The relatively weightless conditions of orbital and interplanetary space are inherently deleterious to human health. Humans and other terrestrial animals have evolved their physical and physiological attributes under our planet's heavy gravitational environment.
On the surface of the Earth, the human heart has to counter the downward pull of gravity in order to pump adequate amounts of blood to the head and torso when people are standing erect, while blood flowing to the lower limbs is aided by the pull of gravity. But under the microgravity conditions of space, human blood disproportionately flows to the head and torso while blood flow to the lower limbs is reduced. This makes the human face look puffy while their legs become thinner.
Such blood flow redistribution can sometimes cause nausea and headaches when astronauts first arrive in orbit but usually disappears after a few days in space. But other minor but annoying problems can be experienced under weightless conditions, including:
1. Weight loss: the less strenuous conditions diminish appetite, resulting in weigh loss which could become excessive if astronauts don't exercise and eat regularly.
2. A degraded sense of smell and taste: your favorite foods could taste a little different under microgravity
3. Clumping of tears and perspiration: there's no gravity to force trickles of water to run off the human body
4. Facial and speech distortions: the face becomes puffy and the voice tone and pitch becomes more nasal
5. Increased flatulence: since digestive gasses no longer rise towards the mouth, their is an increase in gas being expelled through the posterior orifice
The problems listed above could be viewed as only a minor inconvenience on short missions into space. But during long interplanetary journeys lasting months or years, such problems could be annoying enough to enhance stress and increase tension aboard ship.
But there are other, more serious health problems that the human body may be subject to during months or years in a microgravity environment:
1. Astronauts can lose between 1 to 1.5% of their bone mass in a single month
2. Without regular exercise, astronauts can lose up to 20% of their muscle mass in just 5 to 11 days.
3. A microgravity environment can reduce cardiovascular fitness
4. Vision problems of varying degrees of severity can occur especially in older men
5. Blood flow redistribution in a microgravity environment can effect medicines ingested or injected into the human body.
6. Fluid loss and bone demineralization in a microgravity environment can increase the blood's calcium concentration, increasing the risk of an astronaut developing kidney stones.
7. The infected spray from the cough or the sneeze an ill person on board floats in the air instead of falling to the floor, enhancing the spread of infection aboard ship-- especially in a confined environment.
Returning to Earth after a few months aboard the ISS, the blood pressure of some astronauts drops abnormally low when they move from a lying position to a sitting or standing position. Some astronauts even have problems standing up, walking, and turning and stabilizing their gaze.
Since conjunction class missions to Mars may take more than seven months, there could be some question as to whether or not astronauts would have the physical ability to perform a mission to the Martian surface. Astronauts would have to endure weightless for several more months during their return trip from Mars to Earth.
One possible solution to the deleterious effects of weightlessness would be temporary periods of high simulated gravity (hypergravity). On Earth, sustained bed rest on a flat surface that is tilted a few degrees backwards can simulate the deleterious effects of a microgravity environment, causing more blood to rush towards the head and less blood to flow into the hind limbs. But if such beds are attached to a short radius centrifuge, high levels of G forces can be experienced at the human foot level.
|Short radius hypergravity centrifuge could help to mitigate the deleterious effects of a microgravity environment on the human body (Credit NASA).|
Hypergravity studies have shown that protein synthesis in the leg muscles can be sustained if 2.5 G is experienced for just one hour a day at the foot level during a 21 day period. This suggest that daily exposure to a brief period of hypergravity in space may be an effective counter measure, mitigating the loss of muscle mass in a microgravity environment.
|Production of the 8.4 meter section of the SLS hydrogen fuel tank. Such components could be used for reusable common bulkhead interplanetary vehicles or for pressurized human space habitats (Credit Boeing Aerospace).|
|SLS hydrogen fuel tank derived Skylab II concept (Credit Griffin).|
However, it has yet to be determined whether short centrifuge hypergravity machines can also alleviate some of the other serious physical and physiological problems associated with a weightless. And temporary hypergravity would have no effect on the enhanced spread of infectious diseases and some of the minor but annoying problems associated with weightlessness.
To eliminate all of the problems associated with a microgravity environment during long periods of space travel, a continuous artificial gravity environment would be required.
In order to mitigate the physiological effects of Coriolis, a habitat capable of producing at least 0.5g of simulated gravity (higher than the gravity on the Moon and Mars) would require a rotation of approximately 2rpm (rotations per minute) and a radius of at least 112 meters. That would require a rotating habitat approximately 224 meters in diameter if twin counterbalancing pressurized habitats were utilized.
|Artificial Gravity Habitat (AGH) located at EML4 or EML5 and radiation shielded with iron enriched lunar regolith to reduce annual radiation levels for inhabitants to below those allowed for radiation workers on Earth.|
|AGH core module featuring elevators and cable attachment rings for the habitat modules.|
Each AGH pressurized habitat module would provide shielded living area equivalent to a small two story homes, providing a spaciously comfortable environment for scientist and astronauts who may have to live in the confined simulated gravity habitats for several months or even a few years.
|AGH habitat module that can be internally shielded with 50 cm of water for interplanetary journeys or 50 cm of iron enriched regolith for permanent space stations.|
|OTV-2 orbital transfer vehicle positioning the second habitat module to be docked with the core module of an Artificial Gravity Habitat (AGH) at EML4 or EML5.|
For interplanetary journeys, the twin habitat modules would be shielded with water 50 cm thick. This would reduce astronaut's exposure to cosmic radiation to approximately 20 Rem per year during the solar minimum while also protecting astronauts from major solar events. Internally water shielding two levels of inhabited area within a pressurized habitat would add approximately 118 tonnes of weight to each habitat. Twin habitats, therefore, would add an additional 236 tonnes of mass to an interplanetary vehicle. So an AGH shielded for interplanetary travel would weigh nearly 300 tonnes, not including the additional mass for food, water, and air for the crew. Over the course of 1000 days, a crew of ten would add at least 50 tonnes of additional mass to the vessel unless their was significant recycling of both air and water.
Delta- V Budget from Cis-Lunar Space to Mars Orbit
EML1 or EML2 to Mars Capture Orbit -- 1.64 km/s
EML4 or EML5 to Mars Capture Orbit - 1.93 km/s
EML1 or EML2 to Low Mars Orbit ------ 3.04 km/s
EML4 or EML5 to Low Mars Orbit ----- 3.33 km/s
LEO to Mars Capture Orbit ---------------- 5.2 km/s
LEO to Low Mars Orbit -------------------- 7 km/s
Liquid hydrogen and oxygen fueled cryogenic propulsion stages have been proposed by SpaceWorks with a fuel capacity of over 450 tonnes but with n inert weight of less than 30 tonnes. The ULA has proposed a cluster of six ACES boosters with a LOX/LH2 fuel capacity of approximately 700 tonnes.
Under this scenario, a common bulkhead LOX/LH2 fuel tank derived from the SLS hydrogen tank technology is utilized for a reusable interplanetary booster in order to minimize development cost. The OTV-400 would be capable of storing up to 400 tonnes of fuel for crewed interplanetary journeys to Mars, Venus, and the near Earth asteroids. The standard 400 tonne fuel tank is also utilized for large fuel depots under this scenario in order to minimize cost. Integrated Vehicle Fluid (IVF) technology would utilize ullage gases for tank pressurization and attitude control. Cryofuel boil-off would be eliminated during interplanetary journeys by using solar powered cryocoolers.
A single SLS launch would be required to deploy the OTV-400 to LEO with enough fuel to travel to an Earth-Moon Lagrange point for refueling. Less than 350 tonnes of fuel would probably be required for a crewed interplanetary journey to high Mars orbit, including two fully fueled Extraterrestrial Landing Vehicles.
Launching human interplanetary missions from the Earth-Moon Lagrange points to high Mars orbit rather than from LEO has several advantages.
1. To travel from an Earth-Moon Lagrange point to high Mars orbit requires less than 2 km/s of delta-v. But traveling from LEO to high Mars orbit would require more than 5 km/s of delta-v.
2. The delta-v requirement to transport water for shielding and fuel to an Earth-Moon Lagrange point is less than 2.6 km/s. The delta-v requirement to transport water to LEO is more than 9 km/s
3. The vehicles required to transport water to from the Moon to the Earth-Moon Lagrange points could be used for at least ten round trips before their CECE engines would have to be replaced or a new vehicle would be required. The vehicles required to transport water from Earth to LEO, however, would be expendable and could only be used once.
|Reusable OTV-400 attached to a contracted AGH during propulsive delta-v maneuvers and a reusable OTV-400 attached to an expanded rotating AGH after the completion of a propulsive delta-v maneuver.|
|An OTV-400 derived fuel depot (WFD-400) capable of producing cryogenic hydrogen and oxygen from stored water. The WFD-400 would be capable of transporting itself anywhere within cis-lunar space or into orbit around Mars or Venus.|
For permanent space stations at the Earth-Moon Lagrange points or in orbit around Mars, the twin habitat modules would require a radiation shield of iron enriched regolith about 50 cm thick to reduce cosmic radiation exposure to less than 5 Rem annually (maximum allowed for radiation workers on Earth) during the solar minimum. That would require nearly 1865 tonnes of mass shielding (932 tonnes of iron enriched regolith for each habitat module). This would require one or two SLS launches of twin reusable regolith shuttles to the lunar surface-- depending on whether or not the CECE engines on the regolith shuttles are replaced after ten round trips.
Delta-V requirements to transport water for fuel, air, drinking, and mass shielding to LEO or to the Earth-Moon Lagrange Points
Lunar surface to EML1 --------------------- 2.52 km/s
Lunar surface to EML2---------------------- 2.53 km/s
Lunar surface to EML4 or EML5 ---------- 2.58 km/s
Lunar surface to LEO (with aerobraking) - 2.74 km/s
Earth's surface to LEO ----------------------- 9.3 km/s
Earth's surface to EML2 ------------------- 12.73 km/s
Earth's surface to EML1 ------------------- 13.07 km/s
Earth's surface to EML4 or EML5 ------- 13.27 km/s
Transporting an iron enriched regolith shielded space station to high Mars orbit would obviously require a much larger vehicle than the OTV-400. A light sail with a surface area of at least 100 square kilometers should be able to transport a few thousand tonnes to Mars within a years time. But if light sail technology is still not available, vehicles capable of transporting a few thousand tonnes to Mars orbit could easily be assembled by clustering four or more OTV-400 tanks around a core tank. A cluster of five OTV-400 vehicles would create an OTV-2000 interplanetary booster. A cluster of seven OTV-400 boosters would create an OTV-2800 interplanetary booster. Large clustered LOX/LH2 fuel tanks have also been proposed by the ULA for their human interplanetary vehicle concepts.
The fuel requirements for such large interplanetary vehicles under this scenario would require one or two SLS launches of reusable water shuttles to the lunar surface. The lunar tankers would then transport water manufactured on the Moon to fuel manufacturing depots at L4 or L5. Again, such lunar tankers should be capable of at least ten round trips before their CECE engines would need to be replaced.
|OTV-2000 is comprised of a cluster of five OTV-400 boosters.|
OTV-2000 and OTV-2800 class of interplanetary boosters could enable human journeys from the Earth-Moon Lagrange points to the asteroid belt to places like Ceres and Vesta, again with WFD-OTV- 2000 or 2800 fuel depots pre-deployed in orbit around such large asteroids.
Marcel F. Williams
© 2013 Mu Omega Enterprises
Links and References
What if you were born in space
Weightlessness and Its Effect on Astronauts
Effect of spaceflight on the human body (Wikipedia)
Artificial gravity maintains skeletal muscle protein synthesis during 21 days of simulated microgravity
NASA Gives Artificial Gravity a Spin
Artificial Gravity (Wikipedia)
Artificial Gravity Visualization, Empathy, and Design
Space Launch System's Liquid Hydrogen Tank Under Construction
Cosmic Radiation and the New Frontier
Utilizing the SLS to Build a Cis-Lunar Highway
An SLS Launched Cargo and Crew Lunar Transportation System Utilizing an ETLV Architecture
A Study of CPS Stages for Missions beyond LEO
A Study of Cryogenic Propulsive Stages for Human Exploration Beyond Low Earth Orbit
Evolving to a Depot-Based Space Transportation Architecture