On Wednesday, NASA announced the two finalists for its next New Frontiers mission, a robotic exploratory mission that will launch sometime in the mid 2020s. From a field of 12 proposals submitted last April, the team has selected the final two candidates: a mission to Saturn’s moon Titan and a sample-return mission to a comet.

New Frontiers is one of the agency's unmanned mission programs. As Van Kane at the Planetary Society wrote earlier this year, NASA flies three types of exploratory missions to investigate the solar system: Discovery, Flagship and New Frontiers.

Discovery missions fall at the lower end of expense, costing around $600 million to $700 million, and are tightly focused on a single goal, like the Dawn mission to explore dwarf planet Ceres. NASA's Flagship missions are the priciest of the bunch, running over $2 billion, and are once-in-a-decade launches with craft carrying an array of instruments. Examples of these missions include hall-of-fame probes like Viking, Voyager, Cassini and the upcoming Mars 2020 Rover. New Frontiers missions are a happy intermediate. Costing around $850 million, these missions hit a sweet spot between budget and exploratory firepower.

Currently, three New Frontiers missions have soared away from Earth. These include the New Horizons craft, which sent back incredibly detailed pictures of Pluto in 2015 and is currently on its way to explore 2014 MU69, a strange-looking space rock in the Kuiper Belt. Also in the group is the Juno spacecraft, which entered orbit around Jupiter in 2016 and is currently beaming back unprecedented images of the gas giant Jupiter. Finally the last in the group is the Osiris-REx mission, currently underway, which is scheduled to grab a chunk of the asteroid Bennu in late 2018 and send it back to Earth.

The fourth New Frontiers mission will either be the deployment of a rotorcraft called Dragonfly to Titan or the Comet Astrobiology Exploration Sample Return (CAESAR), a mission to grab a bit of the comet 67P/Churyumov-Gerasimenko and return it to Earth. According to a press briefing, NASA will fund the projects till the end of 2018 so the teams can develop their concepts before deciding which of these missions to pursue in the spring of 2019. They hope to launch the new probe before the end of 2025.

“This is a giant leap forward in developing our next bold mission of science discovery,” Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate says in the press release. “These are tantalizing investigations that seek to answer some of the biggest questions in our solar system today.”

The second finalist is the Dragonfly project, led by Elizabeth Turtle from the Johns Hopkins University Applied Physics Laboratory. As Turtle explains, Titan’s dense atmosphere is full of complex hydrocarbons and the surface is studded with lakes of methane. The goal of Dragonly is to land on the surface of the moon and explore just how far that prebiotic chemistry has come to help researchers understand how organic molecules could progress toward life.

Earlier this year, researchers found that carbon chain anions—the first building blocks of the more complex organic molecules that are the foundations of life—exist on Titan. While Turtle says the craft will spend most of its time on the ground to conduct its sampling, its rotors will allow it to fly tens of or even hundreds of kilometers to new areas of the moon’s surface. If launched by 2025, Dragonfly could make it to Titan by 2034.

The CAESAR initiative is currently led by Steve Squyres of Cornell University. The project's primary goal is to send a probe to 67P/Churyumov-Gerasimenko and grab 100 grams of material from the comet’s surface and send it back to Earth in a special capsule. As Squyres explained during the briefing, exploring comets is important since they are where Earth's water and organic materials originated and are fundamental in the building of planets. So understanding comets could help researchers understand the process of how planets form. If the 2025 launch goes as scheduled, he says the sample would return to Earth by 2038.

If the seven-mile-long 67P/Churyumov-Gerasimenko sounds familiar, that’s because it’s the same comet that the European Space Agency’s Rosetta probe orbited for almost two years, between 2014 and 2016. Squyres explained that going back to a comet we already understand and have great maps of simplifies the CAESAR mission and makes its likelihood of success even greater.

“Comets are among the most scientifically important objects in the solar system but they’re also among the most poorly understood,” he said. “I think it’s going to produce groundbreaking science for decades to come.”

Venus has missed the final cut, but along with one other mission, it will receive funds to continue to develop new technologies. The team is planning to continue work on the Venus In situ Composition Investigations (VICI) mission, which was designed to investigate minerals on the planet's surface. The device uses a specially hardened camera to operate in the rough conditions and lasers to analyze mineral composition.

Much focus has been cast to Venus in recent years, with some researchers even calling for human exploration of the planet. It is very similar in size, mass and distance from the Sun but has a hellish life history. By better understanding how Venus became so inhospitable, researchers believe they can learn more about the ingredients necessary for habitable environments on other planets.

The second mission to not make the cut (but still receive funds to continue development), is the Enceladus Life Signatures and Habitability (ELSAH) concept to probe Enceladus, Saturn’s icy moon where hydrogen gas, a potential feast for microbes, was recently found venting into space. That team will continue to develop cost-effective techniques that limit spacecraft contamination to help researchers avoid carrying terrestrial microbes into space and to help detect life on other worlds.

While not all the missions are a go, NASA did limit its proposal to six themes, indicating where it would like to aim its next batch of New Frontiers craft in the future. The themes included comet surface sample return, lunar south pole-Aitken Basin sample return, ocean worlds (Titan and/or Enceladus), Saturn probe, Trojan asteroid tour and rendezvous, and Venus in situ explorer.