Re: are ultrahigh energy cosmic rays a threat to life on earth from 60mly?

Hello Galina,
I'm answering two of your questions about "ultra-high-energy cosmic rays": would a nearby galaxy cluster collision irradiate Earth with them? Do individual UHECR pose any risk to spaceships? They are both interesting questions, so I've written a long answer. Bear with me.

Ultra-high energy cosmic rays are interesting because we don't know much about them. We don't know how they're accelerated, or by what objects, or from how far away. We don't even know whether they are photons, protons, neutrinos, or something else. Does a galaxy cluster collision create UHECR? Nobody knows. If it does, what is the luminosity? Where is the source? What is the energy spectrum? What is the maximum energy? Again, nobody knows.

Rather than giving up, let's do something very physicsy. Let's see if we can figure out the maximum imagineable UHECR flux. We know that the two clusters have a lot of gravitational potential energy. As they fall together, this is converted into kinetic energy. When they collide, some of this energy goes into shock waves, and some of the shock wave energy becomes cosmic rays; perhaps some of the cosmic rays are UHECR. So, it is hard to imagine that the total UHECR energy is greater than the total gravitation energy available. Similarly, it's hard to imagine that the UHECR flux comes out all at one time, when it takes millions of years for the clusters to slowly meet.

You can calculate the gravitational potential energy for yourself. Let's suppose we have two small clusters (each 10¹⁶ solar masses); let's pretend that they are point masses which fall from infinitely far apart and end up 10M light years apart. The gravitational energy is about 10⁵⁸ Joules. Let's say the energy is released over at least 10¹⁵ seconds (that'd about how long it would take the clusters to cross each other at the speed of light! It probably takes 100--1000 times as long!) So the total power available in the collision is of order 10⁴³ watts---not so different than a gamma-ray burst's (GRB) power, although a GRB can only keep it up for a few seconds. OK, now this power has to travel 60 million light years in order to reach Earth. That's 10²³ meters. The UHECR will not be "beamed" towards Earth; they'll evenly illuminate a sphere surrounding the source, with an area (at 60 Mly) of 10⁴⁷ meters². So the flux at Earth amounts to 10^-4 Watts per square meter. If you want real UHECR, with more than a joule per particle, that's 100 UHECR per square kilometer per second. (For comparison, experiments have seen 1 UHECR per square kilometer per century!!)

And that's basically the maximum; that's if all of the cluster's collision energy goes into UHECR. This is not true:

• The cluster collision doesn't "stop" the infalling galaxies. They retain a lot of kinetic energy for a long time. We can see this by looking at distant galaxy clusters; many of them are distorted, suggesting that they are still "relaxing" after a merger.

• Some energy will go into "intracluster gas" and shocks, which is a good place to accelerate cosmic rays. However, a lot of this energy goes into ordinary heat.

• We understand quite a lot about lower energy cosmic ray acceleration. It's a statistical process; cosmic rays wander around in a region of space filled with moving, magnetized gases, and each partile receives a "kick" whenever it meets a large-scale magnetic field moving in a particular direction. Suppose that a particle has a 10% chance of being "kicked" at any time. If you start off with a million particles, 100,000 of them will be kicked once, and become low energy cosmic rays. Of those, 10,000 will be kicked again, gaining more energy. 1,000 will be kicked three times, and so on, so high-energy CR are always scarcer than low energy CR. (UHECR may just be the ultra-rare particles that happen to get thousands of kicks.) In any case, this is another energy drain on the system. Low-energy CR are usually trapped in the cluster and do not reach Earth.

OK, so, are UHECR a danger to astronauts? That's an interesting question. A single UHECR may have 10²⁰--10²¹ eV, or 10--100 Joules. That's as much energy as Mariano Rivera's cut fastball. However, a UHECR can't break your finger, or brush Kevin Millar back from the plate. Radiation causes damage by passing through things, and depositing energy along its path. As it passes through matter, a particle creates a violently-changing electric field which can rip electrons off of atoms. The strength of this field, and the number of ionizations in a given distance, depends on the velocity of the particle, not its energy! UHECR are traveling very, very, very near the speed of light, but ordinary cosmic rays are traveling pretty near light speed. A UHECR proton passing through your body will do about the same damage as an ordinary cosmic-ray muon---and you're absorbing about 10 of those per second right now!

The difference between UHECR and ordinary high-energy particles (let's say protons) is after they've passed through a layer of material. A 2 GeV proton will stop after 10--20 centimeters of rock; it might knock off a neutron or a pion before it runs out of steam. A 200 GeV proton will stop after ten meters of rock; it will not only knock off neutrons and pions, but those neutrons and pions have enough energy to create more, and so on. It's called a "hadronic shower". UHECR, of course, create huge showers. That's how we detect them sometimes; UHECR showers are so big, with up to 10⁹--10¹⁰ particles in them, that they make a visible flash or light in the atmosphere! But it takes time and area for a shower to develop. Not much energy is deposited in any one place, or any one bit of material.

So, in terms of astronaut protection, the best UHECR shielding is no shielding at all. With no shielding, the original CR particle will hit the astronaut's body, do very little damage, and keep going. With shielding, the original particle may initiate a shower in the shielding; each particle in the shower can cause radiation damage. In practice, most of the risk to astronauts is from the lowest-energy radiation: x-rays and 1--100 MeV cosmic rays from the Sun. Higher energy cosmic rays (mostly 1--2 GeV), the ones which are hard to shield against, still represent a very dangerous radiation dose for an astronaut going to Mars. (On the Space Station, the Earth's magnetic field keeps these cosmic rays away.)

Hope this was interesting. If you want to do more order-of-magnitude calculations, like the one I did for the Fornax gravitational energy, it's helpful to have a good sense of how big things are. (here's a more astro-specific list). If you want to think about cosmic-ray effects at Earth, take a look at these research papers: Terrestrial Ozone Depletion Due to a Milky Way Gamma-Ray Burst and Climatic and Biogeochemical Effects of a Galactic Gamma-Ray Burst, which speculate that astronomical events (very near Earth---within 10000 light years) can affect the Earth's climate.

-Ben