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Interstellar Deceleration: Can We Ride the ‘Bow Shock’?

Interesting things happen at the edge of the Solar System. Or perhaps I should say, at the boundary of the heliosphere, since the Solar System itself conceivably extends (in terms of possible planets) further out than the 100 or so AU that marks the heliosphere’s boundary at its closest. The fact that the heliosphere is pliable and reacts among other things to the solar cycle in turns means that the boundary is a moving target. It would be useful if we could get something like JHU/APL’s Interstellar Probe mission out well beyond the heliosphere to help us understand this morphology better.

But let’s think about the heliosphere’s boundaries from the standpoint of incoming spacecraft. Because deceleration at the destination system is a huge problem for starship mission planning. A future crew, human or robotic, could deploy a solar sail to slow down, but a magsail seems better, as its effects kick in earlier on the approach. Looking at the image below, however, suggests another possibility, one using the interactions between stars and the interstellar medium to assist the slowdown. And then the question arises: Does our own Sun produce a similar kind of bow shock?

Image: A multi-wavelength view of Zeta Ophiuchi. Credit: X-ray: NASA/CXC/Dublin Inst. Advanced Studies/S. Green et al.; Infrared: NASA/JPL/Spitzer.

Here we’re looking at a star, Zeta Ophiuchi, that is some 440 light years from Earth. It’s about 20 times as massive as the Sun, and evidently was once in a tight orbit around another star that became a supernova perhaps a million years ago. As a result, Zeta Ophiuchi was ejected from its binary orbit, and we have data from the Spitzer Space Telescope as well as the Chandra X-ray Observatory depicting the spectacular after-effects. The shock wave consists of matter blowing away from the star’s surface, slamming into gas. In the above image, the shock wave is in vivid red and green.

The latest work on Zeta Ophiuchi comes from a team led by Samuel Green (Dublin Institute for Advanced Studies, Ireland), with a paper laying out computer modeling of the shock wave and running the data against observational data obtained at X-ray, optical, infrared and radio wavelengths. Their results are interesting, as what can be found in data on the X-ray emissions shows that it is brighter than the modeling suggests. The bubble of X-ray emissions shows up in blue around the star in the image above. Its brightness indicates further modeling including turbulence and particle acceleration is needed.

I’ll send you to the paper for more on Zeta Ophiuchi, whose position – enveloped by the nebula Sh2-27 and pushing through dense dust clouds – makes it a natural for studying what happens when a shock wave develops. But let’s cut back to more mundane interactions, such as what happens when the Sun’s solar wind encounters the interstellar medium. Does a bow shock form here? Depending on the relative velocity of the heliosphere and the strength of the local interstellar magnetic field, such a phenomenon may or may not occur, as suggested by Voyager data as well as earlier findings from the Interstellar Boundary Explorer spacecraft (IBEX). A bow shock had been assumed, but we’re learning that these interactions are complicated.

While we investigate our heliosphere’s interactions with the interstellar medium, we can point to numerous bow shocks especially associated with more massive stars. In fact, a citizen science effort called The Milky Way Project is all about mapping bow shocks, building our catalog of these interesting astrophysical features. Learning more about how bow shocks form will clearly take us into the influence of interstellar magnetic fields as they roil the outflowing stellar winds they encounter. The density and pressures of the medium and the speed of the star’s astrosphere determine the result.

Image: Stars travel through the galaxy surrounded by a bubble of charged gas and magnetic fields, rounded at the front and trailing into a long tail behind. The bubble is called an astrosphere, or — in the case of the one around our Sun — a heliosphere. This image shows a few examples of astrospheres that are very strong and therefore visible.
Credit: NASA/Goddard Space Flight Center.

All of this has implications for our thinking about certain kinds of interstellar missions. If a star does form a bumper of plasma and higher density gas at the edge of its astrosphere, then as Gregory Benford has suggested (in correspondence some years back), we are looking at an obvious place to slow down an incoming starship. As Benford noted, the bow shock produces 3D structures, surfaces within which one can move while shedding speed, perhaps braking via a magsail. Each star would produce its own unique deceleration environment, allowing us to brake where possible along the bow shock, the astropause (cognate to the heliopause) and the termination shock.

We are talking about long, spiraling approaches to a destination system with continual magsail braking – decelerating from interstellar velocities is not going to be fast or easy. But it seems clear that one kind of precursor mission before we send missions that are more than flybys to other stars will be to visit our own shock environment at the edge of the Solar System, where we can learn more about using shock surfaces to slow down. I like the way Benford put it in an email: “As a starship approaches a star, sensing the shock structures will be like having a good eye for the tides, currents and reefs of a harbor.” For more, see 2012’s Starship Surfing: Ride the Bow Shock, where I assumed the existence of a solar bow shock.

All of this reminds us that the interstellar medium is anything but uniform. If the Sun is currently near the boundary of the Local Interstellar Cloud (and its exact position within it is unclear), the Alpha Centauri stars appear to be outside that cloud in the direction of the G cloud, another variation in the medium. So we have another kind of boundary crossing to consider. Different hydrogen densities play havoc with the Bussard ramjet concept, too. Robert Bussard assumed hydrogen densities in the range of 1 hydrogen atom per cubic centimeter, but move outside denser clouds and that figure should drop precipitously. If you’re flying an interstellar ramjet, pay attention to the clouds!

The Zeta Ophiuchi paper is Green et al., “Thermal emission from bow shocks. II. 3D magnetohydrodynamic models of zeta Ophiuchi,” in process at Astronomy & Astrophysics (abstract).

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{ 22 comments… add one }
  • Alex Tolley August 2, 2022, 10:30

    At first glance, I see some issues with using the bow shock to decelerate.

    1. The bow shock as shown in the images is directional, so presumably, it is only useful as a denser medium for target stars where the bow shock is facing in our general direction.

    2. Just as with atmospheric braking, the trajectory must be along the bow shock surface. This implies that the probe must have a trajectory that best aligns with the bow shock surface, rather than the target star. This will then requires a delta-V to alter the probe’s now slower velocity, towards the star. [Unless this can be provided by “surfing the bow shock” to cross the bow shock.]

    The OP mentions “spiraling in” for a deceleration. This seems to require entering the bow shock that is approximately perpendicular to the probe’s trajectory and spiraling around the front surface of the bow shock with its “pole” as the spiral center. Whether or not this can work, at least it offers a continuous circular path for the probe to follow in the denser medium. As with point [1], this requires the bow shock to be orientated correctly for this to work.

    It has been a decade since the 2012 post on ” Starship Surfing: Ride the Bow Shock” that suggested using a magsail to decelerate and navigate using the bow shock. What progress has been made to determine if this is viable and under what conditions (bow shock density, probe velocity, magsail performance parameters, etc.)? [This would seem to be a prime investigative domain for the Plasma Magnet. It would follow in the tracks of Slough’s suggestion of using the PM technology to aerobrake.]

    • Paul Gilster August 2, 2022, 15:38

      Like you, Alex, I’m curious about the Plasma Magnet in this regard. Since the 2012 post, I think the biggest news has been the realization that the Sun’s bow shock is, if it exists, minimal.

      • Dave McDysan August 2, 2022, 16:55

        When I first read about the plasma magnet about a year ago, the performance was impressive. After looking at it further the magnetic field falloff rate originally assumed as 1/r in 2006 by Slough (and Winglee for M2P2 in 2000) and later updated to 1/r^2 in 2012 by Slough/Kirtley render the predicted performance much more modest. The large, massive Zubrin magsail has comparable performance for acceleration/deceleration in the solar wind. Variants of the M2P2 approach dubbed the MagnetoPlasma Sail (MPS) that inject plasma to achieve a 1/r^2 falloff rate also have comparable performance and are still being researched.

        I have reviewed the papers that cite the Slough plasma magnet papers and have found only ones that include it as part of a literature survey, except for the 2012 Kirtley/Slough magneto aero-capture paper that changed the magnetic field falloff rate.

        I have been updating the Wikipedia magnetic sail page, https://en.wikipedia.org/wiki/Magnetic_sail, which contains a summary of the citations for the above comments.

        • Alex Tolley August 2, 2022, 20:21

          Very nice, useful Wikipedia page. Bookmarked.

          If I have any references that you do not have, I will try to get them to you.

          Best,
          AMT

          • Dave McDysan August 3, 2022, 16:08

            Thank you, Alex.

            In writing the Wikipedia article, I tried to use on-line citations. I joined AIAA to get a discount on some publication purchases and help contribute to that cause.

            I have encountered some potentially relevant citations in papers, particularly older ones for which there appears to be no on line version available; for example, the following which appears relevant to this discussion:

            Fujita, K., Particle Simulation of Moderately-Sized Magnetic Sails, The Journal of Space Technology and Science, Vol.20, No.2, (2004), pp.26-31, ISSN 0911-551X

            Thanks!

      • Dave McDysan August 6, 2022, 21:36

        To Paul Gilster’s point regarding existence of a bow shock upwind from the Solar system, a 2021 paper on the JHU/APL Interstellar Probe mission:

        https://www.sciencedirect.com/science/article/pii/S0094576522003484?ref=cra_js_challenge&fr=RR-1

        cites the following 2012 paper that analyzed IBEX measurements

        https://www.researchgate.net/publication/224951247_The_Heliosphere%27s_Interstellar_Interaction_No_Bow_Shock

        that (as the title suggests) concluded there is likely no bow shock, but a bow wave since the Sun is moving too slowly in the Local ISM to create a shock. From Figure 3 , the most likely proton density n_p 0.3 (nT). From Figure 4, the ISM headwind velocity is 23.2 km/s.

        So, not good bow surfing conditions right now ;-(
        – but that could change in a few millennia according to the JHU/APL paper.

        • Paul Gilster August 7, 2022, 5:58

          Absolutely right. The 2012 findings point to how different each bow shock situation is going to be; we evidently can’t assume bow shock without taking into account the stellar environment within or outside of an interstellar cloud, etc.

    • Dave McDysan August 2, 2022, 16:45

      I have been reading a 2017 paper by Claudius Gros, “Universal scaling relation for magnetic sails: momentum braking in the limit of dilute interstellar media,” https://arxiv.org/abs/1707.02801 predicts that braking performance for the classic large loop (e.g., 100 km) Zubrin magsail at high velocities (e.g., 10% c) will be substantially less than that predicted by Zubrin in 1991.

      I believe the effective sail area model based upon numerical analysis of proton motion near the current loop according to the Biot-Savart law of Equation (8) is correct. It is as a function of velocity, plasma density, coil size and current carried. Force is proportional to the effective sail area for magnetic sails.

      Section 4.1 reports that a magnetic sail radius of 1,600 km at a mass of only 1500 tonnes is necessary to decelerate on approach to Proxima Centauri. The estimated total travel time is 58 years with 2/3 of that spent in decelerating over a distance of 0.37 light years.

      There may be issues with the optimization approach of section 3.2 by reducing coil current below that supported by a superconductor that could change the above results, which require a much smaller sail radius at comparable mass. For example, see the 2015 paper by Freeland, https://bis-space.com/shop/product/mathematics-of-magsails/ that reported use of a 260 km sail radius at 1232 tonnes taking 19 years to decelerate on approach to Alpha Centauri.

      These are relatively detailed papers. I have been updating the Wikipedia magnetic sail page, https://en.wikipedia.org/wiki/Magnetic_sail, which contains a summary of highlights from these papers.

      • Adam Crowl August 2, 2022, 20:15

        Hi Dave McDysan
        The difference found by Gros vs the Mag-Sail behaviour in Zubrin-Andrews is due to the different assumed behaviours of the plasma environment – in one scenario the medium is assumed to be non-interacting particles, in the other they do interact with each other to form a magnetosphere. Andrews discusses the difference in his recent book “Chasing the Dream” – there’s a certain size range where the magnetosphere will form, as I’m sure even Gros would agree.

        • Dave McDysan August 3, 2022, 15:55

          Hi Adam Crowl,

          I did not find a difference except at very high velocities after comparing the results as detailed below.

          In the last bullet of section 3 Gros compares his numerical result using a particle model to that from the 1989 Andrews/Zubrin paper covering magsail and interplanetary travel that also used a particle model and notes a discrepancy. Gros did not use the same parameters as Andrews/Zubrin and in 2015 Freeland published results indicating that their force results were optimistic by a factor of 3.1. Making these corrections, the Gros and Zubrin results align quite closely, as illustrated in the figure in https://en.wikipedia.org/wiki/Magnetic_sail#Magsail_kinematic_model_(MKM)

          Andrews/Zubrin in 1989 reported that the particle and fluid model yielded similar results for the parameters they evaluated.

          The Gros model predicts poorer performance only when the ion gyroradius at magnetopause field strength becomes significantly larger than the magnetopause radius as seen in the figure, which occurs when spacecraft velocity exceeds approximately 10% of light speed. In Appendix B, Gros does question formation of a bow shock when the ion gyroradius is near the magnetopause radius. Other researchers have reported a similar result. (Note that Gros evaluates the ion gyroradius near the coil, which is smaller due to the larger magnetic field for a different purpose.)

          I looked at the description of the Dana Andrews book on Amazon and the 5-star review by Greason and put it on my reading list. Thanks!

          • Adam Crowl August 5, 2022, 19:22

            Hi Dave McDysan

            I must thank you for an excellent Wikipedia page. Well done!

            I discussed Gros’ paper with the author when it first appeared on the arXiv, as I was using his model for my study of near-Sun launching of mag-sails for high-speed missions to Planet 9. It’s a very helpful work for mag-sail studies, especially in the high dynamic pressure regime – for example, Jim Benford’s ultra-high acceleration particle beam pushed mag-sail.

            • Dave McDysan August 6, 2022, 15:46

              Hi Adam Crowl,

              Thank you for the comment on the Wikipedia page – it is still a work in progress.

              I had downloaded your paper from 2017 but had not yet reviewed it. I gave it a first read this morning:

              https://www.researchgate.net/publication/321005582_High-Speed_Magnetic-Sail_Interstellar_Precursor_Missions

              If I have any questions is the Email address at the top of the above citation still current for you?

              It appears to address several concerns I have with the Gros 2017 paper, in particular section 3.2 equation (13) and the mass calculations of section 4.2 where the equations are inconsistent with the example numerical values, with the coil radius example differing by an order of magnitude. Some of the reasoning of his paper was difficult to follow, but your description is much clearer. A nicely written paper!

              Interesting mission profile that combines acceleration away from the Sun with deceleration in the Local ISM on approach to Planet 9.

              I plan to reorganize the Magsail (MS) section on the Wikipedia article and move the summary of Freeland 2015 results and a summary of the design and mission profile a citation of your 2017 paper into a new subsection at the end of the section named “Specific designs and mission profiles.”

              Gros’s equations would be retained, and I had already removed the substitution of his Equation (13) optimization into the more general preceding equations entered by another contributor years ago. Gros’s equations prior to (13) appear consistent. The description of optimization resulting in your equation (16) is more intuitive and removes the dependency on relative velocity of his Equation (17). I would include a summary with a few of your equations at the end of the “Magsail kinematic model” section citing your paper as well.

              Thank you for resolving the issues I had with the Gros 2017 paper.

              • Adam Crowl August 9, 2022, 1:26

                Hi Dave McDysan,

                Email address from the paper is fine. Alternatively there’s my Gmail address, which is on my blog here in obfuscated form.

                Adam

                • Dave McDysan August 13, 2022, 14:41

                  Hi Adam Crowl,

                  Great! We’ll take this discussion offline then, unless someone wants to have it here.

                  Dave

        • Dave McDysan August 6, 2022, 20:29

          Wasn’t easy to find a place to purchase “Chasing the Dream” by Dana Andrews. I found this site that sells it:

          https://www.retiredrocketdoc.com/product-page/chasing-the-dream-book

    • Brett Bellmore August 3, 2022, 6:12

      I’m not sure how you “spiral in” in space. It’s not like atmospheric travel, where you can deploy airfoils to produce forces perpendicular to your motion; In space, “lift” is a very limited thing indeed.

  • James M. Essig August 2, 2022, 17:42

    The stellar bow shocks might also be good for spacecraft acceleration to highly relativistic velocities.

    Due to the higher density of hydrogen and helium in the bow shocks in regions where the solar wind or background primordial gas gets compressed, a number of interstellar ramjet configurations might work.

    For example, a CNO-bicycle fusion system may work.

    Alternatively, aneutronic fusion cycles that produce a large charged fractional component of fusion products could perhaps work. The charged species would be intaked into a deceleration chamber where the kinetic energy of the charged particles would be bled off by reverse particle cyclotrons and the synchrotron radiation collected and recycled back into the propulsion system. The charged particles where exothermically fusionable could then be fused and the charged fusion products exhausted as a reaction mass further accelerated by the recycled energy.

    A system of multiple layers of heat exchangers, photovoltaic and photoelectric cells, thermo-electric cells, and multicycle turbo-electric systems could convert heat sourced from drag energy back into electrical energy. The system may include evacuated areas with appropriately designed and located reflective and highly emissive coatings to capture the waste heat.

  • Michael August 3, 2022, 3:41

    Perhaps the E-sail would do well here.

    https://www.electric-sailing.fi/

    • Michael August 3, 2022, 16:27

      Could anyone ask Pekka Janhunen as i dont have his details, if it could do the trick of slowing down. Multiple of these probes could shotgun a star system for better coverage.

  • Robin Datta August 3, 2022, 5:37
    • Michael August 3, 2022, 16:09

      Great the probe will move through an extended X-ray machine !

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