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

By Ryan F. Mandelbaum on at

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 (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 theorised Oort cloud, an icy sphere of comets orbiting the Sun a third of the way to our nearest neighbouring 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. [GRL]