New Research Enables Space Physicists To Gain Insights Into Heliosphere

For some time  space physicists and solar physicists have pondered the hypothetical properties of the heliosphere - the protective bubble created by the solar wind and depicted in the image above.  Understanding the physics at the bubble's edge- called the heliosheath -  is not easy.  This is given it's in constant flux and pushes out against the broader interstellar magnetic field that permeates our corner of the Milky Way.

For reference, the heliosheath occurs at the far edge of the heliosphere. More technically, it occurs between the "termination shock" and the "heliopause".    The heliopause  is the interface where the solar wind is stopped by the interstellar medium. One can therefore think of it as a three dimensional region or surrounding “envelope” at which the solar wind's strength is no longer sufficient to overcome the stellar winds of the external stars. In technical terms, this absence of counter-pressure signals the end of the solar system.   Thanks to data from the Voyager 1 and Voyager 2 spacecraft,  we now know the outer boundary of the heliosheath  is located roughly 18 billion kilometers from the Sun. Or 119 times the distance from the Earth to the Sun -  right where Voyager 2 found it in November, 2018.

Now,  Dialynas et al.  have combined Voyager data with observations from NASA's Cassini mission - which orbited Saturn from 2004 to 2017- to  gain much more insight.  Basically, the researchers recognized that the missions, although launched 20 years apart, had collected complementary data. Voyager 1 and 2 had instruments that measured energetic ions as the craft crossed the heliosheath and exited the solar system. Cassini, meanwhile, was able to remotely observe energetic neutral atoms  arriving in all directions from the heliosheath.


The energetic neutral atoms come from the heliosheath, where fast solar wind protons collide with neutral hydrogen atoms from interstellar space and “steal” an electron from the interlopers. The Voyager probes took in situ measurements of the parent heliosheath proton distributions as they passed through this region. Meanwhile, the protons with newly added electrons become energetic neutral atoms and shoot off in all directions.

The synergy among the spacecrafts’ observations allowed the researchers to use Voyager data from the heliosheath to ground transmissions and thereby calibrate energetic neutral atom data from Cassini, which was more sensitive to lower energetic particles than Voyager. Together, the spacecraft extended data on the intensity of both energetic neutral atoms and ions to include a broader range of energies, which gave the team a window into the physics in the heliosheath as the solar wind and interstellar medium press against each other.

The researchers found that in the energy range considered in their study (>5 kiloelectron volts), lower-energy ions with energies between about 5 and 24 kiloelectron volts played the largest role in maintaining the pressure balance inside the heliosheath. This allowed the team to calculate the strength of the magnetic field and the density of neutral hydrogen atoms in interstellar space—about 0.5 nanotesla and 0.12 per cubic centimeter, respectively.

Their finding that the lower-energy ions dominate the pressure balance in the heliosheath means that space physicists will have to rethink their assumptions about the energy distribution of such particles in the heliosheath..  See e.g. Geophysical Research Letters, https://doi.org/10.1029/2019GL083924, 2019.

Interestingly, pressure balance considerations also entered in early models and computations to do with another part of the heliosphere:  the solar corona .   In the past century an interesting question was whether the corona was static or not. In a static case its boundary would be more or less fixed, so there'd be no expansion even in times of high solar activity.  For this to occur there would need to be a consistent pressure balance, i.e. between the outward coronal  (& solar wind) pressure and inner directed pressure, from the interstellar medium.

 A static corona  superficially appeared  to be quite reasonable.  And so it was that the father of space physics,  Sydney Chapman,  first assumed a condition for hydrostatic equilibrium applied:

dp/ dr = - r {GM_s/ r²}

Where G is the usual Newtonian gravitational constant, and r defines the plasma density for the corona, with n the number density for protons, e.g. 

r = n(m_p) 

while M_s is the mass of the Sun, and r the distance from the solar center:

The coronal pressure (p) is given by:

p = 2 n T

Provided both protons and electrons are assumed to have the same temperature.

Ultimately, detailed computations showed the static corona model could not be accurate.   If the static model were accurate, the pressure at infinity, i.e.

p(¥) = p(R_o) [exp – 7k/5 * 1/ T(R_o) R_o] 

 Should be zero  (p(¥)   = 0) , not a small finite pressure that’s effectively equal to the coronal base pressure. This finding led to the further investigations that disclosed a solar “wind” had to flow outwards from the corona. 

See also:

http://onlinelibrary.wiley.com/doi/10.1002/2016GL068607/full


And:

New Research Into The 'Slow' Solar Wind Sheds Ligh...

The Parker Solar Probe - Why It's A Monumental Engineering Feat - And The First Image Delivered

Artist's conception of the Parker Solar Probe which will approach within 4 million miles of the Sun.   

The level and rate of successes - including newly processed imagery- of the latest solar space craft are almost too numerous to tally. One can then understand-  even if one is not a solar physicist - why the Parker solar probe had been renamed from the original "Solar Probe Plus".   In this post I want to examine why the Parker solar probe has been such a success, especially in terms of the engineering components and the associated experiments to be carried out over its 7-year mission. (Okay, actually 6 years and 321 days)

In terms of solar physics there is perhaps no more towering personage than Eugene Parker. Indeed, it was reading his original research papers on the magnetic structure of sunspots - portrayed as assemblies of multiple flux tubes - that enticed me to go into the field of solar physics 40 years ago.

Why is Eugene Parker so special ?  Well, to many of us who've invested segments of our lives into solar research he is seen as the father of solar physics. It was while Dr. Parker was still a budding young astrophysicist at the University of Chicago that he wrote a seminal paper in 1958 about the solar wind and its association with the interplanetary magnetic field. (Parker, E.N. : Dynamics of the interplanetary gas and magnetic fields,” 128, 664, Astrophys. J., 1958.) The paper can be accessed at the link below for those interested:

http://adsabs.harvard.edu/full/1958ApJ...128..664P

Fast forward some 21 years, to ca. 1979. Measurements over decades of the so -called Evershed effect showed the plasma motions to be radial and inwards. There did not appear to be any 'escape hatch' for the rising gas columns represented by the umbral dots. This being the case sunspots ought to heat up and reach equilibrium with the surrounding photosphere after a few days, and yet spots with umbral dots were observed to last weeks.

And so the "multiple flux tube" model of Eugene Parker was born (cf. Astrophys. J., 230, 905-13). In the diagram shown below note the geometry of the field lines extending from beneath the photosphere (in the convective zone) to far above it. The 'flaring field' on top is buoyant for reasons that have to do with the stratification of the solar atmosphere. The Wilson depression is shown as the indentations at the umbral surface on either side.

Parker in his paper (ibid.) showed that the downdraft velocity needed to remove heat from beneath a sunspot  (at a depth of 2500- 5000 km) is on the order of the Alfven velocity  e.g.

v  _A   = B_o  / [m _o  r _o]  ^{1/ 2}

for this region, where   B_o  is the equilibrium magnetic field, m _o   is the magnetic permeability of free space,  and r _o   is the plasma density. This leads to v  _A   =  about 2 kilometers per second. This then is adequate to provide the observed umbral energy flux of 0.2 F _o  where F _o  denotes the normal photospheric flux.

The full paper can be accessed here:
http://articles.adsabs.harvard.edu//full/1979ApJ...230..905P/0000905.000.html

A key fact relevant here is that heat flux and magnetic field strength is independent of sunspot area. The parameter that best helps to explain this is the vertical distance 'x'  which the model predicts is characteristic of all sunspots whether they be 4,000 km or 40,000 km across. Calculations by Parker show x = 1150 km approximately. It is the limiting distance below which an instability would occur in a single flux tube.

From all these points of view, the adoption of Eugene Parker's name for this one of a kind  solar craft - the first ever long term mission to our nearest star- is welcome and quite understandable. Indeed, the mission is a culmination of Prof. Parker's research in the fields of solar physics and heliophysics (the latter distinguished from the former on the basis of the extent of the solar wind, e.g. into the heliosphere - or the magnetically affected region that extends beyond Pluto's orbit).

Now what makes this solar mission so extraordinary?  First, back in November last year the craft made the first of 24 "oscillating" sweeps into the solar atmosphere. By that I mean the flight path carried it in then back out. In the November approach it came to within 15 million miles of the Sun's surface or photosphere. This is far closer than any earlier craft has ever gone. The previous record was set by Helios B in 1976 and broken by Parker on Oct. 29 — and this maneuver has exposed the spacecraft to intense heat and solar radiation in a complex solar wind environment. 

 Second, Parker features marvelous engineering designs which enable it to make such close passes.  (And bear in mind this was the first of 24 such oscillating in and out orbits, where the sunlit sie can reach as high as 1370 C.)   The main component here is a 2.5 meter wide, 72.5 kilogram heat shield, made of carbon foam sandwiched between two carbon sheets.  The whole shield is only 11. 5 centimeters thick (about 4. 6 inches).  It's coated on the Sun facing side with white ceramic paint to reflect as much sunlight as possible. Even then that side is expected  heat up to more than 1300 degrees Celsius.   Behind it, the bulk of the craft will coll to just 30 C on average, or about 86 F - about like a balmy spring day in Barbados.  

Other features of the Parker solar craft worth noting:

1) A component experiment for 'Integrated Science Investigation of the Sun' which is designed to detect solar particles across a wide range of energies.  This will allow solar physicists to decipher how the Sun accelerates the solar wind.  One detector will search for low energy particles, while a different one will search for high energy ones.  

  2) A component designated WISPR (Wide -field Imager for Solar Probe) will take images of the solar corona, solar wind, shocks and solar flares.  These images will help scientists properly interpret data from the other instruments.   

3) The craft features two solar panels mounted on a movable joint to control how much sun light (radiant energy) the panels absorb.  In close passes to the Sun - such as last November-  the panels will fold behind the heat shield, leaving only a last row of solar cells to absorb energy.   

4)The craft will be able to expel heat through a set of radiators composed of black material, which will be worn like a collar between the heat shield and the bulk of the space craft. Tubes of water will carry heat from the solar panels to the radiators where the heat can then escape into space.  At all temperatures the probe's solar panels will need to stay cool and the design of this system ensures it - especially the fact the panels are threaded with 'veins' to carry cooling water. 

4) Another experimental component, designated FIELDS, is comprised of five antennas to measure electric and magnetic field strengths in the solar neighborhood.  These measurements ought to help us to understand what makes the solar corona so hot (at nearly 2 million K).  To this end, four of the antennas are made of special material to protect them from direct sunlight..  

5) At the back of the probe will be the SWEAP experiment, for "Solar Wind Electrons Alphas and Protons".   This is designed to catch charged particles including electrons, and alpha particles ("alphas") from the solar wind and also determine their temperature, density and speed.  It will be exposed to sunlight 475 times as intense as felt on Earth.  

We can expect much data as well as imagery to be forthcoming in the coming months and years, as this monumental exploration of our Sun proceeds. Already, Parker has captured the image below of the Sun's corona, taken on December 8th.

The Parker solar adventure is just beginning, stay tuned!


See also:

Parker Solar Probe Heat-Resistant Technology - Spacecraft ...

New Research Into The 'Slow' Solar Wind Sheds Light On Its Source.

The solar wind is a  1, 610,000 km/h stream of charged particles constantly emitted by the Sun in all directions. However, solar wind dynamics are maddeningly complex as the original contributors to the Solar-Terrestrial Predictions Workshop held in Meudon, France (1986) learned.

For example, fast solar wind can attain speeds in excess of 500 km/s and emerges from coronal holes. These latter comprise dark holes visible in coronal imagery indicating where the Sun's magnetic field lines open up and extend into space thereby providing an escape channel for hot solar plasma. In the coronal image below these holes, as well as magnetic field arches, can clearly be seen:

This fast solar wind  is differentiated from the much slower moving stream of solar wind that floods the solar system continuously, and defines the "slow wind". It defines what we call the heliosphere which is essentially the solar wind's" bubble of influence.

The heliosheath occurs at the far edge of the heliosphere. More technically, it occurs between the "termination shock" and the "heliopause".

Thus, it marks a boundary at the edge of solar system space. In May 2005, NASA announced that Voyager 1 had crossed the termination shock and entered the heliosheath in December 2004, at a distance of 94 AU. 

While we can tie the fast solar wind to coronal holes, the origin of the slow solar wind has proven far more difficult to untangle.  One way to possibly resolve the issue is to examine data from the slow wind  intercepted by several near Earth spacecraft.

In fact, this has been reported now by Kepko et al (Geophysical Research Letters, 2016)  see e.g.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL068607/full


who have used such data to trace the slow wind's origin. This entailed examining its charge state ratios. For example, it has been found  that the ratio of charged oxygen atoms (ions)  O+7/ O+6,   well reflects and tracks the level of entropy and conditions at the solar surface (cf. Wimmer-Schewingruber et al, 1997, J. Geophysical Research)  Thus, the slow solar wind exhibits higher  O+7/ O+6  ratios than the fast wind.  The slow wind also displays high  O+7/ O+6 variability (cf. Zurbuchen et al, 2000, op. cit.).

The point is that these charge state ratios as well as elemental composition can serve as "fingerprints" for the specific conditions that generate the solar wind. The authors used measurements of charge state abundance and composition measurements at 12 minute intervals made by the ACE (Advanced Composition Explorer)  and WIND spacecraft.  The research team found that the plasma measurements fluctuated at roughly 90 minute cycles.  (This agreed with previous research on charge state and composition variability)

Thereby, Kepko et al discovered that the charge state and composition properties of the slow solar wind oscillate in nonrandom patterns. The oscillations occurred between patterns more typical of fast solar wind and more typical of slow wind. Specifically, the proton (H+) density within the plasma was three times higher at the midpoint of the cycle before decreasing. Meanwhile, the helium, carbon and oxygen abundances peaked late in the 90 minute cycle.

This pattern of oscillation indicated the charge state and composition of the slow wind originate in the solar atmosphere itself, not in transit from the surface. How can this be? Well, only magnetic reconnection at the solar surface could cause the solar wind to alternate properties characteristic of both fast and slow aspects. In other words, what we have is strong evidence for a magnetically-driven slow wind source with the potential to overturn previous models. As the authors put it (op. cit.):

"The major conclusion from our study is that the slow solar wind exhibits a quasiperiodic variability with a time scale of ∼90 min, with a repeatable, systematic, elemental abundance signature. Further, on this time scale, the wind speed shows no correlation with the repeatable charge state or elemental abundance signatures.

This result has far-reaching implications for understanding slow wind origin. First, it rules out all quasi-steady models such as the expansion factor. In the quasi-steady models the solar wind plasma properties in any flux tube are set by the geometry of the flux tube in the corona, which determines the heating and momentum deposition along that flux tube."

In the above quote what Kepko et al are referring to are the "expansion factor" wind models that predict the slow wind issues from the tube like stretches of space containing the solar magnetic field as it peels off into space away from the Sun at the edges of coronal holes. Thus, the authors' model offers instead strong evidence for a magnetically driven slow wind source..

Why is it important which model is correct? Because we will then  be able to more accurately forecast solar wind phenomena including the occurrence of magnetic substorms at Earth's magnetophere.  In effect, we would be better able to protect infrastructure, including power grids on Earth, from the vagaries of space weather.

Magnetized Collisionless Shock Waves Measured in Lab for First Time

Portion of  the debris cloud in the Cygnus Loop supernova remnant wherein collisionless shock waves occur

In space plasma physics, shock waves are of special interest, particularly what we call collisionless shocks, because they're observed throughout the universe. For example, they are detected in the heliosphere e.g.



which is the bubble -like region of space in our solar system governed by plasma (including the solar wind) blown outwards from the Sun.  The presence of collisionless shocks also appears in the expanding debris clouds of supernova remnants such as those for the Cygnus Loop depicted in the graphic at the top. What makes these shocks unique is the low density plasma in which they occur.

To fix ideas, in a normal shock wave as the gas compresses, particles collide, i.e. they get so close to each other that their respective electric fields cause them to change direction.

In the space physics setting we often consider shocks as being incited by what we call micro-instabilities. Often cited is the case of an ion-acoustic instability triggering a soliton and shock, for which one has the electron temperature T _e  ¹ 0 and ion temperature,   T _i = 0 ("cold plasma" conditions), so electrons, with ions are streaming at same speed.

The electron thermal speed is then:  v_e (o)² = 2 T _e / m_e

 And the ion sound speed: c(s) =  T _e / m_i

So that: n(i) =  n(e)/  [1 – (2 e j)/ m_i u(o)² ] ^1/2

Where the last is derived from continuity relation: n(i) v_i (o) = n(e) v_e (o) = const.

(With the numbers of ions and electrons identified respectively).

Given the non-dimensional constant:

F = e j/ T_e

Then one can obtain:   {2 e j)/ T_e }/[ m_i u(o)² )/ T_e] =  2F/ { u(o)² / c(s)²} =  2F/ (M _s )²

 Where (M _s ) is the sonic Mach number.

By contrast to normal shocks, collisionless shocks have a gas (plasma) density too low for the particles to interact. They are instead pushed about by turbulent waves that act on much smaller scales.

By creating collisionless shocks, say in a laboratory setting, we can conceivably make measurements at a micro-scale that would otherwise be impossible to do in space. And while space plasma physicists have numerically modelled shock formation in the past they have hitherto not been able to confirm the model-based predictions in situ - or through experiments.  Previous lab experiments used pistons to generate shocks in plasmas but these were limited to perpendicular motion and alas, did not propagate away from the pistons. (Later experiments created shocks in lasers or photo-ionized plasmas but encountered problems with magnetizing or ionizing the background).

Now, Niemann et al (Geophysical Research Letters, 2014) have succeeded in making measurements of collisionless shock waves in the lab. The authors effectively created an experiment for the Large Plasma Device at the University of California in Los Angeles that used a high power laser to blow up  piece of plastic, creating a cloud of debris exploding into a stationary magnetized plasma.

The collisionless interactions created transferred momentum from the debris cloud to the ambient ions, thereby incepting a shock wave that propagated away from the piston.  The UCLA team was then able to plot the shock waves and compare them to their numerical simulations.

Make no mistake this is huge because too often, as in space plasma physics and solar physics, we get beautiful numerical simulations, but no real cross checks in physical reality.  In this case though, the measurements collected by Niemann et al validated the models that predicted that collisionless interactions accelerated the ambient ions to the debris velocity.

The data also indicated that microinstabilities (e.g. ion acoustic) play a minor role in shock dissipation. Of course this work will be useful in future studies of the microphysics of magnetized shocks, and we also are confident that now similar in situ experiments can be performed - perhaps using different materials.

Stay tuned!

Joe 'Idiot' Barton: "Great Flood was an Example of Climate Change" - But Humans Had Nada to do with it!

Graph of radiocarbon (C14) excess over C12 over ~ 2,000 yr. period. The zero level is an arbitrary norm referenced to 1890. Arrows mark persistent features identified as possible solar anomalies. (From Eddy, J. 'Evidence for a Changing Sun' in The NEW SOLAR PHYSICS, AAAS Symposium 1978)

According to another fundie nitwit, in this case a congress critter,  "Noah's Flood" was an example of climate change but it also shows - since so few humans inhabited the planet then, climate change isn't human-caused or to do with hydrocarbons.

 Buzzfeed reports that during a Subcommittee on Energy and Power hearing to discuss the Keystone XL pipeline, Joe Barton blabbed:

"I would point out that if you’re a believer in the Bible, one would have to say the Great Flood is an example of climate change and that certainly wasn’t because mankind had overdeveloped hydrocarbon energy,”

Well, the point is there is no hard and fast evidence for any "great flood"! At least that has been published in any recognized oceanographic or geological journal.  Barton went on to insist:

“I think you can have an honest difference of opinion of what’s causing that change without automatically being either all in that’s all because of mankind or it’s all just natural. I think there’s a divergence of evidence.”

But in fact the "divergence" in evidence of which he speaks is essentially non-existent! In their analysis of the extent of scientific consensus on global warming (Eos Transactions, Vol. 90, No. 3, p. 22) , P. T. Doran and M. Kendall-Zimmerman found that (p. 24)

“the debate on the authenticity of global warming and the role played by human activity is largely non-existent among those who understand the nuances and scientific basis of long-term climate processes.”

In their analytic survey for which 3146 climate and Earth scientists responded, a full 96.2% of specialists concurred temperatures have steadily risen and there is no evidence for cooling. Meanwhile, 97.4% concurred there is a definite role of humans in global climate change. The authors concluded (p. 24) :


“The challenge appears to be how to effectively communicate this fact (non-existent debate among real climate specialists) to policy makers and a public that continues to mistakenly perceive debate exists among scientists”

In truth, Barton's bollocks is just one more in a long line for agnotology, to provide cover for the fossil fuel burner industry. See, e.g. http://www.brane-space.blogspot.com/2012/01/as-i-noted-in-earlier-blog-httpbrane.html

To go back and compare records from 2,000 years ago, using the same physical factors, has been the problem but it was resolved in the 1960s when a series of papers demonstrated that radiocarbon (C14) in plant cellulose could be used as an indirect or proxy register of solar activity.  In general, C14 is produced in the upper atmosphere via the impact –interaction with high energy cosmic rays, say from galactic sources. Solar activity in turn modulates the intensity of these cosmic rays via the action of the heliosphere which deflects a fraction of the intense cosmic ray flux and other harmful interstellar radiation.

At times the Sun is more active, so also will the heliosphere be stronger, shielding the Earth from more intense cosmic rays the effect of which is to reduce the C14 produced in the Earth’s upper atmosphere. Conversely, when the Sun is less active – as it’s been from 2000- 2008 then the shield is weaker and more intense cosmic rays penetrate to our upper atmosphere yielding more C14 produced. It follows from this that if a record could be obtained of the ratio of say C14 to C12 then one would have a proxy indicator of solar activity for any time (with the C14 to C12 ratio extracted from tree rings or other plant tissue). If such a record showed falling C14 to C12 then we’d deduce higher solar activity and if increased C14 to C12 lower solar activity. If the same ratios were obtained in the modern era it might be feasible to normalize all the results to compare them and draw conclusions.

Most importantly and crucially, since the Sun isn’t the exclusive modulator of cosmic rays (clearly if there’s an anthropogenic effect that would also impact the high atmosphere and modulate intensity and C14 production, as would a changing magnetic moment for the planet) then the C14 record embedded in tree rings would have to be expected to be inscribed with other histories too.

Fortuitously, a 2000-year record of C14:C12 deviations has been compiled by P.E. Damon ('The Solar Output and Its Variation', The University of Colorado Press, Boulder, 1977) and this is shown in the accompanying graph. To conform with solar activity the plot is such that increasing radiocarbon (C14) is downward and indicated with (+). The deviations in parts per thousand are shown relative to an arbitrary 19th century reference level.

As John Eddy observes concerning this output and the data (Eddy, 'The New Solar Physics',  p. 17):

“The gradual fall from left to right (increasing C14/C12 ratio) is…probably not a solar effect but the result of the known, slow decrease in the strength of the Earth’s magnetic moment.[1] exposing the Earth to ever-increased cosmic ray fluxes and increased radiocarbon production.

The sharp upward spike at the modern end of the curve, representing a marked drop in relative radiocarbon, is generally attributed to anthropogenic causes—the mark of increased population and the Industrial Age."

Assuming the validity of the arbitrary norm (zero line or abscissa) for 1890, then it is clear that the magnitude of the Middle Ages warming period (relative C14 strength of -18),  as well as that for 0 AD (about the same) are both  less than about ½ the relative effect attributed mainly to anthropogenic sources in the modern era (-40).  Again, this era commences with the hydrocrabon -pumping Industrial Age.

So Barton and his agnotologists are wrong again. Or maybe not so much wrong, as wiley like a fox.  After all, Barton, is one of Congress’s top recipients of oil company campaign donations, and is perhaps best known for apologizing to BP for what he called a White House ”shakedown,” after the Obama Administration announced that it would set up a $20-billion escrow fund to benefit victims of the 2010 BP oil spill.

By the way, excellent documented footage on the current rapid melting of glaciers world wide can be found at this link:



http://video.pbs.org/video/1108763899


It ought to get even the most blinkered global warming denier's attention!

Has Voyager I Left the Solar System?

A burning question on some astronomy forums concerns the Voyager I spacecraft (see photo) and whether it has left the solar system. Only recently the JPL team behind this marvelous craft celebrated its 35th anniversary, all the more remarkable given its longevity and what it’s done: delivered some of the most amazing images of the planets before the arrival of the Hubble Telescope. (Voyager I reached Jupiter in 1979, and Saturn in 1980)

It is also notable for being the first space craft that played a lead role in a movie: this was in the film “Star Trek I” (“Star Trek: The Motion Picture") from 1979 which launched the famous 1960s science fiction series film run. In the film, the Voyager I appeared as “V-GER” an apparently alien entity that poses a challenge to the crew of Enterprise, and which they only surmount when Spock figures out it’s actually Voyager I – but with an alien ‘remake’ that confers a fairly nasty "disposition".

Though now 11.3 billion miles from Earth, and far beyond Pluto (which many take to be the ‘boundary’ of the solar system, the fact is that Voyager is still at least two years from actually being considered to have left the solar system. The reason has to do with its trajectory through the heliosphere, or the region associated with the Sun’s magnetic influence including the extent of its charged particle stream called “the solar wind”.

A particular boundary called the “heliopause” is the one of most interest in determining the departure of a craft under powered flight (and under gravitational influence). The heliopause is the interface where the solar wind is stopped by the interstellar medium. One can therefore think of it as a three dimensional region or surrounding “envelope” at which the solar wind's strength is no longer sufficient to overcome the stellar winds of the external stars. In technical terms, this absence of counter-pressure signals the end of the solar system.

From the computations of celestial mechanics, we expect Voyager 1 to cross the heliopause by 2014. The exact time of crossing should be indicated by a sharp drop in the temperature of charged particles, a change in the direction of the magnetic field, and an increase in cosmic rays (Voyager detect 9% enhancement in one month), following a more gradual increase of 25% from Jan. 2009 to Jan. 2012) All this suggested it was approaching the heliopause.

According to JPL Director Edward Stone, once this threshold is crossed Voyager I will become the first craft to “leave the bubble”.

This is remarkable! It means that despite the fact manned space missions have likely been abandoned forever (no money, and when money is available it'll likley be used for more weapons, wars) we have actually dispatched a craft that will have - after 37 years - departed from the solar "bubble" into the distant stellar firmament. Will actual aliens pick up on it, traks it and then track it back to us?

Who knows? But it's kind of an exciting prospect to consider....assuming those ETs are friendlies!