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Voyager Update: Still in Choppy Waters

The continued explorations of our two Voyagers have earned these tough spacecraft the right to be considered an interstellar mission, which is how NASA now describes their journeys. Neither will come anywhere near another star for tens of thousands of years, but in this context ‘interstellar’ means putting a payload with data return into true interstellar space. Right now the Voyagers are still within the heliosphere, that great bubble opened out around our system by the Sun’s solar wind, and the signs are multiplying that a transition is soon to occur.

Three measurements are going to mark the boundary crossing, and we’re seeing that two out of the three are in a state of rapid change. This JPL news release points out that on July 28, Voyager 1’s cosmic ray instrument showed a jump of five percent in the level of galactic cosmic rays the craft was encountering. In the second half of that same day, the level of lower-energy particles flowing from inside the Solar System dropped by half. Both measurements had recovered to their former state within three days, but you can see that Voyager 1 is moving through the chop and froth that marks a boundary somewhere up ahead.

Image: The Voyager interstellar mission, pushing up against the edge of the Solar System. Credit: NASA.

The third factor is the direction of the magnetic field, which researchers expect will change direction when true interstellar space is encountered. We should have an early analysis of the latest magnetic field readings some time in the next month. At some point, all three indicators are going to switch over to a more definitive state, but even then we’ll have to see how long the back-and-forth continues in what could be a ragged boundary area.

Noting the gradual increase of high-energy cosmic rays over a period of years and the corresponding drop in lower-energy particles, Voyager project scientist Ed Stone can only say: “The increase and the decrease are sharper than we’ve seen before, but that’s also what we said about the May data. The data are changing in ways that we didn’t expect, but Voyager has always surprised us with new discoveries.” In any case, the flow of lower-energy particles is expected to drop close to zero when the final transition occurs.

As of this morning, Voyager 1, the more distant craft, is 16 hours 46 minutes and 28 seconds light-travel time from Earth, corresponding to 121.479 AU. We’re used to thinking of today’s spacecraft as being far more complex than those of previous decades, but bear in mind that the two Voyagers each contain some 65,000 individual parts, their continued functioning a testament to the skill of the scientists and engineers who designed them. What will eventually silence them is a lack of power as their radioisotope thermoelectric generators lose their punch.

Looking forward, the ultraviolet spectrometer is expected to function until mid-2013, when it will be turned off to save power. But as long as the spacecraft are still operational, the cosmic ray subsystem, the low-energy charge particle instrument, magnetometer, plasma subsystem, plasma wave subsystem and planetary radio astronomy instrument should continue to operate. We’ve got years of data return ahead and can hope for a window between the crossing into interstellar space and the loss of power around 2020 in which to see what surprises Voyager may yet spring about the environment future interstellar craft will have to move through.


Comments on this entry are closed.

  • Christopher Phoenix August 7, 2012, 17:48

    It’s rather exciting to live in the century when the first automated spacecraft will forge their way into interstellar space. Hopefully, a dedicated interstellar precursor probe will someday join the Voyagers as the first probes to investigate the environment future starships will travel through…

  • A. A. Jackson August 7, 2012, 19:43

    An odd thing. Looking up heliosphere , I notice the termination shock is at , roughly, 100 AU from the Sun. The whole structure is sometimes called ‘the sphere of influence’ of the Sun.
    I don’t know how this term for the heliosphere (it’s not really a sphere) … came from.
    ‘Sphere of influence’ , when I was knee high to grasshopper meant the transition bounty , in a multi-body gravitational arrangement where one body could be considered dominant and smaller bodies within it were considered to be in a perturbed two body situation (as a good approximation).
    There are various ways to specify, a famous one is the Hill’s Sphere, the Tisserand criteria , (even some others) all differing a little, but usually lumped into The Gravitational Sphere of influence.
    The Sun can bind an object in an approximate Keplerian orbit up to nearly 1 light year. In the last 30 years or so it has become known the Galactic Tide plays a role in long period comet orbit which are , in general, Oort cloud comets, so the situation is complicated.
    In any cast , for a space craft, just what defines ‘interstellar space’ , there are four spacecraft that have attained escape velocity from the Sun , but it will be a long time before they detach from the gravitational influence of the Sun.
    Are the Oort cloud comets in interstellar space?
    Kind of a very thin distinction, but one I wonder about.

  • Daniel Suggs August 7, 2012, 22:24

    The Little Engine(s) That Could. I read that book to my grand-kids, maybe someone will do a rewrite with the voyagers.

  • Lepton August 8, 2012, 13:59

    To Jackson,

    IMHO, interstellar space more or less implies particle physics. The ‘the sphere of influence’ is more about solar wind than sun itself. I have never linked interstellar space with gravity until I read your posting. Oort cloud comets in interstellar space? That is really an interesting point of view indeed.

    Also, just nitpicking, I think apart from P1/2, V1/2, NH is the fifth spacecraft achieved solar escape velocity.

  • Astronist August 8, 2012, 19:22

    From the point of view of a hypothetical dweller on Sedna (aphelion 940 AU), the Voyagers are still only in the inner Solar System!

    Clearly, just as there is no sharp dividing-line between a planet and a non-planet, there is no sharp boundary between interplanetary and interstellar space. The precision of language is misleading in this case.

    Where one places the dividing-line depends upon one’s particular interest. Thus a particle physicist may speak of interstellar space beginning at 120 AU from the Sun, whereas an orbital dynamicist will place that boundary 500 times more distant, at around a light-year. And both are right, in their own ways.

    Oxford, UK

  • ljk August 21, 2012, 10:23


    Voyager at 35: Break on Through to the Other Side

    August 20, 2012

    Thirty-five years ago today, NASA’s Voyager 2 spacecraft, the first Voyager spacecraft to launch, departed on a journey that would make it the only spacecraft to visit Uranus and Neptune and the longest-operating NASA spacecraft ever. Voyager 2 and its twin, Voyager 1, that launched 16 days later on Sept. 5, 1977, are still going strong, hurtling away from our sun. Mission managers are eagerly anticipating the day when they break on through to the other side – the space between stars.

    “Even 35 years on, our rugged Voyager spacecraft are poised to make new discoveries as we eagerly await the signs that we’ve entered interstellar space,” said Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena.

    “Voyager results turned Jupiter and Saturn into full, tumultuous worlds, their moons from faint dots into distinctive places, and gave us our first glimpses of Uranus and Neptune up-close. We can’t wait for Voyager to turn our models of the space beyond our sun into the first observations from interstellar space.”

    Voyager 2 became the longest-operating spacecraft on Aug. 13, 2012, surpassing Pioneer 6, which launched on Dec. 16, 1965, and sent its last signal back to NASA’s Deep Space Network on Dec. 8, 2000 (It operated for 12,758 days.)

    Scientists eagerly awaiting the entry of the two Voyagers into interstellar space have recently seen changes from Voyager 1 in two of the three observations that are expected to be different in interstellar space. The prevalence of high-energy particles streaming in from outside our solar system has jumped, and the prevalence of lower-energy particles originating from inside our solar system has briefly dipped, indicating an increasing pace of change in Voyager 1’s environment. Voyager team scientists are now analyzing data on the direction of the magnetic field, which they believe will change upon entry into interstellar space.

    Notable discoveries by Voyager 2 include the puzzling hexagonal jet stream in Saturn’s north polar region, the tipped magnetic poles of Uranus and Neptune, and the geysers on Neptune’s frozen moon Triton. Although launched second, Voyager 1 reached Jupiter and Saturn before Voyager 2, first seeing the volcanoes of Jupiter’s moon Io, the kinky nature of Saturn’s outermost main ring, and the deep, hazy atmosphere of Saturn’s moon Titan. Voyager 1 also took the mission’s last image: the famous solar system family portrait that showed our Earth as a pale blue dot.

    Voyager 2 is about 9 billion miles (15 billion kilometers) away from the sun, heading in a southerly direction. Voyager 1 is about 11 billion miles (18 billion kilometers) away from the sun, heading in a northerly direction. For the last five years, both spacecraft have been exploring the outer layer of the heliosphere, the giant bubble of charged particles the sun blows around itself.

    “We continue to listen to Voyager 1 and 2 nearly every day,” said Suzanne Dodd, Voyager project manager at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “The two spacecraft are in great shape for having flown through Jupiter’s dangerous radiation environment and having to endure the chill of being so far away from our sun.”

    Dodd and her team have been carefully managing the use of power from the continually diminishing energy sources on the two spacecraft. They estimate that the two spacecraft will have enough electrical power to continue collecting data and communicating it back to Earth through 2020, and possibly through 2025.

    While no one really knows how long it will take to get to interstellar space, Voyager scientists think we don’t have long to wait. And, besides, the first 35 years have already been a grand ride.

    A public lecture about the journey of the twin Voyager spacecraft will be held at JPL on Sept. 4. More information is available at

    http://www.jpl.nasa.gov/events/lectures_archive.cfm?year=2012&month=9 .

    The Voyager spacecraft were built by JPL, which continues to operate both. JPL is a division of the California Institute of Technology. The Voyager missions are a part of the NASA Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate in Washington.

    For more information about the Voyager spacecraft, visit:

    http://www.nasa.gov/voyager and http://voyager.jpl.nasa.gov

    Jia-Rui Cook 818-354-0850
    Jet Propulsion Laboratory, Pasadena, Calif.


  • ljk September 19, 2012, 9:37


    Cosmic Rays at Voyager 1

    Credit: Voyager Project, NASA

    Explanation: Launched on a grand tour of the outer planets in 1977, by good fortune the twin Voyager spacecraft were also headed in the general direction of the Sun’s motion relative to nearby stars. Thirty five years later, Voyager 1 appears to be nearing the boundary of the Sun’s heliosphere and interstellar space.

    Of course the heliosphere is the realm of the Sun defined by the influence of the solar wind and the Sun’s magnetic field. But how can you tell when your spacecraft crosses the boundary into interstellar space?

    One clue would be a sudden increase in the detection of energetic cosmic rays. The high energy particles stream through interstellar space accelerated by distant supernovae in our galaxy, but are normally deflected or slowed by the heliosphere.

    Covering a 12 month period (September 2011 to 2012), this plot does show a dramatic increase in the rate of cosmic ray particle detection in past months by the Voyager 1 spacecraft.

    Voyager 1 is now 18 billion kilometers (17 light hours, 122 Astronomical Units) from the Sun and may soon be the first spacecraft from Earth to enter the realm of the stars.

  • ljk December 3, 2012, 15:18

    3 December 2012

    Text & Images:



    NASA’s Voyager 1 spacecraft has encountered a new region on the outskirts of our solar system that appears to be a magnetic highway for charged particles. Scientists believe this is the final region Voyager has to cross before reaching interstellar space, or the space between stars.

    Scientists call this region the magnetic highway because our Sun’s magnetic field lines are connected to interstellar magnetic field lines. The connection has allowed lower-energy charged particles that originate from inside our heliosphere — the bubble of charged particles the Sun blows around itself — to zoom out, and higher-energy particles from outside to stream in.

    Before entering this region, the charged particles bounced around in all directions, as if trapped on local roads inside the heliosphere. Thinking the particles might be colliding against the gaseous boundary of the solar system, scientists operating Voyager’s low-energy charged particle detector wondered if the spacecraft had reached the last stop before — or even crossed into — interstellar space. Data indicating that the direction of the magnetic field lines has not changed, however, leads the Voyager team to infer that this region is still inside the solar bubble.

    The new results will be described today at the American Geophysical Union meeting in San Francisco.

    “If we were judging by the charged-particle data alone, I would have thought we were outside the heliosphere,” says Stamatios Krimigis, principal investigator of the Low-Energy Charged Particle (LECP) instrument, based at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “In fact, our instrument has seen the low-energy particles taking the exit ramp toward interstellar space. But we need to look at what all the instruments are telling us and only time will tell whether our interpretations about this frontier are correct. One thing is certain — none of the theoretical models predicted any of Voyager’s observations over the past 10 years, so there is no guidance on what to expect.”

    Since December 2004, when Voyager 1 crossed a shockwave known as the termination shock, the spacecraft has been exploring the heliosphere’s outer layer, called the heliosheath. Here, the stream of charged particles from the Sun — known as the solar wind — abruptly slowed down from supersonic speeds and became turbulent. Voyager 1’s environment was consistent for about five and a half years, but then the spacecraft detected that the outward speed of the solar wind slowed to zero. The intensity of the magnetic field also began to increase.

    “The solar wind measurements speak to the unique abilities of the LECP detector, designed at APL nearly four decades ago,” Krimigis says. “Where a device with no moving parts would have been safer — lessening the chance a part would break in space — our team took the risk to include a stepper motor that rotates the instrument 45 degrees every 192 seconds, allowing it to gather data in all directions and pick up something as dynamic as the solar wind. A device designed to work for 500,000 ‘steps’ and four years has been working for 35 years and well past 6 million steps.”

    In fact, for the past several months, the entire Voyager spacecraft was commanded to rotate periodically by 70 degrees so the LECP instrument could measure the solar wind flow in the up-down direction, or north-south according to the ecliptic plane on which the planets orbit the Sun. In theory, with the flow in the ecliptic plane having dropped to zero, the plasma should have been headed north at Voyager’s position. But the measurements, reported Sept. 6 in the journal Nature, showed that the flow was consistent with zero.

    “This was a real surprise,” says LECP Co-investigator Rob Decker, of the Applied Physics Laboratory (APL), “because most models were expecting the northward speed to be at least as high as 25 kilometers per second.”

    Full article at link at top.