Re: The Extinction Challenge (fwd)

Robert J. Bradbury (
Sat, 31 Jul 1999 09:47:31 -0700 (PDT)

I have refrained from commenting on this previously because it is a very complex topic and to *really* accurately discuss it we need a spreadsheet that details the energy exposure at a distance from a SN/GRB at a specific wavelength. If we had that we could discuss this topic without all of the hand-waving that seems to be occuring.

That being said, I'm going to inject some comments:

> John Clark <> wrote:
> Billy Brown <> Wrote:
> >If it arrives as IR or UV it is completely harmless.
> True.


You can't comment very much on the IR/UV exposure without knowing exactly how much dust is between you and the radiation source. UV/Visible will be absorbed by dust and re-emitted in the IR.

If you get enough IR radiation, you vaporize the oceans and all water-based life forms on the planet (leaving only bactera in subterranean ("insulated") locations.

If you get enough UV radiation, you produce enough thymine dimers to either make the next generation mutation rate go ballistic or even sterilize the existing surface dwelling population entirely.

Energy "density" does count!

> >X-rays or gamma-rays will be completely blocked by a planetary atmosphere
> True, but they and the high speed electrons and protons that also come from
> the supernova would produce radioactive isotopes in the atmosphere that
> would be no fun at all. The ozone layer would go to hell too.

The ozone layer gets axed primarily by UV I believe. Whether X/Gamma-rays are blocked by the atmosphere depends upon the proximity and the energy released by the event. An interesting comparison would involve how close a "typical" SN/GRB would have to be to sterilize/kill (a) people on the surface, (b) people on the 1st floor of a skyscraper; (c) people in underground bunkers designed to withstand nuclear blasts. Then you have an estimate as to real extinction potential.

> >and any space habitat capable of surviving a solar storm will likewise be
> >unharmed.
> That is very far from obvious.

I agree, you have to show me the radiation frequency curve of a solar storm. Since you have to put a lot of mass into a space habitat radiation shield and mass in space is generally expensive, I'd vote to be under the atmospheric blanket or in an undergound bunker. Most of the space radiation exposure curves I've seen try to reduce the levels to a "nuclear industry worker" (which are presumed to be "non-hazardous).

There is a *big* debate going on right now about how to properly assess radiation hazards (there is a National Research Council panel that is supposed to provide the standards for this). There are two very opposed camps: (1) that radiation hazards are "linear" with dose; and (2) that radiation hazards below a certain level are "essentially" harmless (i.e. you have biological systems designed to cope with these "natural" levels).

Just for "comparison" purposes, I would point out that ionizing radiation sufficient to produce 5 double strand DNA breaks in most bacteria/eukaryotic cells is "lethal" to the cell (never mind cancer), the radation-resistant bacteria Deinococcus radiodurans, on the other hand, can tolerate radiation sufficient to produce 100 double strand DNA breaks.

That is a factor of 20 in exposure tolerance, which I presume translates into a significant difference in distance.

> >If it were all neutrinos an average human would stop a grand total of maybe
> >10^-17 % of that flux, which amounts to around 10^-7 electron volts -
> >in other words, less than one interaction.
> I have no idea what you're talking about. You seem to saying that a human
> body would not absorb even one neutrino from a nearby supernova, but
> you can't possibly mean that.

The neutrino absorption/interaction rate depends entirely upon the neutrino density where the human is when the neutrinos hit. This depends on the neutrino density generated by the SN/GRB, the distance to the Earth, and the mass between the individual and the SN (interstellar dust, atmosphere, oceans (for those in submarines), ground (for those in bunkers), the Earth itself (for those on the side "away" from the SN/GRB, etc.

The question is how many neutrinos go through the human? That largely depends on distance. I would presume that the reason that most of the energy in SN/GRB is released as neutrinos is that they are the only particles that can easily escape through the overlying material. Since the "mass" between a human and a SN/GRB is probably much greater than the human, you have to take into account the probabilities for interactions with that mass and the probability that those secondary products would interact with the human.

> >It doesn't matter what exotic form you want the energy to arrive in,
> >because there simply isn't enough of it to do anything.
> To repeat myself, energy has little to do with danger, just one X ray
> photon could kill you if it hits in the right place and in fact that very
> thing is a major, perhaps the major, cause of the most feared disease
> of the 20'th century that kills millions of people every year.

Energy "density" (and type) is the basis for the danger. Cancer in a single cell probably requires 5+ mutations. Even if you have those mutations, there is a good chance that one of the natural defense systems in the body may remove the mutated cells. The only way to resolve this question is to use the radiation levels an average person would receive from a SN/GRB relative to the radiation levels received from atomic bomb blasts (or nuclear workers and/or flight attendents).

Even then, the calculations are very difficult, because most of the radiation exposure from which disease rates are caculated *do not* have neutrinos as the radiation source.

> >The target civilization will be exposed to much higher levels of
> >every type of radiation (including neutrinos) simply by living near
> > their own sun.
> Once more I must say that you can't possibly mean what you seem to be saying.

Billy can say that if he assumes a minimal distance to a SN/GRB. We know that SN/GRB occur all the time. Most of the time they have relatively little effect on Earth. I happened to be in a plane and observed the "Northern lights" that resulted from the atmospheric ionization from one of the larger the GRB/CosmicRays last August. I'm still alive to recall the event. Whether the secondary radiation I received was sufficient to cause a future cancer is anyones guess. For the simplest approximation it comes down to the energy of the event and how distant you are from it.

> >Do I really need to write up a complete treatment of all the calculations here?
> Yes, I rather think you do.

To be perfectly frank, I don't think it can be done (without a huge amount of work). You have to determine the particle types that result from SN/GRB, determine their density when they reach Earth, determine their reduction (and byproduct production) for intervening mass, determine their interaction rates with the elements in the human body, determine the mutation rates resulting from those exposures, etc.

You could come up with numbers, but the assumptions would probably be open to question. The best you can probably do is a simple calculation that says SN/GRB are harmless at greater distance X, and SN/GRB are guaranteed to be fatal when closer than distance Y.