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Three Mars transit vehicle scenarios are presented here: the SpaceX Starship, Victoria – a Low Earth Orbit (LEO) assembled multi-module vehicle, and a Dragon/Orion transit vehicle with a habitat in combination with a Mars Ascent Vehicle (MAV) and an Earth Return Vehicle (ERV) that are both previously sent to Mars. The common thread for all three scenarios is the transit vehicle begins the mission in a circular low Earth orbit with an altitude of 350 kilometres and an associated orbital velocity of 7.7 km/s. Escape velocity for each of the ships is 12.1 km/s, assuming an additional factor for course corrections. Radiation mitigation, cognitive radio and artificial intelligence are common to all three with varied artificial gravity scenarios.
The SpaceX Starship is a methane and liquid oxygen (LOX) propellant 390-foot spacecraft with reusable first and second stages. The Starship portion is the 160-foot second stage. It is capable of lifting 150 MT to LEO and of landing with a 50 MT payload. On this mission, it will employ a crew of three to carry additional payload and 3500 kg of consumables in order to support a 3-person crew up to 3 years. The crew of three will enjoy five decks consisting of payload, consumables, command and control deck, laboratories, crew habitat and radiation protection deck.
Starship is refuelled in LEO by a heavy lift Falcon. Another fully fueled heavy lift Falcon, acting as a travelling fuel depot, upon leaving LEO, will join the Starship and connect via a tether. The travelling fuel depot of methane and LOX is a safety measure to lower the risk of not being able to produce methane and LOX in situ on Mars. It also serves as a counter mass for artificial gravity (AG). This ensemble will spin about a central axis to create an AG environment. Upon reaching Mars orbit, the Starship will untether from its counter mass, and deorbit to land on Mars leaving the fuel depot stage in Mars orbit. The Starship enters the Mars atmosphere at 7.5 km/s at a 60-degree entry profile, levels off horizontally bleeding off 99% of its speed aerodynamically. Upon reaching stall speed, the Starship initiates a pitch over to a supersonic retro propulsion (SRP) and rotates for a vertical landing at a site with previously sent habitats, a 10-kW power generator for fuel production, a fuel chemical reactor using Mars’ atmosphere of 95% carbon dioxide, and fuel storage.
Mars ascent is eventually performed upon refuelling which may be required from the journey to Mars and landing. It is assumed that the Starship will have sufficient fuel remaining to reach Mars orbit in order to reach the orbiting fuel depot stage if LOX and methane are not produced in situ.
Physiological Response to Lack of Gravity
A mars expedition transit in zero-G and one to two years at 38% Mars gravity that of the Earth, exceeds the experience of the physiological effects of crews in Earth LEO. The Mars crew may be physiologically compromised especially with muscle loss and decreased bone density at a loss rate of 10% per 6 months. As an effect of decreased bone loss, the risk of kidney stone formation is present, not to mention the risk of bone breakage. There may be some vision deterioration which may be permanent.
Artificial gravity represents a novel and integrated approach to addressing the detrimental effects of reduced gravity on the human body for Mars transits and to overcome the accumulative effects of reduced Mars gravity. The crew on the International Space Station (ISS) exercise 13% of their waking hours to slow the deleterious effects of microgravity. Extending this protocol to a Mars transit, one may assume that 33 days of a 256-day Mars transit will be spent exercising to reduce the effects of zero-G, time which could be used for experimentation, spacecraft maintenance and mars mission preparation. The amount of exercise in zero-G will not alleviate the risk of vision deterioration as well as other effects that cannot be mediated by exercise.
Starship Artificial Gravity System
In order to reduce the effects of space on human physiology, it requires a holistic systems architecture approach. Only addressing the deleterious effects of zero-G in Mars transit does not accommodate the risks upon arrival, such as management of inflight zero gravity sequelae. During both the transit to Mars and the return to Earth, the mission will create a gravity environment of at least Mars equivalent 0.38g to Earth equivalent 1.0g. To generate this artificial gravity, the Starship spins about the centre of mass, tethered to a refuelling stage. The Starship is tethered nose-to-nose by a 170m non-conducting tether continuously rotating the entire complex at two revolutions per minute with a rim speed of 17.8m/sec. The fuel tanker will remain in orbit around Mars to be tethered for artificial gravity on the return to Earth transit. The rotation speed may be increased to Earth gravity equivalent by extending the tether to 447m at 2 revolutions per minute and a rim speed of 46.8m/sec. The tether length is lengthened or shortened to meet the gravity equivalent needed and where the centre of the mass resides as a mass of the two ships vary throughout the course of the mission.
The habitat modules’ orientation is an important consideration in their rotation radius, Coriolis force, and g-levels. Because the centrifugal acceleration varies directly with the distance from the centre of rotation, the gravity strength will vary between the spacecraft axis and the crew habitat in a radial orientation. In a radial orientation, there are large gravity gradients with tangential and transverse Coriolis effects. Crewmembers climbing up the ladder toward the vehicle’s centre of rotation will lose weight with each step. The orientation creates gravity gradients within the Starship with greater gravity force toward the end of the Starship and lesser on the nose end and tether. The comfort zone for the crew members is based on the number of revolutions per minute and the rim speed. Tangential Coriolis forces expose the crew to g-forces affecting their pseudo weight depending on the direction of motion with respect to the vehicle spin axis. Coriolis forces do not occur moving vertically between Starship levels. When climbing towards the center of rotation, the crew is pushed sideways in the direction of the Starship spin. The opposite effect is felt when climbing down the ladder between Starship levels. With a combination of AG, exercise, and pharmaceutical aids, the effects of low gravity may be greatly minimized.
The tether is composed of a nonconducting material made of carbon nanotubes that have the highest tensile strength to weight ratio. It must be able to absorb an occasional impact from space debris and not break.
To deploy a tether to enable two ships to spin about an axis to create artificial gravity, the tether must be under constant tension. If a tether loses tension during deployment, it becomes slack and its resulting motion is unpredictable. Loss of tension allows such mishaps as the tether jamming inside the reel mechanism or becoming tangled as it deploys. To keep tension on a tether during linear deployment, constant thrust on both the Starship and the counter stage in anti-parallel directions along the tether line. This is done after the ships have left Earth LEO, and onto their interplanetary trajectory, to avoid possible impact from LEO space debris. After the tether is connected to both ships and the tether is deployed by the reel mechanism, the reaction control system thrusters are used to spin the docked system to the desired rate.
Victoria Mars Transit Spacecraft
The 256-foot-long Victoria Mars 3-person crew transit vehicle is assembled in LEO at an altitude of 350 km comprised of a Dragon/Orion crew capsule, propulsion module, folding photovoltaic arrays, folding thermal radiators for crewed areas and electronics, a centrifuge habitat, a primary communications dish, logistical stores of methane and hydrogen fuel and water, a radiation habitat, and a Mars Ascent/Descent Vehicle (MADV) docking port. The Victoria assemblage is architected to be an Interplanetary Transport System (ITS) whereby it may serve multiple and varied space missions to the Moon, asteroids, and Mars. Victoria also has an Environmental Control and Life Support System (ECLSS) module that provides a life support system that controls atmospheric pressure, fire detection and suppression, oxygen levels, waste management and water supply. The ECLSS consists of two key components, the Water Recovery System (WRS) and the Oxygen Generation System (OGS). The WRS provides clean water by recycling crew urine, cabin humidity condensate and wastes.
The spacecraft’s command and control are located in a centre node module where the communications systems and other critical life support systems are controlled. For external maintenance, a Remote Manipulator System (RMS) is located on an Integrated Truss Structure rail track on Victoria, that may be deployed anywhere along the length of the spacecraft to capture, deploy and manoeuvre various payloads, modules and assist in crew external vehicular activity.
The Dragon/Orion crew capsule serves a dual purpose as an Earth ascent vehicle joining the already assembled modular spacecraft in LEO and serves as an Earth descent vehicle upon return to Earth. The MADV, single-stage methane and LOX propelled 100 MT spacecraft, has been previously sent to Mars orbit with a fuel depot stage whereby it docks with the arriving Victoria at a 500 km orbit about Mars for the 3-person crew to transport to the Mars surface while Victoria remains in Mars orbit.
The use of the MADV affords adjustments in mission planning and allows for greater safety of early Mars surface missions as the crew can return to Victoria at almost any time if equipment issues, medical concerns, or events that could not be predicted/managed occur. This includes the ability to abort to orbit at any point during Mars entry and descent. Once undocked from Victoria, the MADV performs a deorbit burn and proceeds to perform direct Mars atmospheric entry at about 4.5 km/s. As the MADV reaches stall speed, it initiates a pitch over to an SRP attitude and performs a powered gravity turn to bleed off the remaining velocity and subsequently deploys its landing gear to land vertically. Upon return to Mars orbit, the MADV docks with Victoria and based on mission needs may remain docked with Victoria as it returns to Earth or may be undocked and positioned into a circular Mars orbit for future crews returning to Mars.
Victoria Artificial Gravity System
The habitat modules’ orientation is an important consideration in their rotation radius, Coriolis force and g-levels. Because the centrifugal acceleration varies directly with the distance from the centre of rotation, the gravity strength will vary between the spacecraft axis and the habitat. Crew members climbing up the ladder toward the vehicle’s centre of rotation will lose weight with each step. Conversely, as crew members approach the ladder to the habitat, they can virtually “slide” down the ladder as their acceleration will increase, as well as their weight, as they approach the habitat.
The potential placement of the habitat modules, shown in Figure 1, consist of an axial, radial, and tangential orientation. All three would require a counterbalance being a mass of some kind or another habitat. The axial habitat module orientation is the most optimized for this spacecraft’s needs. It provides a minimal gravity gradient with the habitat, it has minimal Coriolis forces effects, which is an apparent deflection of a moving mass resulting in a crewmember being pushed sideways in the direction of the spin when climbing the ladder, the sleeping bunks are parallel to the spacecraft spin axis as well as workstations. However, it has a long transfer tunnel than the radial orientation. The radial orientation habitat has gravity gradients with the habitat with greater gravity force toward the end of the habitat and lesser on the end towards the axis. There may be levels within the habitat to separate the gradients placing ladders within the habitat and consuming valuable habitat space. The tangential orientation is similar to the radial orientation in that it has minimal gravity gradients and a long transfer tunnel. However, it has greater Coriolis effects.
The AG system on Victoria consists of two living habitat modules spinning about the spacecraft axis positioned in an axial or parallel orientation. The spin rate is 3.5 rpm providing an AG of 5.05 m/s2, approximately half of Earth’s gravity, to maintain crew muscle mass while remaining in a comfort zone preventing dizziness or other uncomfortable sensations due to rapid movement and Coriolis effects. This spin rate will slow bone loss and is further supplemented with exercise and pharmaceutical remedies.
The habitats’ volume is 400 m3 to easily accommodate a crew of 4 per habitat. With two habitats and a crew of 3, there is sufficient space for additional crew requirements. The two habitats, as well as their modest size, afford personal time improving psychological effects of close proximity to other crew members for long durations. The habitat module is 37.6m from Victoria’s main axis. The crew transit tunnel between the habitat module and the main axis of Victoria is large enough for the crew to float partially through grabbing onto a ladder whereby they essentially slide down into the habitat as the force of gravity increases as they approach the habitat. Returning to the main axis of Victoria from the habitat will require some ladder climbing with each rung becoming easier as the force of gravity slips away.
The habits and their truss tunnel system are spun about the spacecraft axis with the use of ion thrusters. The ion thrusters, situated with a gimbal system on the habitat ends, have a nominal performance of 5.8 m/s2 in 34 hours to either spin up or spin down the axis.
Mars Direct Modified Spacecraft
The Mars Direct Modified (MDM) is a 3-event logistical plan, Dragon/Orion capsule supported by a Falcon heavy Earth launch that can deliver 53 tons to LEO and 17.5 tons to Mars. In event 1, a Dragon/Orion Earth Return Vehicle (ERV) with a methane and LOX propellant stage is parked in Mars orbit with an uninflated space habitat. In event 2, a Mars Ascent Vehicle (MAV), a single-stage rocket delivering 11 tons of payload, with methane and LOX propellant lands on Mars with 2.6 tons of methane without LOX. The LOX will be made in situ from the carbon dioxide that makes up 95% of the atmosphere. A chemical reactor and a 10kw power generator are sent in the MAV and weigh about 2 tons. The remaining 11-ton payload consists of other necessary supplies and gear.
In event 3, the Falcon Heavy launch 3-person crewed Dragon/Orion, bound for Mars, also carries a Mars methane-powered ground vehicle and a drone, developed specifically for Mars atmospheric density. A space habitat is deployed once in LEO. The 8,600 kg, 2-deck inflatable space habitat is 8.7m long by 6.3m in diameter with a volume of 180 cubic meters supporting a crew of 3. It consists of 2 solar arrays and 2 thermal radiators with walls made of layered Nextel and Kevlar that will absorb and prevent small space debris from entering the habitat.
Artificial gravity is established with an expired stage and the Dragon/Orion docked habitat. Upon entering Mars orbit, the Dragon/Orion detaches from the habitat and tethered expired stage leaving it in Mars orbit. The event 3 spacecraft lands near the already present event 2 MAV. Upon landing, the mars surface vehicle, drone and other supplies are unloaded and LOX production capability is begun.
Upon Earth return, the MRV will launch into Mars orbit docking with the already in orbit ERV. The crew will transition to the ERV and establish its 8600 kg habitat and tether with the in-orbit expired stage for artificial gravity.
MDM Artificial Gravity System
During both the transit to Mars and the return to Earth, the mission creates a gravity environment of at least Mars equivalent 0.38g to Earth equivalent 1.0g. To generate this artificial gravity, the Dragon/Orion and inflatable habitat spins tethered to an expired trans-Mars injection stage by a non-conducting tether continuously rotating the entire complex at 2 revolutions per minute with a rim speed of 17.8m/sec. The expired stage and inflatable habitat will remain in orbit about Mars to be tethered for artificial gravity on the return to Earth transit. The rotation speed may be increased to Earth gravity equivalent by extending the tether to 447m at 2 revolutions per minute and a rim speed of 46.8m/sec. The tether length is lengthened or shortened to meet the gravity equivalent needed and where the centre of the mass resides as a mass of the two ships vary throughout the course of the mission.
Cognitive Communications and Artificial Intelligence
Previous crewed missions have been confined in LEO and the Earth-Moon system whereby speed-of-light communications delays between crew and ground support are, for practical purposes, nonexistent due to the close proximity of the crew to Earth controllers to provide near real-time support. As collection of space data increases, the complexity of the spacecraft increases, the delay of Earth communications due to greater distance and in this case for 2033 an average of 4 minutes one way or 10 minutes minimum to receive a knowledgeable support reply to a problem, requires increased autonomy of the crew from Earth based support. To ease the burden on the crew and to provide support in lieu of Earth support, intelligent autonomy spacecraft and habitat systems are necessary and will require greater autonomy from human interaction due to their complexity. To facilitate communications due to expected delays due to distance and interference from space events such as SPEs, cognitive radio (CR) and the infusion of artificial intelligence (AI) with space communications networks is integrated into the communications systems of the spacecraft to meet demand and increase efficiency.
The Mars communication system may fail to communicate due to:
- The Earth or Mars spacecraft or Mars base receiver encounters interference or non-optimal antenna pointing resulting in loss of lock
- Mars mission hardware degradation due to radiation or design, reducing transmit power or increasing receiver noise
- Throughput is maximized
- Bandwidth is minimized
- Transmit power is minimized due to available power and distance
In scenarios of potential communication failures, the traditional remedy is to keep transmitting or follow a predetermined protocol until the result improves. These real-time problems will result in a loss of mission data or systems failure. The benefit of CR is spacecraft onboard, real-time, autonomous mitigation of the problem as well as its root cause determination to be avoided in the future.
The objectives of cognitive links, networks, and systems are to provide increased autonomy and reliability for the Mars communication architecture. The communication system onboard the spacecraft and on Mars provides communication, processing and storage instead of simply providing point-to-point communications and links. Due to the nature of Mars mission human spaceflight, autonomy technology is robust and resilient, reacting both to changes in the space and Martian environment and system faults and failures. Cognitive radio is able to restrict the interference of communications by adapting to the dynamic nature of space weather by modifying its transmission frequency or cancelling out distortions with the help of AI.
The CR system is data-driven and uses non-deterministic methods and decision-based algorithms to improve communications autonomy optimizing communication links of bandwidth, frequency, data routing, antenna pointing, and asset management. The CR system is constantly aware of the communications environment and adapts to the operational parameters to improve on past performance to optimize future performance.
AI is engaged in the variety of mission planning functions which in turn provides greater autonomy for the crew. The systems onboard the spacecraft use AI to replace the Earth ground controllers to expedite spacecraft maintenance, mitigate failures, and prevent future failures. The AI maintenance system is designed to operate without crew interaction and communicates with Earth systems to provide continuous monitoring, maintenance and software repair of spacecraft systems to reduce crew workloads as well as lack of trained experts in the myriad of systems.
Human space exploration has been limited to various durations in Low Earth Orbit (LEO) and too short visits to the Moon. For short-duration missions, the adverse effects of radiation exposure outside of the Earth’s protective magnetic field on the human body are minimal. There have been only a few astronauts in LEO for more than 6 months and very few more than one year. Mars missions are planned to be from one to three years in length and will have a large impact on crew health due to the effects of space exposure and the lack of environmental protections enjoyed by Earth surface dwellers.
The space radiation environment is significantly different from that found terrestrially. Space radiation primarily consists of Galactic Cosmic Radiation (GCR) and Solar Particle Events (SPE), consisting of protons, alpha and heavier particles. These high energy particles inflict greater biological damage than that resulting from typical terrestrial radiation hazards. Space crew exposures will exceed exposures routinely received by terrestrial radiation workers.
Astronaut exposure to GCR within an unprotected or thinly shielded spacecraft over the expected Mars transits is sufficient to exceed current NASA exposure guidelines. Galactic cosmic radiation originates from outside the solar system. It consists of ionized charged atomic nuclei from hydrogen (88%) and helium (10%) to heavier ions such as uranium. They are characterized by extremely large kinetic energies.
For terrestrial radiation workers, additional protection against radiation exposure can be provided through increased shielding. Additional shielding against space radiation exposure may not be practical or efficient. GCR is extremely penetrating. Dose equivalent exposure rates behind thin shielding are reduced rapidly at first, but plateau with increasing thickness. Thus, thicker shields become less efficient. The additional mass added purely for reducing radiation exposures becomes a substantial mass penalty for transportation vehicles and therefore may dramatically increase mission cost. This is the result of the production of a large number of secondary products including neutrons from nuclear interactions between the GCR and shield nuclei. The ionization of materials temporarily increases their conductivity, potentially permitting damaging current levels in spacecraft electronics resulting in permanent damage. The electronics are made to be “radiation hard” for space applications to resist such effects through design, material selection, and fabrication methods.
SPEs are the most difficult to predict and manage to reach the Earth in less than 30 minutes. SPEs are the result of coronal mass ejections that originate from disturbed magnetic regions of the sun’s surface. The solar cycle is 11 years with a period of 4 years of relative inactivity and 7 years with a higher amount of SPEs. Solar flares occur without much warning with the magnitude and intensity of a flare are difficult to determine until the event is in progress. SPEs are directional as well. They may impact the Earth but not the Mars transit crew or vice versa. Fortunately, most SPEs are a problem for 1-2 days, which allows for relatively small areas of confinement for the Mars transit crew. To minimize exposure, the crew would be restricted to the habitat storm shelter during the most intense portion of the SPE, which may last for several hours. Shielding of approximately 20 g/cm2 or more of water equivalent or hydrogen-rich material will provide sufficient shielding to protect the crew. For relative comparison, the aluminium equivalent of the ISS US Lab is 10 g/cm2, ISS average is 5.26 g/cm2, a space suit is 1.22 g/cm2, and the STS-66 cargo bay is 15 g/cm2.
Although there are different types of radiation, ionizing radiation is measured in Sievert (Sv). It is a unit of a dose of ionizing radiation and is a measure of the health effect of low levels of ionizing radiation on the human body resulting in genetic damage or cancer. One Sievert represents a 5.5% probability of developing cancer. An astronaut’s career exposure to radiation is limited to not exceed 3% of the Risk of Exposure-Induced Death (REID) from fatal cancer. NASA policy is to assure that this risk limit is not exceeded by the cumulative effective dose that is received by an astronaut throughout their career. The relationship between radiation exposure and risk is both age and gender-specific due to latency effects and differences in tissue types, sensitivities, and life spans between genders.
One milli-Sievert (mSv) of space radiation is approximately equivalent to receiving one abdominal x-ray. An average CT scan is 10 mSv. Earth surface dwellers receive an average of 3 mSv every year from background radiation alone. For relative comparison, 6 months onboard the International Space Station, a crew member receives approximately 75 mSv or 0.49 mSv/day. A 180-day transit to Mars is approximately unshielded 325 mSv, and 500 days on Mars is about unshielded 325 mSv with a return to Earth of approximately 180 days for a total of 710 mSv. In Mars orbit, a crew member can expect an average radiation of 1.07 mSv/day. For a 35-year old male and female crew member, the radiation limit is 2.50 Sv and 1.75 Sv respectively. The total accumulation is approximately 3.9% of cancer probability. This does not exceed the REID requirement, but the crew members would certainly appreciate the ability to return to space eventually upon Earth arrival and reduce their probability of cancer due to radiation accumulation. A mitigation habitat architecture is achievable without the high cost attributed to large mass shield structures.
Ionizing radiation is mitigated by materials with high amounts of hydrogen. Hydrogen is effective at fragmenting ions such as are found in GCR, stopping protons such as are found in SPE, and slowing down neutrons formed as secondary interaction particles when the GCR and SPE ions interact with the spacecraft hull. Materials with high amounts of hydrogen that may be used are an ethylene polymer (CH2) containing a lot of hydrogen and is a solid material, but is not a high strength load-bearing material for aerospace applications. The space industry uses aluminium alloys for primary structures, augmented with polyethene or water (H2O) for radiation shielding. Water is very heavy and would require a thickness of 30 cm or 42 metric tons to provide sufficient radiation shielding.
The Mars mission Starship, as well as crew member clothing, have an integrated hydrogenated boron nitride nanotubes (BNNT) made of carbon, boron, and nitrogen, with hydrogen interspersed throughout the empty spaces between the nanotubes. BNNT has a high strength-to-weight ratio, high-temperature resistance to over 950°C whereby aluminium melts at 660°C. BNNT’s, which are a molecular combination of boron and nitrogen in a rectangular polymorph structural form (about one-thousandth the thickness of a human hair), provide a nanoscale structural reinforcement in a variety of materials. BNNT is an excellent absorber of ionizing radiation and secondary particles making hydrogenated BNNTs an ideal shielding material. It has good structural properties for load-bearing and high-temperature stability. Further radiation mitigation is afforded by weaving crew member clothing with BNNT for onboard the spacecraft, as well as their extravehicular spacesuits.
The crew habitat modules’ walls are integrated with BNNT to mitigate omnipresent radiation and SPE events. The BNNT density is sufficient to mitigate GCR ions and average intensity of SPE ions. In the case of more severe SPE events, the cabin crew module with the additional layering of BNNT, water cells used for recycling as well as the spacecraft hull serves as a radiation shield.