*Written by OC Wanderer, author of Destiny Among the Stars, a LitRPG sci-fi series set in the real stellar neighborhood. Published October 16, 2026.*
Our map of [the Lupus Tunnel](https://ocwanderer.com/blog/what-is-the-lupus-tunnel-and-is-it-a-viable-corridor-for-interstellar-travel) shows a thin patch in the gas that nobody has yet measured for dust or hot plasma. Newer maps suggest several possible openings in the Local Bubble. Imagine sending a scout into one and getting a promising report back: very little gas, little dust, manageable radiation. Would that emptiness make it the route to choose?
I'd be tempted, if my ship carried its own fuel and could stop with its own engine. The interesting catch is that the same gas that batters a fast ship could also feed a scoop or give a magnetic sail something to brake against. Choosing the emptiest lane changes what we have to bring aboard.
## Forty times less gas than our own cloud
Space is already staggeringly thin where we live. A cubic centimeter of room air holds about 25 billion billion molecules.<sup class="footnote-ref" data-footnote="true" id="fnref-1"><a class="footnote-link" href="#fn-1">1</a></sup> The warm cloud around the Sun holds about a quarter of one hydrogen atom in that same volume: models based on starlight and measurements inside the solar system give 0.23 to 0.27.<sup class="footnote-ref" data-footnote="true" id="fnref-2"><a class="footnote-link" href="#fn-2">2</a></sup> The hot gas filling much of the Local Bubble is thinner still, roughly 0.005 to 0.007 hydrogen atoms per cubic centimeter, about forty times fewer.<sup class="footnote-ref" data-footnote="true" id="fnref-3"><a class="footnote-link" href="#fn-3">3</a></sup> Imagine choosing between those two environments after leaving Earth's atmosphere. Both seem empty; to a ship crossing light-years, the difference adds up.
A navigator needs to count everything the bow will meet along the whole route. Multiply density by distance and you get the column: atoms per square centimeter of the ship's front. Over a 4.4 light-year hop, the warm-cloud example gives about 10¹⁸ atoms per square centimeter; the hot-gas example gives about 3 × 10¹⁶.<sup class="footnote-ref" data-footnote="true" id="fnref-4"><a class="footnote-link" href="#fn-4">4</a></sup> That makes a detour an intriguing possibility. A longer path through thinner gas can expose the ship to fewer atoms than the direct route through a denser cloud. The shortest line on the chart may ask more of the hull.
## What those atoms do to a fast ship
At twenty percent of light speed, a hydrogen atom hits with about 19 million electron volts, several million times the few electron volts holding a chemical bond together.<sup class="footnote-ref" data-footnote="true" id="fnref-5"><a class="footnote-link" href="#fn-5">5</a></sup> The atom is tiny, but the speed turns an ordinary constituent of space into a shielding problem. In the warm cloud, the bow meets about forty times more atoms each second than it would in the hot gas.
A model of a Breakthrough Starshot craft at that speed found narrow damage tracks from heavy atoms such as oxygen and iron, reaching about 0.1 millimeter into quartz after a gas column of 2 × 10¹⁸ atoms per square centimeter.<sup class="footnote-ref" data-footnote="true" id="fnref-6"><a class="footnote-link" href="#fn-6">6</a></sup> In our two density examples, accumulating that column takes about 8.5 light-years in the warm cloud and about 325 in the hot gas. I like what that does to the route map: a choice of environment can buy the same shielding much more travel before it reaches the modeled damage level.
The measured gas column toward Alpha Centauri, about 6 × 10¹⁷ atoms per square centimeter, is below that benchmark.<sup class="footnote-ref" data-footnote="true" id="fnref-7"><a class="footnote-link" href="#fn-7">7</a></sup> For longer journeys, choosing thin gas becomes more valuable. A designer could consider a thinner sacrificial layer on the bow, or keep the layer and gain more range. Dust still has a separate vote in that decision.
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## What if the ship needs the gas?
The Bussard ramjet is a wonderfully tempting idea: scoop up interstellar hydrogen with a magnetic field and use it as fuel. You can see why empty space changes the bargain. The amount passing through the scoop is density times area times speed. At 0.2c, a one-square-kilometer scoop encounters about 0.8 kilograms of hydrogen a year in the warm cloud, and about 21 grams in the hot gas, before collection losses.<sup class="footnote-ref" data-footnote="true" id="fnref-8"><a class="footnote-link" href="#fn-8">8</a></sup> Twenty-one grams after a year of racing through space! The map that promises gentler wear on the bow offers very little to fill the tanks.
The scoop's scale makes the problem vivid. A simulation of the Fishback magnetic-scoop design, seeking 10 million newtons of thrust in gas at about one atom per cubic centimeter, required a funnel about 4,000 kilometers across and 150 million kilometers long, roughly the Earth–Sun distance.<sup class="footnote-ref" data-footnote="true" id="fnref-9"><a class="footnote-link" href="#fn-9">9</a></sup> Our warm cloud is four times thinner than that assumed gas, and the hot gas about 150 times thinner. Even that enormous funnel belongs to the more generous fuel environment; the thin lane makes the collection problem harder.
Then imagine approaching your destination with the tanks nearly dry, hoping a magnetic sail will slow you against the ionized gas. A study of dilute interstellar gas found that a sample fast transit to Proxima Centauri would demand a sail system of about a thousand tons.<sup class="footnote-ref" data-footnote="true" id="fnref-10"><a class="footnote-link" href="#fn-10">10</a></sup> Magnetic braking remains a possibility, but its mass becomes a serious part of the mission. Carrying propellant for a rocket brake is another option. Suddenly the [Kuiper Belt depot](https://ocwanderer.com/blog/is-the-kuiper-belt-a-dead-end-or-a-refueling-stop-for-deep-space-missions) we explored has a job to do: supply the ship before it commits to a lane with little to collect along the way.
## What the gas map leaves for the scouts
Dust can change the shielding calculation dramatically. The Starshot model estimated about 0.5 millimeter of surface erosion after a gas column of 3 × 10¹⁷ atoms per square centimeter, assuming the usual dust-to-gas ratio.<sup class="footnote-ref" data-footnote="true" id="fnref-11"><a class="footnote-link" href="#fn-11">11</a></sup> That assumption is exactly what I'd want a scout to test along a proposed route.
An eROSITA X-ray survey found brighter Local Bubble emission where the dust column was lower, and traced dust cavities filled with hot plasma.<sup class="footnote-ref" data-footnote="true" id="fnref-12"><a class="footnote-link" href="#fn-12">12</a></sup> Those cavities are promising places to investigate. Their appearance on the map gives us a lead, while the actual route's dust abundance and grain sizes still need measuring. A ship meets grains individually, and a rare large one can do disproportionate damage, the concern explored in [the Oort Cloud article](https://ocwanderer.com/blog/can-a-spaceship-safely-travel-through-the-oort-cloud) and [the grain of sand article](https://ocwanderer.com/blog/what-would-hitting-a-grain-of-sand-do-to-a-ship-traveling-at-20-percent-light-speed). I'd want the dust report beside the gas report before choosing a cruising speed.
Cosmic rays add another question: what radiation will the crew encounter inside that inviting cavity? Their intensity depends on particle sources and the magnetic fields steering them. A model of superbubbles, cavities excavated by clusters of massive stars, found that the highest-energy particles escaped quickly after each supernova, giving intermittent intensities inside the modeled bubble.<sup class="footnote-ref" data-footnote="true" id="fnref-13"><a class="footnote-link" href="#fn-13">13</a></sup> That is a reason to investigate radiation independently of gas density, rather than a forecast for our particular lane. [Our Local Bubble also has a supernova history](https://ocwanderer.com/blog/could-the-local-bubble-be-unusually-good-or-unusually-bad-for-habitability); measuring its present radiation environment belongs in the scout's assignment.
## So would I take the empty lane?
With reassuring dust and radiation reports, and a ship equipped to arrive under its own power, I'd be keen to explore it. The prospect of making the same shielding last much farther is compelling. A ramjet designer or a crew relying on magnetic braking would read that same map with a different question: where is the gas we need? Empty space makes the ship's design part of the navigation decision.
Zoom out from that route and the Local Bubble becomes a small cavity within the Milky Way. All those destinations orbit together, held by far more mass than their starlight reveals. [What is dark matter, and why are physicists so confident it exists without having seen it?](https://ocwanderer.com/blog/what-is-dark-matter-and-why-are-physicists-so-confident-it-exists-without-having-seen-it)
> [!lore] If you want to follow a crew that plots its way between the stars, [start *Destiny Among the Stars*](https://ocwanderer.com/storytime/story/destiny-among-the-stars).
## Endnotes
1. <span id="fn-1"></span> NIST CODATA, [Loschmidt constant](https://physics.nist.gov/cgi-bin/cuu/Value?n0=): 2.687 × 10²⁵ molecules per cubic meter at 273.15 K and 1 atmosphere, which is 2.687 × 10¹⁹ per cubic centimeter. Dividing by 0.25 atoms per cubic centimeter gives a ratio of about 10²⁰. <a class="footnote-backref" href="#fnref-1" aria-label="Back to reference 1">↩</a>
2. <span id="fn-2"></span> Jonathan D. Slavin and Priscilla C. Frisch, “The Boundary Conditions of the Heliosphere: Photoionization Models Constrained by Interstellar and In Situ Data,” *Astronomy & Astrophysics* 491 (2008): 53–68, [arXiv:0704.0657](https://arxiv.org/abs/0704.0657). Abstract: total hydrogen n(H) = 0.23–0.27 cm⁻³, neutral hydrogen 0.19–0.20 cm⁻³, electrons 0.07 ± 0.01 cm⁻³. <a class="footnote-backref" href="#fnref-2" aria-label="Back to reference 2">↩</a>
3. <span id="fn-3"></span> Katia M. Ferrière, “The Interstellar Environment of Our Galaxy,” *Reviews of Modern Physics* 73 (2001): 1031–1066, [arXiv:astro-ph/0106359](https://arxiv.org/abs/astro-ph/0106359), Table I and section on hot ionized gas: hydrogen density about 0.0065 cm⁻³ for the Local Bubble, from the Snowden et al. (1998) ROSAT model. W. Liu et al., “The Structure of the Local Hot Bubble,” *The Astrophysical Journal* 834 (2017): 33, [arXiv:1611.05133](https://arxiv.org/abs/1611.05133), adopt an electron density of 4.68 × 10⁻³ cm⁻³ from Snowden et al. (2014). The ratio of about forty is 0.25 ÷ 0.0065 = 38; using the electron density it is about 53. Electron density stands in for hydrogen here because the gas is ionized. <a class="footnote-backref" href="#fnref-3" aria-label="Back to reference 3">↩</a>
4. <span id="fn-4"></span> Author calculation: column = density × length, with 4.37 light-years = 4.13 × 10¹⁸ cm (Hoang et al. give Alpha Centauri at 1.34 parsecs). At 0.25 cm⁻³ (midpoint of the Slavin and Frisch range) the column is 1.0 × 10¹⁸ cm⁻²; at 0.0065 cm⁻³ it is 2.7 × 10¹⁶ cm⁻². <a class="footnote-backref" href="#fnref-4" aria-label="Back to reference 4">↩</a>
5. <span id="fn-5"></span> Author calculation. Atoms struck per second per square centimeter = density × speed, with 0.2c = 6.0 × 10⁹ cm/s: 0.25 × 6.0 × 10⁹ = 1.5 × 10⁹, and 0.0065 × 6.0 × 10⁹ = 3.9 × 10⁷. Kinetic energy of one hydrogen atom, (γ − 1)m_pc², with γ = 1.0206 at 0.2c, is 19.3 MeV. A carbon–carbon single bond is about 346 kJ/mol, or 3.6 eV ([LibreTexts, Bond Energies](https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Chemical_Bonding/Fundamentals_of_Chemical_Bonding/Bond_Energies)), so the ratio is about 5 million. <a class="footnote-backref" href="#fnref-5" aria-label="Back to reference 5">↩</a>
6. <span id="fn-6"></span> Thiem Hoang, A. Lazarian, Blakesley Burkhart and Abraham Loeb, “The Interaction of Relativistic Spacecrafts with the Interstellar Medium,” *The Astrophysical Journal* 837 (2017): 5, [DOI](https://doi.org/10.3847/1538-4357/aa5da6), [arXiv:1608.05284](https://arxiv.org/abs/1608.05284). Abstract: damage by track formation from heavy elements; about 0.1 mm in quartz “after traversing a gas column of N_H ~ 2 × 10¹⁸ cm⁻² along the path to α Centauri”. <a class="footnote-backref" href="#fnref-6" aria-label="Back to reference 6">↩</a>
7. <span id="fn-7"></span> Author calculation: 2 × 10¹⁸ ÷ 0.25 = 8.0 × 10¹⁸ cm = 8.5 light-years; 2 × 10¹⁸ ÷ 0.0065 = 3.1 × 10²⁰ cm = 325 light-years (1 light-year = 9.46 × 10¹⁷ cm). Hoang et al., section VI and figures 6 and 11, take the measured gas column toward Alpha Centauri from Linsky and Wood as log N_H = 17.80 ± 0.30, about 6 × 10¹⁷ cm⁻² (range 3 × 10¹⁷ to 1.3 × 10¹⁸), and give the average density along that line of sight as 0.07–0.24 cm⁻³. The 2 × 10¹⁸ in the abstract is therefore the benchmark for 0.1 mm of damage, and it is higher than the measured column, which would need an average density of 0.48 cm⁻³ over 4.37 light-years. <a class="footnote-backref" href="#fnref-7" aria-label="Back to reference 7">↩</a>
8. <span id="fn-8"></span> Author calculation, mass rate = density × hydrogen atom mass (1.67 × 10⁻²⁴ g) × area (10¹⁰ cm² for 1 km²) × 0.2c (6.0 × 10⁹ cm/s) × 3.156 × 10⁷ s per year: 0.79 kg per year at 0.25 cm⁻³ and 20.6 g per year at 0.0065 cm⁻³. This is the gross amount crossing the scoop, before any losses. The idea is from R. W. Bussard, “Galactic Matter and Interstellar Flight,” *Astronautica Acta* 6 (1960): 179–194. <a class="footnote-backref" href="#fnref-8" aria-label="Back to reference 8">↩</a>
9. <span id="fn-9"></span> Peter Schattschneider and Albert A. Jackson, “The Fishback Ramjet Revisited,” *Acta Astronautica* 191 (2022): 227–234, [DOI](https://doi.org/10.1016/j.actaastro.2021.10.039); abstract: computer simulations prove the scoop's functionality “albeit with absurdly long magnetic coils when reasonable thrust is to be achieved.” The 4,000 km, 150 million km and 10 million newton figures, and the density of about one atom per cubic centimeter, are from the TU Wien press release, [“Science Fiction Revisited: Ramjet Propulsion.”](https://www.tuwien.at/en/all-news/news/science-fiction-nachgerechnet-der-ramjet-antrieb) Ratios: 1 ÷ 0.25 = 4 and 1 ÷ 0.0065 = 154. <a class="footnote-backref" href="#fnref-9" aria-label="Back to reference 9">↩</a>
10. <span id="fn-10"></span> Claudius Gros, “Universal Scaling Relation for Magnetic Sails: Momentum Braking in the Limit of Dilute Interstellar Media,” *Journal of Physics Communications* 1 (2017): 045007, [arXiv:1707.02801](https://arxiv.org/abs/1707.02801). Abstract: densities “of the order of 0.1 particles per cm³, or lower”; for a high-speed transit to Proxima Centauri, “daunting mass requirements of the order of 10³ tons”. <a class="footnote-backref" href="#fnref-10" aria-label="Back to reference 10">↩</a>
11. <span id="fn-11"></span> Hoang et al. (2017), abstract, as in note 6: for v = 0.2c, dust erodes “a surface layer of ~0.5 mm thickness after the spacecraft has swept a column density of N_H ~ 3 × 10¹⁷ cm⁻², assuming the standard gas-to-dust ratio of the interstellar medium.” <a class="footnote-backref" href="#fnref-11" aria-label="Back to reference 11">↩</a>
12. <span id="fn-12"></span> Michael C. H. Yeung et al., “The SRG/eROSITA Diffuse Soft X-Ray Background. I. The Local Hot Bubble in the Western Galactic Hemisphere,” *Astronomy & Astrophysics* 690 (2024): A399, [DOI](https://doi.org/10.1051/0004-6361/202451045). Abstract: the Local Hot Bubble emission measure “shows clear anti-correlation with the local dust column density”; “tunnels of dust cavities filled with hot plasma, potentially forming a wider network of hot interstellar medium”; the three-dimensional model assumes constant density. <a class="footnote-backref" href="#fnref-12" aria-label="Back to reference 12">↩</a>
13. <span id="fn-13"></span> Thibault Vieu, Stefano Gabici, Vincent Tatischeff and Sruthiranjani Ravikularaman, “Cosmic Ray Production in Superbubbles,” *Monthly Notices of the Royal Astronomical Society* 512 (2022): 1275–1293, [DOI](https://doi.org/10.1093/mnras/stac543), [arXiv:2201.07488](https://arxiv.org/abs/2201.07488). Abstract: high-energy particles “escape quickly after each supernova explosion, which makes both their intensity inside the bubble and injection in the interstellar medium intermittent”. <a class="footnote-backref" href="#fnref-13" aria-label="Back to reference 13">↩</a>