New Horizons Spacecraft Sees Possible Hydrogen Wall at the End of the Solar System

Artist’s impression of New Horizons
Artist’s impression of New Horizons
Illustration: NASA

As it speeds away from the Sun, the New Horizons mission may be approaching a “wall.”


The New Horizons spacecraft, now at a distance nearly four billion miles from Earth and already far beyond Pluto, has measured what appears to be a signature of the furthest reaches of the Sun’s energy—a wall of hydrogen. It nearly matches the same measurement made by the Voyager mission 30 years ago, and offers more information as to the furthest limits of our Sun’s reach.

“We assume there’s something extra out there, some extra source of brightness,” study author Randy Gladstone from the Southwest Research Institute told Gizmodo. “If we get a chance with New Horizons we can maybe image it.”

The Sun’s light sends charged particles outward, causing hydrogen particles in the space between planets to release characteristic ultraviolet light. But eventually, the Sun’s energy should wane, creating a boundary where interstellar hydrogen piles up at the edge of the outward pressure caused by the solar wind’s energy.

The boundary of the solar system
The boundary of the solar system
Graphic: NASA/JPL-Caltech

Scientists took a 360-view of this ultraviolet emission using New Horizon’s Alice instrument. When they looked into the distance away from the Sun, they saw an added brightness to the signal. This could be from hydrogen particles beyond the Solar System interacting with the furthest reaches of the solar wind, creating what appears to be a boundary in the distance, according to the paper published this week in Geophysical Research Letters.

The Voyager probe measured a similar signature three decades ago. Recent re-analysis demonstrated that Voyager’s scientists probably overestimated the signal’s strength. But once the Voyager data was corrected, New Horizon’s results looked almost exactly the same.


Perhaps the signal is something else, said Gladstone, but the corroboration of the data at least adds credence to its existence, whether it’s coming from the hydrogen wall or some other feature. Scientists plan to observe the signal perhaps twice a year, according to the paper.

New Horizons is currently prepping for its visit to Kuiper Belt Object 2014 MU69, a roughly 30-kilometer-wide rock, and then it will continue toward the edge of the Solar System.


The Solar System’s boundary is a hard-to-define location—after the end of the solar wind’s influence, there’s still the theorized Oort cloud, an icy sphere of comets orbiting the Sun a third of the way to our nearest neighboring star.

New Horizons will continue on, first past 2014 MU69 and perhaps past other Kuiper Belt Objects, provided it gets NASA’s approval. Then it’s onward—but space is really big. It will still take until the late 2030s before New Horizons reaches Voyager’s distance today. We’ll likely be dead before it truly leaves the influence of our Sun.



Former Gizmodo physics writer and founder of Birdmodo, now a science communicator specializing in quantum computing and birds



“The Alliance uses subtle emissions in the common bands to mask energy and visible light photons in certain directions near the boundary shock. Its how we allow young races to evolve. Our manipulations are usually lost in ‘the glow’ - and thus, remain hidden until a species breaks free of their solar confines. Usually, when a new species takes their first steps into the larger universe, we help them out, give them the basics... and then they have to do for others what was done for them.” N’vor said looking on as others watched, “By giving them the basics, we thereby impose the last test - their ability to come together as a people and understand their place in the universe and their responsibility as caretakers of it. Not all make it past this stage - for the technology we give is also a curse of knowledge unearned. It is always easier to destroy than it is to create.”

N’vor’s eyes widened slightly, then closed as a brief flash was seen in the sky, and a star ended its life. “It is always a great burden, to know you have done everything right - have invited them, guided them to the best of your ability - and yet they still choose the wrong path.”

The young Na’tal looked at N’vor. “Who were they? Were they important?” Sixteen cycles, so young.

N’vor looked up not unkindly. The horror of watching a species consume itself in fire and war written on their innocent faces. The fear, the anger, and the impotent sense of loss. He could taste it on the very air he breathed. He allowed an expression full of sadness to overtake his features: “Not anymore.”