Deep into the Kuiper Belt, New Horizons is still doing science

New Horizons is now nearly twice as far from the Sun as Pluto, the outer planets are receding fast, and interstellar space is illuminated by the vast swath of the Milky Way ahead. But the spacecraft’s research is far from over. Its instruments are all functioning and responsive, and the New Horizons team has been working hard, pushing the spacecraft’s capabilities to carry out new tasks.

Since its launch in January 2006, the New Horizons spacecraft has traveled over 5 billion miles, passed by the moons of Jupiter, and surveyed the scaley frozen methane ice of its target planet Pluto. In January 2019, it buzzed by Arrokoth, another billion miles beyond Pluto—the most distant object to have ever been visited by a spacecraft. The data it returned from this intact remnant of our Solar System’s formation has given us important new insights into how that process happened.

But New Horizons’ mission is far from over. While it may never have another close encounter with an orbiting object, the team that operates the spacecraft is working out ways to put its instruments to new uses.

Budgets and power budgets

As New Horizons has gotten further from the Sun, piloting the spacecraft requires not only patience but a revised focus. Led by Alice Bowman—the mission’s version of Star Trek’s Scotty—engineers start building a command load three months in advance, then run them on a simulator at the Applied Physics Laboratory to check that they’re sound. Transmitting the commands currently takes eight hours to reach the craft from Earth and requires booking a slot on NASA’s Deep Space Network—three huge radio dishes located in California, Australia, and Spain, which handle communications with multiple space missions. So, like getting a table at a popular restaurant, bookings are required months in advance unless there’s an emergency.

New Horizons spins as it races through space, and while some instruments (like its particle detectors) operate best in spinning mode, to use its imagers, the craft has to be de-spun and pointed, using precious fuel. Power comes from an RTG (radioisotope thermoelectric generator), essentially a nuclear battery made from plutonium-238, which has a half-life of 87.7 years. It’s not currently known how long that power will last.

The two Voyager spacecraft, which already left the Solar System ahead of New Horizons, are still operational but have had to switch off some instruments, including the onboard cameras, which were "power hogs," so now they run just a few instruments with a low power demand, then send back the data. As with the Voyagers, the more power-hungry instruments on New Horizons (e.g., the imagers) that need heaters to keep them at operating temperatures will likely be switched off first. It’s hard to predict when that will be, though, because the RTG’s lifespan is continually being extended by the engineering team, which keeps inventing ever more ingenious tweaks to eke out the power.

The mission also needs to continue paying those engineers. Happily, NASA recently announced that funding will continue for New Horizons through at least 2028 or 2029.

A new view of KBOs

One of the spacecraft’s missions is to continue to explore the Kuiper Belt, which extends from Neptune’s orbit at 30 AU to beyond 50 AU from the Sun. It consists of chunks of rock, ice, comets, and dust. Since leaving the largest Kuiper Belt Object (KBO) Pluto behind, the geology team has been using the spacecraft’s designed capabilities to study other KBOs, so far finding more than 100 new ones and passing almost 20 KBOs close enough to reveal surface properties, shapes, rotational periods, and close-in orbiting moons.

The Kuiper Belt holds the key to a big puzzle. Why did all the planets accrete from clouds of interstellar dust and gas rather than just smashing into each other in mutual annihilation? Asteroids are too battered and reshaped by multiple collisions to retain traces of their formation. So when the geology team learned the spacecraft would fly by a large KBO, they got very excited.

Sweeping past the contact binary Arrokoth at a distance of just 3,500 kilometers (2,198 miles) in 2019, the images that New Horizons returned appeared to an untrained eye to resemble an unspectacular lumpy potato. But its lonely location in the outer Kuiper Belt has kept Arrokoth intact, essentially a fossil from the early days of Solar System formation. Modeling the detailed data New Horizons obtained of this 36-kilometer-long (22-mile) by up to 20-kilometer-wide (13-mile) object, shows that the larger side was assembled from 8 to 10 smaller components, which all had to be moving quite slowly to successfully "dock." “If they’d come together faster, their outlines would have been smooshed by the impact,” said Will Grundy, head of the mission’s Planetary Geology team at Lowell Observatory, where Pluto was discovered in 1930.

The team now thinks that accreting dust formed "pebbles" around the size of golf balls. Just like the lead cyclist in a peloton works harder than the riders behind, trailing particles were shielded from gas drag by those ahead of them. “This created what’s known as a streaming instability, where many particles can accumulate near enough to each other for the force of gravity to take over and cause them to collapse into a single large object,” explained Grundy. “Evidence of the events preserved on Arrokoth has never been seen before—it’s a key advance in understanding how planetesimals, then planets, formed in our early Solar System.”

With Arrokoth providing such spectacular insights, the team would love to find another KBO to have a close encounter with before exiting the Solar System. It won’t be easy—finding Arrokoth took four years. To speed up the process, J.J. Kavelaars at the University of British Columbia and Wes Fraser at the Canadian National Research Council have turned to AI. Using technology that would have astounded Pluto’s discoverer Clyde Tombaugh, the pair have been applying deep-learning algorithms to observations made with two of the world’s largest telescopes located in Hawaii and Chile.

The software has dramatically increased KBO detection rates over the last two years and significantly reduced "false" candidate sources. Human vetting of an entire night’s worth of search data now takes just a few hours rather than weeks, and AI reanalysis of 2020 data turned up 67 more KBOs than unassisted human searches. “Sooner is always better for finding a new target. It takes time to refine the target's orbit and navigate the spacecraft to the flyby—we had about 4.5 years lead time for Arrokoth,” said Grundy.

Checking for dust

New Horizons launched with seven instruments onboard, all optimized to explore Pluto. But despite the lack of an object to fly by, only LEISA (Linear Etalon Imaging Spectral Array, part of a larger detector used specifically for imaging objects on a flyby) is not currently in use.  All seven main instruments are still working well and active. The key to getting the New Horizons "old dog" to do new tricks has been improvisation—and the spacecraft’s unique location.

REX is the Radio Science Experiment, and it relies on the main dish used to communicate with Earth. It has already measured thermal emissions during flybys of Pluto and Arrokoth, and the researchers are now evaluating whether they can point it back along the ecliptic and detect the thermal signature of zodiacal dust. Tests are also underway to see if REX’s huge (83-inch diameter) dish antenna can help increase the sensitivity of the spacecraft’s main dust particle detector.

Meanwhile, the visible and infrared imager and spectrometer "Ralph"—the spacecraft’s main “eyes” during the Pluto rendezvous—has been used by Grundy recently to image Neptune and Uranus. “Lit from the side, we’re getting a view analogous to seeing exoplanets around other stars,” said Grundy. In September, Ralph collected photometric data on both ice giants while the HST simultaneously imaged them from Earth orbit. “It’ll be a great test to see how many of the variations HST sees in surface light and color caused by cloud activity on Neptune and Uranus, can be discerned by New Horizons,” said Grundy. “But the data will take several months to arrive—our download rate is under 2k Baud, a lot slower than that 36k Baud home modem you might’ve once owned.”

The Planetary Geology team has also been studying Kuiper Belt dust, fragments of rock and ice shed by comets or formed when KBOs collide with each other or with interstellar dust particles zooming in from space at speeds of 40–100 kilometers per second (25–60 mps). The collisions emit sprays of dust grains typically 10–100 microns across, many of which begin a long slow spiral into the Sun, like an immense dusty bathtub draining.

The New Horizons instrument measuring this dust is the Student Dust Counter (SDC), built and run by students at the University of Colorado as a way for them to contribute to the mission at a near-professional level. One of those graduate students, Andrew Poppe, now an associate research scientist at the Space Sciences Laboratory (University of California, Berkeley), recently took over as the leader of New Horizons’ Heliophysics team.

The SDC consists of 12 detectors made from thin (30-micron) rectangular pieces of special plastic about the size of a Hershey’s chocolate bar, coated with aluminum. “They’re lined up in two rows, facing forward on the front of the spacecraft—a lot like a hood ornament,” said Poppe. When a particle hits a detector, it’s registered as a tiny electric pulse, with the magnitude of the pulse depending on the particle’s size and speed, from which mass and approximate size can be inferred. The density of the dust is incredibly low. “We only get a couple of hits a week, so it’s a bit like hunting for needles in a very large haystack. The combined surface area of the detectors is small, but we’re traveling at quite a clip—around 30,000 miles per hour—so that helps,” Poppe said. “From this data, we’re building a picture of how much dust is out there and where it's concentrated.”

The edge of the Solar System

The SDC has been collecting data for most of the mission. Now at 60 AU from the Sun, it’s become the most remote dust counter ever run. And it has turned up something strange. Earth-based observations predict that Kuiper belt dust should peak at 40–50 AU, but recent SDC data confounded this expectation. “The spacecraft should be showing a declining dust count this far out, but we’re not seeing that,” said Poppe. “We’re scratching our heads about why. It’s a preliminary but intriguing finding.”

Understanding how the uppermost layer of the Sun’s atmosphere, evaporating away from the solar surface at supersonic speeds, thins out and changes with distance is another major focus for the Heliophysics team. This solar wind of charged particles (97 percent protons, two percent helium ions, and a sprinkling of heavy ions) carves out a protective bubble in space, shielding Earth from lethal ionizing radiation. New Horizons measures the direction of travel and energy of charged particles using SWAP (solar wind around Pluto) for energies of up to 10 keV, and PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) for more energetic charged particles (between 10 keV and 106 keV).

“The exciting thing with New Horizons is that we can study the transition from where the Sun is in charge of things to where interstellar space and neutral particles dominate,” said Poppe. Unaffected by electromagnetic fields, these uncharged particles continuously drift into the heliosphere. If they become ionized, New Horizons can detect them with SWAP and PEPSSI. “We call these newly ionized arrivals "pickup ions." The energy of each particle tells us what likely happened to it over its lifetime—you can think of them as "individual messengers from interstellar space," Poppe explained.

A distant telescope

Finally, researchers are taking advantage of the craft’s onboard telescope. What it lacks in resolving power, it compensates for with location.

“What’s exciting about where the spacecraft is now is the ability to do things that are completely impossible with Earth-based telescopes. They’re so close to the Sun, it’s like sitting next to a roaring campfire and trying to spot the glowing eyes of small animals well beyond the firelight,” said Grundy. “It’s impossible to see small, faraway objects, even with Earth-orbiting space telescopes, because the light reflected from far-flung objects dims exceedingly quickly—by the distance (r) to the fourth power (r4). Meaning they get completely washed out by sunlight reflected from nearby dust.”

For the Astrophysics team headed by Tod Lauer, while New Horizons lacks the power to probe deep space of the Hubble or James Webb Space Telescopes, it has the huge advantage of prime dark sky real estate. “Where we are now, the Milky Way is spectacular, stretching all the way around the sky. With Earth lost in the glare of the fading Sun, our equator is now defined more by the band of the galaxy than Earth-based notions of north and south,” said Lauer.

He has been using LORRI (Long Range Reconnaissance Imager) a panchromatic high-magnification imager that focuses visible light onto a charge-coupled device (CCD) and registers the data in black and white. Originally designed as a backup for Ralph during the Pluto flyby, LORRI has no filters or shutter. Essentially equivalent to an 8.2-inch reflector, it’s slightly less powerful than the telescopes most amateur astronomers use. But unlike amateur scopes, the optical truss is made of silicon carbide. designed to be stable at low operating temperatures.

And it’s in a spectacular location, which Lauer is taking full advantage of. “It’s awfully dark where we are now, giving us the power to do something that no one else can do. We can measure precisely how dark space itself is,” he said. Based at the National Science Foundation’s aptly named NOIRLab in Tucson, Arizona, Lauer has been measuring the background visible light in the cosmos, known as the Cosmic Optical Background (COB).

To make the measurements, Lauer’s team had the spacecraft maneuvered to keep the Sun behind it, allowing LORRI to look between the glowing Milky Way and other galaxies ahead. “Peering into the darkest areas of space, it feels like we’re taking images of the face of the Universe itself,” said Lauer, “and we’re seeing something we don’t understand.”

In 2021, his team photographed a dark patch of sky and digitally removed all known light sources in the Universe. What remained—the estimated COB—is roughly twice as bright as expected. “Our test field was far from the Milky Way, bright stars, dust clouds—anything that would wash out the fragile darkness of the Universe, yet that mysterious glow is still there. It’s like being in an empty house out in the countryside, on a clear moonless night, with all the lights turned off, and finding it’s not completely dark,” said Lauer.

The team has exhaustively examined all possible sources of light contamination—sunlight bouncing off bulkhead equipment, a CCD "dark current," a faint radioactive glow from the RTG, double-checking that there were no blinking navigation lights. “It was intense, miserable work,” said Lauer. “It’s true I’m using LORRI to do things it was never designed for. I wish we had a dark shutter to see what the camera returns when we think no light is getting in, but I did demonstrate that a zero-second exposure returns a zero signal.”

Despite all his careful checking, when Lauer wrote the paper announcing the COB anomaly, he continued to worry that the team had missed something obvious. “I felt exposed—like I was walking out of the house without my pants on!” Over the next month or so, 15 additional fields, also selected for exquisite darkness, will be measured to see if initial COB results are repeatable.

How long there will be enough power to LORRI is not currently known. ”Once heat to the instrument is shut off and it goes below a critical temperature, it’s unlikely it can ever be revived,” explained Lauer. But the Astrophysics team also has ALICE, a low-power imaging spectrograph, which the researchers are using to go after the cosmic UV background. They’re also using that instrument to make a full-sky map of Lyman-alpha emissions, the most important spectral line associated with ionized hydrogen, to give a whole-sky map of how hot and shocked the cosmos is.

One remaining uncertainty is where and when New Horizons will be able to collect critical data at a key location: upon its exit from the heliosphere. It will be at least a decade, although the exact timing is uncertain. That’s partly because, unlike the Voyager probes, New Horizons is flying close to the ecliptic—the plane of Earth’s orbit. Also, the boundary of the giant lumpy, bubble-shaped heliosphere fluctuates with the 11-year solar activity cycle. “It’s as if the whole heliosphere breathes in and out in three dimensions,” said Andrew Poppe. “Is that bubble ovoid or more of a deformed sphere? We don’t know yet.”

When it does make that epic crossing, New Horizons will be only the fifth human-made probe ever to enter the vast unknown territory of interstellar space.

Diane Hope, PhD, is a former research ecologist turned writer and audio producer. She has covered topics ranging from the nocturnal life of a research telescope operator to the culture and science behind makgeolli, Korea's most historic beverage. You can hear her adventures in sound around the world on Instagram: @inthesoundfield