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

Five good reasons why their aren’t alien visitors

Imagination travels where bodies can’t.  There are three very good reasons why aliens haven’t visited this planet.  One is the dose of radiation in a long space journey would prove lethal, and this cannot in a practical way be prevented.  Two time.  It takes too long.  At one tenth the speed of light (assume that can be reached) it would take 42 years to travel from the closest star (Proxima Centauri).  This brings us to the third problem energy to move at that speed.  It takes scads of energy to get a proton in a atomic generator to send a proton at a speed near that of light.  Consider how much more to send a group of people.  The fourth is that of dust.  Grains of sand would penetrate a vehicle traveling at that speed.  Fifth, If there was a way of producing a 20 teslas magnetic force to deflect cosmic rays, the equipment would generate sufficient disruptive electromagnetic forces so as to do more damage than the background radiation experienced in space.  And as for hearing them, the use of energy gobbling broads will be replace by smoothing more efficient--jk.