NASA’s New ‘To Do’ List Is Published This Week. Will It Go To Mars, An ‘Ice Giant’ Or Saturn’s ‘Slam Dunk’ Ocean Moon?

Where should NASA go next?

Next week, on April 19, 2022, the space agency will get a new “to do” list when the National Academy of Sciences finally publishes its Decadal Survey for Planetary Science and Astrobiology.

Compiled by scientists at the cutting edge of their fields and specifically designed to be fair and honest, it will set out the priorities for NASA for the next 10 years. Congress usually follows its recommendations. What will it say?

A decade ago it recommended NASA prepare as astrobiology mission to Mars as its top priority. The details changed, but the Perseverance rover is now on the red planet. The second priority was a mission to Jupiter’s icy moon Europa, one of the most promising environments in the solar system for supporting life. NASA’s Europa Clipper is scheduled for launch in October 2024.

However, the third priority was deemed to be a Uranus Orbiter. Err …

That last mission never got off the ground. Will Uranus or its “ice giant” companion in the Solar System, Neptune, get a second mention 10 years later? Or will mission concepts to visit the “ice giants” be consigned to history? There have been myriad other mission concepts for the Decadal Survey to peruse, from landing on Saturn’s ocean moon Enceladus to launching a planetary science telescope to replace Hubble.

However, there’s a problem for all of those mission concepts. Mars. More specifically, our continued obsession with it.

With Perseverance now collecting samples of Martian rock and carefully placing them in boxes on the surface, a mission to go collect them and bring them home—dubbed the Mars Sample Return—seems virtually self-selected as NASA’s next flagship mission.

There’s already a website for it and NASA has even been announcing contracts.

As is stands the exciting plan is for NASA to deliver a Mars lander close to Jezero Crater, where Perseverance will by then have collected and cached samples.

A Sample Retrieval Lander would carry a NASA Mars Ascent Vehicle rocket along with ESA’s Sample Fetch Rover. The latter will gather the cached samples and put them on the ascent vehicle. If it’s close enough then Perseverance could help out. The samples would them launch a container into orbit to rendezvous with an ESA spacecraft to return to Earth.

It’s a concept that’s fleshed-out and, frankly, expecting to be green-lit.

“This groundbreaking endeavor is destined to inspire the world when the first robotic round-trip mission retrieves a sample from another planet,” said Bill Nelson, NASA Administrator. “America’s investment in our Mars Sample Return program will fulfil a top priority planetary science goal and demonstrate our commitment to global partnerships, ensuring NASA remains a leader in exploration and discovery.”

Could that mission possibly be delayed by the Decadal Survey to allow something …new?

“I would love to see the decadal survey discuss the pros and cons of sending the Mars Sample Return as the next flagship mission,” said Abi Rymer, a Program Officer at NASA who acted as Principal Investigator for the Neptune Odyssey concept mission. “It’s almost like it’s already selected, but it really does require a flagship level effort in the next decade.”

It is certainly possible that the Mars Sample Return mission—which seems inevitable in the medium-term—could be paused. “I’m hoping to see an open discussion about delaying that mission in order to get the Neptune mission, or any ice giant mission, stood up now so that we can get that window for the Jupiter gravity assist,” said Rymer.

Some of the proposed missions do require a pause on Martian ambitions. Celestial mechanics dictate that if NASA is to send a spacecraft to Uranus or Neptune it needs to go by 2032 so it can sling-shot around Jupiter.

The main reason to visit Neptune or Uranus is to figure out how the Solar System formed.

That’s important because as astronomers look at other star systems they see different kinds of planets to those in our own. “The most abundant exoplanets that we’ve discovered are ‘mini-Neptunes’ and ‘super-Earths’ and we don’t have either in the Solar System—we have Uranus and Neptune,” said Dr. Jackie Faherty, Senior Scientist and Senior Education Manager jointly in the Department of Astrophysics and the Department of Education at the American Museum of Natural History. “What exoplanet scientists need to know more about are the ice giants—Mars isn’t a world they talk about.”

“I definitely think there will be conversations about Uranus or Neptune, or both,” said Erin Leonard at NASA’s Jet Propulsion Laboratory and lead author of a proposal for a NASA New Frontiers mission to visit the Uranian System, which is now under review by the Decadal Survey.

Each Decadal Survey is always about yielding the most scientific benefit per dollar spent, and with the Mars Sample Return on everyone’s minds a lot of the mission concepts have been scaled-down.

After all, NASA’s budget is tight.

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“The Mars Sample Return mission is one reason to be looking at lower costs,” said Leonard, whose concept of a mission to Uranus would cost under the $1 billion cost-cap for a New Frontiers mission (flagship missions like Cassini and Juno tend to cost over $2 billion). “There’s no guarantee that anything will be left after Mars Sample Return—and no guarantee of what the budget for the next decade will look like,” said Leonard.

While flagship missions are often called “Christmas Trees” because they have every kind of science instrument on them and can study everything, the Uranus Orbiter would be more focused. The plan is to study the rings of Uranus, how the solar wind interacts with its magnetosphere (a kind of “missing link” in solar physics) and four the planet’s largest, innermost moons. It’s suspected from Voyager 2 data that Ariel, Umbriel, Titania and Oberon are both geologically active and could host subsurface oceans.

“I would love to see an ice giants mission,” said Shannon M. MacKenzie, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory and author of the Enceladus Orbilander Mission Concept. “I think the success of Cassini at Saturn has really whetted everyone’s appetite for repeating Cassini at the other giant planets.”

Her own plans to have a spacecraft visit Enceladus include a 200 days orbit to fly through the plumes of water known to be spewing into space from “tiger stripe” cracks on the tiny moon’s icy shell. The spacecraft would then follow so it can sample the material falling onto the surface. “The search for life on ocean worlds is important and very different from what we’re doing at Mars,” said MacKenzie.

After all, if life has taken hold on Enceladus then it will have had to happen completely independently from life on Earth. That’s not something anyone will be able to say for certain if traces of life are found on Mars.

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With liquid water and a heat source, Enceladus is one of the most exciting places in the Solar Systems from an astrobiological point of view. “If I had to put my money on something in the Solar System that’s a slam-dunk for life it would be Enceladus,” said Faherty. Meanwhile, while Uranus and Neptune—virtually unknown and analogs for the thousands of similarly exoplanets astronomers keep on finding in the Milky Way—surely deserve some kind of mission.

It’s possible they’ll get a mention, but it still seems likely that Mars Sample Return mission will get the lion’s share of attention in the Decadal Survey.

“Mars seems to always win and part of why it wins is because we’ve gone there so many times,” said Faherty, who thinks that it’s the ice giants that carry in them the secrets of how the Solar System formed. However, she knows how this works: “When you go up against Mars, most of the time you lose.”

Wishing you clear skies and wide eyes.

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