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NASA’s Voyager 1 Spacecraft May Not Be Near Edge of Solar System after All [Updated]

The views expressed are those of the author and are not necessarily those of Scientific American.

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Voyager 1 and Voyager 2 in relation to the solar system

Credit: NASA

It’s been a long, strange trip out of the solar system for NASA’s Voyager 1 spacecraft, and it may be a bit longer still.

Voyager 1, which launched 35 years ago today, has ventured farther from Earth than any other spacecraft in history. Voyager 1 is now 18.2 billion kilometers from Earth—so distant that it takes radio commands traveling at the speed of light nearly 17 hours to reach the spacecraft. (The twin Voyager 2 probe launched first but took a more leisurely route through the solar system. It is now 14.8 billion kilometers from Earth.)

That’s quite an odometer reading, and all indications were that Voyager 1 was well on its way to an astonishing feat—exiting the solar system and venturing into interstellar space. But a new study suggests that the spacecraft is farther from taking that unprecedented step than had been assumed.

Nearly eight years ago, the spacecraft crossed into the heliosheath, the outer region of the solar system where the solar wind (plasma from the sun) begins to slow due to pushback from interstellar plasma. And in 2010, the velocity of the solar wind at Voyager 1’s back unexpectedly dropped all the way to zero. Researchers expected that as Voyager 1 drew near to the boundary between the heliosheath and interstellar space, known as the heliopause, it would encounter solar plasma that been deflected by interstellar plasma flows and begun  to turn northward (in relation to the sun’s axis of rotation).

But in the September 6 issue of Nature, Robert Decker of the Johns Hopkins University Applied Physics Laboratory and his colleagues report that Voyager 1 has measured the speed of poleward-flowing plasma at five intervals since March 2011. And the velocity of northbound plasma at the spacecraft’s location is basically zero, within the uncertainties of the measurements. (Scientific American is part of Nature Publishing Group.)

No one knows the thickness of the heliosheath, so no one knows how soon Voyager 1 might reach its outer edge. By one previous estimate, the heliopause could be just ahead of humankind’s most well-traveled emissary, or it could lie as many as four billion kilometers beyond—about seven years’ travel time. The new findings seem to favor the latter possibility. Decker and his colleagues conclude from the plasma measurements “that Voyager 1 is not at present close to the heliopause, at least in the form that it has been envisioned up to now.”

UPDATE (9/6/2012): In an email, Decker notes that the Nature paper was based on data collected before February 2012 and that newer observations further complicate the question of when Voyager 1 will leave the heliosphere and enter interstellar space. “I should point out that data received from Voyager 1 within the past two months show very interesting variations in the numbers of charged particles that enter our heliosphere from the interstellar medium and in the number that leave our heliosphere and enter the interstellar medium,” Decker notes. “Whether these very new data are another feature of the broad transition region that Voyager 1 has been in for the past two years, or a new region or boundary of the heliosphere, remains to be seen.”

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. dblatner 5:54 pm 09/5/2012

    There is also the issue of what constitutes the “edge of the solar system.” The sun’s gravitational influence theoretically stretches far past the heliosphere to the (theoretical) Oort Cloud, perhaps a thousand times farther away.

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  2. 2. bfchicago 12:23 am 09/6/2012

    If it’s seven years away, I hope #1 makes it out before the fuel goes!

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  3. 3. Na g n o s t ic 6:46 pm 09/6/2012

    Oh well, we’re now reduced to celebrating achievments gained with the use of 35 year old equipment. Too bad Americans 35 years hence will not have the same opportunity.

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  4. 4. EHDIII 7:21 pm 09/6/2012

    Voyager I is helping answers that the LHC cannot, as no other project can either. We should have launched a new Voyager every decade or so and equipped them with the best sensors possible designed for the greatest speeds possible. This could be a quite cost effective international project that should be done.

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  5. 5. Postman1 11:32 pm 09/6/2012

    EHDIII Couldn’t agree more. Computer programs tell us blah, blah, blah….Direct observation continues to find new and different facts.

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  6. 6. Owl905 10:20 pm 09/9/2012

    ‘Fascinating’ said the Vulcan.
    The outward push dropped to zero two years ago, but instead of recording deflection wind, there’s nothing.
    ‘Fasten seat belts’ said the Kirk.

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