LAUREL, Md. — NASA and the science team for a spacecraft in the outer reaches of the solar system are locked in a dispute about the future of that mission and the science it can perform.
The uncertainty about the future of the New Horizons mission started last year when NASA reviewed a proposal from the mission’s science team for a second extended mission. The spacecraft, launched in 2006, flew by Pluto in 2015 and the Kuiper Belt object Arrokoth in early 2019, and will continue to traverse the Kuiper Belt through 2028.
The project team had proposed a multidisciplinary science mission for New Horizons, conducing a mix of astrophysics, heliophysics and planetary science research. The focus is “only things that can be done by dint of being at that great distance or in the Kuiper Belt,” said Alan Stern, principal investigator for New Horizons, at a meeting of the Outer Planets Assessment Group (OPAG) here May 3.
NASA, as part of the planetary science senior review that considered proposals from New Horizons and other spacecraft seeking extended missions, only approved funding for two years, rather than three as requested. The agency elected to fund New Horizons through fiscal year 2024 as part of the planetary science division, then have the mission compete in a separate senior review for the heliophysics division for fiscal year 2025 and beyond.
The agency’s rationale was that the planetary science that New Horizons could do was less compelling than astrophysics and heliophysics. The senior review gave the overall proposal a score of “excellent/very good”, the second-highest possible score, but the planetary portion was rated “very good/good”, two levels lower.
“The proposed Kuiper belt object (KBO) studies are unlikely to dramatically improve the state of knowledge,” the senior review report stated. New Horizons would be able to observe several KBOs at a distance, and at viewing angles not possible from the Earth, but the report concluded that those observations would not be competitive with ground-based observations.
“We think that this is shortsighted,” Stern said. “It was the only mission ever sent and the only mission planned to study the Kuiper Belt, and we’re still there.”
He said that while the mission was invited to submit a proposal to the heliophysics senior review, it has decided not to do so. His concern was that New Horizons would become an “infrastructure” mission for heliophysics without a dedicated science team but instead teams that run the spacecraft’s instruments. “I dub them ‘zombie’ teams.”
“Writing a proposal to walk the plank, if you will, writing a proposal for the entire science team to be disbanded, did not look like something that we wanted to do,” Stern said. “We were afraid that the proposal would be accepted.”
That puts the future of New Horizons after fiscal year 2024 in limbo. “We are in a quandary. I don’t know what we’re going to do about it,” said Curt Niebur, lead scientist for flight programs in NASA’s planetary science division, at the OPAG meeting. NASA had hoped the mission would accept the senior review outcome. “That path is broken.”
He acknowledges, though, there had been a “miscommunication” on NASA’s part about the ability of New Horizons to do planetary science, particularly if scientists find a Kuiper Belt object in range of the spacecraft for a close flyby. “Should a KBO be found, yes, let’s talk about it. Let’s see if it’s reachable,” he said. “Let’s continue the search and we’ll take that up at the time that we find it.”
However, another KBO flyby appears unlikely. “It’s a needle-in-a-haystack problem. We’re looking,” he said, efforts that have included upgrading an instrument on the Subaru Telescope in Hawaii to better look for targets. “The odds are against us.”
That search, he argued, would become impossible if NASA’s plans for New Horizon go forward. “I doubt we’ll get any telescope time in 2024 because the planetary portion of the mission is ending,” he said, as observatories decide to allocate telescope time to others.
In March, NASA issued a request for information for a potential “New Horizons Interstellar Mission” that would continue the mission after fiscal year 2024. It sought ideas for heliophysics and other science the mission could do in 2025 through 2027 “to gauge the level of interest of the wider science community in pursuing the next phase of science leadership for the mission” and the costs to do so.
The document stated that “mission operations would be terminated at the end of the second extended mission” at the end of fiscal year 2024, but both Niebur and Stern emphasized at the OPAG meeting that there are no plans by NASA to turn off the spacecraft.
“The senior review did not suggest that the mission be truncated,” Niebur said. “NASA is not suggesting we turn off New Horizons.”
“NASA is not planning to turn the spacecraft off, simply to terminate the planetary mission and to dismiss the planetary team,” Stern said.
While NASA’s planetary programs have struggled with cost issues, from potential increasing costs for Mars Sample Return to the delay in the Psyche launch that has pushed back the VERITAS Venus mission by three years, New Horizons takes up a minute part of the overall planetary budget. NASA requested $9.7 million for the mission in its fiscal year 2024 budget proposal, less than 0.3% of the overall planetary science budget of $3.38 billion.
“We need to finish the Kuiper Belt,” Stern said, “We finally got a spacecraft here. We’re going to leave the Kuiper Belt in a few years. Why so impatient over pennies out of the planetary budget?”
Some at the OPAG meeting suggested that a solution would require some improved cooperation among NASA’s science divisions, including perhaps a more equitable sharing of mission costs. “Meritorious science can be achieved in heliophysics, astrophysics and planetary science but science optimization will require creative problem solving and cross-divisional leadership,” the senior review report stated.
“The longer you explore space, the more cross-divisional it becomes,” said Pontus Brandt of the Applied Physics Lab, who joined the New Horizons science team last year to support heliophysics studies. “Nature doesn’t really care about divisions and we need to figure out how to support that.”