For 13 years, NASA’s Cassini spacecraft sent back captivating observations of Saturn, and its rings and moons, solving some mysteries but raising plenty of new questions.
With the spacecraft’s demise on Friday, the stream of data from Saturn has dried up.
“Until we go back, that’s a very distant world now,” Linda Spilker, the project scientist for Cassini, said during a news conference on Friday.
“The details of the rings, and those small moons snuggled in so close — those are all gone until we go back.”
NASA currently has no plans to return to Saturn, but that could change. In the latest round in a scientific competition called New Frontiers, NASA specified categories of missions it would consider.
Those include a probe to study Saturn’s atmosphere or a mission to go to Titan or Enceladus, two moons known to have oceans.
The New Frontiers program solicits ideas for missions from teams of scientists and engineers. These projects can be ambitious, costing up to about $1 billion.
Earlier proposals included Juno, now orbiting Jupiter, and Osiris-Rex, currently en route to the asteroid Bennu.
NASA may announce finalists by the end of the year. A winning mission is to be selected by summer 2019 for launch around 2025.
At least five submitted proposals take aim at Saturn, Titan or Enceladus.
As a spacecraft, Dragonfly would be an oddity: It would have propellers, like a helicopter — “a nuclear quadcopter to look for life on Saturn’s moon, Titan,” Peter Bedini, a program manager at Johns Hopkins Applied Physics Laboratory, said in a recent talk.
Proponents of this concept say a quadcopter would be an ideal way to explore Titan, Saturn’s largest moon. The air is thick there, thicker than on Earth.
The landscape is varied, interspersed with obstacles — rivers, lakes and seas of liquid methane — that could prove inaccessible for a rover.
The booming popularity of flying drones in recent years makes the technology potentially feasible for interplanetary exploration, too.
In the past, scientists have suggested exploring the moon with balloons and airplanes. But Titan’s geology — sand dunes, eroded gullies — is more interesting than what is in the air.
Dragonfly would fly from place to place, but would spend most of its time performing experiments on the ground.
A second Titan proposal, Oceanus, is led by Christophe Sotin, the chief scientist for solar system exploration at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., which was Cassini’s home base.
The Oceanus spacecraft would study the moon from orbit, potentially identifying habitable regions for life.
Jonathan I. Lunine, a planetary scientist at Cornell University, was a member of the science team managing the Huygens probe, which traveled to Saturn with Cassini and landed on Titan.
He would be the principal investigator on a proposed mission to revisit Enceladus, a small moon just 313 miles wide.
The discovery of geysers shooting from its south pole was a stunning surprise, and now the moon is considered a prime place for look for life.
The proposed spacecraft, called Enceladus Life Finder, would fly through the plumes like Cassini did but with more sophisticated instruments capable of identifying a wide variety of molecules including amino acids, which would hint at signs of life.
The Saturn Probe Interior and Atmosphere Explorer would essentially do what Cassini did on Friday: descend into the planet’s atmosphere. But it would go much deeper.
The main part of the mission would end quickly — in about 90 minutes, as the probe parachuted into the atmosphere. It would take measurements of certain elements like helium that are hard to measure.
The ratio of helium to hydrogen is a crucial measure indicating how far from the sun a planet formed in the early days of the solar system.
Cassini attempted to measure that in its final plunge, but that data, from high in the atmosphere, will not be conclusive.
NASA’s Galileo spacecraft dropped an atmospheric probe into Jupiter in 1995, and this proposal is the “same exact idea as the Galileo probe,” said Amy Simon, an expert on planetary atmospheres at NASA Goddard Space Flight Center in Greenbelt, Md.
She would be the principal investigator for the mission.
To understand how the solar system formed, it’s crucial to understand its biggest fixtures. Saturn, of course, is the second largest planet, after Jupiter.
“The two of them together tell you a lot of what happened in the early solar system,” Dr. Simon said. “It will answer those few fundamental questions that we could not do with Cassini.”
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