered by a chunk of plutonium, Dragonfly would take advantage of recent technological advances in flying drones. The craft would spend most of its time making measurements on the ground, but it would be able to fly tens or hundreds of kilometers through Titan’s thick atmosphere to study other geologic terrains, said Elizabeth Turtle, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory who is the principal investigator for Dragonfly.
“In this way, we can evaluate how far prebiotic chemistry has progressed in an environment that we know has the ingredients for life — for water-based life or potentially even hydrocarbon-based life,” Dr. Turtle said.