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The Sun’s Exotic Neighborhood

We think about our interstellar neighborhood in terms of stars, like Alpha Centauri and Tau Ceti, but the medium through which our relative systems move is itself a dynamic and interesting place. The Sun is currently passing through a shell of material known as the Local Interstellar Cloud. And that cloud is, in turn, located at the edge of a vast region known as the Local Bubble, scoured of material by supernova explosions in the nearby Scorpius-Centaurus and Orion Association star-forming regions. Within the past 105 years, the Sun emerged from the interior of the Local Bubble; it now moves obliquely in the direction of the high-density molecular clouds of the Aquila Rift, a star-forming region that itself reminds us how energetic ’empty’ space really is.

Ulysses Spacecraft

If we’re ever going to send fast missions outside the Solar System, we’re going to need plenty of data about the materials through which our vehicles move, particular as velocities mount to the point where collision with even small particles can be devastating. As of now, we’re only piecing together a broad idea of the galactic environment within 500 parsecs of the Sun, and remain in need of more detailed, in situ results from regions much closer in. Fortunately, space-based data have begun to accumulate.

Image: An artist’s take on the Ulysses mission, now approaching the end of its life, but a workhorse that has taught us much about the interstellar medium. Credit: Jet Propulsion Laboratory.

Take the Ulysses mission, which Harald Krüger (Max-Planck-Institut für Sonnensystemforschung) and Eberhard Grün (Max-Planck-Institut für Kernphysik) examine in their recent paper. The scientists take a hard look at the Sun’s position and its relation to nearby interstellar dust, noting interesting clues like the Arecibo radar findings about micron-sized interstellar meteor particles, which seem to radiate from the direction of the Geminga pulsar. Are these evidence that the supernova responsible for the Geminga pulsar created the Local Bubble itself?

At this point, we simply don’t know, but space-based studies of these exotic materials should help to clarify things. First examined during the 1930s, when astronomical evidence of starlight scattering and weakening gave clear signs of its existence, interstellar dust became available for close study when spacecraft began flying dust detectors. It became clear thirty years ago that interstellar dust grains can cross the heliospheric boundary and make their way toward the Solar System. In the 1990s, Ulysses’ dust instrument identified the mass, speed and approach direction of impacting grains as they swept through the heliosphere.

Krüger and Grün write about using future Ulysses studies to learn more, but recent NASA news seems to rule that scenario out, as the spacecraft’s power systems are in a state of terminal decline. Future Ulysses studies would have been useful for two reasons: The spacecraft’s data would have covered the entire 22-year range of a solar cycle, offering a unique set of measurements. And further readings from the outer heliosphere might have made it possible to explain the origin of an odd thirty degree shift in the flow direction of the dust Ulysses monitored.

For data from the period 1996 to 2000 showed a specific impact direction from interstellar dust, which little prepared investigators for the following:

Six years later, when Ulysses was travelling through almost the same spatial region and had an almost identical detection geometry for interstellar grains, the situation was vastly different: first, the range in approach directions of the grains was somewhat wider…; second, and more noticeable, in 2005/06 the approach direction of the majority of grains was shifted away from the helium flow direction. Preliminary analysis indicates that this shift is about 30◦ away from the ecliptic plane towards southern ecliptic latitudes… At the moment, we do not know whether it is a temporary shift limited to the time period stated above or whether it continues to the present time. Furthermore, the reason for this shift remains mysterious. Whether it is connected to a secondary stream of interstellar neutral atoms shifted from the main neutral gas flow… is presently unclear.

So even within the Local Interstellar Cloud, through which we now move, the concentration of dust is little understood. Properly interpreted, these dust grains may be able to tell us more about how heavy elements are moved about within the interstellar medium and how they are affected by the five distinct clouds of gaseous material within five parsecs of the Sun. Data from the recent Stardust mission should be of help, their interpretation aided by what Ulysses has found so far. What we lack, of course, are direct observations of interstellar dust outside the Solar System, something that future missions like Innovative Interstellar Explorer may remedy if funded for launch some time in the next decade.

The paper is Krüger and Grün, “Interstellar Dust Inside and Outside the Heliosphere,” submitted to Space Science Reviews and available online.

Comments on this entry are closed.

  • Hans Bausewein February 28, 2008, 17:08

    I have long wondered, whether we would see a supernova bubble from inside. Most radiation is usually seen from the side of the bubble, because that’s the longest path through the shell. From inside that path is quite short.

  • tacitus February 28, 2008, 18:15

    Within the past 105 years, the Sun emerged from the interior of the Local Bubble; it now moves obliquely in the direction of the high-density molecular clouds of the Aquila Rift, a star-forming region that itself reminds us how energetic ‘empty’ space really is.

    It’s funny, my initial reaction upon reading this was “uh-oh” could we be in for a spot of bother once we reach the Rift, at least in the sense of hampering interstellar travel. But then I realized, of course, that the time frame involved means that it will be many millions of years before this becomes an issue, and if we haven’t escaped the confines of the Solar System by then, we probably never will anyway.

  • Bilal M February 28, 2008, 22:30

    I have always wondered about interstellar particles…rocks…dare I say “bodies?”

    Is there a chance that the solar system could hit a patch of really large objects, say asteroid sized or greater.

    Is the chance of a Jupiter sized planet (ejected from dead or binary star system) completely nil? Even if that kind of object doesn’t come close to earth, even if it passes only through the Oort cloud , it could pose serious danger as it could fling comets and bodies from the Oort cloud to the inner solar systems.

    That kind of threat would be nearly impossible to mitigate without a seriously large telescope array and a very large successor to Arecibo.

    I would like to see the raw data about interstellar particles. If there are large numbers of “micron” sized particles, there is some probability that we need to be concerned.

  • tacitus February 29, 2008, 16:54

    I’m guessing that most statisticians would tell you that since the Solar System (and more importantly Earth) has survived over 4 billion years without being destroyed by rogue planets or asteroid fields, then the odds of it happening in the future are likely equally as slim. Stars are a much bigger threat, given their huge mass, and yet no star has passed close enough to the Sun in its lifetime to rip the planets away from it.

    We should know a lot more once about the odds once we have surveyed all the planets in the star systems around us. If it turns out that ours is one of the few systems with a full gamut of planets then we might begin to consider ourselves very lucky!

  • ljk March 8, 2008, 17:37

    Oxygen isotope anomalies of the Sun and the original environment of the Solar system

    Authors: Jeong-Eun Lee, Edwin A. Bergin, James R. Lyons

    (Submitted on 5 Mar 2008 (v1), last revised 5 Mar 2008 (this version, v2))

    Abstract: We present results from a model of oxygen isotopic anomaly production through selective photodissociation of CO within the collapsing proto-Solar cloud. Our model produces a proto-Sun with a wide range of Delta_17O values depending on the intensity of the ultraviolet radiation field. Dramatically different results from two recent Solar wind oxygen isotope measurements indicate that a variety of compositions remain possible for the solar oxygen isotope composition. However, constrained by other measurements from comets and meteorites, our models imply the birth of the Sun in a stellar cluster with an enhanced radiation field and are therefore consistent with a supernova source for 60Fe in meteorites.

    Comments: 27 pages, 10 figures. Accepted for publication in M&PS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.0692v2 [astro-ph]

    Submission history

    From: Jeong-Eun Lee [view email]

    [v1] Wed, 5 Mar 2008 16:15:18 GMT (2822kb)

    [v2] Wed, 5 Mar 2008 22:34:09 GMT (2822kb)


  • ljk March 10, 2008, 23:28

    Cluster observations of the mid-altitude cusp under strong northward Interplanetary Magnetic Field

    Authors: Renyu Hu, Yulia V. Bogdanova, Christopher J. Owen, Claire Foullon, Andrew N. Fazakerley, Henri Reme

    (Submitted on 8 Mar 2008)

    Abstract: We report on a Cluster multi-spacecraft cusp observation lasting more than 100 minutes. We determine the cusp boundary motion and reveal the effect on the cusp size of the interplanetary magnetic field (IMF) changing from southward to northward. The cusp shrinks at the beginning of the IMF rotation and it re-expands at the rate of 0.50$^{o}$ ILAT per hour under stable northward IMF.

    Based on plasma signatures inside the cusp, such as counter-streaming electrons with balanced fluxes and quasi-periodic enhancement of ion fluxes with energy E$>$5keV, we propose that pulsed dual lobe reconnection operates during the time of interest. SC1 and SC4 observations suggest a long-term regular periodicity of the pulsed dual reconnection, which we estimate to be $\sim$ 1-5 minutes. Further, the distances from the spacecraft to the reconnection site are estimated based on observations from three satellites. The distance determined using SC1 and SC4 observations is $\sim$ 15 RE, and that determined from SC3 data is $\sim$ 8 RE. The large-scale speed of the reconnection site sunward motion is $\sim$ 16 km s$^{-1}$. We observe also a fast motion of the reconnection site by SC1, which provides new information about the transitional phase after the IMF rotation.

    Finally, a statistical study of the dependency of plasma convection inside the cusp on the IMF clock angle is performed. The relationship between the cusp stagnation, the dual lobe reconnection process and the IMF clock angle is discussed.

    Comments: JGR Space Physics, in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.1206v1 [astro-ph]

    Submission history

    From: Renyu Hu [view email]

    [v1] Sat, 8 Mar 2008 02:41:43 GMT (2285kb)


  • ljk March 24, 2008, 9:33

    Mars, Earth And Moon From ‘Unique Planetary Nursery’

    ScienceDaily (Mar. 20, 2008)

    A study of meteorites suggests that Mars, the Earth and the
    Moon share a common composition from ‘growing up’ in a
    unique planetary nursery in the inner solar system.

    The finding could lead to a rethink of how the inner solar
    system formed.

    In the journal Nature the international team of scientists,
    which includes Professor Alex Halliday from Oxford University’s
    Department of Earth Sciences, report how they analysed 16
    meteorites that fell to Earth from Mars. They found that the
    amounts of neodymium-142 these contain are subtly different
    from those of objects found in the asteroid belt. This isotopic
    fingerprint is proof that the chemistry of the inner solar system
    was different even for elements that are hard to vapourise.

    Full article here:


  • ljk April 24, 2008, 22:28

    Is the Sun Embedded in a Typical Interstellar Cloud?

    Authors: P. C. Frisch

    (Submitted on 23 Apr 2008)

    Abstract: The physical properties and kinematics of the partially ionized interstellar material near the Sun are typical of warm diffuse clouds in the solar vicinity. The interstellar magnetic field at the heliosphere and the kinematics of nearby clouds are naturally explained in terms of the S1 superbubble shell. The interstellar radiation field at the Sun appears to be harder than the field ionizing ambient diffuse gas, which may be a consequence of the low opacity of the tiny cloud surrounding the heliosphere. The spatial context of the Local Bubble is consistent with our location in the Orion spur.

    Comments: “From the Outer Heliosphere to the Local Bubble”, held at International Space Sciences Institute, October 2007

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0804.3798v1 [astro-ph]

    Submission history

    From: Priscilla Chapman Frisch [view email]

    [v1] Wed, 23 Apr 2008 20:05:50 GMT (176kb)


  • ljk April 28, 2008, 22:56

    Exotic clouds in the local interstellar medium

    Authors: Snezana Stanimirovic (UW Madison)

    (Submitted on 26 Apr 2008)

    Abstract: The neutral interstellar medium (ISM) inside the Local Bubble (LB) has been known to have properties typical of the warm neutral medium (WNM). However, several recent neutral hydrogen (HI) absorption experiments show evidence for the existence of at least several cold diffuse clouds inside or at the boundary of the LB, with properties highly unusual relative to the traditional cold neutral medium. These cold clouds have a low HI column density, and AU-scale sizes. As the kinematics of cold and warm gas inside the LB are similar, this suggests a possibility of all these different flavors of the local ISM belonging to the same interstellar flow. The co-existence of warm and cold phases inside the LB is exciting as it can be used to probe the thermal pressure inside the LB. In addition to cold clouds, several discrete screens of ionized scattering material are clearly located inside the LB.

    The cold exotic clouds inside the LB are most likely long-lived, and we expect many more clouds with similar properties to be discovered in the future with more sensitive radio observations. While physical mechanisms responsible for the production of such clouds are still poorly understood, dynamical triggering of phase conversion and/or interstellar turbulence are likely to play an important role.

    Comments: 10 pages, refereed, accepted for publication in the proceedings of the “From the Outer Heliosphere to the Local Bubble: Comparisons of New Observations with Theory” conference, Space Science Reviews

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0804.4236v1 [astro-ph]

    Submission history

    From: Snezana Stanimirovic [view email]

    [v1] Sat, 26 Apr 2008 16:45:01 GMT (81kb)


  • ljk May 20, 2008, 11:56

    Iron-60 evidence for early injection and efficient mixing of stellar debris in the protosolar nebula

    Authors: N. Dauphas, D.L. Cook, A. Sacarabany, C. Frohlich, A.M. Davis, M. Wadhwa, A. Pourmand, T. Rauscher, R. Gallino

    (Submitted on 16 May 2008)

    Abstract: Among extinct radioactivities present in meteorites, 60Fe (t1/2 = 1.49 Myr) plays a key role as a high-resolution chronometer, a heat source in planetesimals, and a fingerprint of the astrophysical setting of solar system formation. A critical issue with 60Fe is that it could have been heterogeneously distributed in the protoplanetary disk, calling into question the efficiency of mixing in the solar nebula or the timing of 60Fe injection relative to planetesimal formation. If this were the case, one would expect meteorites that did not incorporate 60Fe (either because of late injection or incomplete mixing) to show 60Ni deficits (from lack of 60Fe decay) and collateral effects on other neutron-rich isotopes of Fe and Ni (coproduced with 60Fe in core-collapse supernovae and AGB-stars).

    Here, we show that measured iron meteorites and chondrites have Fe and Ni isotopic compositions identical to Earth. This demonstrates that 60Fe must have been injected into the protosolar nebula and mixed to less than 10 % heterogeneity before formation of planetary bodies.

    Comments: 15 pages, 5 figures, ApJ in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.2607v1 [astro-ph]

    Submission history

    From: Nicolas Dauphas [view email]

    [v1] Fri, 16 May 2008 19:37:34 GMT (520kb)


  • ljk June 4, 2008, 7:44

    Ulysses, the end of an extraordinary mission

    ESA PR 29-2008. After more than 17 years relentlessly exploring
    the effects of solar activity on the space that surrounds us, the
    Ulysses mission is now approaching its end.

    Representatives of the media are invited to a joint ESA/NASA
    press conference on 12 June 2008 taking place at ESA Headquarters
    in Paris, France, to hear about the achievements that will form the
    Ulysses legacy.

    More at:


  • ljk June 12, 2008, 11:00

    Ulysses mission to end on July 1, 2008

    The joint ESA/NASA solar mission Ulysses has forever changed
    the way scientists view the Sun and its effect on the surrounding

    The mission’s major results and the legacy it leaves behind have
    been presented today at ESA Headquarters in Paris, in view of
    the impending conclusion of the mission on 1 July 2008.

    More at:


  • ljk July 3, 2008, 11:25

    Ulysses hanging on valiantly

    3 July 2008

    The Ulysses spacecraft, whose mission was expected to end
    on 1 July 2008, is hanging on valiantly as spacecraft controllers
    wait for a sign of the fuel freeze that would end the mission.

    This could happen any time now.