Proxima Centauri b seems to be the closest exoplanet in its habitable zone. At a distance of $4.2$ lightyears, travel to this planet within a human lifetime is not impossible prima facie, however doing so requires a mean velocity that is an appreciable fraction of the speed of light, i.e. around $0.1 c \approx 3 \times 10^4 \text{km/s}$.

My questions concern the problem to travelling at this speed posed by the presence of an interstellar medium due to (1) the damage caused by collisions with microscopic particles and (2) the loss of momentum to the interstellar medium.

Firstly the problem of collisions. At this speed it is plausible that collisions with even microscopic particles pose a problem. A crude equation of the incident kinetic energy with energy of required to raise steel to its boiling point from absolute zero, suggests that a collision of a dust particle $1\text{mg}$ ($\approx$ a grain of sand) travelling at $0.1c$ would be enough to vaporize about $0.6\text{kg}$ of steel from the surface of a spacecraft, $$ \frac{(1\text{mg})\cdot(0.1 c)^2}{2 \cdot (420 \, \text{kJ} \cdot\text{kg}^{-1} \cdot\text{K}^{-1})\cdot (1643 \text{K})} = 0.65\ldots \text{kg} $$ and transfer about the same momentum as a bowling ball thrown from a moving car. Collisions with objects that are hard enough to start to penetrate steel before vaporising are potentially even more serious problems with the penetration depth of a hard hypervelocity projectile growing proportional to the incident kinetic energy (see e.g. Walker 2001).

Interstellar medium is of course very empty. However to be empty enough that such collisions could be neglected requires that dust particles large enough to do damage have a number density $n \ll 10^{-19} \text{m}^{-3}$, (roughly the inverse of the volume traced out by a small ($10\text{m}$ by $10\text{m}$) spacecraft travelling $4.2\text{ly}$. Is the density of potentially damaging particles in the interstellar medium low enough to permit interstellar travel at speeds $0.1c$?

The second problem is the problem of viscosity. Neglecting the potentially damaging nature of collisions with larger particles, the fact space is not empty implies that work must be done against it to maintain a constant velocity. Does presence of a viscous interstellar medium place any practical restrictions on the speed of interstellar travel?


2 Answers 2


The density of the interstellar medium varies hugely, so the specific problem of Sol-to-Proxima-Centauri travel is different from the general problem of interstellar travel.

According to WP:

In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of $10^6$ molecules per $cm^3$ (1 million molecules per $cm^3$). In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as $10^{−4}$ ions per $cm^3$.

The Local Interstellar Cloud, within which is our solar system, is somewhere in the middle of this enormous density range:

...not very dense, with 0.3 atoms per cubic centimetre. This is less dense than the average for the interstellar medium in the Milky Way (0.5 per $cm^3$, though six times denser than the gas in the hot, low-density Local Bubble (0.05 per $cm^3$) which surrounds the local cloud.

The Alpha/Proxima Centauri system is inside the neighboring G-Cloud; I didn't find any density estimates for the G-Cloud in my extensive five-minute survey, but David Hammen asserts in a comment on this answer that its density is similar to the LIC.

Wikipedia also tells us that:

By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium... By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements.

With the given mass breakdown, the overall mass density is straightforward to calculate; if I haven't screwed up it works out to something like $ {6 \times 10^{-25} g}$ per ${cm^3}$ in the Local Interstellar Cloud.

Without knowing more about the distribution of dust particle sizes, it's difficult to calculate the odds of a single catastrophic impact, but the mass density should give some idea of the continuous "wearing away" rate as well as the "drag" resistance of the ISM.

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    $\begingroup$ This is useful. Your density suggests a viscous drag at 0.1c is $\approx 10^{-6} N m^{-2}$, which is not to hard to correct for. Regarding the first question, do you know if mass distributions of interstellar particles are unknown, or simply that I/we have been unable to find them? $\endgroup$ Jul 26, 2021 at 1:40
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    $\begingroup$ I found a couple of papers that might be informative, but nothing easily digestible. Kim, Martin, Hendry 1993 ; Landgraf, Baggaley, Grün, Krüger, Linkert 2000 $\endgroup$ Jul 26, 2021 at 1:49
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    $\begingroup$ @ComptonScattering I doubt it will change the numbers much, but I don't think viscous drag is a good description for the interaction with the interstellar medium. At the low density/high-velocity we are talking about, it will behave more like molecule-sized bugs hitting your windshield, i.e. individual collisions with no relevant effect on the surrounding medium. $\endgroup$
    – mlk
    Jul 26, 2021 at 17:27
  • $\begingroup$ @mlk You are correct that the lack of local thermodynamic equilibrium means there is no resistance to shear deformation - maybe this is how you are saying the term "viscous drag" should be precisely used? In which case I agree my usage is incorrect. In the question though I have been looser, and (incorrectly?) used the term to refer to the rate of momentum transfer to the spacecraft that results from its relative velocity with the medium. $\endgroup$ Jul 26, 2021 at 18:29

UAPs got here somehow.

So, about a month ago, the United States government has issued a report on Unidentified Aerial Phenomena (UAPs), which is basically the modern US government term for UFOs, stating that they're real, they're not American, they do things beyond our current understanding of physics, and they don't know what they are. The actual wording of the report seems to be couched in deliberate understatement, but that's what the facts as presented by the report boil down to. Indeed, the information in the report was significant enough that Avi Loeb, the professor in charge of Astronomy at Harvard University, has begun a project to place meter-wide automated telescopes across the country in the hopes of capturing high-resolution images of them.

If the UAPs are extraterrestrial (rather than extradimensional, time travelers, a prehuman species living in habitats in unexplored areas on the bottom of the ocean, or something similar), then presumably they got to Earth somehow. This implies that interstellar dust isn't a problem for interstellar space flight, or at least whatever form of interstellar flight that they originally used to arrive here.

There's a lot of open questions about what UAPs are, how they work, and where they come from, but I suppose you could consider their existence evidence that interstellar travel may be possible.

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    $\begingroup$ "they're not American, they do things beyond our current understanding of physics"; the report does not say either of these things $\endgroup$
    – Dancrumb
    Jul 26, 2021 at 16:44
  • $\begingroup$ @Dancrumb Yes, it did. "In a limited number of incidents, UAP reportedly appeared to exhibit unusual flight characteristics." "In 18 incidents, described in 21 reports, observers reported unusual UAP movement patterns or flight characteristics." "We were unable to confirm, however, that these [American] systems accounted for any of the UAP reports we collected." $\endgroup$
    – nick012000
    Jul 26, 2021 at 16:51
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    $\begingroup$ Your answer says that the report "[states] that [the UAPs] are not American". In fact, it very clearly states "Some UAP observations could be attributable to developments and classified programs by U.S. entities. We were unable to confirm, however, that these systems accounted for any of the UAP reports we collected.". This is saying that they don't know if they are American... not that they know that they are not American. $\endgroup$
    – Dancrumb
    Jul 26, 2021 at 16:57
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    $\begingroup$ Also "unusual flight characteristics" is not remotely equivalent to "they do things beyond our current understanding of physics". Indeed, the word "Physics" appears exactly 0 times in the report. $\endgroup$
    – Dancrumb
    Jul 26, 2021 at 16:59
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    $\begingroup$ A report that has no solid conclusion, and that in fact mostly emphasizes the lack of data, is not a good basis for an answer to this question. The US government's inability to identify something is not evidence of extraterrestrial life! Please remind yourself what the "U" in "UAP" stands for. $\endgroup$
    – Harabeck
    Jul 26, 2021 at 17:07

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