After 321 days of sleep, a NASA spacecraft woke up in the cold outer fringes of the solar system.
New Horizons, the spacecraft that flew past Pluto in 2015, entered hibernation on purpose to save energy as it drifted deeper into the Kuiper Belt, a disk beyond Neptune of comets and tiny icy worlds.
At some 6 billion miles from Earth, a wakeup call from mission control would have taken nine hours to travel through space. Instead, the spacecraft woke itself up on June 23 from a previously scheduled command.
New Horizons needs its strength. The intrepid explorer is expected to continue operating into the 2050s over a distance perhaps 100 times farther from the sun than Earth. During its travels, it will visit one more Kuiper Belt rock, following its flyby of Arrokoth seven years ago. The team is now using the Vera C. Rubin Observatory to figure out its next destination, said Alan Stern, the mission's principal investigator, in a comment on X.
Scientists have used New Horizons to measure how solar wind — the stream of charged particles flowing away from the sun — changes as it moves toward the edge of the solar system.
They found that the solar wind gradually slows as it travels outward because it collides with atoms drifting in from interstellar space. New measurements show the solar wind is 13 to 15 percent slower out there than it is near Earth. The spacecraft's data was published in The Astrophysical Journal.
Scientists predict the solar wind will slow much more dramatically when New Horizons reaches the so-called "termination shock," where the sun's particles suddenly lose speed. Voyager 2, which is now 13.3 billion miles away from Earth in interstellar space, previously measured a 46-percent drop in solar wind speed when it had reached that point. It was then about 84 times farther from the sun than Earth.
Learning how the solar wind changes helps scientists map the edge of the sun's influence, said Heather Elliott, a researcher at the Texas-based Southwest Research Institute. And despite the Voyager twins having already left the solar system, the "edge" of the heliosphere — this region in space affected by the sun — is still mysterious.
"Not only do we learn more about how the sun's influence ends," said Elliott, first author of the paper, in a statement, "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."
New Horizons launched 20 years ago and flew past Jupiter in 2007 for a gravity boost and scientific observations. It became the first spacecraft to explore Pluto and its moons. New Horizons also detected a previously unknown population of distant objects beyond the known Kuiper Belt with its long-range camera. The discovery could mean the belt extends much farther than expected or that a second, more-distant belt of icy bodies exists beyond it.