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Voyager’s Exit To The Stars…In 17,000 Years

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


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Recent debates over whether or not the Voyager 1 spacecraft has ‘left the solar system’ typically leave out some critical details. The limits of the Sun’s particle radiation is not the physical edge of the contents of the solar system, but it is the point of changeover to the exceedingly tenuous atmosphere of matter and magnetic fields that fills the space between the stars in our galaxy.

It’s been a hot topic recently because the Voyager 1 spacecraft, after 36 years of cruising away from us, now seems to be passing through a zone described as the ‘magnetic highway’, where the Sun’s magnetic field lines connect to those pervading interstellar space in our galaxy. This marks a transition where Voyager is going to move beyond the heliopause – where the endless flood of particle radiation streaming from our sun – stuff like electrons and protons – has become sufficiently diluted that it no longer pushes through the tenuous particle radiation and matter between the stars. This is the entry point to the interstellar medium, in a sense the atmosphere of the Milky Way.

But this has led so some misleading comments about ‘the end of our solar system’. NASA recently released a statement on March 20th, 2013 to try to clear up the confusion about Voyager’s whereabouts:

“The Voyager team is aware of reports today that NASA’s Voyager 1 has left the solar system,” said Edward Stone, Voyager project scientist based at the California Institute of Technology, Pasadena, Calif. “It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space. In December 2012, the Voyager science team reported that Voyager 1 is within a new region called ‘the magnetic highway’ where energetic particles changed dramatically. A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed.”

However, I feel compelled to write this post because even NASA’s statement doesn’t quite manage to explain what’s different between ‘leaving the solar system’ or ‘reaching interstellar space’ – and there is a difference, for this solar system or any other.

Something like this is happening, Voyager 1 (upper spacecraft icon) is in a messy region close to the interface (NASA/JPL-Caltech)

Like any normal star, the Sun creates what is in effect a bubble of its own effluvia. It does this by exerting a pressure against the interstellar atmosphere, but the further away you get the weaker this pressure. Exactly where the solar pressure become equal to the surrounding pressure depends on a lot of factors. It depends on things like magnetic fields that interact with electrically charged matter, it also depends a great deal on exactly where we are in the galaxy and the local density of this interstellar atmosphere – which is always changing as we follow our lengthy galactic orbit, a journey of about 230 million years.

Regions of denser interstellar atmosphere, or medium, are expected to drastically shrink the size of the Sun’s bubble, bringing the heliopause in close. Variations in the Sun’s activity will also effect this. In other words, the heliopause is not some fixed ‘edge’ to our solar system, and it just happens to occur out beyond all the major planets at this time.

It is however the place at which the Sun’s extended atmosphere gives way to the atmosphere of the galaxy at large. So while one can say that beyond this is interstellar space, that’s a little confusing. It doesn’t mean that one is leaving the solar system, just that one is now bathed in a different medium – the interstellar medium, not the solar medium.

But if the heliopause, and Voyager 1′s present location just within it, is not the edge of the solar system, where does the solar system actually end?

It’s a good question. In my opinion the most physically sensible marker is the distance from the Sun at which the Sun’s gravity can no longer sustain objects in long-term stable orbits. In other words it’s the distance at which the gentle tug of other stars, and even the subtle variations in the net gravitational field of all the matter in the Milky Way, can perturb and destabilize the trajectory of any lonely, cold, bodies out there.

The problem is, much as with the heliopause, that this may not always be at the same distance from the Sun. Again, our movement around the galaxy and the hither-and-dither of other stars and objects result in a slowly shifting gravitational landscape. Nonetheless, rough estimation suggests that it’s about 1 light year away. Not coincidently this is also the speculated outer edge of the Oort cloud – a vast structure consisting of trillions of icy chunks of detritus flung outwards while our planets formed some 4.5 billion years ago.

A (somewhat old) artist's impression of the Oort cloud (Credit:NASA)

The Oort cloud is the likely origin of very long-period comets – objects that come falling inwards with orbits that may take anywhere from hundreds of years to millions of years to complete. For example, Comet West was last seen in 1976. If you want to see it again you’ll need to wait about 6 million years, since it’s now swinging back out to its far point of some 1.1 light years away.

All of which means that while poor old Voyager may have begun to taste a little fresh galactic breeze in its face, it’s still a very, very long way from passing beyond the wealth of islets that separate us from truly open interstellar space.

In fact, since one light year is about 63, 240 astronomical units (AU) from the Sun, and Voyager is currently about 124 AU from the Sun, moving away at approximately 3.6 AU a year, it will take another 17, 500 years (give or take) for it to move beyond the outer Oort cloud.

Reports of Voyager having left the solar system are therefore a little premature.

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His latest book is 'Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos', and he is working on 'The Copernicus Complex' (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

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





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  1. 1. EricBourland 1:33 pm 04/8/2013

    >>>it’s own effluvia around itself.

    Grammar?

    Also … effluvia? As in multiple exhalations of noxious vapor?

    Link to this
  2. 2. vagueofgodalming 1:37 pm 04/8/2013

    This is the kind of ‘wrong on the internet’ post that I like.

    Link to this
  3. 3. Caleb A. Scharf in reply to Caleb A. Scharf 1:38 pm 04/8/2013

    oop, thanks for catching that.

    yes, effluvia – mainly because I like the sound of it, but also I think it’s a fair description of a hot plasma of electrons and protons coming from a star that has its share of flares and mass ejections….

    Link to this
  4. 4. Acoyauh2 5:28 pm 04/8/2013

    We had a discussion on the ‘end of the solar system’ some years ago – I think due to some Voyager’s change of phase on its trip.
    There was no consensus, but the two main fields agued for the Kuiper belt as the end of the planetary disk, thence the system. I backed the side of the Oort cloud as the end of the Sun’s gravitational influence. Dont’ remember anyone discussing particle zones or magnetic fields, though.

    Link to this
  5. 5. jtdwyer 6:01 pm 04/8/2013

    Very well done! I think the most important underappreciated point here is that there is no ‘edge’ to the Solar system, the Oort cloud or the interstellar medium.

    BTW, while still being collectively dominated by the Sun’s diminished gravitation, surely the objects comprising the vast Oort cloud must also to some extent be self-gravitating…

    Link to this
  6. 6. Caleb A. Scharf in reply to Caleb A. Scharf 7:22 pm 04/8/2013

    Thanks. I’ve seen estimates of the *total* Oort cloud mass of as low as 1 to 2 times Earth’s mass – to as high as 100 times. But given the scale over which this is spread it’s not going to be self gravitating (in the sense of holding together against external perturbations or the motion of the pieces) – even a total of 100 Earth masses is still small compared to the 333,000 Earth masses of the Sun, so I think the Sun will still dominate.

    Link to this
  7. 7. Acoyauh2 8:10 pm 04/8/2013

    I think the point is that, whatever reference you take, out there the ‘lines’ will probably move too dang much for the poor little guy to ever get a checkered flag in its race. Still, it’s an interesting debate that you put a good perspective on – thank you.

    Link to this
  8. 8. rshoff 8:40 pm 04/8/2013

    Thanks, very interesting. Also a reminder of how small we are and how far we are from the center of things way out here in the hinterlands of the Milky Way….

    How long until it reaches Inter Galactic territory?

    Link to this
  9. 9. matthew393 10:40 pm 04/8/2013

    One does not simply leave the solar system.
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  10. 10. jtdwyer 3:10 am 04/9/2013

    Caleb A. Scharf – thanks. I don’t do math – what is the gravitational influence of the Sun at the distances of the Oort clouds?

    The question is: what gravitational influences affect each object within the Oort cloud. While very sparsely populated, I understand that the outer cloud is thought to contain several trillion objects > 1 km, and the inner cloud perhaps hundreds of times as many objects. I think the Oort cloud has few external gravitational influences and many internal ones, even if they are of low magnitude… Most seem to be quite stable out there, unless perturbed by one another.

    Link to this
  11. 11. bucketofsquid 1:08 pm 04/10/2013

    Thanks for giving me a much better perspective on the scale of the solar system. I hadn’t realized on an emotional level just how close and yet really far even the outer planets are until I read this blog post.

    I do have one pet peeve though. There is only 1 star named Sol so there is only 1 Solar system. Sure you can refer to Puffs or Best Choice facial tissue as Kleenex and people know what you mean but they really aren’t the same thing. It is the same thing with stars and the associated planets and such. I’m not really clear on if the proper term is stellar system or not but I think it is. We have more stellar systems than we can count but so far only 1 star named Sol so only 1 Solar system. Failing to capitalize Solar doesn’t make it a generic term, it just reinforces bad grammar.

    Link to this
  12. 12. spitfirepilot 11:12 pm 04/14/2013

    Ican’t get enough of this material in scientific American, it’s just great research and editorial food for my scientific appetite.
    Thankyou all for a great website to log onto.
    yours truly
    rob clode,
    Australian high country
    cheers bye.

    Link to this

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