Illustration courtesy Caltech/NASA

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This artist's concept shows the general locations of NASA's two Voyager spacecraft.

Illustration courtesy Caltech/NASA

Voyager 1 Leaves Solar System, NASA Confirms

NASA’s far-flung spacecraft passed into interstellar space last year.

It’s official: Voyager 1 has slipped from the solar system.

Launched in 1977, Voyager 1 traveled past Jupiter and Saturn and is now more than 11.66 billion miles (18.67 billion kilometers) from the sun, becoming the first spacecraft to enter interstellar space. Proof of this long-anticipated milestone for the storied spacecraft comes in a study released Thursday by the journal Science and announced at a celebratory NASA headquarters briefing.

"We made it! We are in interstellar space," said Voyager scientist Ed Stone of the California Institute of Technology in Pasadena, speaking at the briefing.

Solar storm aftershocks at the edge of the solar system provide confirmation that the Voyager 1 spacecraft made the passage on August 25, 2012, space agency scientists said Thursday.

On that date, Voyager 1 passed beyond the fringes of the sun's outward-flowing solar wind and into the interstellar space between the stars.

"It is an incredible event, to send the first human object into interstellar space," says study lead author Donald Gurnett of the University of Iowa in Iowa City. "It’s not quite the moon landing, but we are where the solar wind ends."

Finding the Solar System’s Edge

The solar wind flows outward from the sun traveling at one million miles (1.6 million kilometers) an hour, a bath of energetic particles that's blasted off the solar surface and into space, where it surrounds our star like a bubble.

At its edges, the solar wind piles up into the "interstellar wind," a cloud of cooler charged particles that suffuse the thin vacuum of space between stars. Since 2004, Voyager 1 had been traveling within the boundary region between the solar wind and the interstellar wind, which is the cooked-off debris of thousands of exploded stars in our Milky Way galaxy.

Knowing exactly where the solar wind ends and where interstellar space begins has been an open question among space scientists for more than four decades, says Stone.

Since an instrument for directly detecting that transition died in 1980, the researchers have had to rely on indirect measures of magnetic and electrical activity from other instruments aboard Voyager 1 to find an answer.

One key to identifying this boundary is the difference in the density of charged particles between the solar wind and interstellar space, as it is about 50 times greater in the latter region.

Looking at a pair of solar storms that caught up to the spacecraft last October and then again last April, Gurnett’s team reported that measured changes in electrical activity around Voyager correspond to interstellar space.

As the storms passed the spacecraft, they triggered spikes in electrical and radio waves that uniquely corresponded in frequency to the spacecraft having entered the more densely charged interstellar space.

Based on that increase, the team extrapolated the entry date for Voyager 1 into interstellar space as August 25, 2012.

"The spacecraft doesn't feel anything traveling into interstellar space. We can only detect the transition because of its instruments," says Stone, who was not on the study team.

The new report confirms an analysis made last year that found that Voyager had entered interstellar space, based on indirect measurements.

Stone finds the new report convincing: "Nature has finally given us a nice set of solar storms which show us that Voyager is now out in interstellar space."

Surprise! Galaxy, Sun's Magnetic Fields Aligned

Scientists were surprised by NASA's finding that the galaxy's magnetic field is apparently aligned in the same direction as the sun's, forming a "magnetic highway." Space scientists had generally assumed that the galaxy's magnetic field would have some other direction.

The alignment had stymied attempts to use magnetic measurements to determine a starting line for interstellar space.

"We have a lot to learn still, I think, is what it means," says Voyager scientist Stamatios Krimigis of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, who reported the magnetic highway results last year.

Along with Voyager 1's measurement of increased galactic cosmic rays (the solar wind serves to partly shield the solar system from these high-powered rays), the new results have Krimigis "absolutely convinced."

"In the same way that Sputnik carried us out of the Earth's atmosphere in 1957, Voyager has now carried us outside the sun's atmosphere," Krimigis says. "It is quite an achievement in the short time that we have had spaceflight."

Given the estimated lifetime of the plutonium battery aboard Voyager 1, its last signals should be heard on Earth around 2025, Stone says. The spacecraft will eventually pass within 1.7 light years (about 16.1 trillion kilometers) of another star in 40,000 years, according to Voyager project manager Suzanne Dodd of NASA's Jet Propulsion Laboratory in Pasadena, California.

The spacecraft's twin, Voyager 2, which explored Jupiter, Saturn, Uranus, and Neptune, is also still kicking, now some 9.55 billion miles (15.36 billion kilometers) from the sun on its own journey.

"It has really been an exciting 40 years for the mission, and the next 10 years should be exciting ones as well," Stone says. "We are still exploring places we have never been."