Far beyond Pluto, beyond even the comets, lies the solar system’s true edge—the heliosheath, where charged particles blowing outward from the sun crash into those flowing from other stars to create a vast protective magnetic bubble.
In September scientists produced the most comprehensive study yet of this distant boundary, finding it improbably dynamic. The observations come from NASA’s Interstellar Boundary Explorer (IBEX), which detects neutral atoms that are sent streaming toward Earth after breaking free from the heliosheath. In 2009 IBEX data revealed a long ribbon of those atoms, with a knot in it, crossing the sky. Just six months later, the knot had unwound.
Mission leader Dave McComas of the Southwest Research Institute in San Antonio says there is no good theory for why the heliosheath would be so jittery tens of billions of miles from the sun despite its tremendous size, or for why the ribbon even exists.
Initially scheduled to end next month, IBEX’s mission was extended so that McComas can monitor the heliosheath over a longer timescale. “We have to analyze this as a dynamic structure—breathing, changing, and evolving,” he says. NASA astronauts would be likely to agree: When the heliosheath is weaker, it provides less protection from interstellar particles that could cause cancer in anyone embarking on a lengthy interplanetary mission.