TIM FURNISS / LONDON
Many medical and psychological hurdles will have to be overcome before astronauts can go to Mars
It is the ultimate sci-fi horror story. An astronaut returns from a three-year flight to Mars. He steps off the spaceship and collapses, his bones unable to support his weight, because, as a result of years in zero gravity, he has suffered total calcium loss.
Unfortunately, this is a little closer to fact than to fiction. "They won't come back as jellyfish, but we shouldn't expect them to squat 450lb [200kg] either," says Tamarack Czarnick, resident in aerospace medicine at Wright State University in Dayton, Ohio.
The greatest goal in manned spaceflight today is a Mars landing. The technology and the budget for such an expedition are within capabilities - but there is a question mark against the medical risks of undertaking a three-year roundtrip in space.
Apart from the usual inconveniences, such as sleep disturbance and motion sickness, the major challenges of long-duration spaceflight are loss of bone minerals, muscular atrophy, heart deconditioning, vestibular hearing dysfunction and psychological stress. The loss of bone calcium, particularly in the legs, was discovered after the first long-duration space missions. Complications such as kidney stones made up of accumulated free calcium, and a greater likelihood of broken bones were also possible hazards of a Mars mission.
Exercise has proved to be effective in reducing bone loss, but results are inconsistent. On a two-man Russian Salyut mission, one cosmonaut lost 10% of vertebral bone density, while the other suffered none. Extra calcium and calcium-retaining drugs seem to have helped. Exercise regimes using high-impact stress - inadvisable on Earth - also seem to stimulate bone calcium retention.
In-flight diagnosis would, of course, help. The National Space Biomedical Research Institute (NSBRI) in Houston, Texas, has designed an advanced multiple-projection dual-energy X-ray absorption meter capable of measuring tissue mass, bone density and bone geometry in flight. "Knowing these measurements will allow astronauts either to increase exercise or take medications to counter the loss of bone and muscle mass due to long-duration microgravity exposures," says Dr Harry Charles, NSBRI head of technology development.
Muscle atrophy, however, may be a different matter. Muscle makes up almost 40% of the human body, and it can weaken and easily become fatigued. Reflexes and some fine motor control can also be affected. Symptoms are more acute than those associated with the bed-ridden: a 35-day orbital flight can cause leg muscles to atrophy by 25%. This partly explains why many Russian cosmonauts must be carried from their descent capsules on return to Earth after long flights, and one in 10 shuttle astronauts has to be carried off the orbiter even after a relatively short flight (the longest shuttle flight lasted 17 days - the longest Mir mission more than 400).
The condition of two cosmonauts who flew a then-record breaking 17-day mission on Soyuz 9 in 1970 was so poor they had to be transported from the craft and kept under observation for two weeks. But, with the right type of exercise, Czarnick says it is unlikely muscle weakening will significantly threaten long-duration spaceflight.
Heart problems
Cardiac deconditioning, caused by a decrease in the heart's workload in a weightless environment, can cause diuresis, lower plasma levels, and prevent the manufacture of red blood cells. Another possible effect is low blood pressure, which can cause dizziness and irregular heart rhythms. Russian drug experiments, however, using magnesium, nitroglycerine and potassium have proved effective, as have lower body-negative pressure-suits and the use of g-suits in combination with exercise. NASA encourages vigorous aerobic training on cycles and treadmills, and recommends high-potassium diets to counter cardiac problems.
Microgravity causes loss of proper functioning of the inner ear, where otoliths - tiny granules of carbonate - form at the end of sound-sensitive hairs, making them rigid and no longer responsive to gravity. This causes a loss of a sense of "up or down", creating sensory confusion in the brain.
Around 40% of astronauts and cosmonauts also experience some gastric discomfort. But after two to seven days, the body seems to adapt - though this is of little comfort to those on week-long missions.
Saline ingestion has also been found to curb space sickness, as does trying to maintain an "image" of up and down along the body axis, says Czarnick. Ironically, the nausea caused by motion stimuli on Earth seems to be reduced by spaceflight. Test subjects flying 0g parabolas after landing from a spaceflight do not get sick as frequently. On returning to 1g, the body adapts, and it normally takes several days before a space traveller recovers. No effective countermeasures to space disorientation have been found, however, and some scientists are concerned at the prospects of a weakened, nauseous, dehydrated and disoriented astronaut trying to land a spacecraft.
Another concern is crew compatibility, particularly on long missions. The psychological stress can cause problems even for a compatible crew. Russian doctors were the first to notice that crew members could become prone to lethargy and apathy, a condition which appeared to worsen the longer the mission went on. Fatigue, loss of motivation, and irritability increases mid-way through a long mission, and if measures are not taken, hypoactivity, feelings of isolation, debility, sleep loss, depression, hostility and a lack of productivity can follow.
An example of what can happen was provided by the "rebellion" of the 1974 US Skylab 4 crew, who went on strike, and the legendary tetchiness of the Apollo 7 crew in 1968. In Russia, a two-man crew refused to talk to each other during a 211-day mission. According to one crewman Valentin Lebedev, the crew of Soyuz T5 badly got on each other's nerves in 1982.
A seriously ill patient on the International Space Station can be immediately returned to Earth - in a Soyuz craft, with a two-man crew - or in the shuttle. On a Mars mission, this would be impossible. Treatment would have to be conducted in flight with help from the ground.
Treatment in-flight
Russia has advised its US counterparts about long flights on the ISS. Russian doctors recognised stress symptoms in Norman Thagard, the first US astronaut to fly on Mir, and recommended he be given more work by colleagues and better integrated into the team - among whom there was still a "them and us" approach.
The experience of later astronauts varied widely: Shannon Lucid and Mike Foale were eager to get involved with the Russian crew, while others found it difficult to adjust to life on a long-duration space station mission. One astronaut, John Blaha, even suffered mild depression.
Private communications with family via e-mail and TV conferencing, medical consultations with doctors, and a wide range of entertainments, including videos and regular exercise, are part of the long-duration routine - and it seems to work. "Psychological countermeasures seem to play a considerable role in maintaining crew health, and can be expected to gain prominence as stays on the ISS lengthen and piloted Mars missions draw closer," says Czarnick.
However, there is one other potential problem for any Mars-bound crew. The sight of the Earth getting smaller and eventually becoming a blue speck in the blackness of space could be traumatic. The sight of the orange-red Mars target getting bigger, however, may be excellent therapy.
Source: Flight International
