(EPA/NASA/APL/SWRI)

Compared with the hustle and bustle of Earth, interplanetary space seems pretty empty. But it's actually awash in particles ejected by the sun. Now, NASA's New Horizons — the spacecraft that flew by Pluto in July and has been speeding into deeper space ever since — has sent back some of the first data on this distant space weather.

Spacecraft closer to home can tell us what kind of solar particles reach our neighboring planets, and the two Voyager probes — one of which has entered interstellar space, with the other soon to follow — show what's going on out in the deep. Now, New Horizons has given us three years of data from a journey through the outer edges of our solar system, allowing scientists to connect the dots. Their observations were published Wednesday in the Astrophysical Journal Supplement.

The sun ejects high-energy particles that form solar wind. On Earth, these particles can sometimes be energetic enough — such as in "solar storms," or coronal mass ejections — that they interfere with electronics and radio communication. Most of the time, our planet's magnetic field protects us, but we sometimes see evidence of these particles interacting with our atmosphere in the form of auroras, such as the Northern Lights.

Those particles reach out much farther than our own planet. The Solar Wind Around Pluto (SWAP) instrument, operated by Southwest Research Institute (SwRI), tracked solar particles and their influence as New Horizons made its way to Pluto — while the rest of the spacecraft's instruments were in hibernation mode. It observed the neutral space particles that become charged by high-speed, ionized particles from the sun. After they're charged, these particles get a speed boost and can actually be twice as fast and four times as energetic as the solar particles themselves.

Researchers believe that these high-energy particles could be the seeds of anomalous cosmic rays. According to a statement from NASA, "anomalous cosmic rays are observed near Earth and can contribute to radiation hazard for astronauts, so scientists want to better understand what causes them." The Voyager spacecraft have observed fully grown anomalous cosmic rays, which are thought to help shape the boundaries between our solar system and interstellar space.

“The Voyagers can’t measure these seed particles, only the outcome,” NASA's Eric Christian said in a statement. “So with New Horizons going into that region, this blank patch in the observations is being filled in with data.”

Studying the origin of these cosmic rays could help scientists figure out how to keep astronauts safe during long-haul space missions, such as to Mars and beyond. And by adding to the scant observations of how solar particles behave as they reach the depths of the solar system, New Horizons will help scientists understand how solar activity populates the vacuum of space.

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