Until now.
Taking lessons from animal hibernators, scientists are using their tricks for medical therapies and may someday adopt them for space travel. Some physicians are employing therapeutic hypothermia — a lowering of the body temperature by a few degrees for several days at a time — to help treat patients with traumatic brain injuries or diseases such as epilepsy. And trials are underway to see if there is a way to lower the body temperature of people, keep them in a sleeplike state for days or weeks and then revive them with no ill effects, something that astronauts may have to do to travel deep into space.
"We see the science has advanced enough to put some of the science fiction into the realm of science reality," says Leopold Summerer, head of advanced concepts team of the European Space Agency, one of the operators of the International Space Station. "It doesn't mean we will have hibernating astronauts anytime soon, but we are learning from nature how to understand some of the things that happen to animals during hibernation, such as preventing bone loss or preventing muscle loss. This is already something that would be a great benefit for long-distance space flight."
A panel of European biomedical researchers, biologists and neuroscientists is expected to deliver recommendations for future lines of human hibernation research and funding soon, according to Summerer. One Italian scientist says he will begin an experiment this month to lower a test animal's body thermostat for a six-hour period as a precursor to human trials.
NASA funded a preliminary study that looked at the idea of putting astronauts into a state of torpor, or hibernation, for weeks at a time. The prospective benefits that were reported last year included a cut in the food and water required on their spacecraft, a reduction in waste products, smaller living quarters and less space needed for supplies, exercise and entertainment. And putting the crew to sleep might minimize their psychological challenges. The idea, however, didn't make it to a second round of funding. John Bradford, head of the company that proposed the human hibernation, says he's hoping to get funding elsewhere.
NASA says it will be using the year-long sojourn that U.S. astronaut Scott Kelly just started at the International Space Station, combined with medical monitoring of his earthbound twin brother, retired astronaut Mark Kelly, to collect clues about protecting humans who leave Earth's orbit for months or years at a time.
Biologists aren't waiting for results from space. They are busy dissecting the neurological and biochemical pathways of such hibernating animals as the Arctic ground squirrel, which sets its internal body temperature at 32 degrees — the freezing point — during the winter, and several kinds of bears that slumber six months at a time without awakening as puny weaklings.
"We think that if we understand how they do it, we can replicate it in humans," said Kelly Drew, a biochemist at the University of Alaska at Fairbanks.
Kelly and her colleagues at the university's Institute of Arctic Biology are looking at how the Arctic ground squirrel can get so cold without dying. She believes she has found the molecule that does the job, the A1 adenosine receptor. While she has learned that stimulating this receptor makes the animal get cold, she hasn't found what triggers it.
"We don't know what the natural signal is for torpor," she said. "We don't know where the signal occurs in the brain — it could be in the brain stem or the hypothalamus."
The next step is to learn how to safely use drugs that stimulate the A1 adenosine receptor and then induce animals that do not normally hibernate to enter and stay in a state of torpor for two to three weeks at a time.
A shorter period of human deep sleep, induced by cooling the body, is already used to help brain-injury patients at the Johns Hopkins University and many other hospitals.
Romergryko Geocadin, a Hopkins professor of neurology and anesthesiology, has used the treatment, known as therapeutic hypothermia, to help patients with severe epilepsy or brain trauma.
"We don't know why it works," Geocadin said, "but we know it slows down metabolism and the inflammation" that occurs in the brain with epilepsy.
Cooling the body gives the brain a chance to repair itself. "You actually lower the need for energy for the entire body," he said. "So you give it time to catch up."
Using ice packs, liquid-filled blankets, caps and even cooling IV fluids, Geocadin lowers a patient's body temperature from its ordinary level of about 98 degrees to about 91 to 93 degrees for up to three days at a time. (This involves a handful of patients each month who are comatose because of cardiac arrest or who are experiencing seizures or brain swelling.) Longer than that, he says, "and the whole house of cards starts to fall apart," meaning that the risk of blood clots, pneumonia and other complications increases.
Meanwhile, scientists in Italy will start a clinical trial in April to lower the body temperature of a pig by inhibiting a part of the hypothalamus that controls energy levels, in effect inducing hypothermia. It's a proof-of-concept trial that could lead to longer durations of torpor for an animal that doesn't normally hibernate.
Inhibiting this region of the brain "seems to work the same way in several different species," said Matteo Cerri, an assistant professor of physiology at the University of Bologna, who is leading the work. "There is some wishful thinking that the same [areas of the brain] could work across all mammals."
Cerri and other researchers hope to apply some of these new medical and pharmaceutical approaches to healthy human volunteers. But that could be a ways off, scientists say, given the moral issues involved in putting humans into hibernation: What if something goes wrong? It's an ethical gulf that perhaps is just as great as the challenge of sending a rocket to Mars.
"If you are not conscious, how are we going to know what to do if something goes wrong?" Cerri asks. Still, he says he'd be happy to be a volunteer. "I would love to try it."
Yet there are skeptics who doubt that any of this will lead toward human hibernation useful for space travel.
Stanford University neurobiologist H. Craig Heller has studied black bears and northern brown bears, both of which hibernate. Lowering their body temperature by only a few degrees, they curl up for months at a time in their dens without urinating or defecating.
Because bears recycle the nitrogen from their waste products, going for months without moving doesn't seem to affect their bones or muscles. Astronauts, on the other hand, can barely walk after returning from a few weeks or months in zero gravity, and they have to keep up rigorous exercise programs to maintain bones and muscles while in orbit.
So while it might be possible to induce humans into deep sleep by cooling the body, Heller said, a months-long spaceflight under such conditions is likely to be too damaging.
"I think it's probably not doable," he said. "The hibernator [animal] has evolved so that all the enzymes and biochemical systems are adapted to run at low temperature. That is not true of animals that don't experience it. We can lower body temperature and survive that for a short period of time; it's unlikely we can allow all of our systems to go to a much lower temperature and continue to function."
Niiler is a freelance writer on science topics.
