Long-term human space flight faces at least three major barriers before continuing on with the proposed Mars mission plans, a Penn State professor says.

"Only eight people have been in space over six months, and that's in all of human history," said James Pawelczyk, associate professor of kinesiology and payload specialist on NASA's STS-90 Neurolab mission, which examined the effects of space flight on the nervous system.

The largest problems that will be faced by astronauts, he said, are bone-density loss, heavy doses of radiation and major psychological issues.

Bone-density loss has been documented in all astronauts who have spent some extended amount of time in space, said Pawelczyk, who was in outer space for 16 days during the Neurolab mission in 1998.

Without the presence of gravity over longer periods of time, bone density decreases, especially in the lower half of the body, because the skeletal structure does not have to support the weight of the body.

"The bone density increases slightly in the skull, but everything else is decreasing -- some areas at a rate upwards of 1.5 percent per month," Pawelczyk said.

This would mean that after spending the proposed 30 months in space on the round trip to Mars, a young, healthy astronaut would return with bone density similar to that of a person who has been bed-ridden his entire life due to a spinal-cord injury. "On a trip to Mars, astronauts could lose almost half of their bone minerals in some areas," Pawelczyk said, "and that's with exercise."

This is assuming the gravity on Mars won't protect against this loss, he added.

One way researchers believe will help combat this accelerated form of osteoporosis is to use drugs similar to those used here on Earth to help prevent the condition, Pawelczyk said. For instance, bisphosphonates are a family of drug that prevents bone erosion, he added.

The bone minerals that were lost can begin to return if the individual is very aggressive with their rehabilitation when they return from space, Pawelczyk said. However, this can happen only if the cortical bone -- the type of bone that is affected by osteoporosis -- has not reached the stage where the webbing inside has completely broken, he said.

The next major hurdle faced will be the massive amounts of radiation astronauts will face during their trip through deep space to Mars.

The levels of radiation that could be faced are only found on Earth in nuclear explosions, Pawelczyk said. "Heavy metal ions hit the craft and break into smaller pieces that have less energy, but are still very damaging," he said.

In a 30-month space flight, every cell in the body could be hit at least once by these ions that can cause direct damage to a person's DNA, he added.

"This leads to an increased risk for cancer," Pawelczyk said.

Radiation levels that would lead to an increase in risk of cancer that are more than 3 percent higher than normal are not acceptable, he said. A healthy male already faces a 20 percent chance of developing cancer in their lifetime.

"This would mean that the crew would be facing a 23 percent chance," Pawelczyk said, "but we could be off by 10 times that number due to the radiation caused by solar flares."

Water is a good way to prevent this exposure to radiation, Pawelczyk said. For that reason, the craft designed for this trip probably will have the water containers surrounding the living and working quarters onboard.

"Even on the surface, astronauts will have to worry about radiation," said James Kasting, professor of geosciences and meteorology. "A lot of near-ultraviolet (UV) radiation gets through Mars' atmosphere. There is almost no UV protection, and astronauts will get harmful doses in terms of seconds to minutes."

In the long term, terraforming Mars -- or making a planet habitable -- may be one way to prevent these massive doses of radiation, Kasting said. However, creating a strong atmosphere on Mars may be a bit of a challenge because no practical way has been found to accomplish the task, he said.

While psychological problems will be a major factor, as they have been for almost all forms of large-scale exploration throughout human history, there are a few special instances that will have to be addressed for space flight, Pawelczyk said.

Astronauts sometimes develop Space Adaptation Syndrome -- disruption of the vestibular system that controls a person's balance, with symptoms similar to that of motion sickness, and even visual illusion or anomalies, Pawelczyk said.

"It would not be safe if the pilot or other crew members began experiencing something like this during the landing process," he said.

Astronauts will also face incredibly tough working environments, along with the "away from home, in a can" issue, he said. Coupled with only having limited human contact, depression and other illnesses could easily set in, he added.

"There have been three instances, none U.S., where the mission was canceled due to the mental health of the crew," Pawelczyk said.

Researchers are working on ways to develop the "groundwork" to solve these problems, Pawelczyk said, "but it's going to be the students of today that will be the ones making the expeditions."