48 ASTRONOMY • APRIL 2018
the nuclei of heavier elements. Their ener-
gies are customarily measured in electron
volts (eV), the amount of energy an elec-
tron gets after moving through 1 volt of
electrical potential.
The cosmic ray spectrum we observe
peaks at a few hundred million eV. That
sounds like a lot, but because it’s less than
the equivalent of the mass of a proton, cos-
mic rays travel slower than the speed of
light. Also, their energy is small enough
that Earth’s magnetic field can def lect
them. Because low Earth orbit lies inside
the field, most of our astronauts have con-
siderable protection. From this energy on
up, the flux of cosmic rays decreases. The
highest-energy ones have an amazing 10^21
eV, but on average go through a given
square meter of area less than once a cen-
tury. Good thing, too: These have the
kinetic energy of a Major League Baseball
pitch packed into one atomic nucleus!
Multiple sources of cosmic rays exist,
but evidence indicates that supernovae are
a major source up to about 10^15 eV. W hat
we detect on Earth is a kind of average over
the many cosmic ray sources. Because
magnetic fields def lect charged particles,
the Milky Way’s magnetic field scrambles
most of their trajectories, so there’s no way
to tell where they came from. However, the
cosmic ray background will increase if
there is a nearby event. Scientists are work-
ing out detailed calculations to decide how
much the background would go up.Cosmic rays and
the atmosphere
Our atmosphere protects us from cosmic
rays even more than Earth’s magnetic field.
The magnetic field tends to def lect cosmic
rays to the poles, but the atmosphere soaks
up a lot of their energy. A typical cosmic
ray has its first collision high in the atmo-
sphere, and it undergoes many more on
the way down. By comparison, the first
collision in Mars’ thin atmosphere happens
near ground level, making the radiation
load there about the same as in space. This
is a nearly fatal flaw in Andy Weir’s excel-
lent novel The Martian and many other
schemes for colonization.
Within Earth’s atmosphere, an air
shower cascade occurs. Particles collidewith other particles, producing new par-
ticles. Energy is lost because molecules in
the atmosphere are ionized, which may
lead to ozone depletion in the stratosphere
described earlier. The result is that here on
the ground, we don’t get much radiation.
One thing we do get is muons. Think of
these particles as a kind of “heavy electron”
penetrating all the way to the ground.
Muons are unstable, and most decay into
electrons within a couple of microseconds.
Still, most of the muons produced in the
upper atmosphere reach the ground, about
10,000 per square meter every minute. They
even penetrate a few hundred meters of
rock or water. They mostly pass through us,
but some interact, and there are so many of
them that they account for about one-sixth
of the total radiation we get, on average.
Most cosmic rays lose their energy by
the time they get into the stratosphere.
However, the ones with the highest energy
penetrate much farther and produce many
more muons, which reach farther still.
Scientists have done computations of
the cosmic ray flux at Earth, assuming dif-
ferent kinds of local galactic magnetic-field
configurations. The case we’ll focus on
here is a weak, tangled magnetic field
assuming that it lies inside a Local Bubble
blasted out by earlier events. We take a
type II supernova of typical energy that
releases cosmic rays up to 10^15 eV. Then we
compute the path of these cosmic rays of
various energies to Earth.
Within 100 years, Earth is inundated by
cosmic rays. The intensity is anywhere
from 20 to 100 times the normal f lux at
high energies, depending on the distance
to the supernova. At low energies, the f lux
doesn’t change much. This is crucial to
understanding the effects.Effects of rays on Earth
As mentioned earlier, the f lux of muons
on the ground goes up. This increases the
radiation load, but not catastrophically. It
might explain an apparent acceleration of
mutations over the last few million years.
It might enable some acceleration of evolu-
tion. And it might increase the cancer rate
a bit, but it would be hard to see evidence
of this in the fossil record.
Our recent computations suggest that
the radiation dose from muons may go up
by a factor of 100 or more for some time.
The biggest change would be for large
organisms in the upper ocean because of
the penetrating ability of muons. It is
tempting to think that there is some rela-
tionship between the supernovae and aThe Crab Nebula (M1) in the constellation Taurus the Bull is perhaps the best-known example
of a supernova remnant. At a distance of 6,500 light-years, however, that supernova could not
have affected Earth in any significant way. NASA/ESA/J. HESTER AND A. LOLL (ARIZONA STATE UNIVERSITY)