Designed for 1 g: The Physical Cost of Spaceflight
- Sheila Thompson, RN CPHQ
- 2 hours ago
- 6 min read
The human body evolved over centuries with one constant: gravity. Without it, our dependence on that ubiquitous pull becomes readily apparent as muscles weaken, bones thin, and even vision is affected.
Many of us held our breath watching the recent return of Artemis II, only releasing it when the four-person crew safely emerged from the capsule. Their smiling, triumphant faces in the images captured afterward reflect the excitement we feel about a successful space mission and fuel the imagination for future space exploration. What their smiles don’t reveal is the physical toll on their bodies. This moon flyby marked the first manned flight beyond low Earth orbit since Apollo 17 in 1972; but has technology advanced to mitigate the negative effects of near-zero gravity on the human body? Let’s look at what happens to humans outside Earth’s gravitational field and how science responds.
What is “Microgravity”?
Microgravity and weightlessness are related but different. Microgravity refers to an environment where the gravity we’re accustomed to is significantly reduced. While in orbit, such as on the International Space Station (ISS), objects and people are still subject to gravitational force but they stay at approximately the same height because the ISS is traveling at such high speed. This state of constant freefall is experienced as weightlessness. While it may look fun to be able to float around or easily lift heavy objects, it takes a toll on the body.
Because our bodies are used to a certain amount of gravitational pull on Earth, they have adapted to ensure a uniform distribution of vital bodily fluids to maintain homeostasis and regulate pressure in the arteries. In microgravity, fluid shifts away from bones, muscles, and tissue and toward the head in a condition known as cephalic fluid shift. This leads to muscle atrophy and bone demineralization, particularly affecting the lower extremity muscles responsible for posture, balance, and movement.
Musculoskeletal System Effects
In addition to atrophy of the thigh and calf muscles critical for posture and balance, the reduced ground reaction force experienced in space affects locomotion through stride length and speed. In microgravity environments, astronauts find hopping more effective than walking and may run at lower speeds. Both strategies increase stability and decrease metabolic impacts.
Stability is another factor in human locomotion, relying on coordination between sensory and motor information from the central nervous system and gaze and head position. Microgravity can affect the vestibular-ocular system that controls balance and gaze, causing disorientation, falls, and injuries.
Most astronauts get taller in microgravity as their spine extends 4 to 7 cm. Unfortunately, spinal lengthening during spaceflight tends to cause pain in the lumbar spine and increases the risk of herniated nucleus pulposus to four times that of non-astronauts. Measures to mitigate back pain include exercise, analgesics, and a knee-to-chest position. Long term effects include microfractures of the spine and inter-vertebral disc degeneration.
Injuries to the hands, shoulders, and back are a common reality during space missions. Repetitive motions required for many tasks are a factor, but perhaps the primary culprits are the necessary protective clothing and other adaptations for microgravity. The pressurized gloves impact grip strength and dexterity, and frequently cause fingertip and fingernail injuries such as abrasions, neuropathies, dislocations, and frostbite. The suit worn for activities outside the vehicle, known as the Extravehicular Mobility Unit or EMU, presents numerous challenges. The pressurized suit has multiple layers, beginning with a garment that regulates temperature and ventilation using circulated water. On the upper torso, a rigid fiberglass covering enables attachment of structural components. The EMU restricts movement, causes fatigue and discomfort, and exacerbates problems such as microgravity-associated low back pain. Friction and strain at the attachment points on the upper torso shell are often the impetus for shoulder injuries.
Neuro-ocular Changes
Changes in eye structure and visual acuity, known as spaceflight-associated neuro-ocular syndrome (SANS), are common in astronauts during missions greater than 1 month in duration. A recent study involving 11 ISS crewmembers found choroid thickening and peripapillary optic disc edema typically associated with SANS bilaterally and in members of both genders, expanding upon previous findings. Ophthalmic assessment also confirmed decreased axial length of the eye that persisted 1 year after the mission. With plans to increase mission length in the future, SANS presents significant concern.
Immune System Monitoring
Neutrophil-to-lymphocyte ratio (NLR) is commonly used to measure subclinical inflammation, as chronic inflammation is an indicator of disease development and prognosis. A study of human and rodent subjects in space and in simulated microgravity found elevated NLR, supporting this as a potential biomarker for monitoring of immune health during spaceflight.
Radiation-induced Health Risks
Astronauts face significantly greater risk of cancer due to ionizing space radiation exposure such as X-rays and Gamma Rays. Those stationed on the ISS are exposed to solar flares and galactic cosmic rays despite some protection from the Earth’s magnetic field. Crew members on a 6-month ISS mission receive 50 to 100 millisieverts (mSv) of radiation exposure (higher if performing tasks outside the vehicle), compared to an annual average of 6.2 mSv for a person in the United States. Experts project exposure greater than 1000 times the annual exposure on Earth if spaceflight expands to Mars in the future. Exposure greater than 1Sv can ultimately be fatal.
The danger of radiation exposure is the resulting increase of free radicals, which contribute to various physiological disorders. Absorbed dose, individual sensitivity, and protective measures such as shielding determine how the damage manifests.
Countermeasures and Mitigation Strategies
Traditional countermeasures centered around exercise, nutrition, and pharmacologic options intended to minimize muscle atrophy and bone loss. While these measures are still foundational strategies, scientists are exploring newer technologies.
Physical Countermeasures have progressed from early versions of resistance bands and treadmills to intense regimens involving sophisticated treadmills, stationary bikes, resistive exercise equipment, and high intensity interval training. While somewhat effective, current strategies fall short, so further improvements or new solutions are needed. One promising new approach involves using electrical stimulation to activate muscles. Researchers are also exploring the use of artificial gravity, and while the Japanese Space Agency reported some success using rodent models, spinning an entire spacecraft presents safety challenges and wouldn’t be cost-effective.
Biomedical Approaches include pharmacological options such as bisphosphonates for bone resorption and low-dose testosterone. Various biomonitoring methodologies are used or are in development to assess physical response to the environment. For example, multiple sensors are embedded in the EMU to monitor heart rate, breathing, blood pressure, skin temperature and moisture, sleep quality, and to measure discomfort and risk of injury. Systems that analyze body fluids, metabolites, hormones, and electrolytes are still in the testing phase but could be used for real-time feedback and personalized adjustments.
Nutritional Strategies are critical not only for energy to live, work, and heal, but also to meet crew psychosocial and morale needs. An innovative Vegetable Production System called VEGGIE can grow fresh food on the ISS while promoting rest and relaxation. Nutritional goals include adequate dietary calcium, protein, sodium, and vitamins C, D, and K to counteract bone and muscle loss caused by microgravity. Minerals including copper, zinc, and selenium, and vitamins A, C, and E can support antioxidant defenses and reduce oxidative stress.
Wearable robotics are gaining attention as an Innovative Technology with applications for spaceflight. Possibilities include assistance with tasks in and outside the space vehicle, mimicking a sense of gravity, or applying tension and resistance to muscles (dynamic muscle loading), which supports strengthening and repair. Using rigid exoskeleton versions in a microgravity environment eliminates the weight constraints prevalent in other industries. In contrast, wearable soft robotics are preferable for post-flight at-home rehabilitation though they can also be useful in assisting complex joints such as the shoulders while performing tasks in space, as they are lighter and less restrictive.
Considerations for the Future
Expansion of spaceflight duration and distance is partially limited by what the human body can tolerate. Current measures mitigate but don’t eliminate the effects of microgravity and are bulky and resource intensive. Continued evolution of the EMU incorporating exercise, rehabilitation, task assistance, sensors for real-time monitoring, computing, and intelligence is essential. Materials development for batteries, electronics, spacesuits, and sensors will be key in realizing those advances. As a bonus, this type of research and development has applications on Earth such as for medical devices and other technologies.
Another challenge is that most of the data and research on the effects of spaceflight comes from low Earth orbit scenarios, such as on the ISS, thus its applicability to deep space and extended missions is limited. Currently available data comes from diverse space programs with small sample sizes and different methodologies so collaboration and standardization would likely accelerate progress. Could travel to Mars be next?
Resources:
Assessed and Endorsed by the MedReport Medical Review Board