Get Ready, Because Voyager I Is *This Close* to Leaving Our Solar System

We're on the cusp of one of the greatest scientific accomplishments of all time, but we may not know when the moment strikes. Or, rather, there may be no moment.

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An artist's rendering of the two Voyager spacecraft at the outer edge of our solar system (NASA)

Last week, in the corners of the Internet devoted to outer space, things started to get a little, well, hot. Voyager 1, the man-made object farthest away from Earth, was encountering a sharp uptick in the number of a certain kind of energetic particles around it. Had the spacecraft become the first human creation to "officially" leave the solar system?

It's hard to overstate how wild an accomplishment this would be: A machine, built here on Earth by the brain- and handiwork of humans, has sailed from Florida, out of Earth's orbit, beyond Mars, beyond the gas giants of Jupiter and Saturn, and may now have left the heliosphere -- tiny dot in the universe beholden to our sun. Had it really happened? How would we know?

We're not quite there yet, Voyager's project scientist and former head of NASA's Jet Propulsion Lab, Edward Stone, told me. The spacecraft is on its way out -- "it's leaving the solar system" -- but we don't know how far it has to go or what that transition to interstellar space will look like.

Voyager launched in 1977. Today, Voyager I is about 121 astronomical units away (one astronomical unit is equal to the rough distance from the Sun to the Earth). That is so far that it takes 16 hours for the radio signals it transmits to reach us. (Voyager II is about 22 astronomical units -- approximately seven years -- behind.) It is traveling at about 17 kilometers per second (38,000 miles per hour), propelled by the slingshot effect from flying by Jupiter and Saturn. ("It's well above escape velocity," Stone said.) The spacecraft's cameras have been turned off since 1990, when it took the pictures for the famous Family Portrait mosaic that captures the planets as they appeared as Voyager I looked back over the solar system it had traveled across.

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Family Portrait (NASA)

Now the data coming back aren't photographs but levels of different kinds of particles in the outer edge of the sun's bubble (the heliosphere), known as the heliosheath, the farthest the solar winds reach, which Voyager I entered in December 2004. And it was some of those data -- the levels of a certain cosmic-ray particle -- that provoked the recent speculation that Voyager I had finally flown the coop.

Some cosmic ray particles enter the heliosphere and we can see them here from Earth. But a slower type has a hard time entering the heliosphere. Last month, the sum of those slower particles, suddenly ticked up about 10 percent, "the fastest increase we've seen," Stone says. But an uptick does not mean Voyager has crossed over, though it does mean we're getting close. When Voyager does finally leave and enter the space "out there where all the particles are," the level will stop rising. The rising itself means that Voyager is not out there, yet. "But," cautions Stone, "we don't know. I mean this is the first time any spacecraft has been there." Since nothing's ever been there before, we don't know what it will look like, which makes it a little hard to recognize "it" at all. "That's the exciting thing," he continues.

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Rebecca J. Rosen is a senior editor at The Atlantic, where she oversees the Business Channel. She was previously an associate editor at The Wilson Quarterly.

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