Shielding

Image credit – Ruth Bamford and John Bradford

During extended missions in space for a year or longer, such as when on a trip to Mars, shielding against radiation presents a very great challenge for spacecraft design. When humans are in spacecrafts in earth’s low orbit they are protected from radiation by the earth’s magnetic field. But out in space away from earth the radiation becomes much more intense and hazardous. What is ideally wanted is active shielding where a force field of some type will block any significant radiation from entering the Enterprise in places where people reside. But shields of this type are only in the early research phase. It should not be assumed that they will be ready in time for the Gen1 Enterprise, although every effort should be made to develop them.

The Gen1 Enterprise, via various technologies, will have extensive shielding against galactic cosmic rays (GCRs), radiation from the sun including during worst case solar storms, and from micrometeoroids or small pieces of space debris. While no shielding is perfect, every effort should be made to maximize shielding to protect the Gen1 Enterprise’s human inhabitants.

The Enterprise shielding will have five elements:

  1. The hulls will be at least tripled walled at all points to implement a Whipple Shield to resist ship damage due to a strike by micrometeoroids or space debris.
  2. For radiation shielding, the multiple walls used in the ship’s hulls will have enough collective thickness in the materials to provide the equivalent of 6.25 grams per cm2 (6.25 g/cm2) of polyethylene minimum at all points inside the gravity wheel pods since this is where humans will spend most of their time. All other areas of the ship will have at least 2.5 g/cm2.
  3. One or more pods within the gravity wheel will have active shielding to dramatically attenuate radiation from space (assuming the technology can be developed in time).
  4. A passive storm shelter with 1000 g/cm2 shielding where all crew members can stay when needed. (This storm shelter is needed for protection against radiation in case the active shields inside the gravity wheel are either not possible, inadequate, or broken down.)
  5. A final element for protection, although not really shielding, is the automated hole-patching system in each compartment of the ship. So if the multiple walls of the Enterprise are pierced, the hole-patching system automatically goes into action and patches the hole with a temporary patch.

The hull of an Apollo command module had 7 to 8 g/cm2 of aluminum for radiation shielding. The space shuttle achieved 10 to 11 g/cm2.  The hull of the ISS, in its most heavily shielded areas, has achieved 15 g/cm2. However, the shielding effectiveness depends on the materials used for shielding. For example water and plastics, at the same g/cm2, make for better shields than aluminum. Aluminum, which is typically used in spacecrafts today, is not bad for a shield but not as good as alternative materials. For the Gen1 Enterprise, we will assume that the inner walls will all be made from composite materials with the equivalent shielding characteristics of polyethylene (a common plastic). Polyethylene-based composite materials are currently being researched by NASA.

The outer material of the Enterprise – which can be thought of as the outermost wall – will be aluminum since aluminum is a proven material that can take the dramatic and continuous cycling of temperatures in space experienced by the exteriors of spacecrafts. The sheets of aluminum covering the outer ship will be rather thin at .125 inch (3.2mm). While this contributes to shielding, the inner walls made of polyethylene-like composites will provide most of the radiation shielding. Below shows the four walls that will exist from the outside of the Enterprise to the inside of a pod in the gravity wheel.

To simplify the following discussion, we will assume that all radiation shielding for the walls shown in the figure above comes from polyethylene-like composite walls. The total material of the four walls, as shown above, provides the equivalent of 6.25 g/cm2 of polyethylene-like composite material. (For the math of how the value of 6.25 g/cm2 is derived, click here for the spreadsheet.)

For understanding the level of shielding achieved by 6.25 g/cm2 of passive shielding from polyethylene-like composite walls, one must consider the shielding against the sun’s radiation and galactic cosmic rays. GCRs, originating from outside of our solar system, are fairly constant over time and thus predictable. However the sun’s radiation can vary dramatically over time. In fact the sun’s radiation can suddenly shoot up to dangerous levels as a solar storm erupts. Fortunately, it’s possible to detect precursor patterns of radiation coming from the sun that indicate a major solar storm is about to occur. This will give time for crew members to get to a well-shielded place in the ship.

We will consider shielding first against GCRs and then shielding against radiation from the sun.

Shielding Against GCRs

As mentioned above, GCR radiation is predictable in space. As a spaceship moves away from the earth’s protective magnetic field into open space, the GCRs become much more intense. So for shielding against GCRs, we are mainly concerned with achieving adequate shielding for when the Enterprise is away from earth.

First, it should be pointed out that 6.25 g/cm2 of polyethylene-like composite is not adequate shielding for long term exposure by humans to GCRs in open space. For example, if an Enterprise mission is two years long, such as during a mission to Mars, with this 6.25 g/cm2 crew members will be exposed to GCR radiation of roughly four times higher than what is considered acceptable. If further measures are not taken to improve shielding from GCRs beyond the 6.25 g/cm2, then the health of crew members will likely be unacceptably degraded. For example, their chances of getting a fatal cancer later in life rise unacceptably.

Long term exposure to GCRs is reduced to acceptable levels on the Enterprise by the crew members spending a significant part of each day in the 1000 g/cm2 passive storm shelter or by staying in a gravity wheel pod that has active shielding. It is important to add that 6.25 g/cm2 of polyethylene-like composite materials is more than adequate for protection against short term exposure to GCRs in open space. It’s only long term exposure that creates a health concern. So if crew members spend half of each day in the storm shelter, then they cut their long term exposure to GCRs in half. If they stay longer in the shelter each day, then they cut their long term exposure even more.

GCR shields thickness v2

To the right is a graph comparing aluminum as a shield against GCRs to polyethylene. It can be seen from the dotted lines that 6.25 g/cm2 of polyethylene provides shielding against GCRs equivalent to 12.5 g/cm2 of aluminum.

A big advantage of using polyethylene-like composite walls in the Enterprise is that this reduces the mass of the Enterprise compared to if aluminum was used for the walls. The walls and hull structures are the dominant mass of the ship, so making them lighter is a key goal for ship development. Using 6.25g/cm2 of polyethylene-like shielding instead of 12.5g/cm2 of aluminum gives a 2x mass savings.

Another advantage of plastic-like materials like polyethylene, not captured in the graph above, is that they produce far less “secondary radiation” than heavier materials like aluminum or lead. Secondary radiation is emitted by the shielding material itself. When particles of space radiation smash into atoms within the shield, they trigger tiny nuclear reactions. This secondary radiation is known to have bad effects on the health of humans, worse than the original radiation itself.

So why not make the polyethylene-like composite walls over the ship much thicker to better protect crew members during long voyages such as during a round trip to Mars? The answer is that moderate amounts of shielding greatly reduce the dose from GCRs due to removal of lower energy components within the GCRs, but the effectiveness of shielding approaches diminishing returns beyond 6.25  g/cm2 for polyethylene-like composites due to the penetration of higher energy components of GCRs. Making the polyethylene-like composite walls all over the ship achieve say 25 g/cm2 would make the ship prohibitively heavy and thus not viable. And even if this is done, the walls of the ship still will not provide adequate shielding against GCRs for crew members during a long mission. A passive storm shelter or active shielding is still needed.

The comparison of 6.25 g/cm2 to 25 g/cm2 can be seen in the graph above. The radiation dose drops by only about 25% (from .137 to .1) due to going to 25 g/cm2 of polyethylene, but as mentioned earlier the dose needs to drop by at least a factor of four before humans are adequately protected from GCRs during a long mission if they never use a storm shelter or active shields. For a dose this low on the y axis in the above graph, the x axis goes off the chart since the shielding would need to be over 100 g/cm2.

If active shields are developed within the 20 year development window of the Gen1 Enterprise, then as long as the crew members stay within the actively shielded pods in the gravity wheel they are protected from long term exposure to GCRs. However, crew members can still regularly venture out beyond the actively shielded pods each day since the 6.25 g/cm2 of polyethylene-like composite walls for the other pods provide more than adequate protection from short term exposure each day to GCRs.

Furthermore, crew members can also venture outside the gravity wheel to other parts of the Enterprise where the walls provide just 2.5 g/cm2 of polyethylene-like shielding. While this sounds like a low number, looking at the graph above it can be seen that this only increases the radiation dose by about 20% (from .137  to .165) over 6.25 g/cm2 walls.

If active shields are found to not be ready technologically, then crew members  on long missions can spend most of their time each day in the 1000 g/cm2 passive storm shelter. The disadvantage of this approach is that to create a shelter with walls thick enough to achieve 1000 g/cm2, the shelter is too large and heavy to put in a pod in the gravity wheel. This storm shelter must be put near the center of the saucer hull where there is no gravity. Spending much of the day in a gravityless environment is not desirable, but if active shields are found not to be ready for prime time, this must be the case.

Shielding Against Solar Radiation

The worst radiation emitting from our sun during a solar storm over the last 500 years occurred in 1859 and is known as the Carrington Event.  If any spacecraft built to date for carrying humans had been out of earth’s orbit and away from the earth’s protective magnetic field at the time, the dose of radiation would have likely been fatal within hours. Far better shielding than has been used in spacecrafts to date will be needed to protect humans (and perhaps electronic equipment as well) from the worst case solar radiation.

Solar storms are said to cause a “solar particle event” (SPE) where the SPE gives radiation levels much higher than what is normally emitted by the sun. To get a sense of how different the radiation level can be from an average day, consider the Carrington Event as described on Wikipedia:

On September 1-2, 1859, the largest recorded geomagnetic storm occurred. Aurorae were seen around the world, most notably over the Caribbean; also noteworthy were those over the Rocky Mountains that were so bright that their glow awoke gold miners, who began preparing breakfast because they thought it was morning. According to professor Daniel Baker of the University of Colorado’s Laboratory for Atmospheric and Space Physics, “people in the northeastern U.S. could read newspaper print just from the light of the aurora.”

Telegraph systems all over Europe and North America failed, in some cases even shocking telegraph operators. Telegraph pylons threw sparks and telegraph paper spontaneously caught fire. Some telegraph systems appeared to continue to send and receive messages despite having been disconnected from their power supplies.

So if the surface of the earth was affected like this, just imagine what it would have been like being in a lightly shielded spacecraft without the advantage of earth’s protective layer of atmosphere and protective magnetic field!

To protect the Enterprise crew members against catastrophic SPEs, the 1000 g/cm2 passive storm shelter will be used. If active shields are ready, perhaps they will provide adequate protection against such a storm, but this should not be counted on. So a 1000 g/cm2 passive storm shelter should be assumed as needed in the Gen1 Enterprise regardless of the state of the active shielding technology at the time of Enterprise deployment.

The passive storm shelter’s location and size are shown in the two diagrams below.

USS Enterprise Passive Storm ShelterOn typical days, when there are no SPEs, the 6.25 g/cm2 polyethylene-like composite walls provide adequate shielding against solar radiation when the Enterprise is out in space away from the earth’s magnetic field. In fact, if no SPEs occurred during a long mission, 6.25 g/cm2 of polyethylene-like composite should provide adequate shielding against solar radiation to protect humans. But counting on no significant SPEs over say a two year mission, and assuming a storm shelter and/or active shielding are not needed, is gambling with the health and even lives of the crew.

Regarding SPEs, increasing the shielding to say 25 g/cm2 will not provide adequate protection from major SPE radiation. It has been estimated that at least around 50 g/cm2 might be adequate, this resulting in 13 inch thick walls for the pods of the gravity wheel. This is not practical due to the large mass involved. Therefore either a breakthrough in active shielding is needed, or crew members had better head to the 1000 g/cm2 storm shelter with its thick walls during a major SPE.

Argon Tanks Storm Shelter v2

The fact that the Enterprise is such a large ship becomes an invaluable asset for shielding against SPE radiation since a 1000 g/cm2 passive storm shelter can be built inside the saucer hull. In this shelter the crew cabins are surrounded by 25 foot thick walls on all sides. Tanks within these walls will hold the 55 million pounds of Argon propellant needed by the engines during extended voyages. This thick layer of Argon will make an excellent shield against even the most severe SPE radiation. Water tanks, oxygen tanks, and various other supplies might also be placed within the walls surrounding the shelter to help add more overall shielding. All tanks will be made from composite materials to avoid creating the unwanted secondary radiation from metals.

solar shields thickness v2

The dotted lines in the graph to the right shows that 6.25 g/cm2 of polyethylene-like composite material gives comparable shielding against solar radiation to 8 g/cm2 of aluminum.

An active shield is desired to avoid people having to spend a lot of time in the gravityless storm shelter. Thus a top priority for early research for the Gen1 Enterprise program will be to find technologies that can be harnessed to implement this active shield. When in earth’s orbit the active shield is not needed unless one spends years in orbit. But on long voyages, when the ship is away from the earth’s protective magnetic shield, the active shield becomes essential to protecting the health of the crew unless the crew members spend much of their time each day in the heavily shielded storm shelter.

Much equipment can be contained in the storm shelter so that crew members can work there part of the time during each day. Furthermore, having key equipment (and backup equipment) inside the storm shelter will protect it from radiation such as during an SPE.

For a significant part of each day, unless a SPE is underway, the crew members can go to the gravity wheel to live and work under 1g gravity conditions. Spending a significant part of each day in a 1g gravity environment is very important for maintaining the health of crew members, as has been learned from the space stations around earth. If humans live in gravityless environments for too long their health is significantly degraded.

Even without the active shields, crew members might still be able to sleep within the sleeping quarters of a pod in the gravity wheel. This could be made possible by putting several feet of shielding around the bed where each person sleeps. While that is a lot of material, it’s packed in a very small space, and thus its total mass is not so large. Also, the several feet of shielding may be formed from items needed for living in the pods such as from tanks of water. So the net mass adder to the Enterprise could be minimal. Sleeping under 1g conditions each night may be found to be important for health reasons and will also improve the comfort of crew members during long missions far from earth.