An Essential For Deep Space Manned Missions

An essential need to for deep space missions, like Mars or long term habitats on the moon, is protection from the radiation environment.  For travel, a possible alternative is a much faster way to get there, a different essential need.  For habitats there's more freedom - perhaps burying the colony could be an alternative if the planet doesn't have a good magnetic field. 

Ars Technica has a long, deep article on the radiation issue, far too involved for me to get into here, so I really want to recommend anyone interested read or at least skim the piece.  

The radiation problems start with the sun, and with the current tendency to blame everything from the AT&T update issue a couple of weeks ago to an unexpected pimple on solar flares, I think everyone's aware that can be a problem.  Ars author Jacek Krywko starts off with some interesting background.

On October 19, 1989, at 12:29 UT, a monstrous X13 class solar flare triggered a geomagnetic storm so strong that auroras lit up the skies in Japan, America, Australia, and even Germany the following day. Had you been flying around the Moon at that time, you would have absorbed well over 6 Sieverts of radiation—a dose that would most likely kill you within a month or so.

X13 is not the biggest flare observed since satellites started monitoring the sun, and is still considerably weaker than the Carrington event of the 1850s. Back in November of  2003, toward the end of cycle 23, there was a super flare that was genuinely scary and the kind of flare to worry about.  It was classed as X28 afterwards - only because it saturated the X-ray detectors on the satellites and they couldn't measure it properly. 

This is why the Orion spacecraft, which will be in space for longer times than the Apollo missions, has a built in, heavily shielded, storm shelter for the crew.  Something important to remember about radiation doses is that they're cumulative.  The Orion shielded retreat is cramped and uncomfortable if they need to use it, and only rated for 30 days. 

Radiation problems start with the sun, but don't end there by far.  Deep space is also the domain of cosmic radiation from faraway sources.  The majority of solar particle events flux is between 30 Million electron Volts to 100 MeV which is what the Orion shelter is designed for.  Cosmic rays and energetic particles from other star systems are relatively rare but some are coming at you all the time from all directions. They also can have higher energies, starting at 200 MeV and going to several Billion electron Volts (they use Giga here; GeV), which makes them extremely penetrating.  The most extreme cosmic rays have an energy measured in exa-electron volt (EeV), or 1 billion billion (10¹⁸) electron volts of energy, which is around a million times more energetic than the fastest particles from human-made particle accelerators. They are rare, but energetic. The most energetic particle ever detected had an energy of 320 EeV and traveled at more than 99.9% the speed of light.

On Earth, we're protected by the earth's magnetic field, which is weak but huge so it operates over long distances.

Anything that makes it through the magnetic field runs into the atmosphere, which, when it comes to shielding, is the equivalent of an aluminum wall that's 3 meters thick. Finally, there is the planet itself, which essentially cuts the radiation in half since you always have 6.5 billion trillion tons of rock shielding you from the bottom.

To put that in perspective, the Apollo crew module had on average 5 grams of mass per square centimeter standing between the crew and radiation. A typical ISS module has twice that, about 10 g/cm2. The Orion shelter has 35–45 g/cm2, depending on where you sit exactly, and it weighs 36 tons. On Earth, the atmosphere alone gives you 810 g/cm2—roughly 20 times more than our best shielded spaceships.  

How can a craft be shielded better?  These are charged particles and that points to three possible implementations of electromagnetic protection:  

In the 1960s, NASA funded multiple studies looking into three active shielding concepts: plasma shields (PDF), electrostatic shields, and magnetic shields (PDF). In 1967, Richard H. Levy and Francis W. French delivered a report saying that plasma and electrostatic shields were promising, but they both needed 60 million volts to work—even by today’s standards, that number is ridiculous.

Magnetic shields looked more enticing. The 1950s brought the discovery of type II superconductors—materials that had virtually no electrical resistance at very low temperatures and could be used to build extremely strong magnetic coils. In 1966, P.F. McDonald and T.J. Buntyn of Research Laboratories Brown Engineering Company reported that there were no magnets strong enough to shield a spacecraft, but “rapid advances in superconducting magnets technology indicate that it will soon be possible to produce necessary high fields with very modest power consumption.”

And that's where I'll refer you to the long article on Ars Technica.  As is the usual routine in the 21st century, I've told people interested in exploring deep space that you have a problem and now I leave it to you to solve your problem.  

Artist's conceptual drawing of NASA's CREW HaT.  CREW HaT stands for Cosmic Radiation Extended Warding Halbach Torus, a way of creating a toroidal magnetic shield around a vehicle to protect it without wrapping it in miles of wire (a solenoid).   Image credit: Aurich Lawson | Getty Images | NASA

Final words to Ars Author Jacek Krywko:

ESA’s career radiation dose limit for astronauts is 1,000 mSv [milliSieverts - SiG]. Reference Mars mission scenarios estimate a total dose at a bit below 1,200 mSv. That’s not that much of a difference—nothing you couldn’t fix by throwing a little more mass here and there in your spaceship. NASA had career limits dependent on sex and age, but you could probably get away with just picking old men for the job.

But then, on January 5, 2022, NASA revised Section 4.8.2 of the Spaceflight Human-System Standard and set the astronauts’ career radiation dose limit to a flat 600 mSv. Active shields offer a roughly 50 percent dose reduction at a cost of huge mass penalty and development efforts. They have always ended up shelved because they were overkill. We just didn’t need that much protection. With NASA’s new standards, we ultimately might.