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NASA will either sample a comet or visit Saturn's moon Titan

Saturn's third-largest moon Dione can be seen through the haze of its largest moon, Titan, in this view from NASA's Cassini spacecraft. Credit: NASA / JPL-Caltech / Space Science Institute

NASA has selected two finalists for the New Frontiers space program:

Why it matters: Both concepts will receive further design funding and one will eventually launch, possibly in the mid 2020s. Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate in Washington, D.C, told Scientific American's Lee Billings, "These are tantalizing investigations that seek to answer some of the biggest questions in our solar system today."

New Frontiers is NASA's mid-range exploration program. It fills the funding gap between lower-cost high-speed missions like the Discovery program and expensive Flagship missions. Past New Frontiers missions include:

What they didn't choose: NASA also selected two semi-finalists, which will receive additional funding.

  • ELSAH, or the The Enceladus Life Signatures and Habitability project will work on a design for visiting Enceladus, the sixth largest moon of Saturn, which is covered in a salty sea beneath a massive ice sheet. The Cassini mission flew through the geysers that erupt form Enceladus' south pole, and discovered molecular hydrogen, which means the planet probably has hydrothermal thermal vents. Since life on Earth may have arisen next to such thermal vents beneath the ocean, Enceladus is considered one of the best places to look for life in our solar system.
  • VICI would send two missions to Venus, Earth's sister planet. Venus was once temperate and Earth-like, but today it has a thick atmosphere with a runaway greenhouse effect. Venus is so inhospitable that we know very little about it: it's notorious for killing the probes we send its way.

One more thing: These missions span generations, and as NASA Scientist Jo Pitesky told us during Cassini's grand finale, they can be incredibly meaningful for the teams that guide them. Sara Hörst, who studies Titan, wrote a thread on Twitter about the emotional significance of doing and sharing this work, and how the NASA's selection left her feeling ecstatic and heartbroken.

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