Where does the solar system end and interstellar space begin? That’s a question scientists have been working to answer using spacecraft traveling beyond the sun’s influence. A team of researchers from the Southwest Research Institute, led by Heather Elliott, is using the Solar Wind Around Pluto (SWAP) instrument aboard New Horizons to track the solar wind in the outer reaches of the solar system.
The instrument measured distinct slowdowns of the solar wind as the spacecraft traveled between 21 and 58 AU. According to Elliott, there’s a good reason for that slowing.
“As the solar wind travels away from the sun at supersonic speeds, roughly 1 million miles per hour, eventually it encounters incoming interstellar neutral gas particles entering the heliosphere,” she said. “These neutral interstellar atoms become ionized via charge exchange with solar wind ions, adding mass to the solar wind by picking up interstellar material that slows the solar wind down.”
Understanding the extent of the solar wind not only gives scientists a greater understanding of our star’s influence, but it also helps them see how winds from other stars play a role in their environments. Astrospheres around distant stars also interact with the interstellar medium. They have many properties in common with our sun, so whatever scientists find out from our own solar wind’s influence is important.
“Studying the heliosphere is like solving a cosmic puzzle,” said Elliott. “Not only do we learn more about how the sun’s influence ends, but we also gain a deeper understanding of the boundary between our solar system and interstellar space—a critical step toward planning future interstellar travel.”
Charting the solar wind slowdown
The solar wind is a continuous outflow of plasma from the sun. It flows from different regions of our star, such as coronal holes. When the wind leaves those areas, it’s moving at around 500 to 800 kilometers per second (310 to 500 miles per second). The average speed that we “feel” here on Earth clocks in at around 400 km per second (250 miles per second).
It makes sense that the wind will slow down as it moves through the solar system, and that’s what scientists want to measure. It also makes sense that the point where it’s no longer blowing is the point where interstellar space begins.
New Horizons isn’t the first spacecraft to detect the solar wind. The Ulysses spacecraft, for example, charted all the “regimes” of the solar wind as it made a nearly pole-to-pole orbit around the sun from 1990 to 2009. Other missions have measured it on their way to the sun (such as the Parker Solar Probe), Mars and Jupiter. The twin Voyagers and the Pioneer missions are the only others to have measured the solar wind at such great distances.
The best way to understand the slowdowns is to compare them to the speed of the solar wind at Earth (1 AU). New Horizons and Voyager 2 measurements between 30 and 43 AU showed the solar wind was 5% to 10% slower than at 1 AU near Earth. New Horizons measurements at 58 AU show the solar wind is 13% to 15% slower than the wind at 1 AU. These measurements of the gradual slowing of the solar wind align with previous models of how interstellar material enters the heliosphere and affects the solar wind. They also demonstrate how the sun’s influence decreases over long distances.
What happens farther out?
As New Horizons continues on its trajectory through the Kuiper Belt and beyond, it will eventually reach a point where the solar wind is pushed back by incoming interstellar material. That’s the point where the heliosphere’s influence stops, called the termination shock.
In that region, some 85 AU from the sun, the incoming interstellar material heavily affects the properties of the solar wind as it nears the outer boundary of the heliosphere. Voyager 2 measured a sharp 46% drop in speed at the termination shock at a distance of 84 AU. So it will be interesting to see what New Horizons measures when it reaches that point, which could happen around 2029.
When the solar wind gets to that point, its shape and boundaries change, according to Elliott. “The shape and properties of these heliospheric boundaries control the amount of Galactic Cosmic Rays (GCRs) that can enter our solar system and reach Earth,” she said.
“Therefore, the data from New Horizons combined with observations from other missions, such as IBEX, IMAP and Voyager, will enhance our understanding of the edge of the solar system.”
Galactic cosmic rays are a concern for astronauts on long-term missions. They are lethal, and some do get through the heliosphere. Understanding the boundaries of the solar wind is an important factor in mission planning for trips to the moon and beyond.
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New Horizons watches the solar wind as it slows down (2026, July 7)
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