# Would a spaceship have to evade interstellar clouds?

Apparently, there are clouds of "dust" between the stars. Would a starship have to fly around those clouds, trying to find "tunnels" between clouds, or are the interstellar clouds harmless for a spaceship?

I'm mainly thinking in terms of abrasion or (micro)collisions, not radiation, but would welcome information on the latter also.

• I don't think this can be answered. We don't know the speed of the ship, or the nature of its hull. – James K Aug 7 '16 at 22:37
• We would have to hypothesise about the nature of the futuristic technology involved, so this is off topic here – James K Aug 7 '16 at 22:39
• Maybe space exploration stack exchange, they might have some answers. In a general sense, yes, I think it would be in the interest of a high speed craft to avoid them. Even simple hydrogen becomes a problem with high enough speed travel. At the speeds we're currently able to reach, it's not been an issue. – userLTK Aug 7 '16 at 22:53
• No, you don't have to hypothesize about future technology. The focus of the question is on the nature of the interstellar clouds. If you assume that all materials can and will be abraded, it does not matter for this question if some materials are slightly more resistant to abrasion than others, especially at the speeds necessary for interstellar travel. But you are welcome to move this question to physics.SE, were I could relate it to the question given in Aaron Franke's anwer. – user2233 Aug 8 '16 at 10:19
• @what As it is right now, you are focusing too much on aspects of engineering for this to be on topic here. Tell me why I shouldn't migrate this to Space Exploration. It may be a question about interstellar clouds, but it is of what effect it would have on a spaceship, which requires a knowledge of engineering. – called2voyage Aug 8 '16 at 14:13

Yes.

As has been commented, the amount of damage taken by an interstellar spaceship depends on its velocity $v$, as well as the number of gas and dust particles that it encounters on its way. This number is usually measured per area, in which case it's called column density $N$, and is equal to the total distance $d$ traveled times the particle density $n$, i.e. $N = nd$. For instance, if a spaceship travels 1 lightyear ($10^{18}\,\mathrm{cm}$) through a region with a density of $10\,\mathrm{cm}^{-3}\!$, each square centimeter of the spaceship will encounter $10^{19}$ particles.

That is, the faster you go, the farther you go, and the more dense regions you go through, the more your spaceship is damaged.

The Breakthrough Starshot project aims to reach our nearest-neighbor stellar system $\alpha$ Centauri in ~20 years, with a gram-sized satellite reaching $0.20c$ by means of a light sail. Today, there was a paper by Hoang et al. calculating the amount of damage taken by such a satellite. The total column density of gas from Earth to $\alpha$ Cen is $\sim10^{17.5\mathrm{-}18}\mathrm{cm}^{-2}$, and assuming (fairly) a dust-to-gas ratio of 1% and a carbonaceous/silicate dust grain population with a Weingartner & Draine (2001) size distribution, they calculate that this journey to $\alpha$ Cen will erode the surface of the spacecraft to a thickness of the order of 1 mm.

Most of the damage is caused by dust, not gas, but in principle gas may slowly heat up the spaceship. However, at $v=0.2c$, as long as the density is $\lesssim10\,\mathrm{cm}^{-3}$, the temperature is insufficient to cause any melting.

Molecular clouds — the dense clouds where stars are born — have densities from $10^2\,\mathrm{cm}^{-3}$ and even up to $10^6\,\mathrm{cm}^{-3}$, i.e. many orders of magnitude higher than the roughly $1\,\mathrm{cm}^{-3}$ found in the more dilute interstellar medium. To reach even more distant stars in a tolerable time, you would have to go faster than $0.2c$, and thus it seems that it is in fact a good idea to evade these clouds.

• Thank you for that. Beautiful. So a spaceship would have to either (a) go slow, (b) evade denser areas, or (c) replace the forward parts of its hulls regularly. Depending on the travel destination, the travel strategy could encompass all three to differing degrees. – user2233 Aug 19 '16 at 10:07
• @what: Yes. I haven't done any calculations, but my guess is that speeds much higher than those 0.2c would be too destructive. Your idea with replacing parts is probably good wrt. the body of the ship, but the sail may be more difficult. – pela Aug 19 '16 at 14:00
• @what There's a potentially easier solution, which might be dubbed "deflector shields". Most interstellar particles are ionized, and so can be deflected by a magnetic field. I believe some experiments have been done on the idea and suggest a fairly weak (and likely reasonable to achieve in practice) field, generated by releasing ionized particles of its own, is sufficient to deflect most particles away. – zibadawa timmy Aug 20 '16 at 0:59
• @zibadawatimmy Wouldn't that have a breaking effect? Does that deflect fast enought at 0.2c? – user2233 Aug 20 '16 at 6:33
• @pela Yes, molecular clouds are mostly neutral diatomic hydrogen, so you'd still be in a lot of trouble with those. I was thinking mostly in an ionized medium, or otherwise mitigating the damage from the ionized particles in a stellar wind. I believe these experiments I'm recalling concerned things like missions to Mars. – zibadawa timmy Aug 21 '16 at 9:59

Let's compare nebulae to the air density where the ISS orbits, at 400 000 meters. According to Wikipedia, air pressure at a given altitude is given by the equation

$$p = p_0 \left(1 - \frac{Lh}{T_0} \right)^\frac{gM}{RL}$$

or $101.325\left(1 - \frac{0.0065×400000}{288.15}\right)^{(9.80665×0.0289644)/(8.31447×0.0065)}$.

Google Calculator doesn't like this ^ but putting it in piece-by-piece gives -5737666.10745. Then, let's find the density with the equation

$$ρ = \frac{pM}{RT}$$

or $\frac{-5737666.10745×0.0289644}{8.31447×2473.15}$ which is -8.08192432875. Unfortunately Wikipedia doesn't tell me what unit this number is in (just that it's a "molar form" and that it's density) so unfortunately I'm completely stuck here and I can't finish answering the question. Hopefully this partial answer helped someone make a full answer.

• voted up for effort. :-) – userLTK Aug 8 '16 at 5:57
• This is an (extended) comment, not an answer I'm afraid. – adrianmcmenamin Aug 9 '16 at 0:32
• @RobJeffries How so? Of course it does! It definitely makes a large difference whether your ship is heading into an area with almost nothing versus an area of relatively high atmospheric density. – Aaron Franke Aug 16 '16 at 22:06
• Perhaps my comment is a bit vague. I mean, where have you estimated or quoted the critical parameters - the ISM density and the size distribution of dust particles? What has the ISS travelling through Earth's upper atmosphere got to do with anything? – Rob Jeffries Aug 16 '16 at 22:43