Cosmic radiation is a collection of many different types of radiation from many different
types of sources. When people speak simply of 'cosmic radiation' they are usually referring
specifically to the cosmic microwave background radiation. This consists of very, very low
energy photons (energy of about 2.78 Kelvin) whose spectrum is peaked in the microwave
region and which are remnants from the time when the universe was only about 200,000 years
old. There are also very old remnant neutrinos in the cosmic radiation. Neutrinos pass
through just about everything with no effect so they are harmless. The photons are too low
in energy to be dangerous.
On top of these there are higher energy particles that are being created constantly by all
luminous objects in the universe. Photons of all different energies/wavelengths are being
created by our sun, other stars, quasi-stellar objects, black-hole accretion disks,
gamma-ray bursts and so on. These objects also produce high-energy massive particles such
as electrons, muons, protons and anti-protons. These higher energy particles are
potentially dangerous, but most of these particles never make it to the earth. They are
deflected by magnetic fields between us and the source, or they interact with other
particles, or they decay in flight.
The particles that do make it to the earth interact with our atmosphere, which acts as a
'radiation shield.' The high-energy cosmic rays bombard us all the time, but they interact
quickly, producing particles of much lower energy which impact the earth harmlessly. If
this was dangerous to us, we wouldn't be here to discuss these things! Some particles, like
neutrinos and high energy muons, are passing through us all the time, but they interact so
weakly that they have no effect on our bodies. Of course, if we were in space without the
protection of our atmosphere then we would need some other type of shielding from the
radiation (spacesuits and protective covering on our spacecrafts).
The radiation to worry about, of course, is the 'cosmic' radiation produced by our sun.
There is only one type of cosmic radiation known to adversely affect us and that's UV
radiation from our sun, which causes skin cancer in millions of people every year.. Again,
our atmosphere serves as a shield, but ultraviolet photons do make it through -- and
without that protective ozone layer which blocks these photons we're all going to need a
lot more sunscreen!
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley