Almost four years ago I wrote a Centauri Dreams entry about Dana Andrews’ views on shielding an interstellar spaceship. The paper is so directly relevant to our recent discussion on the matter that I want to return to it here. Andrews (Andrews Space, Seattle) believes that speeds of 0.2 to 0.3 c are attainable using beamed momentum propulsion. That being the case, he turns in his “Things to Do While Coasting Through Interstellar Space” paper to questions of human survival.
Particles with a Punch
Collision with interstellar dust becomes a major issue when you’re traveling at speeds like these, a fact Andrews is quick to quantify. For a starship moving at 0.3 c, a typical grain of carbonaceous dust about a tenth of a micron in diameter should have a relative kinetic energy of 37,500,000 GeV. Our hypothetical star mission with human crew moving at a substantial fraction of light speed will run into about thirteen of these dust particles every second over every square meter of frontal area.
This gets interesting when put in the context of cosmic rays. Most galactic cosmic rays, which as Andrews notes are completely ionized atoms accelerated to extremely high energy states, have energies between 100 MeV and 10 GeV. You can see the overlap. Travel fast enough and even small grains of dust behave like energetic cosmic rays as our vessel encounters them. Clearly, dust between the stars is something we have to reckon with on any interstellar journey.
Absorbing Particles (and the Cost)
In my post on Friday, I looked at the Project Daedalus dust shield and its role in the journey to Barnard’s Star. A key question: Do we want to absorb cosmic rays and dust particles, or redirect them? The former can be envisioned in terms of a human crew surrounded by layers of supplies and equipment, with an outer shell for the spacecraft composed of 25 cm of multiple layers of polyamides, metal foils and polyethylenes to assist in radiation protection. The mass of structure and shielding obviously becomes a major factor. As Andrews writes:
The striking facts from this mass statement are the large masses associated with structure and shielding, and the small masses associated [with] things like food and water. This is because all waste and CO2 is recycled through growing plants and algae to provide clean air, food, and water.
The author is figuring that three percent of food intake would have to be derived from stored supplies to offer the necessary trace minerals and nutrients. He adds a 33 percent margin to that number and bumps the stored food percentage up to four percent, along with an extra year’s food supply as margin in case of intermittent problems with the life support system along the way.
As to protection from galactic cosmic rays (GCR), we can create a workable but quite bulky shielded environment using these methods:
Since 99% of the GCR is ionized hydrogen or helium, it’s obvious why hydrogen makes the best shielding (because like masses scatter better). Hence, plastics with high hydrogen content were selected for the hull and shielding materials. The total shield thickness for the hull alone is about 20 gm/cm2, which will cut the dose rate to about 25 rem/year. Assuming the sleeping quarters are 3 meters by 4 meters and 2.3 meter high we can shield the walls with approximately 30 gm/cm2 of water storage (top and bottom shielded by dry storage and machinery), which should reduce the annual dose to about 15 rem. That’s three times the recommended yearly dosage for radiation workers in the USA, but within (barely) the overall guidelines for astronauts.
A Magnetic Shielding Option
But there is another way to solve the problem. Magnetic shielding, using large current loops of superconducting wire to create a protective magnetic field, could reflect or deflect charged particles around the habitat. And the advantages of using the magnetic approach are considerable.
Get this: Although the mass of the magnetic shielding support structure is close to that of the hull shielding option we looked at above, the living quarters go from a space seven meters high and ten meters long (in three levels) to a habitat 200 meters in diameter with over 6000 cubic meters of usable volume. Foil bumpers are used to break down incoming dust into atoms and ionize the result, which can be then handled by the magnetic field. We arrive at a design like the one below:
Image: Magnetically shielded interstellar habitat (to scale). Credit: Dana Andrews.
Clearly we have a long way to go before building such a vehicle, considering our own halting attempts at sustainable life support (Andrews points to the problems of Biosphere 2, and to the challenging environment aboard the International Space Station). But there is nothing to prevent us from refining these technologies, while magnetic shielding and dust particle ionization is well within the realm of conventional physics. So there are ways around the dust and radiation problem if we do venture between the stars.
Just how a workable life support system could be maintained is the subject of an intriguing appendix. The paper is Andrews, “Things to Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004.