At the edge of the solar system, there are no signs that proclaim, "You are now entering interstellar space."
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.
They also expected that the direction of the magnetic field would change as Voyager 1 emerged from the Sun's magnetic bubble. That did not.
"Nature is far more imaginative than we are," said Stamatios M. Krimigis, a scientist at the Johns Hopkins Applied Physics Laboratory who is the principal investigator of an instrument that records charged particles hitting Voyager 1. Dr. Krimigis is an author of one of the papers in Science.
Last July, the spacecraft -- which is roughly 1,600 pounds and would fit inside a cube about 13 feet on each side, according to NASA -- observed a momentary dip in the intensity of the solar wind. "It was exciting," said Edward C. Stone, the project scientist for the two Voyagers. "We had never seen such a drop before. It happened in less than a day. Then five days later, it was back up."
In mid-August, there was a deeper momentary dip.
Then, on Aug. 25, the solar wind dropped by a factor of more than a thousand, vanishing to imperceptible levels, and it has remained at essentially zero since. At the same time, the number of cosmic rays from outside the solar system jumped by 9.3 percent.
"It looked like we were outside," Dr. Stone said.
But the magnetic field has steadily, stubbornly pointed in the same direction, indicating that Voyager 1 is still ensconced within the Sun's magnetic field. Scientists guess that in this region the magnetic fields of the solar system partly connect to those of the surrounding interstellar space, allowing the solar particles to escape. (Charged particles travel along magnetic field lines.) They have named the zone through which Voyager 1 is hurtling the heliosheath depletion region.
"I think it's clear we do not have a model which explains all of this," Dr. Stone said.
Voyager 2, which is moving slightly more slowly and is not as far from the Sun, has not yet encountered this region.
Dr. Stone noted that when the two Voyagers launched in 1977 on a grand tour of Jupiter, Saturn, Uranus and Neptune, the space age was just 20 years old, and there was no way to know that NASA had built something that would last 35 years, long after it passed the planets. But the designers of the mission were prescient to be prepared if they lasted that long.
"It turns out that in fact we designed the cosmic ray instrument specifically for this phase of this mission," Dr. Stone said. "We were planning, and it really paid off. We've begun to see what's outside even though the magnetic field says you're not outside."
As for actually reaching the outside of the solar system, Dr. Stone said, "it could be a few months, or it could be several more years."
This article originally appeared in The New York Times.