Mystery as Nasa satellite Voyager 1 hits edge of the solar system
Nasa's Voyager 1 spacecraft, launched more than 35 years ago and now 18.5 billion kilometres from earth, 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 expect it to do. But there has been at least one false alarm.
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 18.5 billion kilometres from earth, 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 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 60,000km/h.
Scientists had expected that Voyager 1 would detect two telltale signs as it passed through the heliosheath, the solar system's outermost neighbourhood, which is thought to abut the heliopause, the actual boundary. The key instruments on Voyager 1, as well as those on its twin, Voyager 2, are still working, 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. They had predicted that at the boundary between solar system and interstellar space, the solar wind - charged particles blown out by the sun - would fade away and Voyager 1 would no longer detect it. That happened. They also expected the direction of the magnetic field to change as Voyager 1 emerged from the sun's magnetic bubble. That did not.
"Nature is far more imaginative than we are," said Stamatios 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. Krimigis is an author of one of the papers in Science.