One is the Kryptonian cell structure, adapted to an older, red star. They act as a solar battery when exposed to the radiation of newer stars, like Earth's yellow sun. Immediately, a Terran would start suffering from a vitamin deficiency missing the vitamins we get from our sun. Is it any wonder Sol acts as a supercharger for Kal-el's people.
Secondly, the gravity of Krypton exceeds that of the earth. No estimate has been given, but considering the size of the planet, it can be anywhere from 2-5 times as much. While a human could survive, it would be a fairly difficult probability that they would enjoy it. A good test would take decades to accomplish, but say we colonized our moon, with it's 1/6th gravity. Now, a couple generations down the pike, a "loonie" decides to visit Earth. At 86Kg earth normal, he's grown up believing that his 14KG equivalent is normal. The 86Kg reality on earth would be the same as him suddenly weighing 516Kg. Even at twice Earth Normal Gravity (2G) the effect would be 172Kg feeling.
So, instead of gaining superpowers, an Earth Human would suffer greatly without special preparation and adaptations just to live life as a handicapped person on Krypton.
Do I sound geek? Well, I've followed the adventures of Superman since Ike was president. There are many stories of various Terrans visiting Krypton or Kandor, so it is not really total immersion. Plus, I did run a comic shop for a few years.
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