Interstellar space is not quite a perfect vacuum, though it is very close. About one hydrogen atom exists per cubic centimeter (as compared to 10^18 atoms per cubic centimeter on Earth at sea level.)
At normal speeds, even normal deep space travel speeds, the impact of these atoms is negligible. However, during interstellar flight starting at a few percentage points of lightspeed, impacts with these atoms grows much more numerous, and thanks to the velocity of the ship they hit with far greater energy. What results is the ship’s hull being worn and eroded away, with the process happening much more rapidly the greater velocity with which one travels.
Impact with anything larger than a hydrogen atom could have catastrophic effects; a dust particle massing a single milligram would hit a ship travelling at one-third lightspeed with the equivalent force of one ton of chemical high explosives.
Also, as one approaches signifcant fractions of lightspeed, plowing through the interstellar medium may also produce a noticeable amount of drag, potentially reducing the efficiency of a ship’s interstellar drive.
Besides simply thickening the hull to prevent erosion, there are several strategies for dealing with this phenomenon. A ship could use one or a combination of the following techniques.
Tech Level: 13
A simple and straightforward strategy that may seem counterintuitive to decades of common wisdom about practical spaceship design. Nevertheless, rounding and tapering the hull and minimizing external hull structures would help to deflect the interstellar medium around the ship rather than letting it smash into the ship directly.
Tech Level: 13
This concept was first introduced in the novel The Songs of Distant Earth by Arthur C. Clarke. Basically, the ship carried a large shield in front of it, designed to absorb the impacts with interstellar atoms and slowly ablate away over the course of a voyage. In the novel, this took the form of a large, rounded, segmented barrier of water ice. The ship had to stop and replenish its worn shield halfway through its fifty light year voyage.
More advanced versions of ablative shielding could take the form of foamed alloys or advanced aerogels.
MAGNETIC FIELD SHIELDING
Tech Level: 14
This is a two part system. The first component consists of a powerful laser, or array of such, that shoots ahead of the ship. The laser imparts enough energy to the hydrogen atoms that their electron flies off. Then, powerful magnetic fields projected ahead of the interstellar vessel deflects the positively-charged nucleii away from the ship. A more powerful variant of this idea is used in the Bussard Ramjet interstellar drive.
Tech Level: 16
An idea that comes from the novel Encounter with Tiber by Buzz Aldrin and John Barnes.
In this scheme, the ship either carries a large store of anti-protons or has the means of manufacturing them in quantity. A linear particle accelerator, much like a spinal mount weapon, runs the length of the ship, accelerating the tiny bits of anti-matter to near-light speed and shooting them out ahead of the ship in a fine, wide-angled spray.
Whenever the heavy particles encountered an atom or speck of dust, it would react with it explosively as well as impact it with tremendous relative velocity. It would be much like setting off a firecracker next to a rock, nudging the interstellar particulate out of the way of the ship. Continuous spraying would ensure an even spread that would protect all of the ship’s forward surface area.
The main disadvantage of this system would be that all that matter/antimatter interaction would create a source of gamma rays at the fore of the vessel that could be potentially damaging to the crew and ship systems. However, it is already assumed that interstellar ships would already have heavy forward shielding, so a little extra to minimize the gamma ray hazard would not be unreasonable.
The Songs of Distant Earth by Arthur C. Clarke
Revelation Space by Alistair Reynolds
Encounter With Tiber by Buzz Aldrin and John Barnes
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