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Voyager: Solar Wind Velocity Zero

When Voyager 2 was passing Neptune back in 1989, I stuck a video tape in the VCR and recorded the coverage — two video tapes, actually, because I wasn’t sure how much coverage there was going to be, and I didn’t want to miss anything. That meant getting up in the middle of the night to change tapes, but I figured the loss of sleep was worth it. Going back to those tapes today, I’m still struck by the same sense of awe that both the Voyagers were simply going to continue, that although the media spoke as if their journeys were over after their encounters at Titan and Neptune respectively, they still had years of power left and would continue talking to us deep into the 21st Century.

Image: Voyager 1 looks back to capture six planetary portraits. These six narrow-angle color images were made from the first ever “portrait” of the solar system taken by Voyager 1, which was more than 4 billion miles from Earth and about 32 degrees above the ecliptic. The spacecraft acquired a total of 60 frames for a mosaic of the solar system which shows six of the planets. Credit: Voyager 1 team/NASA.

The spacecraft nearing Neptune on my tapes was roughly 30 AU from the Sun. As of this morning, twenty-one years later, Voyager 2 is 94.03 AU out, and Voyager 1 (which left the ecliptic to make a flyby of Titan in 1980) is at 115.53 AU. The distances play with the imagination and offer a useful perspective. Voyager 1 is sixteen light hours from Earth — to be reasonably precise, 16 hours, 7 minutes as of 1202 UTC on the 14th. We’ve never sent anything sixteen light hours out before, but even at that distance, we’re only now penetrating the edge of the Solar System.

I have more to say about the size and scale of things, but I’m going to hold most of that for tomorrow in the form of a discussion of an interesting new book. For today, let’s talk about what Voyager 1 has found at 17.4 billion kilometers from the Sun. Out there, at the edge of the heliosphere, the spacecraft has moved into a region where the velocity of hot ionized gas — plasma — from the Sun has slowed to zero. With no outward movement of the solar wind, Voyager is seeing signs that the pressure of the ‘interstellar wind’ has turned the Sun’s wind sideways.

Voyager 1, then, is getting close to interstellar space, a crossing that will be marked by a sudden drop in the density of hot solar wind particles and an increase in the density of cold particles. The velocity of the solar wind has slowed at a rate of about 20 kilometers per second every year since August of 2007, at which point it was moving outward at some 60 kilometers per second. Readings over the last few months show an outward speed of zero since June. Rob Decker (JHU/APL) is a Voyager Low-Energy Charged Particle Instrument co-investigator, working with the instrument that provides the data fueling these results. Says Decker:

“When I realized that we were getting solid zeroes, I was amazed. Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again.”

The thinking among Voyager scientists is that Voyager 1 is still within the heliosheath, the outer shell of the heliosphere (the bubble formed by the solar wind as it fills nearby space). After reaching the termination shock, the solar wind slows down and heats up in the heliosheath. Current models of the heliosphere’s structure, as discussed at the ongoing American Geophysical Union meeting in San Francisco, will be tuned up by the new data, allowing us a better estimate of Voyager 1’s entry into true interstellar space, now thought to be about four years away.

“In science, there is nothing like a reality check to shake things up, and Voyager 1 provided that with hard facts,” said Tom Krimigis, principal investigator on the Low- Energy Charged Particle Instrument, who is based at the Applied Physics Laboratory and the Academy of Athens, Greece. “Once again, we face the predicament of redoing our models.”

As we tune those models, we might ponder, along with the distance of the Voyager duo, their speed. Voyager 1 is the faster craft because of gravity assists at Saturn and Titan. It’s moving at about 17 kilometers per second, with Voyager 2 at 15 kilometers per second. Put that into interstellar terms and the scale of things again seizes the imagination. Launched in 1977, the Voyagers have had thirty-three years to get to where they are today. If Voyager 1 were pointed at Alpha Centauri, it would face a journey of 41.5 trillion kilometers. At 17 kilometers per second, that works out to 76,476 years.


Comments on this entry are closed.

  • Alex Rosslyn December 14, 2010, 12:31

    That’s it. We did this.

    Voyager is in the zone where the Sun’s material influence slows down and effectively stops. By God. By all the Gods, Voyager has reached the border to interstellar space.

    At its current velocity it will plow through the Heliopause, one light-day from the Sun, stealthily and brushing away the fine specks of interstellar matter, and it will become a true interstellar probe.

    After all these thousands of years of military conflicts, fake saints and true morons, confident religions and ideologies and billions of people who walked across this Earth and died, after all that, we made it. We sent a ship fluttering out among the stars.

    We made it.

  • Thomas December 14, 2010, 14:55

    “We’ve never sent anything sixteen light hours out before”
    If Pioneer 10 and 11 were alive, they would be most displeased.

  • Thomas December 14, 2010, 14:58

    Correction, you’re right. Voyager 1 overtook the Pioneer probes.

  • Michael December 14, 2010, 17:42

    I wonder if the probes could do some more useful science such as using the onboard camera to image stars and then using parallax measurements to find their distances to greater accuracy.

  • Andy the geek December 14, 2010, 18:23

    The scale of this journey certainly does the imagination good. Of all the things we’ve accomplished as a species, this has got to be near the top of the pile.

  • Tarmen December 14, 2010, 21:19

    In a century (or three), we can pick up those Voyagers again for a museum. Voyager 2 will be well into the Kuiper Belt . We may as well get it before the black marketers do.
    Voyager 1 may be way off the beaten path to anywhere. And in 2310 it will be away farther out, a light week and more! In the Oort cloud with the comets

  • Scott G December 14, 2010, 21:33

    What’s even more daunting than realizing that Voyager 1 would take 76,476 years to arrive at Alpha Centauri (if pointed there) is knowing that while moving at the same speed, it could cross the continental United States in just under 4 minutes… And something even more daunting than that is realizing that the same spacecraft (if continuing at the same speed in a straight line) will travel the diameter of the Milky Way galaxy (9.5×10^17 kilometers) in about 1.8 billion years — approximately the amount of time that has passed since eukaryotes first appeared.

  • Wedge December 14, 2010, 22:43

    If you lived in a tent and every day you packed up your tent and walked for an hour or two then made camp again in a new spot you could easily circumnavigate the Earth in your lifetime. If you got into a car and drove on a magical freeway to the Moon it would take you an entire year of continuous driving before you arrived. If you climbed into an SR-71 and were somehow able to zoom off into space at mach 3 and you started at the time when Kepler discovered that the Earth orbits the Sun rather than vice versa you would still not be as far from Earth as Voyager 1.

    It’s almost impossible to put the scale of our Solar System, let alone nearby interstellar space, let alone the galaxy or the Universe into human terms.

    These probes are a remarkable accomplishments of mankind, on the scale of the great pyramids of Giza, Shakespeare’s plays, etc. We cannot yet even fully comprehend what these missions mean for mankind.

  • Mark December 15, 2010, 0:48

    I predict we’re going to get one last transmission from Voyager 1 before it goes dark. A really loud TING! as it slams into the crystal sphere that contains all the stars.

  • Adam December 15, 2010, 4:32

    Hi All

    Alpha Centauri is approaching the Solar System and will reach its nearest point at ~3.0 lightyears in about 28,700 AD. A probe travelling twice as quick as Voyager should arrive in time if we launch in the next century or so.

  • Darrell E December 15, 2010, 9:51


    At an average speed of 80 mph it would take just over 130 days of continuous travel on your imaginary freeway to travel 250,000 miles.

    Let me guess, you are an astronomer right (smiley)? I’ve noticed that with astronomers an order of magnitude (or two, or three) either way seems to be close enough. Cosmologists are worse.

    More on topic, I wonder when Voyager is going to encounter that black hole? Oh, never mind. That was Voyager 6, not 1 or 2.

  • Ric Capucho December 15, 2010, 11:03

    1.8 billions years to cross the galaxy at 17 km/s? I think the sun orbits the galactic centre faster than that. Hmm. My plan’s to stay here and wait for the other side of the galaxy to swing by. Hare and tortoise teaches us much.


  • Marc T December 15, 2010, 16:26

    I believe that the reason Voyager 2 is behind Voyager 1 is because of the geometry of its flyby of Neptune, not because of Voyager 1 getting any extra boost at Saturn — and certainly not Titan! In order to send Voyager 2 past Triton, they had to reverse the geometry of previous encounters, so that the spacecraft passed over Neptune and “backwards,” thus robbing Voyager 2 of some of its momentum. If Triton had been in a different place, it would be Voyager 2 at the head of the pack right now.

    And Darrell E — slow down! You must live in Montana. at 55 it would take 190 days.

  • ljk December 17, 2010, 1:40

    Here is an interesting video showing the size of celestial objects from our Moon up to the largest known star:


    And of course the famous Powers of Ten video to put things in perspective:


    As for the ultimate fates of the Pioneers and Voyagers, here are some possibilities:




  • ljk June 1, 2011, 9:41

    Mapping Mayhem Where Sun’s Magnetic Influence Wanes

    Source: American Geophysical Union Posted Tuesday, May 31, 2011

    When Voyager 1 passed into the heliosheath in 2004, it became the first man-made object to explore the remote edge of the Sun’s magnetic influence. The heliosheath, from 1.5 to 15 billion kilometers thick (930 million to 9.3 billion miles) and starting roughly 14 billion km (8.7 billion mi) from the Sun, is where the outgoing flows of solar wind start to be pushed back by interstellar particles and magnetic fields that are heading toward the solar system.

    While passing through the heliosheath, Voyager 1 experienced many sudden and drastic changes in the surrounding magnetic field driven by structures called current sheets.

    Using Voyager 1’s ongoing measurements of the magnetic field, Burlaga and Ness identify three distinct types of current sheets. The structures, appearing as proton boundary layers (PBLs), magnetic holes or humps, or sector boundaries, were identified by characteristic fluctuations in either magnetic field strength or direction as the spacecraft crossed nearly 500 million km (310 million mi) of heliosheath in 2009. PBLs are defined by a rapid jump in magnetic field strength, with one observed event resulting in a doubling of the field strength in just half an hour.

    Passing through a sector boundary led to a sudden change in direction of the magnetic field. Magnetic holes saw the field strength drop to near zero before returning to the original background strength. Magnetic humps consisted of a sudden spike in strength and then a return to initial levels. The firsthand detections made by Voyager 1 are likely to be extremely important for researchers trying to decide between current leading theories for the source and structure of current sheets.

    Source: Journal of Geophysical Research-Space Physics,

    doi:10.1029/2010JA016309, 2011


    Title: Current sheets in the heliosheath: Voyager 1, 2009

    Full article here: