New Horizons: A New Era for Heliospheric Science

If you visit the New Horizons mission website often, you will see that the spacecraft keeps sending new data as it travels through the Kuiper Belt. After a recent hibernation period, the probe woke up healthy and is now sending the data it collected back to Earth. New Horizons first flew past Jupiter in 2007, then visited Pluto in 2015, and later studied the Kuiper Belt object Arrokoth in early 2019. Two mission extensions in 2016 and 2023 have let the probe keep working for many more years.

The probe was launched in 2006 on a powerful rocket. After its Jupiter flyby, it used the planet’s gravity to speed up toward the outer Solar System. The 2016 extension gave it a new set of instruments and a fresh power supply. The 2023 extension added more fuel and a new antenna, allowing it to keep sending data for another decade.

A big part of the mission is a continuous study of the heliosphere, the bubble of solar wind that surrounds the Sun and extends to the edge of the Solar System. Two instruments on board—PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) and SWAP (Solar Wind at Pluto)—measure the Sun’s influence all the way out. These instruments are newer than those on the Voyager probes and can do science that no other spacecraft can do.

The PEPSSI instrument counts energetic particles that come from the Sun and from space. SWAP measures the speed and density of the solar wind, the stream of charged particles that flows from the Sun. Together, they give scientists a picture of how the Sun’s energy moves through the Solar System.

The heliosphere is shaped like a comet, with a long tail that points away from the Sun. The boundary where the solar wind slows down and mixes with interstellar gas is called the heliosheath. Studying this region tells us how the Sun’s magnetic field protects Earth from cosmic rays.

The heliosphere is a bubble that protects the planets from the harshest parts of space. Its outer edge, the heliopause, marks the point where the solar wind meets the interstellar medium. Studying this boundary helps scientists understand how the Sun interacts with the galaxy.

Alan Stern, the principal investigator of New Horizons, says the data from PEPSSI and SWAP are essential. "There is an almost continuous trace of data across the solar system from Earth outward from these instruments since launch," he says. "Next, New Horizons will go through the heliosphere’s termination shock in the next couple of years and pass the heliopause in the next ten years or so; and this data from SWAP and PEPSSI are the only chance we or anyone else will have to measure those events and then to study interstellar space after Voyager."

During its second extended mission, New Horizons gives equal priority to planetary science and heliophysics. The spacecraft studies Kuiper Belt objects and also collects data about the solar wind and high‑energy charged particles throughout the Solar System. The instruments collect data all the time, even when the probe is in hibernation. The data are stored on board and sent back to Earth when the spacecraft wakes up.

The amount of data is growing. As the probe moves farther from Earth, it takes longer for the data to travel back. Planetary science has been getting its data, and heliophysics has been getting its data, but the heliophysics budget has not covered the yearly $2 million needed for instrument operations and data recovery since fiscal year 2024.

New Horizons overall funding will continue in the 2027 budget, but Stern says there has not been enough money in the past few years to cover all of its mission‑control costs. These costs include retrieving more PEPSSI and SWAP data and sending it to Earth. "We couldn’t staff mission control 12 months a year," he says. "NASA’s Planetary Science budget was funding us $10 million a year, and the Heliophysics budget had been funding us $2 million a year, until the heliophysics money stopped coming," Stern explains. "As a result, every year more and more data is now backlogged on the spacecraft recorders. To get that backlogged data to Earth, every year we’ve asked for a restart of the heliophysics funding, but it’s never come."

The $2 million yearly cost covers the power needed to run the instruments, the data storage, and the communication with Earth. Without this money, the instruments cannot be turned on, and the data they collect would stay on the spacecraft forever.

In plain terms, the budget can no longer cover heliophysics science from the spacecraft. "Now we’ve reached the point where we can no longer afford to take heliospheric data," Stern says. "Beginning in October, we have to suspend using those instruments. They won’t be collecting any data until helio or someone can again support the two million per year to allow SWAP and PEPSSI data collection and transmission to Earth."

It’s Not a Political Fight, It’s a Money Issue

Stern stresses that this is not a dispute between NASA divisions; it is simply a lack of money to staff mission control and run the heliospheric instruments. Fran Bagenal, a New Horizons team member and planetary scientist, says the team really needs to keep the heliophysics science going. "It’s worth it," she says. "I understand the concern that Helio has at NASA. They’ve got other instruments that are making measurements around the Earth and the L1 point measuring the solar wind and space weather, and they’re short of money."

Stern points out that there is no other source of data on the outer heliosphere right now but New Horizons, which is now having to pause because of that lack of funding. Unless the budget issue is solved, the only choice facing the mission team is to shut down the two heliospheric instruments. That would end a program that has measured the solar wind and particles ever since launch.

Bagenal explains that tracking changes in the solar wind throughout the Solar System is a unique advantage that New Horizons has with its newer instruments. "I’m a magnetospheres person," she says, "but obviously, it’s extremely important to know and understand the environment around the planets and out through the Kuiper Belt into the interstellar medium."

The termination shock is the point where the solar wind slows from supersonic to subsonic speeds. It is a key marker that shows where the Sun’s influence ends. New Horizons will cross this shock in the next few years, giving scientists a chance to measure it directly.

The heliopause is the outermost edge of the heliosphere. It is where the solar wind meets the interstellar medium. Voyager 1 crossed it in 2012, but New Horizons will cross it later, providing a fresh look at this boundary.

Scientists expect the termination shock and heliosheath—the boundary between the solar wind and interstellar space—to be revealed in new detail from measurements that SWAP and PEPSSI on New Horizons will make. In particular, heliophysics scientists want to improve on the Voyager 1 and 2 measurements.

It seems absurd that we’re focusing on trying to keep the money flowing to keep these instruments working when it’s so obvious that we can keep getting heliophysics data at relatively small cost," Bagenal says. "This really ought to be dealt with by people at NASA, like Administrator Isaacman, to step in and say, ‘Look, we’re NASA and only the United States can do this heliospheric science at the frontier of our solar system, so let’s do it."

The New Horizons spacecraft itself is in good shape, according to the mission team. It has enough fuel and power to keep going until the early 2050s, gathering information about the heliosphere and then interstellar space, just as the Voyagers did.

The mission team hopes that the funding will be restored soon. If it is, New Horizons will continue to send data until the early 2050s, when it will likely run out of power and fuel.

If the instruments are shut down, scientists will lose a unique window into the outer heliosphere. No other spacecraft will be able to fill the gap until a new mission is launched, which could take many years.

The data from New Horizons will complement the data from Voyager, giving a more complete picture of the outer Solar System. It will also help future missions plan their trajectories and instruments.

James Green, a heliophysicist who spent 12 years as head of planetary science at NASA and then two years as NASA chief scientist, is a strong supporter of the overall mission and also emphasizes the importance of this heliophysics science and the need to keep funding it on New Horizons. He explains that NASA does respond to opinions outside the agency when challenges like this come up.

The current discussion is, ‘Should they do that or should they not do that?’ he says. "Well, the ability for that decision to be made, which sometimes happens in this way, is for the community to say, yes, we think this is a really good idea. It’s well worth the $2 million to get unique data."

Green also says that the New Horizons heliophysics data has importance beyond just our Solar System. "Every star has magnetic fields. Most stars have planets. Some of these planets are habitable, and these stars that are moving into and through interstellar clouds are experiencing the same thing we are. And these stars, instead of calling them heliospheres, these stars have what we call astrospheres," he says. "By understanding the physics going on with ours, we can help see how astrospheres of other stars are affecting the habitability of those planets. So these kinds of measurements just cut through several really important areas. It’s heliophysics, but it’s also astrophysics."

Green is circulating a public petition to send to NASA, explaining the importance of New Horizons heliophysics data and why the funding to continue getting that information is necessary. He has sent it to the planetary science and heliospheric communities and has already gathered several hundred signatures. Members of the public who are interested in supporting the effort can sign at Change.org under the title 'Heliophysics Community Support for Continued Measurements by NASA’s New Horizons mission.'