NASA’s Voyager 1 spacecraft, launched more than 35 years ago and now 11.5 billion miles from where it started, is closing in on this boundary. In recent years scientists have been waiting eagerly for it to become the first artificial object to leave the solar system and enter the wider reaches of the Milky Way, which they fully expect it to do. But there has been at least one false alarm.

On Thursday, scientists reported that, no, Voyager 1 still had not reached interstellar space, but it had entered a region that no one expected and no one can yet explain, a curious zone that is almost certainly the last layer of our Sun’s empire — technically speaking, the heliosphere. Three papers published in the journal Science describe in detail the sudden and unpredicted changes encountered in the surroundings of Voyager 1, which left Earth about three months after the original “Star Wars” movie was released and is heading for the cosmos at 38,000 miles per hour.

Scientists had expected that Voyager 1 would detect two telltale signs as it passed through the heliosheath, the outermost neighborhood of the solar system, which is thought to abut the heliopause, as the actual boundary is known. Happily, the key instruments on Voyager 1, as well as those on its twin, Voyager 2, are still working after all these years, and its nuclear power source will last until at least 2020.

Last summer, one of the two events occurred, but not the other, leaving scientists perplexed. Scientists had predicted that at the boundary between solar system and interstellar space, the solar wind — a stream of charged particles blown out by the Sun — would fade away, and that Voyager 1 would no longer detect it. That happened.