Tuesday, September 8, 2015

Starship Design – Dust Shield

The main constituents of the Interstellar Medium are gas, mostly hydrogen, and dust grains, little motes of silicate or metal-rich dust. The gas has been around for ever, but the dust grains are made of material that had to be processed in at least one stellar furnace. In the early universe, Population I stars had only hydrogen and some helium to burn, but they made everything else, and since then there has been dust. It’s not perfectly clear how dust forms, if it forms by itself, but some of it probably comes from supernova explosion shockwaves hitting the planetary materials orbiting around the star. The violence of these shockwaves is made clear by the size of the particles, typically 0.1 micrometer. It is an inconceivably monster explosion to take a planet down to dust this fine.

It wouldn’t be very promising for interstellar travel if running through the dust eroded any dust shields and then proceeded to damage the ship. If we assume that the ship’s speed is 5% of light speed, we can calculate the damage that hitting just one mote of the estimated size. It’s an easy calculation to see how much energy is deposited, and then to assume that the dust shield is iron and is melted and blown off by the encounter. A hemispherical hole about 80 times the diameter of the dust mote goes away. But the dust mote is so small, that the hole would be almost invisible.

This erosion would be continual however, with each dust mote adding to the iron removal. If we figure a density of 1 mote per million cubic meters, which is the current estimate, and figure that the trip is 100 light years long, after it was over only a fraction of a millimeter of iron would be gone. In other words, dust erosion is not a problem, and a three centimeter dust shield over everything would be just fine.

The galaxy is not a uniform place, and there are many different kinds of regions, each with different amounts of material floating around in them. There is a range of densities, with the most dense region having ten thousand times as much material as the average. These regions are only a fraction of a percent of the total volume of the galaxy, so any competent stellar navigator could avoid them.

Two recent astronomical observations indicate there is even more diversity out there in the galaxy. A probe called StarDust, which was sent to take samples from the vicinity of a comet, also pulled in samples of what is believed the first collected interstellar dust. They found some of the dust particles of the size mentioned above and used in the calculations. But they also found two particles that were a thousand times more massive, still unbelievably small, than the ones listed. There was no calculation or deduction yet as to where these came from, how they got into the vicinity of the comet that StarDust visited, how prevalent they are, or if they actually are interstellar grains or something from our own solar system. If it turns out that there are regions where these larger grains abound, and if they are in the same number density as the earlier particles, then the dust shield would have to be larger, perhaps a meter of metal, if the whole trip was going through a region where these larger particles were standard. No information yet exists on the prevalence of these, and earlier astronomical investigations of the content of the Interstellar Medium do not include them.

The other recent astronomical observations indicate that there may be regions where serious problems awaits any interstellar ship moving at high speed. They obtained some evidence of centimeter sized ‘pebbles’ in the highest density gas clouds in the Orion Nebula’s star-forming clouds. These clouds are not something that a starship would want to run through anyway, but if it happened, in the Orion Nebula or in some other nebula somewhere, hitting one of these pebbles at 0.05 c would destroy the dust shield and probably some of the supporting structural members holding it in place.

Obviously, this is on the cutting edge of astronomical research today, so no firm conclusions can be drawn. But it certainly is interesting to see new results beginning to answer questions that are needed to predict if star travel is possible.

There is one other thing to be considered in navigating the galaxy. High density star-forming clouds like nebulas or Bok globules should be avoided, but so should solar systems. A solar system has a large amount of debris floating around in it. We worry about larger asteroids hitting the Earth and producing another extinction event as the Chicxulub asteroid did. But for every large asteroid, there are a huge number of smaller rocks flying around the star in orbit. Some of them appear as meteors here on Earth, but the rest of the volume of the solar system has them as well as near-Earth space. Any starship moving at high speed would want to make a wide berth around the Oort cloud surrounding the star. We are not sure that there are Oort clouds around other stars, but it is a safe bet to assume that the process of solar system formation is roughly the same for other stars, and if we have one, they probably do also.

Our Oort cloud gradually diminishes in number of objects as the distance to the star increases. One estimate of the edge is 0.2 light years, but others are farther. Steering the starship a half-light year away from any star would greatly reduce the chance of a collision with a large Oort cloud object. There have been no observations yet to tell us if there are many small objects also located there.

The final maneuver of a star ship would be to reduce its speed and come toward the destination star. If a constant thrust propulsion system is being used, the starship would already be at low speed when it neared the outer edge of the Oort cloud of the destination star. Perhaps it would have been possible to use astronomical observation to locate all the larger objects of the destination Oort cloud, but certainly not the smaller ones. Exactly how the starship would maneuver into the planetary destination needs to be figured out. If it is traveling at a slow speed, for example orbital speed of the planet, maybe 0.0001 c, there would be plenty of time to scan the volume in front of the ship and take whatever actions were needed to avoid collisions with any substantial mass. At these low speeds, pebbles are not a problem.

As more is learned about the galaxy, it becomes clear that navigating a starship requires a large amount of information, and perhaps some specialized equipment to observe the surroundings. If we can deduce that some active transmissions are necessary, perhaps they can be used in our monitoring programs.


  1. That's solid dust shields . I would guess that any civilisation evolved enough to travel at significant fractions of c would also be able to develop sophisticated force fields to carry out this function. Raises the question of power though, already a bane of the propulsion system required to get the ship to factions of "c"

  2. Would not the technology necessary to reach fractions of light speed not provide potent and more efficient ( and safer ) force field shields rather than physical barriers? Energy would be an issue but presumably solved by whatever is powering the propulsion system. I guess an interstellar arkship could reach such speeds over millennia but if perhaps a hollowed out asteroid (with rotation for simulated gravity) then it's Kim's thick walls would act as its own screen. A great post that raises the practical issues of interstellar travel that sci fi conveniently sidesteps such as precision navigation over light years not to mention collision warning systems generally lumped in Star Trek like as "sensor scans show...".