Sunday, June 2, 2013

The Mars Mission and Radiation

We're taking a little break from our dialog on time to talk about science in the news. It seems as there has been quite the buzz over the past week regarding the potential exposure to radiation exposure of astronauts on any potential Mars mission.  (read here)

Science or Science Fiction?
What's surprising perhaps is that we've known about excess cosmic and solar radiation ever since space travel began. Our Apollo mission astronauts were exposed to substantial amounts of radiation over the course of multi-week missions and the astronauts on the International Space Station are already close to reaching the amounts projected for the Mars mission (as noted in the article).

So basic question #1 - why is radiation exposure associated with Space travel?
The Earth's Magnetosphere protects us from both solar and cosmic radiation. This consists of high energy particles hurtling at us from nearly every direction. The magnetosphere is powered by what is thought to be a huge amount of molten magnetic material in motion within the earth's core. 
This is what allows life on Earth to exist
Basic question # 2 - why aren't spaceships and the space station shielded from radiation?
First, because of the gravity well - sending extra materials, such as lead, into orbit would be prohibitively expensive. Second, providing an energy source powerful enough (read 'nuclear') to generate an "artificial magnetosphere" would potentially expose astronauts to as much radiation as the unshielded passage through solar and cosmic radiation.
As one can see, this poses a major obstacle to any possible attempts to support long-distance missions away from earth (either to planets or in self-contained environments). We haven't had to worry about it too much primarily because most of our ambitions for space travel were prematurely 'paused' during the 1970's. For those who are curious how much 1,000 millisieverts is check out this chart. If that level of exposure were to occur all at once it would lead to serious radiation sickness. Of course, 1,000 millisieverts would not be the limit of exposure for people who spent years on a space station or perhaps living on the surface of Mars or the Moon. Even if Mars were terra-formed ( a popular science fiction theme) it wouldn't solve the radiation problem unless we also figured out how to create a Magnetosphere on Mars.

So, let's return to the original question or issue - can we effectively travel to Mars if our astronauts are exposed to that much radiation? What are our alternatives to solving the problem? Solutions might include the following:

  1. Increasing the velocity of spacecraft by a factor of 2. Here is data on the speed of the Mars Science Laboratory (MSL) mission. Relative speeds of the rocket are around 58,000 mph. This allows for about 9 months on each leg of the trip (to and from).  So this means we'd need a propulsion system that supports an average speed of over 100,000 mph. 
  2. Develop lite-weight radiation shielding materials. This cannot add more than about 5% of the total weight of the spacecraft and remain feasible (for a craft launched from the surface of Earth). 
  3. Build our Mars missions craft in two stages; segments on earth first and then assembly in orbit. This would support use of more shielding materials (and heavier materials) without incurring gravity well limits (as they would be distributed over multiple launches).
  4. Develop a non-nuclear power source for generating an artificial magnetic field around the spacecraft.
Of the four options, the last one is the most intriguing as it potentially resolves all of the longer-term issues associated with allowing humans to work and live in space. However, this option is not without risk either, as studies have shown that exposure to Magnetic fields can cause cancer as well. So perhaps what we're looking at is a combination of shielding materials and engineering mitigations as well as artificial fields. If you look again for a moment at diagrams showing the Earth's Magnetosphere you'll note that the magnetic fields extend outward from the poles. This configuration if it were approximated for a spacecraft might manifest itself as fields emanating from the sides of the craft or from the front or back. The idea would be to segment the portions of the craft generating the fields away from crew living / control quarters so that the field would work more or less the way it does on earth. (and what the field does specifically is deflect oncoming / incoming solar and cosmic particles, but the field extends outward away from earth rather than flowing through / over us). 

We will dedicate another post soon to exploring what types of energy options might be available to help solve this problem.

copyright 2013, Stephen Lahanas


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