*Written by OC Wanderer, author of Destiny Among the Stars, a LitRPG sci-fi series set in the real stellar neighborhood. Published October 14, 2026.*
[Sol is the system](https://ocwanderer.com/blog/how-resourceful-is-sol-compared-to-a-typical-scorpius-centaurus-star) whose resources we can vouch for, since its supply is measured body by body. A Scorpius-Centaurus star could beat it only for a settlement that makes what it needs on site for less than shipping it, and a survey ship has to arrive before anyone can test that. I imagine a navigator facing Scorpius-Centaurus and asking which direction has the least in the way.
One direction on the old maps has a wonderfully suggestive name: the Lupus Tunnel. I can picture a navigator tracing it toward Scorpius-Centaurus, hoping the hull will meet less material along the way. But what does a tunnel on an astronomical map actually promise a ship? I think it gives us a direction worth investigating. Establishing a usable corridor would mean finding out what fills the passage, how far it runs and what waits at its exit.
## Reading a passage in starlight
Starlight can show us where a passage might run. Neutral sodium absorbs a particular yellow color; measuring that dip in many stars' spectra lets astronomers reconstruct where neutral gas lies between us and the stars. A 2003 map built from about a thousand stars showed the Local Bubble, the cavity around the Sun, linked to neighboring cavities by narrow tunnels.<sup class="footnote-ref" data-footnote="true" id="fnref-1"><a class="footnote-link" href="#fn-1">1</a></sup> Along those directions, the light had crossed little neutral gas. It is an intriguing way to scout a passage without ever sending a probe through it.
The Lupus name comes from this tradition of sodium-absorption mapping. A 2024 X-ray study traces the discovery of the Lupus tunnels to those earlier maps.<sup class="footnote-ref" data-footnote="true" id="fnref-2"><a class="footnote-link" href="#fn-2">2</a></sup>
There is also a tantalizing hole in the infrared glow of the Lupus dark clouds. A study of 205 stars' colors confirmed a region depleted of interstellar material, and the hole shows up in ROSAT's soft X-ray view too. Its surrounding material lies about 195 to 326 light-years away.<sup class="footnote-ref" data-footnote="true" id="fnref-3"><a class="footnote-link" href="#fn-3">3</a></sup> That is a real feature to investigate in the Lupus direction. Matching it to a continuous navigable passage through the older gas maps would take more than sharing a name on the sky.
## Where would the passage come out?
Newer three-dimensional dust maps let us see individual clouds around the Bubble.<sup class="footnote-ref" data-footnote="true" id="fnref-4"><a class="footnote-link" href="#fn-4">4</a></sup> The dusty wall [we met when the Bubble came up](https://ocwanderer.com/blog/could-the-local-bubble-be-unusually-good-or-unusually-bad-for-habitability) has a wonderfully irregular shape: its surface averages 554 light-years from the Sun, reaching as near as about 230 light-years in some directions and beyond 1,950 in others. A northern extension looks like a chimney, and several tunnels may connect to adjacent cavities.<sup class="footnote-ref" data-footnote="true" id="fnref-5"><a class="footnote-link" href="#fn-5">5</a></sup>
For an imagined navigator, that shape makes the exit as interesting as the entrance. Turn the ship and the nearest boundary could be hundreds of light-years closer. The dust study does not single out a confirmed Lupus passage among its candidate openings, though, so it cannot yet give that navigator an exit point. And it traces a dusty surface, whereas the older sodium map traces neutral gas; laying the two together means comparing different views of the surrounding material.
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## A pale dust map can still hide hot gas
Suppose the dust map shows a promising gap. I would want to bring the gas maps up beside it before imagining a crew setting course. Dust dims starlight, neutral gas reveals itself through sodium absorption, and hot plasma at around a million degrees glows in X-rays. Each observation adds a different part of the route. The hot gas filling the Local Bubble is extraordinarily thin, at about 0.0065 hydrogen nuclei per cubic centimeter.<sup class="footnote-ref" data-footnote="true" id="fnref-6"><a class="footnote-link" href="#fn-6">6</a></sup>
The X-ray view makes that extra layer matter. eROSITA found two tunnels with little dust along them, toward Canis Major and Centaurus, that appear filled with hot plasma. Where the dust thins, the X-ray glow brightens, suggesting a longer stretch of hot gas along the line of sight. Lupus lies along the Galactic plane, where this analysis has trouble separating its emission from the rest, so it leaves the tunnels' hot-gas content unmeasured.<sup class="footnote-ref" data-footnote="true" id="fnref-7"><a class="footnote-link" href="#fn-7">7</a></sup> A pale strip on the dust map could still lead through plasma. How much of it would the ship actually cross?
In Book 4, a symposium asks Danny whether the *Triumph* passed through stellar regions Dr. Caldwell flagged. He answers from the route itself: "Our route took us through the Oort Cloud Passage," and "We didn't transit any of the candidate stellar regions Dr. Caldwell indicated." His reply leaves me picturing two different sets of marks on the same chart: the route a ship actually flew and the places someone hopes it investigated. That distinction matters just as much when a promising tunnel becomes a proposed expedition.
## A thin passage is only part of the journey
For a fast ship, every stretch of the journey contributes to wear. A model of a small craft heading toward Alpha Centauri at one-fifth of light speed found gas damage about 0.1 millimeter deep in a quartz shield, the kind of numbers [we worked through for the grain of sand](https://ocwanderer.com/blog/what-would-hitting-a-grain-of-sand-do-to-a-ship-traveling-at-20-percent-light-speed).<sup class="footnote-ref" data-footnote="true" id="fnref-8"><a class="footnote-link" href="#fn-8">8</a></sup> In that model, damage depth grows with the number of atoms swept up per square centimeter: gas density multiplied by distance flown. A map's beautifully clear patch matters because it could reduce that accumulated exposure.
The Bubble's hot gas contains roughly forty times fewer hydrogen nuclei per volume than the cloud around the Sun. That makes it tempting to imagine a long cruise through the thinner material. Now suppose the passage ends against something as dense as a molecular cloud, at least 100 hydrogen nuclei per cubic centimeter. Just one light-year through that material would put as much hydrogen in the ship's path as about 15,000 light-years through the hot gas.<sup class="footnote-ref" data-footnote="true" id="fnref-9"><a class="footnote-link" href="#fn-9">9</a></sup> This is a hypothetical density comparison, not a measurement of the Lupus exit. It shows why I would want the survey to keep going when the apparently easy stretch ends.
## So is the Lupus Tunnel a viable corridor?
I think the Lupus direction is worth surveying, with a corridor still waiting to be established. A ship would need a full-route inventory of dust, neutral gas and hot plasma, including whatever boundary it eventually crosses. Low amounts along the whole route could make the passage useful; a dense exit could change the calculation. I imagine a survey ship logging those layers together, and I wonder how far the thin stretch runs before the wall. If a lane like that were confirmed, a ship inside it might be flying through almost nothing. What would that cost it as well as save it? [What happens to a ship traveling through a region of near-zero interstellar medium density?](https://ocwanderer.com/blog/what-happens-to-a-ship-traveling-through-a-region-of-near-zero-interstellar-medium-density)
> [!lore] If you want to follow a crew that has to choose its route through unmapped space, [start *Destiny Among the Stars*](https://ocwanderer.com/storytime/story/destiny-among-the-stars).
## Endnotes
1. <span id="fn-1"></span> R. Lallement, B. Y. Welsh, J. L. Vergely, F. Crifo and D. Sfeir, "3D Mapping of the Dense Interstellar Gas around the Local Bubble," *Astronomy & Astrophysics* 411 (2003): 447–464, [DOI](https://doi.org/10.1051/0004-6361:20031214). Neutral sodium D-line absorption toward about 1,005 sight lines with Hipparcos distances, out to about 350 parsecs (1,140 light-years); a Local Bubble about 150 parsecs (490 light-years) in diameter linked to surrounding cavities by narrow tunnels. Abstract: [ADS record](https://ui.adsabs.harvard.edu/abs/2003A&A...411..447L/abstract). <a class="footnote-backref" href="#fnref-1" aria-label="Back to reference 1">↩</a>
2. <span id="fn-2"></span> M. 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, [arXiv:2410.23345](https://arxiv.org/abs/2410.23345), Sect. 4.6: the "Lupus tunnels first discovered from Na I absorption lines (Welsh et al. 1994; Lallement et al. 2003)." <a class="footnote-backref" href="#fnref-2" aria-label="Back to reference 2">↩</a>
3. <span id="fn-3"></span> G. A. P. Franco, "On the Infrared Void in the Lupus Dark Clouds," *Monthly Notices of the Royal Astronomical Society* 331 (2002): 474–482, [DOI](https://doi.org/10.1046/j.1365-8711.2002.05228.x). Strömgren photometry of 205 stars; the void also appears in the ROSAT soft X-ray background; surrounding material at 60 to 100 parsecs (195 to 326 light-years). <a class="footnote-backref" href="#fnref-3" aria-label="Back to reference 3">↩</a>
4. <span id="fn-4"></span> G. Edenhofer et al., "A Parsec-Scale Galactic 3D Dust Map Out to 1.25 kpc from the Sun," *Astronomy & Astrophysics* 685 (2024): A82, [arXiv:2308.01295](https://arxiv.org/abs/2308.01295). Gaia BP/RP-based estimates for 54 million stars; 516 distance bins from 69 to 1,250 parsecs (225 to 4,077 light-years). <a class="footnote-backref" href="#fnref-4" aria-label="Back to reference 4">↩</a>
5. <span id="fn-5"></span> T. J. O'Neill et al., "The Local Bubble is a Local Chimney: A New Model from 3D Dust Mapping," [arXiv:2403.04961](https://arxiv.org/abs/2403.04961) (2024): peak extinction surface at an average distance of 170 parsecs (554 light-years), spanning 70 to more than 600 parsecs (228 to more than 1,957 light-years); "several tunnels to other adjacent cavities in the interstellar medium may be present." <a class="footnote-backref" href="#fnref-5" aria-label="Back to reference 5">↩</a>
6. <span id="fn-6"></span> K. 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: hot ionized gas at about 10⁶ K and a hydrogen density of about 0.0065 cm⁻³ (Local Bubble); molecular gas at 10²–10⁶ cm⁻³. J. D. Slavin and P. C. Frisch, *Astronomy & Astrophysics* 491 (2008): 53–68, [arXiv:0704.0657](https://arxiv.org/abs/0704.0657): total hydrogen 0.23–0.27 cm⁻³ in the Local Interstellar Cloud. Ratio: 0.25 ÷ 0.0065 = 38. <a class="footnote-backref" href="#fnref-6" aria-label="Back to reference 6">↩</a>
7. <span id="fn-7"></span> Yeung et al. (2024), [arXiv:2410.23345](https://arxiv.org/abs/2410.23345), Sects. 3.4, 4.6 and the conclusions: two low-column tunnels that appear filled by hot plasma, toward β Canis Majoris and toward (l, b) ≈ (315°, 25°) in Centaurus; the local hot bubble's emission measure anti-correlates with dust; the Centaurus feature "might be another channel connecting to the Loop I superbubble"; the Lupus tunnels "are located on the Galactic plane and, thus, are difficult to identify in our spectral analysis." <a class="footnote-backref" href="#fnref-7" aria-label="Back to reference 7">↩</a>
8. <span id="fn-8"></span> T. Hoang et al., "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): about 0.1 mm of damage in quartz after a gas column of about 2 × 10¹⁸ cm⁻², and about 0.5 mm of dust erosion after a column of about 3 × 10¹⁷ cm⁻², at 0.2c. Damage depth was read from the full text as proportional to the column swept. <a class="footnote-backref" href="#fnref-8" aria-label="Back to reference 8">↩</a>
9. <span id="fn-9"></span> Author calculation, column = density × path length, using the values in note 6. 0.0065 hydrogen nuclei/cm³ × 100 light-years = 0.65 (hydrogen nuclei/cm³)·ly, equal to 0.25 hydrogen nuclei/cm³ over 2.6 light-years (2.4 to 2.8 across the 0.23–0.27 range). A molecular cloud at 100 hydrogen nuclei/cm³ × 1 light-year = 100 (hydrogen nuclei/cm³)·ly, equal to 0.0065 hydrogen nuclei/cm³ over about 15,400 light-years. Densities count hydrogen nuclei for this illustrative comparison. The molecular-cloud density is the low end of Ferrière's range, so the comparison is conservative; the density of the Bubble's wall itself is not measured here. <a class="footnote-backref" href="#fnref-9" aria-label="Back to reference 9">↩</a>