# [ExI] Will cosmic rays stop human space travel? (BillK)

Flexman, Connor connor_flexman at brown.edu
Fri May 29 06:29:51 UTC 2015

```On Thu, May 28, 2015 at 3:46 AM, Stuart LaForge <avant at sollegro.com> wrote:

> Maybe I am missing something obvious here, but if the problem is high
> energy charged particle radiation like protons, then why not just give the
> spacecraft a magnetic field?
>
> If the earth's magnetosphere is sufficient to protect the ISS, then some
> neodynium magnets stuck around the hull should be enough. After all, the
> earth's magnetic field is about 100 times weaker than a typical
> refrigerator magnet at the earth's surface and should fall off inverse
> square-wise out where it matters.
>

I have thought of this too, so I'm sure it's been considered by actual
aerospace technicians.
A back of the envelope calculation:
x=.5at^2
F=*qvB*=ma=*2mxt^-2*
Using orders of magnitude, 10^(-19+8+logB)=10^(-27+16) gives us B=1 Tesla.
This assumed a cosmic ray of a third the speed of light being displaced 1
meter under a field uniform for a meter out (t being the time spent in that
1 meter). As you predictively stated, this is about the strength of a
neodymium magnet at its edge. A lot closer to reasonable than I had
intuited.

Unfortunately, the biggest issue seems to be extending the field far from
the spacecraft. As you noted, the field falls off as inverse square of the
distance, so unfortunately the field 1 cm from the center of the neodymium
magnet is reduced by 4 orders of magnitude a meter out. Then even a 1 inch
neodymium coating all over would be woefully inadequate. I have little
experience with actual magnetic configurations, so I'm not sure what our
options are here, but it seems like the problem is less "subleties and
complications" than raw power. I think superconducting solenoids a la LHC
work at about 10 Tesla for almost a meter, so we'd need something like that
all around the spacecraft. We would also need to make sure our field was
confined, otherwise our shipboard electronics will be entirely destroyed.
If I'm wrong or overlooking easy fixes, someone else can help me out here.

Further, this a run of the mill .3 v/c cosmic ray; their flux drops off as
a power law with energy, but there are still many within two orders of
magnitude that wouldn't be blocked. All in all, while this would be a nice
simple fix if it worked, it looks to me like it would fail any practical
cost-effectiveness measure. At some point, instead of building a magnetic
field a meter out, you might as well start stacking lead.

Connor
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