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Voyagers Look at the Edge of the Solar System

We always cite the Mars rovers as examples of missions that perform far beyond their expected lifetimes, but the two Voyager spacecraft are reminding us once again that we have instrumentation at the edge of the Solar System that is still functioning after all these years. Both Voyagers are now in the heliosheath, the outermost layer of the magnetic bubble we call the heliosphere. With Voyager 1 crossing into the heliosheath in late 2004 and Voyager 2 in the summer of 2007, we get an estimate of the size of the heliosphere, a useful finding because it tells us something about what lies beyond.

What’s out there has been known for some time. Indeed, the interstellar medium (ISM) houses some ten percent of the visible matter in the Milky Way, mostly in the form of hydrogen gas. The ISM is patchy, enough so that astronomers have been able to isolate a Local Interstellar Cloud through which our Solar System is moving, a cloud flowing outward from the Scorpius-Centaurus Association, a region of star formation. About thirty light years wide, this cloud is colloquially called the ‘Local Fluff.’

Image: An artist’s concept of the Local Interstellar Cloud, also known as the “Local Fluff.” Credit: Linda Huff (American Scientist) and Priscilla Frisch (University of Chicago).

The Voyagers have yet to reach the cloud, but they’re closing in on it, and therein hangs a tale. For what determines the size of the heliosphere appears to be the balance between the inflation of the ‘bubble’ by the solar wind and the compressive forces of the Local Interstellar Cloud. In a new paper in Nature, Merav Opher (George Mason University) uses Voyager data to study this balance. Some of the pressure exerted by the cloud is magnetic, and Opher’s measurements of the magnetic field help us to understand how the cloud continues to exist despite forces that should tear it apart.

The problem is that the ‘Local Fluff’ should have been dissipated by the effects of nearby supernovae that exploded some ten million years ago. These hot gases would break up the cloud were it not for its strong magnetic field, argues Opher, who goes on to phrase the issue starkly:

“Using data from Voyager, we have discovered a strong magnetic field just outside the solar system. This magnetic field holds the interstellar cloud together and solves the long-standing puzzle of how it can exist at all.”

The Local Interstellar Cloud is thirty light years across and, given its temperature and density, should not be able to resist the effects of the supernova gases around it. Opher’s finding is that the cloud is much more strongly magnetized than had been previously thought, between 4 and 5 microgauss. This is roughly twice previous estimates. “This magnetic field,” adds Opher, “can provide the extra pressure required to resist destruction.”

: Voyager flies through the outer bounds of the heliosphere en route to interstellar space. A strong magnetic field reported by Opher et al in the Dec. 24, 2009, issue of Nature is delineated in yellow. Image copyright 2009, The American Museum of Natural History.

The field is found to be tilted between 20 and 30 degrees from the interstellar medium flow direction (determined by the Sun’s motion), and is at an angle of 30 degrees from the galactic plane. The conclusion: The interstellar medium is turbulent, at least in the vicinity of our Solar System. If other nearby clouds are similarly magnetized, the heliosphere should vary in size as the Sun moves through them (on a timescale of hundreds of thousands of years), varying the protection the heliosphere offers the inner system against galactic cosmic rays.

The paper is Opher et al., “A strong, highly-tilted interstellar magnetic field near the Solar System,” Nature 462 (24 December 2009), pp. 1036-1038 (abstract).


Comments on this entry are closed.

  • NS December 30, 2009, 14:25

    I don’t see it mentioned on the website, but is there any chance the New Horizons mission will last long enough to return similar info?

  • Wayne Farmer December 30, 2009, 14:49

    There’s anthropic wonderfulness in that picture. Why did life arise and continue to live on Earth? Because:

    (1) Gravity causes matter to coalesce, counteracting the dissipation of entropy, thus forming stars from some of the matter;

    (2) Gravity continues to press inwards on those stars, causing some to go supernova and produce carbon (but only because the triple-alpha energies are just right!) and other heavier elements;

    (3) Gravity causes those heavier elements to coalesce, forming planets;

    (4) Gravity presses inwards on stars too small to go supernova, but does cause them to fuse and provide continuous energy to any nearby planets, making anti-entropic chemical reactions possible in the mix of lighter and heavier elements, thus allowing matter to organize itself into life;

    (5) That same fusion creates a solar wind and a magnetic field that protects the new life on the planets from rbeing destroyed by receiving too much disruptive radiation.

    And now that all the above conditions have created our own sentient form of life, with the ability to build or destroy this assemblage of matter and life, what shall we decide to do with that ability in the time we have before our planet becomes uninhabitable for us?

  • jim moore December 30, 2009, 19:45

    It can’t be a magnetized cloud of interstellar hydrogen gas. Hydrogen gas does not respond to magnetic fields. It has to be a diffuse strand of interstellar plasma, were at least some percentage of the hydrogens have been separated from their electrons. Protons and electrons will respond to magnetic fields, neutral hydrogen will not.

  • Wayne Farmer December 31, 2009, 0:02

    Minor correction on my previous post: Carbon isn’t produced explosively by supernova; it’s formed as the ash of helium burning as a star ages. That’s still wonderful. :-)

  • Kenneth Harmon December 31, 2009, 2:25


    Does anybody see any implications for Human Interstellar Travel over the next couple of centuries given the presence of “Local Fluff”? It is also interesting to note that from the pictoral diagram Alpha Centauri seems to be about to enter the Local Fluff and will join us. Will this be a help or a hinderance to Human Interstellar Travel at least between Sol and Alpha Centauri, and how long will we both be in this Local Fluff Cloud? It may be that if Alpha Centauri has a similar Heliosphere and it is being distorted like our own our two Heliospheres may actually overlap, at least as long as we are in the “Local Fluff”. Any thoughts on the implications of all of this?

  • nate December 31, 2009, 2:38

    @wayne, good comment, seems like you know about our priorities. now to get humanity to realize we’re being had by forces that seek to continue to control us simply to remain “on top.” as more people look at the universe that way more people will question the apparatus of control… why must we have privitized currency, and why must we have middlemen between government, production, science, technology, and development? why must things be framed in terms of interest rates and monetary limits being set by private interests against the common good? why should resources and the common good be rationed by a privatized money system and our path be dictated by the few ruling families which have been on top for the past thousands of years? just because they ruled over us for “modern” history doesn’t grant them the right. it’s time we awakened and choose man’s common interest, spread to the stars, and continue the divine role we were ordained with by our sentience.

  • Duncan Ivry December 31, 2009, 15:36


    You ask a lot of questions — rhetorical ones, and because of this not really questions, but statements of your — sorry, to say — sentiments and prejudices.

    But I will answer some of your questions: “why must we have privitized currency, and why must we have middlemen between government, production, science, technology, and development? why must things be framed in terms of interest rates and monetary limits being set by private interests against the common good?”

    The answer is: … because it is much more efficient having these institutions, than having them not.

    If “we” want to explore the universe and travel through space some time in the future, then the best thing we can do is using our resources as efficient as we can. Your opinions are competely yesterday, and they are even dangerous: they will lead to not having much modern science and technology and to not having space travel at all.

  • Terraformer (a.k.a Tobias Holbrook) January 1, 2010, 17:13

    Does this affect the performane of interstellar cyclers in any way? What about Ramjets?

  • ljk January 3, 2010, 0:46

    Apparent Faster-Than-Light Pulse Propagation in Interstellar Space: A new probe of the Interstellar Medium

    Authors: F. A. Jenet, D. Fleckenstein, A. Ford, A. Garcia, R. Miller, J. Rivera, K. Stovall

    (Submitted on 13 Sep 2009 (v1), last revised 29 Dec 2009 (this version, v2))

    Abstract: Radio pulsars emit regular bursts of radio radiation that propagate through the interstellar medium (ISM), the tenuous gas and plasma between the stars. Previously known dispersive properties of the ISM cause low frequency pulses to be delayed in time with respect to high frequency ones.
    This effect can be explained by the presence of free electrons in the medium.

    The ISM also contains neutral hydrogen which has a well known resonance at 1420.4 MHz. Electro-magnetic theory predicts that at such a resonance, the induced dispersive effects will be drastically different from those of the free electrons. Pulses traveling through a cloud of neutral hydrogen should undergo “anomalous dispersion”, which causes the group velocity of the medium to be larger than the speed of light in vacuum. This superluminal group velocity causes pulses containing frequencies near the resonance to arrive earlier in time with respect to other pulses. Hence, these pulses appear to travel faster than light.

    This phenomenon is caused by an interplay between the time scales present in the pulse and the time scales present in the medium. Although counter-intuitive, it does not violate the laws of special relativity.

    Here, we present Arecibo observations of the radio pulsar PSR B1937+21 that show clear evidence of anomalous dispersion. Though this effect is known in laboratory physics, this is the first time it has been directly observed in an astrophysical context, and it has the potential to be a useful tool for studying the properties of neutral hydrogen in the Galaxy.

    Comments: Accepted by Astrophysical Journal

    Subjects: Galaxy Astrophysics (astro-ph.GA); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:0909.2445v2 [astro-ph.GA]

    Submission history

    From: Fredrick Jenet [view email]

    [v1] Sun, 13 Sep 2009 20:30:28 GMT (25kb)

    [v2] Tue, 29 Dec 2009 13:57:49 GMT (29kb)


  • nate January 4, 2010, 1:02

    @duncan – it is in fact your opinions that are yesterday. we deviated from policies that created our space faring abilities and you propose to continue the existence of entrenched interests. per capita production has fallen globally since globalization. yes, profits have increased, if that’s what you happen to be referring to when you speak of “efficiencies”… having private interests capable of telling nations what they may or may not do scientifically is less efficient despite however many years you may have been groomed to believe otherwise. i’m not sure what sort of radical and dangerous philosophy you subscribe to but i am frankly worried.

  • nate January 4, 2010, 20:41

    @duncan –
    this is an example of your “efficient” use of capital: please take a moment to ponder the intelligence of putting indoor ski slopes and underwater restaurants in the middle of the desert (recall further that dubai world is bankrupt).


    we can return to FDR and JFK style economics and quite quickly industrialize the moon and mars. under these types of imperial monetarist policies that you promote we will never get off planet again, and certainly not voyage to the interstellar realms.

  • Henry Harris February 14, 2010, 19:42

    Several years ago I had a conversation at JPL with an expert on the local interstellar medium (sorry, I forget her name) and she said there was a danger that when we enter the aforementioned cloud, interstellar hydrogen might combine with oxygen in our atmosphere in sufficient amounts to produce torrential rains that might pose a danger to human life. Since then, I’ve not heard any comments on this possibility. Any thoughts?

  • Eniac February 16, 2010, 10:34

    Henry: Not a chance. The ISM stays outside the solar system, andeven if it didn’t, it is much too thin to have any affect at all on the atmosphere.