One of the problems you’d still face after you’ve gotten thru all the hurdles of *accelerating to some noticeable percentage of the speed of light is the danger of an impact with ‘cosmic dust.’
So, how bad would it be? Let’s say I’m sailing along at a modest 10% of the speed of light, and I hit a one micron dust particle. Is it catastrophic?
*And, yeah, I know about the problems with that. Hence the ‘thought experiment.’
Not considering relativistic effects, and assuming a particle density comparable to granite, the energy of the impacting particle is equivalent to a .22 bullet traveling all of 22 meters per second. Not too incredible.
But it scales real quick with size. One hundred microns? A million times the energy (like a .22 at 70,000 m/s). 1mm? A billion times the energy of your original particle.
I saw a photo once of a satellite that had been grabbed from orbit by the space shuttle. The photo showed a close-up of where a small paint chip had struck the satellite at orbital speeds (~17,000 MPH). It blew a pretty damn big hole in the satellite’s outer insulation.
At 10% the speed of light you are still 99% Newtonian so energy will be mv^/2
If your particle was of water in a cube 1 micron in each dimension it would have a mass of about 10^-15 kg.
So the energy from the impact would be about 0.5 kg(m/s)^2. Not a whole lot. This is about the equivalent energy as dropping a baseball from 8 feet. So energy-wise hitting several thousand such particles hitting your ship every second won’t hurt. If they reached millimeter size then we will have more of a problem.
For the micorn size particles the biggest concern is the micron sized holes they leave in everything as they pass through the ship. Without knowing the structure of your ship I can’t tell you whether this would be a problem. I also don’t know if the occasional micron size hole in a person would be a problem (any biologists out there?).
However since you have the technology to accelerate your ship to this speed maybe you also have an superdense unobtainium shielding that can absorb the impacts.
It seems kind of odd that one would be speaking of aerodynamics in space, but your high-speed probe would have to be shaped ideally for traveling through the interstellar medium. Its forward face would have a needle-shaped long shield that would create a shadow for the probe’s body, that would have a strong electrostatic field to capture/divert smaller particles, possibly retaining a dirty, gaseous layer of significant depth. Your fast probe would have smooth surfaces, it would not resemble the gangly solar system probes with shit sticking out all over the place.
If it has a specific destination, deceleration becomes somewhat problematic: if you just turn it around to fire the engine backward, you are exposed to dust and dangerous stuff at high speed for quite a while.
The real problem would be communication. Assuming it is an unmanned probe, it is only useful if we can get information from it. Given the beam spread of coherent light (a laser), for each lightyear of distance, a pinprick point source spreads out by about 278 million miles, meaning what we would see from a laser would be about 278 million times fainter than the source. All you can do to harden/shield the probe is of minimal value if we cannot listen to what it has to say (presumably it would have sophisticated logic so it would not have to rely on us for direction).
Arthur C. Clarke posited a forward-facing shield made of ice in Songs of Distant Earth. The interstellar ship has to stop in planetary systems at regular intervals to replenish the shield with fresh ice. I forget the % of c the book’s Quantum Drive allows the ship to travel, the Wiki page for the book doesn’t mention it either. I’m sure ACC covered the problem of how to stop the ship at the destination system while keeping the ship shielded, I’m sure he explained it somehow.
You could polish the front of the probe and use a huge big laser beam to slow it down as it swings around the destination star.
If I remember correctly, the spaceship in that story has a “vacuum energy drive” which uses quantum energy fluctuations as a power source. But I don’t think it was a reactionless drive like Rama. So presumably, during deceleration, the exhaust from the engine serves as a shield.
Of course, we have built an interstellar probe. Voyager has entered interstellar space, which is characterized by a much higher density of particles.
The space it’s in is interstellar (by some definitions, at least), but the probe itself isn’t yet, and won’t be for a very, very long time.
As a matter of fact, the concept for the Project Daedalus probe, with a top speed of 12% c, did include an impact shield for the interstellar medium—a “beryllium disk up to 7 mm thick and weighing up to 50 tonnes.”
Wiki also mentions the plan calling for a protective dust cloud dispersed robotically ahead of the probe, for larger objects.