Why there aren’t alien visitors
1. HAZZARD OF PROLONGED SPACE TRAVEL INSURMOUNTABLE: Cosmic rays are ionized atoms traveling at near the speed of light--primarily protons. It takes a vertical column of about 70 grams—about 1/14 the distance through the atmosphere, achieved
at an altitude of 20 to 25 kilometers (60-8000 feet)—before the average incoming proton hits the nucleus of an atom
in the air. The rest of the atmosphere serves to absorb the shower of high-energy
gamma rays and pi mesons, or pion, particles. Each gamma ray propagates deeper
into the atmosphere and ends up producing an electron and its antimatter counterpart, a positron. These two particles annihilate each other, yielding less energetic gamma rays, and so the cycle continues
until the gammas become too weak to create particles.
The annual cosmic radiation dose of about 0.03 rem is equivalent
to a couple of chest x-rays.
Outside the atmosphere,
the cosmic-ray bombardment is approximately one proton or heavier nucleus per second passing through your finger nail every
second, or 5,000 though your body—each one leaving a trail of broken chemical bonds and triggering the same cascade
that occurs in the atmosphere. It is estimated by NASA that
about one-third of the DNA in an astronaut’s body would be cut by cosmic rays every year. In space they would receive more than 80 rems per year—the legal dose for a nuclear power-plant worker
in the U.S. is 5 per year. One in 10 male astronauts would eventually die from cancer [attributable to their exposure], and one in
six women (because of the greater vulnerability to breast cancer).
A severe solar flare, such as that of February
23, 1956, would deliver in excess of a couple of hundred rem over an hour or so—a lethal dose. Protective shielding, either magnetic or matter require too much materials to justify construction on earth
and launching into space. A field
of 20 teslas (600,000 the strength of the earth’s field at the equator) would (if it could be achieved) have significant
health consequences. It would take about 500 grams of material to shield 1 cm
square. Thus long-term space travel is beyond us.
Scientific American, March 06