The next “giant leap” for humans may be a trip to Mars, but having enough oxygen-carrying red blood cells for the journey might present a challenge, new research suggests. Even space tourists lining up for short trips might have to stay home if they are at risk for anemia, or red blood cell deficiency, researchers said. Astronauts are known to experience “space anemia” but until now it was thought to be temporary. One NASA study called it “a 15-day ailment.” Doctors attributed it to destruction of red blood cells, or hemolysis, resulting from fluid shifts as astronauts’ bodies accommodated to weightlessness and again as they re-accommodated to gravity. In fact, anemia is “a primary effect of going to space,” said Dr. Guy Trudel of the University of Ottawa, who led a study of 14 astronauts funded by the Canadian Space Agency. “As long as you are in space, you are …