Space exploration has diverse challenges and the exploration of Mars possess numerous challenges whereas the Mars exploration has been a source of inspiration to global space firms for decades. The inspiration for Mars began from the conceptual and architectural designs first put forwarded by Wernher von Braun. To date, we have extensively demonstrated numerous space technologies efficient for executing human class missions to Mars and beyond. Similarly, we have successfully explored the red planet with numerous autonomous spacecrafts and planetary probes. So, from the perspective of sustainable and efficient human-crewed mission, it is substantial to consider the natural challenges caused by the phenomena of the space environment and technological challenges ensued from the artificial technologies and that’s what Biswal and Annavarapu studied in their paper. They have studied and emphasized every possible challenge that the crew may experience during their interplanetary spaceflight from the Earth to the Mars except the Entry, Descent, and Landing challenges, as it was already technically reviewed by R.D. Braun. They stratified overall challenges into terrestrial, earthbound, interplanetary, Mars-bound, and planetary challenges under the simplified categorization of terrestrial, interplanetary, and planetary challenges outlined in Fig 1 below.
Let’s start with..
I. Terrestrial Challenges
A. Consideration of feasible future technology
From the perspective of efficient human-crewed Mars mission, it is significant to consider simple, robust, feasible, and affordable future technologies. Don’t suspect researchers assumptions towards extreme or massive technology but to have substantial technology that can be engineered with our existing pieces of machinery. (For example: if we propose to use SEP (Solar Electric Propulsion) for the space transportation system, we might be having hardship in fabricating massive solar panels, enfolding together to place in LEO with the launch vehicle and deploying in interplanetary space. Further, the challenge for this type of propulsion system is the availability of solar irradiance for powered propulsion that gradually decreases as we move far apart from the sun with a variance of 1360 W/m² (at Earth) to 590 W/m² (at Mars). Alternatively, Electric Propulsion, NTR, NEP, and NIMP for transit to Mars can be exploited with the technology of controlled propulsion and management. It minimizes the risk of mass constraints.
Similarly, other than powerful propulsion systems, we need to develop and demonstrate advanced inflatable heat shield to ground large scale masses, power spacesuits to keep the astronauts safe against hostile Mars environment, the concept of sub-surface habitats to maintain thermal stability and the habitat on wheels to enable surface mobilization, uninterrupted power generation employing radioisotope thermoelectric generators or fission power systems, and advanced laser-guided communication system to stay tethered from Earth to receive more information and to stay updated about the mission strategies.
B. Mission Design and Architecture Selection
Mars enthusiasts and scientists have proposed more than 70 human Mars mission’s architectures since 1952. Analyzing through hundreds of pages, comparing with feasible technologies, and executing the mission plan with the right budget seems complicated. Because each and every proposal have their desired goal. So it is preferable to sort out simple, robust with economic standard and feasible plans. Hence, Biswal and Annavarapu have selected (12) architectural strategy for budget comparison and reliability shown in Fig 2.
Among these strategies, SpaceX’s Mars expedition, Mars One, NASA’s Design reference mission has the potential technology to attempt for a human-crewed mission to Mars within the time frame of 2040 and 2060s. Further, based on the current state of matured technology, these strategies were found to be feasible and inexpensive as compared to other mission plans.
C. Consideration of the Heavy Lift Launch Vehicle
The primary progression step towards the Mars expedition is the launch of heavy mass (cargoes, crews, and necessities) to low earth orbit (LEO) as part of initial mass low earth orbit (IMLEO). Delivering of large mass facilitates the reliability of mission thereby expanding the number of cargoes required for the crews and planetary exploration, size of the crew, and possible payloads for spaceship rendezvous in low earth orbit (LEO). But the technology of launch vehicle to deliver large scale mass has made a significant impact to execute a mission. Numerous architectures on launch vehicle enhancement were proposed like Soviet Union’s N1 Rocket, Aelita, United States Saturn MLV, Comet HLLV, Ares, Sea Dragon, and SpaceX’s Interplanetary Transportation System. These launchers may have the potential to deliver huge IMLEO mass with an extent from 100-1400 metric tons. But the task of construction, launch, testing, and validation may cost expensive and infeasible. Hence technologically feasible launchers such as Saturn V, Ares V, SLS, Falcon Heavy, Long March, Starship, New Glenn are considerable for manned mission. It is because some are under development and some are technologically proven in past decades. Moreover, if these launchers come accessible, they can lift a mass variant from 40-200 metric tons to LEO and may pave a way for the affordable human class Mars MISSION. In fig 3, Biswal and Annavarapu showed launchers and their respective payload mass to LEO and launch cost comparison.
D. Crew Size Limitation
The most crucial part of the Human Mars Expedition is the commission of crew and crew size that determines the extent of mission accomplishment. The size of the crew defines the quantity of cargoes desired over the mission period that affecting the initial mass (IMLEO) allocated for launch. Comparably a limited number of crew influences the state of psychological fitness and the ability to accomplish the scientific goal. So it is substantial to sort out each crew with multitudinous skills (For example a crew capable of accomplishing his/her assigned work along with the skill of medical first aid, managing spaceship, rectifying the damaged instruments and miscellaneous). It is always desirable to have stability between the crew and the commodity. Hence, Biswal and Annavarapu interpreted that the limit of crew size from 4 to 6 seems to be ideal for mission robustness.
E. Simulated Training of Crew
Before executing human class Mars mission in a challenging space environment, the astronaut (future Martians) undergoes a various and complex process of sub-orbital training to validate the rate of mission accomplishment during their stay in mission. For this intention, various space and Mars firms have effectuated human analog missions to simulate various environmental aspects. Some of the human Mars analog training centers with their locations and objectives are shown in table 1.
It is worth and satisfactory to have testing, validation, and training in a planetary simulated environment. But the results are incomparable to the actual space environment. Because some results may go erroneous. In a longduration space mission, the first and foremost priority must be given to the crew safety and surviving requirements. For every emergency, there must be a back-up plan and mission abort option that can be proceeded. We cannot put life at risk on a voyage of the search for life. It is not that the mission can be accomplished without crew, instead, it is with the crew. It is significant to make avail every crew necessity at a remote distance.
II. Earth-Bound Challenges
A. Hazards of Space Debris
The prevalence of sub-orbital space debris of variable sizes around earth rises the potential danger to all space missions and vehicles. There are the circumstances where the spacecraft experiences collision with the debris and can lead to the Kessler Syndrome. NASA Keeps tracking the debris and data of sub-orbital debris with the aid of the Department of Defense and Space Surveillance Network. Employing the data Joint Space Operation Center contribute to the interpretation of conjunction assessment to meet the human spaceflight criteria thereby reporting to the Johnson Spaceflight Center and Goddard Space Flight Center. Even though debris is carefully tracked, the threats come as a result of untraced debris of small sizes. For the manned mission, we need to launch and strand large seized orbital vehicles in LEO and have an increased chance of vulnerability to sub-orbital debris. Nations and their quantity of debris are shown in Fig 4.
B. Spaceship Rendezvous
In the scenario of the manned Mars mission, we need to launch an enormous number of spaceship segments into LEO to enable the assembly of Crew Transportation Vehicle. Launching a massive space vehicle aboard launchers is unsustainable due to the limitation of launch vehicle technology to assemble a transportation vehicle for Mars excursion. Hence the assembly requires refined orbital rendezvous and docking technology. We have successfully demonstrated this technology in the 1960s when Gemini 6A and Gemini 7 achieved a technological milestone. Since 1960 astronauts have gone through manual rendezvous (during Apollo and Shuttle Program) and automatic rendezvous (during ISS mission) by making use of Radar (Radio Detection and Ranging) and Lidar (Light Detection and Ranging) technology. And the Lidar has a better insight into future docking and proximity operations. Further, focusing on manual and autonomous rendezvous, both can be preferred in LEO (but autonomous is best). But in the case of Mars orbital rendezvous, autonomous proximity operations and docking is always preferred than manual method (considering the risk of crew safety and health fitness). Because a small misstep can lead to mission loss and space disaster. So, according to the statement of Dr. Dennehy (NASA GNC Technical Fellow) “Autonomous rendezvous and capture will be an integral element of going to Mars”, it is absolute fact and for this, Biswal and Annavarapu recommended to execute autonomous rendezvous and proximity operations in Mars orbit than manual.
C. Orbital Refueling
Refueling of space vehicles and reusing of vented tanks or launcher components are the key technology of manned mission to drive the cost down. In refueling operations, significant criteria like fluid transfer, pressure control, pressurization, gauging, zero-boil off storage, mixing desertification, passive storage, and leak detection are to be considered for fuel management. The low gravity in space greatly influences the Deep Space Refueling process. Additionally, the technology of fluid transfer, liquid acquisition, and mass gauging have a low technology readiness level and need to be matured. So, we need to conduct more experiments and enhance refueling technology despite past experiments shown in table 2.
D. Recycle and Reusable Technology
Currently, we have extended our technology to reuse the launch vehicle components (For example Past space shuttle – first reusable launch system). To date, several space firms have demonstrated the reusable technology. Some of them were tabulated by Biswal and Annavarapu in table 3. Considering economic standard and low-cost space access, we need to extend and enhance the reuse and fabrication technology to space vehicles, fuel tanks, and degraded satellites (solar panels, batteries, and some reliable components).
E. On-Orbit Construction and Assembly
On-Orbit constructions and assembly are one of the greatest challenges for future deep space exploration mission as well as Mars expedition. The manual on-orbit assembly has numerous threats and challenges like the effect of zero-gravity on physical health, exposure to solar irradiance, solar flare and eruptions, cosmic radiations, dynamics of astronauts, health, and energy aspects. Space firms like NASA, ROSCOSMOS, ESA are involved in developing artificial intelligence robots for on-orbit constructions to redress the above challenges. Hence for future missions (where large spaceships and space platforms are required), robotic on-orbit constructions assure 100% safe and secure assembly eliminating every on-orbit challenges. Levels of radiation exposure are shown by Biswal and Annavarapu in Fig 5.
F. Achieving maximum delta-v (∆v)
Achieving maximum delta velocity (∆v) relies on possible orbit-raising maneuvers with the aid of chemical thrusters (thruster fairing on-off), gravity assist, and advanced propulsion system. Exploiting chemical propulsion systems, we can achieve a minimal delta velocity of 5.08 km/s (variable) and may take up to 180 days to transit from Earth to Mars. Longer transit time has increased exposure of crew to hazardous space environmental conditions. However, exploiting the most preferred approach of NTR (Nuclear Thermal Rocket or Propulsion) proposed in many human mission architectures with maximum delta velocity of 8 km/s can minimize the transit time to 120-130 days approximately. The transit time and delta-velocity relation are shown in Fig 6 by Biswal and Annavarapu and it shows the increase in delta velocity decreases the interplanetary transit time from Earth to Mars.
G. Mars Trajectory
We have several trajectory options and are classified into conjunction class and opposition class. Beneficial human class mission requires more scientific goals and its extent of accomplishment. So conjunction class trajectory is more favourable than the opposition class trajectory. Because conjunction class has Mars surface stay time of approx. 350 days higher than the opposition class approx. 30 days and it minimizes the crew exposure to galactic cosmic radiation and solar flares being sheltered under the Martian environment. Opposition class trajectory has maximum exposure to cosmic hazard throughout Trans-Mars as well as Trans-Earth transits due to shorter surface stay period and additional requirement of Venus flyby (that makes closure vulnerability to the Sun and its hazardous elements). Opposition class mission increases the mission budget parallel to launch mass and propulsive energy, but in case of conjunction class mission – it follows the least energy path with minimal energy requirement (Hohmann’s Transfer Trajectory). Hence, conjunction class mars mission is decidedly recommended for Human Mars Expedition and it is found to be the most proposed approach in various mission strategies. Comparison of the duration of crew exposed to various states of Mars expedition (both Conjunction and Opposition class trajectory) is shown in Fig 7 by Biswal and Annavarapu.
So that’s all about challenges we have to face on Earth.. We will discuss further challenges in next part of this series..
Reference: Malaya Kumar Biswal M and Ramesh Naidu Annavarapu, “Human Mars Exploration and Expedition Challenges”, Aerospace Research Central, 2021. https://doi.org/10.2514/6.2021-0628 https://arc.aiaa.org/doi/abs/10.2514/6.2021-0628
Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us