Nearly 4 billion miles from Earth, NASA's New Horizons spacecraft has detected evidence of a glowing wall of hydrogen at the edge of the solar system. Writing in the journal Geophysical Research Letters, the New Horizons Team says the discovery may help prove the existence of a region where the sun's solar wind and interstellar forces interact.
"We're seeing the threshold between being in the solar neighborhood and being in the galaxy," team member Leslie Young of the Southwest Research Institute told Science News.
First detected in 1992 by the two Voyager spacecraft, the hydrogen wall has been theorized to exist at the very edge of the heliosphere. This bubble-like region of space is composed of cosmic rays — solar wind particles emanating from the sun. This is proven through data the Voyager spacecrafts are sending back to NASA. Currently, Voyager 2 is measuring an increased rate of these rays as it approaches the outer boundary of the heliosphere.
As the rays race towards the outer reaches of our solar system, they begin to encounter interstellar forces that slow its velocity. At an estimated distance of 9.3 billion miles from the sun, just where the heliosphere wanes, it's believed that uncharged hydrogen atoms colliding with solar wind should scatter ultraviolet light in a distinctive way.
A solar map of where the hydrogen wall is thought to exist at the edge of the heliosphere. (Photo: NASA/JPL/Wikimedia Commons)
Between 2007 and 2017, New Horizons used its Alice instrument seven times to scan the sky for ultraviolet wavelengths. Analyzed over time, the data collected showed the distant presence of ultraviolet light consistent with observations recorded by Voyagers I and II nearly 30 years earlier.
According to the researchers, the signals picked up by the spacecraft is either the hydrogen wall or possibly ultraviolet light from some other unknown source. The team says they plan on having New Horizons scan the sky twice a year for possibly as long as the next 10 to 15 years as the spacecraft moves deeper into the outer solar system.
Prepping for a close encounter with 'Ultima Thule'
An illustration of New Horizons' flyby of 'Ultima Thule,' a Kuiper belt object that has changed little since its formation billions of years ago. (Photo: NASA/Wikimedia Commons)
In addition to discovering the secrets of the heliosphere, New Horizons is also drawing closer to its New Year's Day rendezvous in 2019 with a primordial rock called Ultima Thule. Formed during the early days of the solar system, Thule is a 20-mile-wide Kuiper belt object of irregular dimensions. As New Horizons completes its flyby at a distance of only 2,200 miles from Thule's surface, its instruments will gather unprecedented details about the object's surface composition and surrounding environment.
According to Alan Stern, principal investigator for New Horizons, the team isn't exactly sure what surprises Ultima Thule has in store.
"We don't know enough about it to predict," he told Discover magazine. "It's certainly ancient and pristine, and we've never seen anything like it."
Editor's note: This article has been updated since it was originally published in August 2018.