After 26 lonely years and about 8 billion miles of travel, the Voyager 1 spacecraft has become the first human-made object to leave the solar system. Maybe.
A group of astronomers announced Wednesday that Voyager 1 had crossed the "termination shock" at the edge of the solar system where the sun's powerful influence wanes and the solar wind drops from supersonic speeds to a relative whimper.
But another group argued that the spacecraft still has a journey ahead before it reaches this outer limit. They contend that the strange readings collected by the aging craft in the last few months came from a "foreshock," and were nothing more than a brief hint of the exotic territory that lies ahead.
The one thing the rivals do agree on is that Voyager 1 has entered a final frontier -- far beyond our system's most distant planet, Pluto -- unlike anything humans or their space probes have encountered before.
When it was launched in 1977 to study the outer planets, its creators at the Jet Propulsion Laboratory in Pasadena hoped it would travel this far -- and possibly much farther. Along with its suite of scientific instruments it carried with it photos of life on Earth, greetings in 55 languages and a collection of songs -- including Chuck Berry's "Johnny B. Goode" -- intended for an extraterrestrial audience that could encounter the craft tens of thousands of years from now as it approaches other stars.
The spacecraft is now cruising through a turbulent realm where radiation pulses are a hundred times more than normal, and the solar winds that speed past Earth at a million mph abruptly slow as they push up against the great celestial winds that travel between the stars. The pioneering spacecraft has entered a region with a different chemical mix that is bathed in streams of peculiar cosmic rays.
"It's like we are piercing a hole in the curtain that separates us from the rest of the galaxy," said Merav Opher, an interstellar space expert at JPL who was not directly involved in the new research. "It's like stretching our arm and touching interstellar space."
The disagreement -- made public at a news conference Wednesday at NASA headquarters and in dueling papers published in today's issue of the journal Nature -- highlights how little is known about the farthest reaches of our solar neighborhood. Despite crisp drawings that fill elementary school textbooks, astronomers are not entirely sure where the solar system ends.
The solar system is encased in a huge magnetic bubble, known as the heliosphere, formed of particles that flow from the sun for billions of miles.
To astronomers, the solar system's first clear boundary lies 88 to 102 astronomical units, or 8 billion to 9.5 billion miles from the sun, where its influence begins to wane. (An astronomical unit is 93 million miles, the average distance between Earth and the sun.) That boundary, more than two times the distance between Pluto and the sun, is called the "termination shock."
Supersonic solar winds abruptly slow as they butt against winds of charged particles blowing toward us from distant stars.
The next boundary is the heliopause, a region 10 billion to 15 billion miles from the sun where the pressure from solar wind is in balance with the interstellar winds. Even farther away, approximately 21 billion miles from the sun, is the "bow shock," a violent wave where the interstellar medium is pushed outward by the heliopause.
Pinpointing where these boundaries lie is a guessing game.
Part of the problem is that the boundaries are not fixed. They wax and wane with the sun's level of activity. Blasts of solar flares and solar wind issuing from the sun, as they did last week, can extend the heliopause out several million miles.
The heliopause breathes and pulses in a roughly 11-year cycle, a kind of solar heartbeat, said Edward Stone, Voyager's project scientist and one of the researchers who believes the craft has not yet left the solar system but is teetering on its fluctuating edge.
"We will likely be surfing the shock for three or four years," said Stone, the former director of JPL and a physics professor at Caltech.
The group who does believe Voyager 1 has left the solar system is headed by Stamatios "Tom" Krimigis, who also heads the space department at the Johns Hopkins University Applied Physics Lab.
Last August, when Voyager 1 was about 8 billion miles from the sun, it detected radiation levels 100 times more intense than normal. He believes this is when the craft crossed the termination shock. Nothing of the kind was recorded by Voyager 2, which lags more than a billion miles behind its sister craft, so "we knew something very special had happened," he said.
Indirect measurements also showed the solar wind had slowed to less than 100,000 mph, "which in this business is a slow wind," Krimigis said. The craft also detected different chemistry in its new environs: a mix of elements more akin to that found in the interstellar medium than that from the solar wind.
If Voyager 1 could measure the solar wind directly, astronomers would have a clear answer about whether it crossed the termination shock. But the solar wind detector broke in 1990.
By this February, when Voyager 1 was 8.4 billion miles from the sun, all of the strange readings suddenly dropped off. Krimigis thinks the solar wind, fueled by solar activity, rushed in front of the craft and enveloped it again, bringing it back into the domain of the solar system where it currently remains.
The rival group, led by Frank McDonald of the University of Maryland used different instruments on Voyager to come to their conclusions. While McDonald's group agrees that Voyager encountered a large increase in high-energy particles, a drop in the solar wind and strange cosmic rays, the group did not see a high number of cosmic rays, precise patterns of streaming particles or high-intensity magnetic fields as they expected. They believe the termination shock still lies ahead.
Likening Voyager 1 to the Lewis and Clark of space exploration, McDonald said Voyager had not yet reached the mountains but was definitely "in the foothills."
In an editorial in Nature accompanying the papers, University of Michigan astronomer Len Fisk said he thought the craft had crossed the termination shock, but said that the termination shock might have a more complex shape than anticipated.
The question is likely to be resolved as Voyager 1 surges forward on its continuing journey. Though designed to last only five years, the craft is still going strong 26 years after it was launched.
It is so distant now that the sun appears to it as only a dim spot in space, and its messages take more than 12 hours to reach Earth.
With a nuclear fuel source, Voyager 1 has enough power to keep running until 2020. At that point, it will be 12.4 billion miles from the sun and definitely outside of the solar system.