Solving the biggest problem of relativistic interstellar spacecrafts

In FTL-free universes, interstellar travel is often done by vessels capable of continuous acceleration (the drives themselves are often magitech), which can use the effects of relativistic time dilation to "shorten" travel times. However, while we often talk about the vacuum of space, space isn't a true vacuum. The interstellar medium isn't a perfect vacuum, per $cm^3$ between $10^6$ and $0.0001$ particles can be found. Crashing into anything, even ionised hydrogen gas, at velocities above $0.9c$ is a very bad idea. The kinetic energy of relativistic objects is given by this formula.

$$E_k=\frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}-mc^2$$

This tells me that a spacecraft with a cross-section of $50 m * 50 m$ moving through dense interstellar medium ($10^6$ particles per $cm^2$) and average interstellar medium ($0.5$ particles per $cm^2$) at $0.9c$ will have to deal with $2.6*10^{14} J/s$ and $6.5*10^7 J/s$ respectively. For comparison, this is about the energy of a $100 kt$ nuclear bomb or a $9 kg$ TNT demolition charge each second respectively. To say that this is bad is an understatement.

Some sci-fi authors have suggested using ice as a shielding to deal with this, but running the numbers shows that this is a ridiculous proposition. Assuming that:

• the sublimation of ice in a vacuum can be viewed as an isobaric process;

• all of the collision energy is absorbed by the ice at 100% efficiency and used to heat it;

• the ice will, according to phase diagrams, sublimate at $213 K$ or $-60 C$;

• the ice is at $5 K$, to begin with; and

• its heat capacity in this temperature range is roughly $4000 J/kg*K$

We'd need about $25*10^9 kg$ of ice for a 10-year voyage at $0.9c$ and for the same voyage at $0.99c$ we'd even need $110*10^9 kg$ of ice. For anyone who wonders, this would be a 10 and 44 km stack of ice respectively. This wouldn't be a spacecraft anymore. It would be an interstellar comet. The "strap your spacecraft to an icy Kuiper belt object and fly to the next solar system" idea sounds interesting but it isn't what I'm looking for.

The Solution Are Energy Shields

While energy shields are pretty much considered to be the antithesis to hard-SciFi, they aren't that fictional. Most of the gas out there is already ionised, and the non-ionised part can be ionised using UV lasers. Rocks and dust grains are vaporised by a point-defense grid before the vapor is ionized as well. The spacecraft itself is covered in superconducting magnets, whose fields deflect the ionised gas away before it hits the spacecraft.

Design-wise I think this would result in kilometre-long spindles, which are thickest at the midpoint. During the acceleration phase, the spindle will plunge through the interstellar medium like an energy dagger. The deceleration phase is slightly more complicated, as the engine would be at the front. I don't think this will be a problem however since an engine capable of continuous multi-gee acceleration will have no problem punching through the interstellar medium. The rest of the vessel is now protected by the magnetic shield and point defense on the lower decks.

Are my assumptions about the damage the interstellar medium will deal with relativistic spacecraft correct? Did I mess up my calculations? Is my idea for dealing with this any good?

posted about 5 years ago

TheDyingOfLight‭

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