Screengrab of NASA's New Horizons spacecraft.
Credit: NASA
By Jonathan O'Callaghan for Scientific American
It’s lonely out there in the desolation that reigns where NASA’s New Horizons spacecraft now cruises on its one-way trip out of our solar system, with little to pass the time besides sniffing whiffs of plasma and stargazing. After nearly two decades of deep-space operations, the probe is currently more than eight billion kilometers from Earth. And much like our planet itself, the mission’s heyday—a historic encounter with Pluto in 2015 and a 2019 flyby of Arrokoth, the most distant object yet visited by a spacecraft—is receding ever further in the rearview.
Back on Earth, a battle has raged over the spacecraft’s future. Pluto and Arrokoth alike reside in what’s known as the Kuiper Belt, a remote and mysterious orbital region of icy objects in the outer reaches of our solar system. New Horizons—humanity’s first and so far only robotic emissary to explore the Kuiper Belt—still traverses its depths, dutifully gathering data and somewhat desperately searching for another object to intercept. Yet last year NASA suggested it would end these investigations in an effort to save money, sparking an outcry from astronomers, given that no other spacecraft will explore the Kuiper Belt for decades.
That decision, it seems, has been partly reversed. In a statement from NASA posted on September 29, Nicola Fox, associate administrator of the agency’s Science Mission Directorate in Washington, D.C., announced some of New Horizons’ Kuiper Belt science would continue. “The agency decided that it was best to extend operations for New Horizons until the spacecraft exits the Kuiper Belt, which is expected in 2028 through 2029,” Fox said. NASA’s statement noted that the agency would “assess the budget impact of continuing the New Horizons mission so far beyond its original plan of exploration” and that other missions may be affected by the decision. “Future projects may be impacted,” the statement added.
Alan Stern, a planetary astronomer at the Southwest Research Institute, who leads the New Horizons mission, welcomed the decision. “It is good news for Kuiper Belt exploration and very much welcomed by our team and also by the planetary science community,” he says. Pontus Brandt of the Johns Hopkins University Applied Physics Laboratory (APL) was similarly jubilant. “The community and I are thrilled that this logjam is finally broken,” he says. “This was the right decision for Kuiper Belt science.” Stern notes that some of the finer details are yet to be ironed out, however. It’s not clear, for example, to what extent New Horizons’ studies of the Kuiper Belt will continue, with NASA’s recent statement noting that the agency’s decision “allows for the possibility of using the spacecraft for a future close flyby” of a Kuiper Belt Object (KBO).
Spectacular Results
NASA launched the nearly $1-billion New Horizons mission in 2006 on its pioneering voyage to Pluto and the Kuiper Belt. The probe’s arrival at the dwarf planet nine years later was a stunning moment in space exploration, with New Horizons returning breathtaking images of a surprisingly complex world of craggy mountains of ice and seas of frozen nitrogen, as well as snapshots of Pluto’s equally enthralling red-tinted moon Charon. The additional visit to Arrokoth was a lucky bonus, achieved by dint of the KBO’s timely discovery when it was still within reach of the approaching spacecraft’s dwindling propellant reserves. The two flybys produced “spectacular results,” says Jane Luu of the University of Oslo, who co-discovered the Kuiper Belt in 1992.
Although New Horizons’ day-to-day operational needs are modest, they add up to a cost of nearly $10 million per year. Last year NASA approved a mission extension—but only through September 2024 rather than 2025, as requested by Stern and his team. At that point, NASA had planned to end the spacecraft’s planetary science studies in favor of a focus on heliophysics by repurposing New Horizons to exclusively examine how our home star shapes conditions in the outer solar system and toward the hazy boundary with interstellar space. That transition would swap the mission from NASA’s Planetary Science Division to its Heliophysics Division. And given that Stern and his team did not heed the space agency’s request to submit a proposal by November 2022 to dedicate New Horizons solely to heliophysics, the transition would remove them from the mission, too. “We refused to write a proposal that terminated the Kuiper Belt science,” Stern says. “It’s outrageous that you would terminate the only mission purpose-built and sent to the Kuiper Belt while it’s still collecting unique data.”
Such a heliocentric shift would have greatly limited the mission’s scientific output, says Jim Green, NASA’s former chief scientist and former head of its planetary science efforts. “It basically pares down the science team to next to nothing and really operates the spacecraft with a minimal cadre,” he says. “From my perspective, if I was the division chief, I would not have made that decision.” He says the reversal was “a good decision” and will “allow the right science for the mission during the right times.”
“Unlikely to Markedly Improve Knowledge”
The decision to halt New Horizons’ Kuiper Belt studies originally emerged in 2022 from NASA’s annual review of most of its planetary science missions, a process in which the space agency assesses their current status and future potential. Although this review acknowledged many benefits of New Horizons continuing its current mission, the report also flagged a key weakness. In the absence of a suitable rendezvous target, the spacecraft can only study KBOs from afar—and in far fewer numbers than what various ground-based telescopes can achieve, perhaps less than a dozen. “The proposed studies of [KBOs] are unlikely to markedly improve knowledge,” the review stated, noting the spacecraft’s priorities “should focus on heliophysics and astrophysics.”
Faith Vilas of the Planetary Science Institute, who led the team that assessed New Horizons for the review, says she and her colleagues did not intend their work to justify ending the mission’s planetary science studies. The team was “being credited, or blamed, for the mission potentially losing the planetary science side of things,” she says. “We didn’t say that. We simply said that all the science together is greater in magnitude than the one portion of science.”
Stern says the mission still has much to offer as it moves through the Kuiper Belt, including feats that cannot be replicated on Earth, such as observing the changing brightness of KBOs as they rotate. “When you do that repeatedly from different angles, you can determine the shape,” he says. “But you can never do that from Earth because you never see the KBOs from significantly different angles.” The spacecraft can also search for binaries—co-orbiting KBOs—in a way Earth-based observers cannot and can collect dust scattered away from distant Kuiper Belt objects. The prospect of visiting a third object remains ever present, too, if a viable target can be found.
The spacecraft is projected to exit the known boundaries of the Kuiper Belt in 2028, at which point Stern agrees the Kuiper Belt science could end. “Then I don’t see a reason to continue a planetary science mission,” he says. By some estimates, the spacecraft could continue operating until 2050, when it will be far beyond the generally accepted boundary of interstellar space. At present, no other spacecraft bound for the Kuiper Belt is in development. The next possibility might be Interstellar Probe, a proposal from APL to send a spacecraft to interstellar space. Optimistically assuming Interstellar Probe becomes a reality and launches in 2036, “that would get you out to the same region of space as New Horizons probably within a decade or so,” says Ralph McNutt, who helms the proposal team at APL, “so potentially up to the mid-2040s.”
Low-Hanging Fruit
In June Green and other members of the space science community signed a letter to NASA urging the space agency to reconsider its decision and noted “alarm” at the proposed abandonment of Kuiper Belt science. “We ... ask NASA, the Administration, and Congress to reverse course,” they wrote. In September the U.S.-based National Space Society made a similar appeal in its own letter. “Continue New Horizons so we don’t miss out on new discoveries from this rare, perfectly positioned, and fully functional mission,” the letter stated.
Not all astronomers agree that New Horizons’ remaining Kuiper Belt investigations will be worthwhile, however. Luu says transitioning the mission to a focus on heliophysics and astrophysics would be “a reasonable decision” because ground-based telescopes can surpass the spacecraft’s Kuiper Belt capabilities in many respects, especially by studying many more KBOs at a much faster cadence. “If you just want to use the spacecraft for monitoring KBOs, I would argue it might be better done from the ground,” she says. And the prospects of a third flyby are becoming increasingly remote because no obvious targets have been discovered. “If they find a new candidate, great, but the low-hanging fruits have been picked,” she says.
Mike Brown of the California Institute of Technology, who discovered the object Eris in 2003, which led to Pluto’s demotion from a planet to a dwarf planet, has similar concerns. “These decisions are always tough,” he says. “There is a spacecraft there! It can do unique things! But ultimately it is a zero-sum cost-benefit analysis. Unless there is a new target for a close flyby, it’s hard for me to see why spending a ton of money is justified. If the science can be done on a shoestring, then perhaps that’s fine. But of course, a shoestring in space is probably many full scientific programs on Earth.”
For now, New Horizons will continue its studies of the Kuiper Belt—and will remain the only spacecraft likely to do so for many years to come. What knock-on effects its ongoing operations will have on “future projects” alluded to by NASA remains to be seen. Far beyond Pluto, one of our most distant emissaries still speeds on into the unknown.