Navigating storm fronts in space

Question

Storm fronts — especially surprise storm fronts — are a popular Sci-Fi trope. Ion storms and radiation storms and neutronic storms and they invariably look 2D, like ribbons in space. But then I read this article about nearby supernovas millions of years ago that showered our planet with radioactive debris and remembered that we've seen the increase in light as the photons from these massive explosions arrived at our planet.

And it got me wondering — how many such "wave fronts" might exist out there that we don't know about because the light hasn't reached us yet¹ and how might that affect space travel? So...

• Given that we're using technology only moderately better than today's space shuttles, save that we have engines that let us move the intragalactic mail.²

• Given that we do not have electromagnetic shielding (whatever solid material known to humanity today that rests between the crew and the vacuum of space is all we have to work with).

• Given that those wave fronts (aka, "storms") are spherical in nature.³

• Accepting the fact that some storms emanated from very distant explosions while others are much "younger" in nature.⁴

• And noting that a "navigational hazard" is anything from interfering with the ship's electronics to killing the crew.

Question: Do these natural storms in space pose, on average, enough risk to our intrepid crew that they represent viable navigational hazards deserving of being charted and monitored?

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_{¹ Or that have already passed us. Ships leaving our system would eventually catch up to the storm fronts that affected our planet millions of years ago — from the back side. Some fronts chase us, others we chase.}

_{² On a galactic scale, we've moved through space not at all at an inconsequential crawl. When we start moving vast distances at near-c/FTL speeds, we might discover storms like this are as common as rain. I'm certainly no expert on the Sci-Fi front (I read as much as I can, but I wouldn't dare claim to have read more than a fraction of what's out there), but I've never encountered this idea before in the literature. Has anyone?}

_{³ Though supernovas are obviously the most likely candidates for a true "storm in space" representing a trackable navigational hazard, the ejecta from a number of objects may be as big a deal in different ways. For example, a supernova creates a spherical shell of matter and radiation that increases forever but has a definable thickness, a pulsar has a less dramatic output that's a constant flow, a conical stream in 3D.}

_{⁴ The most dramatic storm I can think of would be the explosion of a galactic core black hole. I have no idea if this is even theoretically possible, but let's run with it. While such an explosion would be spherical in nature (and dangerous in its own right), it would also be strong enough to push the planets (both destroyed and intact) of the galaxy outward in an ever-expanding band (combined with their existing motion, the expansion would be reminiscent of pouring paint on a spinning board). That's as close to a 2D storm front as I can get, and the natural rate of expansion would probably make it no different than any other storm front the ship encountered. But it's cool to think about.}

Answer 1

The answer to your question depends strongly on the supernova rate in the galaxy. The Milky Way is current not undergoing an episode of significant star formation; therefore, it isn't producing an extreme amount of massive stars and, by extension, supernovae. Energetic events like gamma-ray bursts and kilonovae are not expected to occur at high rates in our galaxy, either (one GRB every 100,000 to 1,000,000 years and one kilonova every 10,000 to 100,000 years). Therefore, we care mainly about the rates of core collapse supernovae.

This turns out to be much easier said than done. Thanks to various observational difficulties, we have yet to tightly constrain the supernova rate. Astronomers are confident in the value to within a factor of a few; typical measurements indicate values of 3-5 supernovae per century (see e.g. Hakobyan et al. 2011), so I'll go with that value.

Let's look at how a supernova shock wave - the "storm front" we're concerned with - evolves over time. There are three phases to its life before it merges with the interstellar medium:

1. Free expansion. The shock has a roughly constant velocity (usually a few thousand kilometers per second) and therefore increases in radius linearly with time. This period lasts for about 500 years.
2. Blast wave phase/Sedov-Taylor phase. Now the shock undergoes adiabatic cooling, and begins to slow down. The velocity scales with time as $v(t)\propto t^{-3/5}$ and the radius scales as $r(t)\propto t^{2/5}$. The shock behaves like this for several tens of thousands of years.
3. Snowplow phase. Eventually, radiative losses become dominant and the Sedov-Taylor solution is no longer valid. A dense shell forms behind the shock, sweeping up matter in the interstellar medium as it expands. We have $v(t)\propto t^{-3/4}$ and $r(t)\propto t^{1/4}$, indicating that the shock is slowing down even quicker. We expect the snowplow phase to end after about one million years, when the speed of expansion drops to the speed of sound in the interstellar medium.

This is an approximation; there are a number of things that can cause some deviations from the model. For instance:

• Asymmetric ejecta would cause departure from a spherical remnant. This is important during the free expansion phase, where the swept-up matter is less massive than the ejecta from the explosion.
• Supernovae, especially those occurring in star formation regions, may exist inside superbubbles caused either by previous remnants or winds from massive stars. This, too, will have effects on the remnant's expansion.

Nevertheless, the normal expansion-blastwave-snowplow model is sufficient in the vast majority of cases. We're only looking for a simple approximation.

Our analysis indicates that the existing supernova remnants in the galaxy should be less than about $\sim10^5$ years old. If they form at a rate of 3 per century, there should be about 3,000 still around - most currently in the snowplow phase, the longest of the three periods. Given that the galactic disk is about 100,000 light-years across, this comes out to a surface density of $3.82\times10^{-7}$ per square light-year, and the nearest one should be, on average, about 1600 light-years away.

Keep in mind, though, that these remnants are big. By the end of the snowplow phase, they may have radii of 100-200 light-years. That said, at that point in time, the shock waves are vanishing into the interstellar medium, and really aren't much of a threat. Earlier on in that phase, though, the shocks can be dangerous, with temperatures and densities in the expanding snowplow shells of $T\sim10^6\text{ K}$ and $n\sim10\text{ cm}^{-3}$ in the worst-case scenario (Cioffi et al. 1988). Interestingly enough, these properties make the shock similar to the solar wind as experienced at Earth's orbital radius.

We should expect to see cosmic ray and x-ray emission from the hot gas, especially during the Sedov-Taylor phase (see Vink 2012, which is actually a really good resource on supernova remnants in general), and I do imagine that this could pose a separate threat to any spacecraft in the vicinity. Turns out that if you heat up gas to $\sim10^7\text{ K}$ ($kT_e\sim1\text{ keV}$, for reference) and send shock waves through it, you get strong x-ray emission! The emission should take several forms:

• Thermal x-rays caused by free-free and bound-free emission in the optically thing plasma
• Line emission, both from collisional excitation and radioactivity
• Non-thermal emission, including x-ray synchrotron radiation and non-thermal bremsstrahlung

Fortunately for us, some of this strong emission should only occur in young supernova remnants - not those in the snowplow phase. That said, essentially all remnants which have not cooled down to temperatures comparable to that of the interstellar medium are still going to be strong x-ray sources. I would recommend avoiding them.

Now, is the expanding shock wave enough to pose a danger to a traveling spacecraft? Perhaps; unlike Earth, a spacecraft has no magnetic field to shield it from the shock. However, I suspect that shocks in the snowplow phase are usually not hot or dense enough to produce cataclysmic effects. There are certainly some supernova remnants that are young and hot and could indeed be dangerous, and from this point of view, it does make sense to map out these potential dangers. That said, keep in mind that these remnants are, on average, over 1000 light-years apart, and it should be easy enough to avoid them.

Answer 2

Barring a 'physics surprise' of something we haven't yet observed, then 'space storms' are unlikely to be too great of a risk to interstellar vessels simply on the basis that anyone who can build such a vehicle is probably going to want to build them to handle the levels of radiation we have seen from such sources.

At this time I am not aware of any observed emission of a power level that could not be overcome with current or near future materials and tech, and our primary limiting factor lies more in infrastructure and energy tech. - That is, we could design a ship currently, but lack the means to gather the resources in a manner that actually makes any remotely economic sense.

Physics and engineering might make use of 'bolt holes' and limiting crew space volume during some periods, to help ensure proper shielding against elevated levels, but I'm not aware of any deep space energy waves having been detected that could 'throw a ship about'.

Keep in mind the effects of the Inverse Square Law - The further you are from something, the lower the energy that reaches you.

Which gives us two counter points to keep in mind:

1. Effects which generate focused energy, that may effectively remain stronger at longer ranges. [and cover narrow regions]
2. Effects that take place while you are close to the generation source. Solar flares in inter-planetary space are going to be far more noticeable, and potentially highly dangerous, while in a solar system, but be far less of an impact mid way between two independent stars.

There does stand the potential for a risk similar to 'rogue waves' in the ocean, where lots of frequencies from countless sources make a sudden energy peak in a highly localized area, but I'm not aware of any hard science that such a thing has ever been detected in reality beyond a mathematical model. - But that would be like a ship hitting a mine, and over and done with in a flash.

Answer 3

Here's my take on the issue of charting 'anomalies'...

1) They're plentiful
2) They move
3) They can be ephemeral
4) The highest risk is more from the things that have just happened, like supernovae

To put this in context, there are several papers available on the theory that one of the major causes of extinction events on the earth is related to its orbital 'wobble', that is to say that the Sun travels up and down along the galaxy's orbital disc in its orbit around the galactic core, and that every time it passes through that disc something happens through the sheer proximity of other stars.

This is despite the fact that the relative location of the Sun in the orbit is very sparsely populated.

If you consider the implications of this, then the primary risk you face in space is being close to... well, pretty much anything.

In that regard, the better solution is protection rather than charting per se. Charting would require constant maintenance insofar as you have to track the threats in real time, not only in relation to their own wobbly orbits, but also in terms of their efficacy, decline, sudden emergence... That's a lot to track and you'd be lucky by the time you get all that in a real time navigational program if it let you cross to the other side of the bridge.

So; how does one deal with the nature of space storms and debris? One of the best solutions I ever read came from Arthur C Clarke (who else?) in his book Songs of a Distant Earth in which he designed a sublight space ship that had hundreds of metres of ice on the front end as a kinetic shield for dust debris and the like.

I'd argue that this would also solve most of the problems of radiation storms et al because the ice would either block or absorb most of that if it's in the path and the only risk you still face is a super nova going off right beside you, throwing a massive lateral flare or storm front your way. For most of what you describe however, the threat is in front of you so the best solution is going to be a shield, and an ice shield as Dr Clarke described is actually one of the most practical solutions to the threats of space at speed that I've ever come across in my reading.

There are some threats that it will be important to chart in the future; black holes, nebulae, suns that we think are at risk of going nova, etc. Being somewhere else when these things are around is always a good idea and as such, we need to know about them in advance of our trip. But, I suspect that the charting of these things will be relatively high level, and that the day to day things in space that can kill us just as dead are better deflected than avoided, especially given that many of them won't be on our radar (so to speak) until the first accident.

Answer 4

1) Ion storms would not exist other than a solar flare, and we do have a way to shield against it. https://chemistry.stackexchange.com/questions/94514/can-gas-be-made-to-block-radiation-better

2) During Interstellar sublight space travel a flight path would be plotted around and away from any star that might cause an Ion storm.

3)This is on topic for Space.SE.