NASA‘s New Horizons spacecraft has helped scientists study a mysterious phenomenon at the edge of the Solar System, where particles from the Sun and interstellar space interact.
This region, about 100 times further from the Sun than Earth, is where uncharged hydrogen atoms from interstellar space meet charged particles from our Sun. The latter extend out from our Sun in a bubble called the heliosphere.
At the point where the two interact, known as the heliopause, it’s thought there is a build-up of hydrogen from interstellar space. This creates a sort of “wall”, which scatters incoming ultraviolet light.
About 30 years ago NASA’s Voyager 1 and 2 spacecraft first detected this wall, and now New Horizons has found new evidence for it. A paper describing its findings will be published in Geophysical Research Letters.
“We’re seeing the threshold between being in the solar neighborhood and being in the galaxy,” Dr Leslie Young from the Southwest Research Institute in Colorado, one of the co-authors on the paper, told Science News.
New Horizons made the detection using its Alice UV spectrometer, taking measurements from 2007 to 2017. It found an ultraviolet glow known as a Lyman-alpha line, which is made when solar particles hit hydrogen atoms.
We see this ultraviolet glow all over the Solar System. But at the heliopause, there appears to be an additional source caused by the wall of hydrogen, creating a larger glow. Beyond the wall there’s more ultraviolet light compared to in front of it, suggesting it's being scattered by the wall.
"This distant source could be the signature of a wall of hydrogen, formed near where the interstellar wind encounters the solar wind," the researchers wrote in their paper.
The theory is not definitive yet. It’s possible that another source of ultraviolet light in our galaxy could be causing this background glow. To find out for sure, New Horizons will continue looking for the wall about twice a year.
At some point, New Horizons will cross the wall, if it exists, so the amount of ultraviolet light it detects will decrease. That would provide some additional evidence that the wall is really there.
Voyager 1 and 2 are both past the wall now, so they’re unable to make any further detections. But New Horizons is only 42 times further from the Sun than Earth, a distance it has taken about 12 years to achieve, and is currently on its way to explore a new target called Ultima Thule having flown past Pluto in 2015.
If our estimates are correct, then by the time the mission ends in about 10 to 15 years, it should hopefully have just about made it to the wall. At that point, we might really know for sure if it’s there or not.