Five years ago, NASA’s New Horizons spacecraft barreled by Pluto for a high-speed encounter that gave humanity its first fleeting close-up look at the distant world, finding glaciers and mountains of water ice. Scientists are now planning how to go back.
New Horizons flew within 7,800 miles (12,550 kilometers) of Pluto on July 14, 2015, more than nine years after departing Earth on a speedy trajectory that made the spacecraft the fastest ever launched up to that time.
The mission snapped numerous images, revealing unexpected geologic activity on Pluto, craggy mountain ranges made of hardened water ice, dune fields containing frozen methane, the largest glacier in the solar system.
The New Horizons mission is the first to visit the the Kuiper Belt, a ring of small, icy worlds beyond the orbit of Neptune. After zooming past Pluto, the largest world in the Kuiper Belt, the plutonium-powered spacecraft journeyed farther from the sun and flew by a peanut-shaped object named Arrokoth on Jan. 1, 2019.
Arrokoth is a billion miles beyond Pluto, and scientists say evidence suggests the two lobes that make up the 22-mile-long (36-kilometer) object likely formed near one another soon after the birth of the solar system 4.5 billion years ago, then merged together at a relatively slow relative velocity. That would make Arrokoth a primordial world that likely has remain unchanged for billions of years.
New Horizons continues deeper into the outer solar system, conducting long-range observations of other objects in the Kuiper Belt and measuring the behavior of the solar wind at ever-farther distances from the sun.
“I think the solar system literally saved the best for last with Pluto,” wrote Alan Stern, principal investigator for NASA’s New Horizons mission. Of course, I’m a little biased — as we all are on New Horizons — but I can’t think of a more beautiful and scientifically richer way to have completed the first era of the reconnaissance of the planets, which NASA started in 1962 with the first visit to any planet – Venus.”
In a matter of a few hours, New Horizons gathered sharp views of Pluto’s mountains and apparent ice flows, observed its tenuous atmosphere, and imaged Pluto’s five moons, including its largest companion Charon.
“It took 16 months to transmit all the Pluto system data back, but by late 2016, the entire haul of precious data was here on Earth,” Stern wrote. “Now, after five years of work to analyze those data, our appreciation and understanding of Pluto and its moons just continues to multiply.”
Data beamed back by New Horizons indicated Pluto likely harbors a subsurface ocean of liquid water, a highly unexpected discovery. A glacier-filled basin named Sputnik Planitia contained icy “cells” that appear to be turning over in a process called convection, evidence that parts of the Pluto’s surface are being regenerated through active geologic processes.
New Horizons also discovered two mountain peaks on Pluto that have deep central pits. They may be signs that volcanoes have erupted on Pluto, but instead of spewing out hot lava, they would have likely discharged slushy, viscous cryoflows of water.
But New Horizons only got a quick look at Pluto, and scientists are eager to send another mission to orbit the distant world. Such a mission would cost billions of dollars, and NASA is awaiting a recommendation from the National Academies of Sciences, Engineering, and Medicine on what robotic planetary missions the agency should begin developing in the next decade.
The National Academies’ planetary decadal survey is expected to produce its report in 2022. NASA’s policy is to follow the decadal survey’s recommendations on which flagship-class planetary science missions the agency should pursue next.
The top two priorities in the last planetary decadal survey report, released in 2011, were a Mars Sample Return mission to collect samples for return to Earth, and an orbiter to visit Jupiter’s icy moon Europa. Those concepts evolved to become NASA’s Perseverance rover, scheduled for launch later this month, to gather the Martian samples, followed by joint U.S.-European missions launching later in 2020s to bring the specimens back to Earth.
The Europa mission concept eventually became the Europa Clipper mission, which NASA is developing for launch as soon as 2024.
NASA has funded 11 planetary mission concept studies for consideration by the next decadal survey panel. The list includes a robotic lander to Mercury, a flagship Venus mission, a network of probes to study the moon’s geology, a long-lived lunar rover that could drive more than 1,000 miles across the moon’s surface, and missions to Mars, asteroids, Saturn’s moon Enceladus, Neptune and its moon Triton, and Pluto.
Carly Howett, a member of the New Horizons science team from the Southwest Research Institute, led the concept study for a potential orbiter that could fly to Pluto. With an estimated cost of $3 billion — not including launch expenses — the mission concept has been named Persephone, wife of Pluto and queen of the underworld in classical mythology.
“We want to go back and explore the Pluto system and the Kuiper Belt,” Howett said June 1 in a presentation to NASA’s Small Bodies Assessment Group, a community of scientists with research interests in asteroids, comets and the Kuiper Belt. “Of course, New Horizons did a great job of exploring the Pluto system in 2015, and I was on that mission … but it was just a single encounter.”
An orbiter would see more of Pluto than New Horizons, and would be able to track changes on Pluto’s surface and in its atmosphere over time.
What’s more, those lengthy travel times assume the Pluto orbiter launches on a huge rocket. Howett said the spacecraft concept from the Persephone study would require a launch on a new version NASA’s Space Launch System with an enlarged upper stage and evolved strap-on boosters, a configuration NASA calls the SLS Block 2.
The first SLS test flight, using a more basic configuration, is currently scheduled in the second half of 2021. An SLS with the enlarged four-engine upper stage could debut a few years later, but the SLS Block 2 with evolved boosters is not expected to fly until the end of the 2020s, at the earliest.
And even the SLS Block 2 couldn’t do the trick by itself. A high-energy upper stage, like United Launch Alliance’s Centaur stage, mounted on top of the launch vehicle would need to give the Pluto orbiter an additional boost.
“We need the kind of the oomph of an SLS Block 2, with a Centaur kick stage,” Howett said. “That’s basically the biggest rocket we can fly with the biggest kick stage we can find.”
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