The Voyager 2 spacecraft burst out of the bubble of gases expanding from the sun and into the wild of the Milky Way a year ago. It was the second spacecraft to cross that boundary and directly observe the interstellar medium. Its faster-moving twin, Voyager 1, made the crossing six years earlier, in August 2012.

Launched 42 years ago , when Jimmy Carter was president, the twin spacecraft have persisted far longer than envisioned, as has their ability to send scientific findings home to Earth.

In a series of papers published on Monday in Nature Astronomy, scientists report what Voyager 2 observed at the boundary of the solar wind’s bubble and beyond.

“We’re certainly surprised,” Edward C. Stone , the mission’s project scientist, said of the probe’s longevity during a news conference on Thursday. “We’re also wonderfully excited by the fact that they do. When the two Voyagers were launched, the space age was only 20 years old. It was hard to know at that time that anything could last over 40 years.”