That humans are not designed for spaceflight is a reality weighing heavy on NASA scientists working against the clock to put astronauts on Mars by the 2030s.

John Charles, the chief of the international science office of NASA's human research program, told The New York Times he thinks NASA is already capable of getting an astronaut to and from Mars. At this point, the trick is making sure he or she arrives in good health.

"My goal," he said, "is to see a program that doesn't deliver an astronaut limping to Mars."

The challenges researchers face are many and cover a broad range in terms of difficulty. The loss of bone density is largely solved through exercise and osteoporosis medication, the Times explains; however, even as scientists succeed in solving one problem, others continue to emerge.

For example, two astronauts aboard the International Space Station in 2009 began to notice they were having trouble seeing things up close. Both physicians, they performed eye exams on one another, and using high-resolution cameras sent by NASA were able to confirm that their eyes had been slightly squashed.

"It is now a recognized occupational hazard of spaceflight," Dr. Michael Barratt, one of the astronauts to notice something was wrong, told the Times. "We uncovered something that has been right under our noses forever."

Perhaps even more worrisome is the problem of radiation, which, according to a study by researchers from the University of Rochester Medical Center, may accelerate the onset of Alzheimer's disease. This is in addition to the known increased risk of cancer cosmic radiation poses.

"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," said Cary Zeitlin, a principal scientist at the Southwest Research Institute in San Antonio and lead author of a paper that looked at radiation levels on Mars.

Such obstacles were admittedly a key reason behind the recent extension of the the International Space Station to 2024. In a joint statement about the station's delayed retirement, NASA Administrator Charles Bolden and Assistant to the President for Science and Technology John Holdren said the extension will allow the space agency to "mitigate fully 21 of the 32 human-health risks anticipated on long-duration missions."

Already, NASA has selected its guinea pig: astronaut Scott Kelly, who will be joined by Mikhail Kornienko for a year-long mission aboard the space station starting in 2015.

Kelly, who was reportedly the one to suggest the idea, previously spent time aboard the space station in 2010 and 2011. Not only that, but Kelly has a twin brother, meaning researchers will have a control to compare any changes to.

Scott's twin Mark told the Times: "My attitude is, I worked at NASA for 16 years and whatever I can do to help, I will."