According to Krimigis, Voyager 1 crossed this boundary around August 1 last year. At that time, he says, the spacecraft picked up a huge influx of radiation. The new material it encountered was depleted in carbon, characteristic of interstellar plasma. Using a low-energy testing device on board the spacecraft, his team also concluded that the plasma was moving far slower than before.
"These three pieces of evidence convinced us that the spacecraft exited the solar wind proper and entered the heliosheath, the transition between our solar system and the interstellar medium," Krimigis said.
Not so, counter another group of scientists, among them Frank McDonald of the Institute for Physical Science and Technology at the University of Maryland in College Park.
In a separate Nature article, McDonald and his colleagues write that the termination shock will accelerate anomalous cosmic rays, and that hasn't happened yet according to their measurements. What Voyager 1 went through for seven months, beginning in August last year, is just a precursor event to the termination shock, they argue.
But if scientists can determine within meters how far the spacecraft have traveled, why can't they say with certainty if Voyager 1 has gone over the edge?
The problem is that the termination shock is changing in size and moving rapidly, sometimes faster than the speed of Voyager 1.
In fact, Krimigis believes Voyager 1 passed into the subsonic region around August 1 last year, then re-entered our solar system about 200 days later.
"Think of the termination shock as a weather front that goes right over the Voyager," said Krimigis. "If the spacecraft were outside, it finds itself inside. When the weather front dissipates, pressure decreases and the termination shock moves inward again and goes over Voyager, which finds itself outside once more. This is bound to happen many times over the next several years."
Scientists say they will be able to learn more about the termination shock as Voyager 1 passes through it, and better understand how the stars interact with their surroundings.
"Now we can calibrate the distances over which these kinds of phenomena occur," said Krimigis. "That's really the principal scientific reason why this is important: to know the span between interstellar space and the solar system."
Greetings from Earth
So where does Voyager 1 go from here? Barring accidents, it is expected to keep going for another 20 to 30 years before running out of power. That means it should eventually encounter the region beyond the termination shock known as the heliopause, where solar material mixes with interstellar material.
Don't expect the spacecraft to stay alive long enough to reach interstellar space, however. First, it would have to pass through a swarm of small cometary nuclei, known as the Oort's Cloud, which is barely held in orbit by the sun's gravity.
It would take Voyager 1 about 20,000 years to reach the middle of that comet swarm, and maybe twice as long for it to pass the outer boundaries of cometary space. By this time, it will have traveled a distance of two light-years or half the distance to Proxima Centauri, the nearest star.
In case it should encounter aliens along the way, Voyager 1 carries a 12-inch (30-centimeter) copper disk containing a greeting from Earth: sounds and images selected by the late astronomer Carl Sagan to portray the diversity of life and culture on Earth, as well as spoken greetings from Earth-people in 55 languages.
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