We don’t talk about the interstellar medium as much as we ought to. After all, if a central goal of our spacefaring is to probe ever further into the cosmos, we’re going to need to understand a lot more about what we’re flying through. The heliosphere is being mapped and we’ve penetrated it with still functioning spacecraft, but out there in what we can call the local interstellar medium (LISM) and beyond, the nature of the journey changes. Get a spacecraft up to a few percent of the speed of light and we have to think about encounters with dust and gas that affect mission design.

Early thinking on this was of the sort employed by the British Interplanetary Society’s Project Daedalus team, working their way through the design of a massive two-staged craft that was intended to reach Barnard’s Star. Daedalus was designed to move at 12 percent of lightspeed, carrying a 32 meter beryllium shield for its cruise phase. Designer Alan Bond opined that the craft could also deploy a cloud of dust from the main vehicle to vaporize enroute particles before they could reach the shield.

Other concepts for dust mitigation have included using beamed energy to deflect larger objects, or in the case of micro-designs like Breakthrough Starshot, simply turning the craft to present as little surface area along the path of flight as possible, acknowledging that there is going to be an attrition rate in any swarm of outgoing probes. Send enough probes, in other words, and a workable percentage of them should get through.

Image: A dark cloud of cosmic dust snakes across this spectacular wide field image, illuminated by the brilliant light of new stars. This dense cloud is a star-forming region called Lupus 3, where dazzlingly hot stars are born from collapsing masses of gas and dust. This image was created from images taken using the VLT Survey Telescope and the MPG/ESO 2.2-metre telescope and is the most detailed image taken so far of this region. Credit: ESO/R. Colombari.

However we manage it, dust mitigation appears essential. Dana Andrews once calculated that for a starship moving at 0.3 c, a tenth of a micron grain of typical carbonaceous dust would have a relative kinetic energy of 37,500,000 GeV.. So understanding what’s around us, and in particular the gas and dust environment beyond but near the Solar System, is essential for future probes. For those wanting to brush up, the best overview on the interstellar medium remains Bruce Draine’s Physics of the Interstellar and Intergalactic Medium (Princeton, 2010).

Of course, most of the interstellar medium is made up of gas rather than dust, at a ratio of something like 99 to 1. The dust effect can be catastrophic, but the presence of this hydrogen and helium has been invoked by people like Robert Bussard because it could theoretically provide fuel to an interstellar ramscoop craft. Current thinking leans away from the idea, but it’s interesting to learn that we have sources of hydrogen far larger than we realized and also much closer to the Sun than, say, the Orion Nebula.

Thus my interest in a new paper out of Rutgers University. An international team working with astrophysicist Blakesley Burkhart has identified a hitherto unobserved cloud that could one day spawn new stars. Composed primarily of molecular hydrogen and dust, along with carbon monoxide, the cloud emerged in studies at far-ultraviolet wavelengths. The discovery is unusual because it is difficult to observe molecular hydrogen directly. Most observations of such clouds have to employ radio and infrared data, where the signature of CO is a clear tracer of the mass of molecular hydrogen.

Image: Blakesley Burkhart, a Rutgers University astrophysicist, has led a team that discovered the molecular hydrogen gas cloud, Eos. Credit: Rutgers University.

The data for this work were collected by FIMS-SPEAR (fluorescent imaging spectrograph), a far-ultraviolet instrument aboard the Korean satellite STSAT-1. Named Eos, the new cloud turns out to be one of the largest single structures in the sky. It is located some 300 light years away at the edge of the Local Bubble, and masses about 3400 times the mass of the Sun. The Solar System itself resides within the Local Bubble, which is several hundred parsecs in diameter and was probably formed by supernovae that created a hot interior cavity now surrounded by gas and dust. The nearby star-forming regions closest to the Sun are found along the surface of this bubble.

If we could see it with the naked eye, Eos would measure about forty moons across the sky. The likely reason it has not been detected before is that it is unusually deficient in carbon monoxide and thus hard to find with more conventional methods. Says Burkhart:

“This is the first-ever molecular cloud discovered by looking for far ultraviolet emission of molecular hydrogen directly. The data showed glowing hydrogen molecules detected via fluorescence in the far ultraviolet. This cloud is literally glowing in the dark.”

And getting back to dust, this work usefully shows us how we’re beginning to map relatively nearby space to locate concentrated areas of such matter. From the paper:

Using Dustribution [a 3D dust mapping algorithm], we compute a 3D dust map of the solar neighbourhood out to a distance of 350 pc. Figure 3 [below] shows the reconstructed map, focusing on the region of the Eos cloud as a function of distance. We see a single, distinct cloud that corresponds to the H2 emission stretching from 94 to 130 pc; no other clouds are visible along the same lines of sight in Fig. 3. This establishes the cloud as among the closest to our Solar System, on the near side of the Sco–Cen OB association [Scorpius–Centaurus Association, young stars including the Southern Cross formed from the same molecular cloud]…

Image: This is part of Figure 3 from the paper. Caption: 3D dust density structure of the Eos cloud. Two-dimensional slices of the cloud at different distances. The colour bar indicates both the total dust extinction density and the total mass density, assuming dust properties from ref. 29 and a gas-to-dust mass ratio of 124… [T]he magenta contour shows the region with high H2/FUV ratio. Credit: Burkhart et al.

And I quite like this video the scientists have put together illustrating the cloud and its relationship to the Solar System.

Image: Scientists have discovered a potentially star-forming cloud and called it “Eos.” It is one of the largest single structures in the sky and among the closest to the Sun and Earth ever to be detected. Credit: Thomas Müller (HdA/MPIA) and Thavisha Dharmawardena (NYU).

The paper is Burkhart et al., “A nearby dark molecular cloud in the Local Bubble revealed via H2 fluorescence,” Nature Astronomy 28 April 2025). Full text.