Radiation Shielding
Now its high time that we expand our reach in the universe beyond the Low Earth Orbit(
LEO). Doing so, will force us to step out of our comfort zone i.e. Earth’s Magnetosphere. It’s not like we have not been to outer space. Missions to moon are a shining example of the same, but they were short duration missions. A mission to mars will be of a comparatively longer duration. Our theme If we are to do so, we must take into account the problem of radiation.
There’s solar energetic particles released by our very own sun which is lower in energy and hence easier to protect astronauts and electronics from it with shielding materials.
There is Van Allen Belts surrounding the earth, but this is not an issue since it only surrounds earth and is completely absent in deep space.
Then there is the big bad brother of all space radiation Galactic Cosmic Radiation(
GCR) and till date there are no space crafts equipped with shielding technologies to deflect or absorb it. Even the NASA’s Deep Space Habitat is vulnerable to it.
There are two ways to protect the craft from radiation. Active protection (using magnetic or electrostatic fields to act as shields) and passive protection (relying on layered material).
All existing spacecraft use the latter one. Aluminum, the main material used in spacecraft construction, does virtually nothing to stop GCR. Computer models and ground based experiment suggests that plastics are much better. Anything with high hydrogen content would work well.
The passive protection method is fairly simple, stashing solid massed between a shielded region and outer space. Increasing mass leads to increasing cost of mission and if we are to protect astronauts from GCR using the same, the mission would be a financial nightmare.
The US National Research Council Committee on the Evaluation of Radiation Shielding for Space Exploration recently stated: “Materials used as shielding serve no purpose except to provide their atomic and nuclear constituents as targets to interact with the incident radiation projectiles, and so either remove them from the radiation stream to which individuals are exposed or change the particles' characteristics – energy, charge, and mass – in ways that reduce their damaging effects.”
So we are left with the option of active shielding which includes magnetic and electrostatic shielding. As of now, this tech does not exist. There are many theories and some undergoing experiments.
For electrostatic shielding, the space vehicle which is to be protected must be kept at a positive potential of
1 or
2 x 10^8 V relative to an outer part of the spacecraft or “infinity”. This doesn’t seem feasible as for now.
So, the most promising option is magnetic shielding.
One of our basic assumption is that the space is empty but is it so? The environment of interplanetary space contains low density (
~10^3 per m^3) plasma of positive and negative charges. I have chosen mini-magnetosphere as a method of shielding from radiation in space.
A magnetosphere is a particular type of “diamagnetic cavity” formed with plasma in solar wind. A plasma is the fourth state of matter where there are approximately equal numbers of positive and negative charges which so hot, that they do not recombine to become neutral particles.
The mean-free-path between physical collisions between the particle is far longer than the system (in solar wind the mean-free-path is about 1 Astronomical Unit). This means the particle collide through their electrostatic charges and collective movements (such as currents) that are guided by or result in magnetic or electric fields.
A plasma is a rapidly responding conducting medium due to the free moving charges. It creates a magnetic field in opposition to an externally applied magnetic field making it diamagnetic, and result in local cavities.
Miniature magnetosphere is fully formed magnetosphere, with collision less shocks and diamagnetic cavity, but the whole structure is very much smaller as compare to the earth’s magnetosphere (
100-110s of km). Mini magnetospheres have been observed associated with the anomalous patches of surface magnetic field which exist on the moon, mars and mercury and also with asteroids such as Gaspra and Ida. It has also been demonstrated that mini-magnetospheres can be formed without magnetic fields, such as artificial comets such as
AMPTE. Mini-magnetospheres are determined by the plasma physics of very small scale which in general has been neglected in the analysis of electromagnetic deflection as a means of space craft protection.
Our destination is somewhere in between the earth and the mars. At that radius, the ultraviolet radiation from the sun is sufficiently high that photon ionization results in very little matter in free space remaining unionized. The medium of space is therefore a plasma belt of very low density.
The radiation encountered in space is a composite of a small percentage of extremely high energetic galactic particles with comparatively lower flux as compare to the solar winds from out sun. For a spacecraft in interplanetary space, the result in intense bursts of radiation of deeply penetrating particles capable of passing through the hull to the crew inside. This result in increase for dose-rates above the specified limits by
NASA.
The principle of active shielding required electromagnetic forces to balance the incoming pressure. There are some expression given which we can’t figure out and hence won’t mention here.
Creating an artificial mini- magnetosphere
An on-board mini-mag system would most likely be comprised of a superconducting coil. Superconductors such as Magnesium diboride (
MgB2) is a superconductor with a critical temperature of
40K and it is highest among the conventional superconductors. In a non-conducting medium, the magnetic field intensity of a dipole magnetic field diminishes rapidly with range. Figure below shows the magnetic field at a distance (far field) where
r >> a (in any direction) is:
|B vac(r)| ≈ |Bo(a/r)|^3
but only when no plasma is present. Or in terms of the current in the coil:
B(r)≈(µoI\ 2a)(a \r)^3
Here
I is the total loop–current of the solenoid
I = NIc, where
N is the total number of turns carrying current
Ic at radius of
a.
Fig. How magnetic field intensity varies in vacuum.
The presence of plasma changes the profile. The prohibitively high power estimate of a magnetic shield is based on the vacuum profile. The effect of plasma environment is not just to extend the range of magnetic field intensity. The effect of magnetic “pile-up” comes with cross field currents in a narrow barrier region (shown in next fig.) some distance from the spacecraft. These currents and accompanying electrical fields alter how the incoming plasma is deflected. Therefore, the efficiency of the shield is found to be much greater than the initial vacuum calculations would have predicted.
Fig. A plot illustrating the difference the plasma environment makes. Sketch of the vacuum magnetic fields when the plasma environment is included.
There is no need to stop the highly energetic particles. The particles only need to be refracted sufficiently away from the central safe-zone. Much like defending against a charging rugby footballer, rather than standing in his way to protect the goal line. The better policy is to deflect the players sideways using a small amount of force.
The mini-magnetosphere barrier interaction with high energy particles is far from simple. The incoming high energy particle not only sees the electric field set up by the interaction of the solar wind and the spacecraft field, it also experiences the usual convective electric field as seen by a charged particle moving relative to a magnetic field. This convective field (
E⊥) is perpendicular to the magnetic field. This results in the particle being deflected by a series of fields in a complex manner. Quantifying the shield performance for specific spectra of high energy particles (like an
SEP) requires a full 3D recreation using a computer simulation, or an experiment either in space or in the laboratory.
Having outlined the principles behind the mini-mag system. The maximum feasible coil radius, ‘
a’ of
3 m (set by the launch rocket cowling)
ic=700A N=8000,. This produces a peak magnetic field of
~6.4T. rs=0.86/(Pin)^1/6. The total power demand limit of 16kW, and 5kW for the cryoplant to keep the system cool. Total mass=
1.5 X 10^3KG.
P.S. : These values resulting from equations that were way above apprehension level. The experimental and observational evidence for the formation of mini-magnetospheres has been established from laboratory using Solar Wind Plasma Tunnels and spacecraft observations of natural mini-magnetospheres on the Moon.
Refrence:
- http://www.space.com/21561-space-exploration-radiation-protection-plastic.html
- https://engineering.dartmouth.edu/~d76205x/research/Shielding/
- http://www.spacesafetymagazine.com/space-hazards/radiation/superconducting-magnets-protect-spacecrafts-space-radiation/
- http://www.sr2s.eu
Authored By Ashutosh Kumar Singh
