A century after the Wright brothers briefly piloted an ungainly aircraft 12 feet above the dunes of North Carolina, humanity is reaching another milestone: getting a craft to the threshold between the solar system and interstellar space.
NASA's Voyager 1 spacecraft may have already entered the inner portion of that boundary. Scientists this week reported readings that suggest the craft had entered a shock wave typical of this region. The shock wave is generated as the solar wind - charged particles that race off the sun at supersonic speeds - slams into the dust and gas of interstellar space.
Not everyone agrees the craft is there yet. The scientists' conclusion has been challenged by other members of the Voyager team, who suggest that the 27-year-old spacecraft has seen only the foothills, not the mountains of this important frontier.
But the dueling sets of data are welcome, notes Voyager's chief scientist Edward Stone.
"We've been inside the bubble for 40 years," he says, referring to an envelope of solar wind and magnetic fields surrounding the solar system. "Now we're in a totally new region."
What they learn here, researchers say, will give them insights into the factors that determine the extended shape of the solar system and the processes that govern how much cosmic radiation the solar system receives from the rest of the galaxy.
Some scientists also suggest that the information gleaned at the solar system's edge could help improve "space weather" forecasts on Earth.
It could provide more data on the interplay between charged particles from the sun and magnetic fields in and beyond the shock wave, which astronomers call the sun's termination shock. This interplay near Earth generates auroras and can silence radio communications, knock out satellites, and trigger electrical blackouts. It also can expose airliners flying over the polar route to relatively high levels of radiation.
For astronomers and astrophysicists, this region of space 10 billion miles from Earth (give or take a few billion miles) represents a milestone. They've seen Hubble Space Telescope images of other stars plowing through dust and gas dubbed the interstellar medium. Now they anticipate the chance to study the process directly.
For some, Voyager crossed the threshold unambiguously, if briefly. Measurements from an instrument designed to detect low-energy charged particles recorded a big jump in particle density that began roughly Aug. 1, 2002. This jump would be expected as the particles get squeezed in a region known as the heliosheath, between the terminal shock and the final boundary known as the bow shock. Voyager 2, some 1.7 billion miles behind, didn't see the jump.
In addition, the particles streamed by in a different direction from those from the solar wind, says Stamatios Krimigis, head of the space department at The Johns Hopkins University's Applied Physics Laboratory. Moreover, indirect measurements suggested the solar wind had dropped dramatically.
By February, however, the measurements returned to previous levels, suggesting that Voyager 1 had reentered the solar wind. Mr. Krimigis and other scientists suggest that the shock boundary may shift with solar activity. Thus Voyager pierced the veil, only to have an energetic blast of solar wind push the veil farther spaceward again.
The second team, which includes Dr. Stone, is less convinced. It notes that the spacecraft's magnetometer failed to detect the shifts in magnetic fields researchers expected to see had Voyager entered the heliosheath. Nor were other indicators present with the characteristics theories predicted should exist.