Previously on “What Challenges Humans Have To Face From Earth To Mars?: Part 1: Earth Bound Challenges”, we saw that Biswal and Annavarapu in their recent paper, discussed about the challenges we have to face from Earth to Mars Exploration. They divided overall challenges into few categories like terrestrial and earthbound challenges which we have to face on the earth, interplanetary Challenges which we have to face on interplanetary space, Mars-bound and planetary surface challenges which we will have to face on Mars. I already discussed terrestrial and earthbound challenges in the last part of this episodic post series. If you haven’t read that article yet, kindly go through it first. In this article, we are going to discuss about..
II. Interplanetary Challenges
So lets start with..
A. Trajectory Option for Mars
Guys, trajectory analysts have proposed numerous pathways for Human Mars Exploration. Biswal and Annavarapu in their paper have discussed the major two types of trajectory classes.
Opposition Class: Opposition class mission is often referred to as a short-stay mission where travelling astronauts spent most of their mission time in interplanetary space (both in outbound and inbound) with a surface stay of about 30-60 days. This class of trajectory is found risky and expensive because its optimized trajectory requires more ∆v (higher energy transfer trajectory for transit back to earth) and longer duration in interplanetary space may subject to the exposure of galactic cosmic radiation. Additionally, the opposition class mission utilizes Venus flyby that highly enables the crew for closure exposure to Sun’s hazardous elements. Further, the departure delta velocity of 7.0 km/s or above may decrease the success probability of Mars orbital capture and this requires maximized backward propulsion with high energy and fuel exhaustion.
Conjunction Class: Contradictory to opposition class, conjunction class is referred to as a long-stay mission where the crew spends most of their time on Mars surface (400-600 days) than stranding in interplanetary space. Because it makes the crew exposure to galactic cosmic radiation. But due to the benefit from the alignment of the planetary position of Earth and Mars, it grasps a minimal delta velocity of 3.36 km/s to follow a minimum energy path (Hohmann’s Trajectory) for Mars vicinity thus affording the simplest way for Mars orbital capture upon Mars approach.
Trajectory Assessment: Several trajectory assessments showed that long-stay mission may expose the crew to harmful cosmic radiation. But being shielded under the Martian environment is safer than spending much time in either Mars orbit or interplanetary space. The threat of radiation exposure on Mars can be minimized by the application of Mars Sub-Surface habitats or deep space habitats. We know that a human mission to Mars is no cheaper than a conventional mission, so the long term effort can be effectively exploited by performing various scientific observations and experiments during their long stay on Mars. Because short-stay mission may limit the long term experiments. Hence, Biswal and Annavarapu recommended conjunction class trajectory is best suited for a manned mission to Mars which is cheaper, safe, and effective in all aspects as compared to the opposition class. Moreover, in case of any emergency abort, the crew can vast-off Mars and follow high energy optimal path or free return trajectories to return back to earth. Optimized trajectory for conjunction class with launch window at 2024 and 2026 is shown in Fig 8, the duration for the human-crewed mission from the 2022 launch window to 2037 is shown in Fig 9 and delta velocity required for various abort options is shown in Fig 10.
B. Trajectory Correction Maneuver
Based on the study of failure analysis of conventional probes by M.K. Biswal, Biswal and Annavarapu have found that 1/4th of Mars probes encounters ignition engine issues caused as a result of the malfunction of the thermal control system. So, Mars Transit aboard a massive space vehicle via interplanetary coast may be subjected to low pressure, low temperature, and zero or microgravity environment directly affecting the zero-boil-off temperature of the cryogenic fuel and fuel pressurization systems. Therefore, improper fuel management and temperature imbalance may result in the inappropriate firing of thrusters plighted for mid-course or trajectory correction and maneuvering. Moreover, employing a larger delta velocity during the Earth departure stage may have serious concerns over trajectory correction and maneuvering of a massive space vehicle. Hence, proper fuel management and optimal delta velocity may curtail these challenges.
C. Spaceship Management
Spaceship management and maintenance are some of the most challenging tasks for voyaging astronauts. Because of being under the microgravity and space radiation environment, the physical health of the astronauts may limit their access to the complete space vehicle and management. As we know that, the hardware and electronic equipment are the prime components advancing the space vehicle design and it gets degraded by the effect of long term exposure of harmful galactic cosmic radiation. This stands the most challenging quest for the crew aboard the spaceship. Hence, the construction of space vehicles with durable stuff (that are tested and validated in our groundbased laboratories with exposure to artificial radiation) and robust electronics are eminently endorsed. Further, it is desirable to employ artificial intelligence-based automated robots for complete management of the space vehicle to minimize the effort of the manual management system and manual detection of damaged or malfunctioned components.
D. Effect of Radiation and Zero-Gravity
Radiation: Radiation and zero-gravity are the confronting challenges of human and interplanetary spaceflight. Astronauts have a greater threat of exposure to galactic and solar cosmic rays (GCR and SCR), and solar particle events (SPE). These are the natural phenomena spontaneously anticipating from our Milky Way galaxy and deep space and a threat to spaceflight safety and security systems. NASA has categorized the serious issues of GCR and SCR exposure into four human diseases. This includes carcinogenesis, cardiovascular disease as part of tissue degeneration, lifetime risks to the central nervous system, and acute radiation syndromes. So the health consciousness of the crew is very much significant for a successful manned mission.
In addition to this space, radiation can cause serious unrecoverable impairment of electronic components and hardware of space vehicles that might result in mission loss. Hence, sufficient thickness of radiation suit and sterling radiation shield is considered to avoid damage and degradation of the onboard circuitry system. It is because interplanetary spaceflight to Mars takes an average of 180-270 days manoeuvring the Hohmann’s transfer trajectory may expose to harmful galactic cosmic radiation in the order of 1.16-1.18 millisieverts (extremely in opposition class trajectory due to additional requirements of Venus flyby). The level of radiation exposure to the number of days is shown in Fig 11 and 12 by Biswal and Annavarapu.
Zero-Gravity/Microgravity: On a Human excursion to Mars, astronauts will experience three sorts of gravitational fields where one is during the interplanetary transit between planets, second is on the surface of Mars, and finally the third when they return back to earth. These three sorts of transition from distinct gravitational fields and zero-gravity can cause discord in brain coordination and functions, improper balance and orientation affecting the spatial orientation of brain, and motion sickness. NASA had performed various experiments aboard International Space Station (ISS) to understand the changes and impact of zero-gravity on the human body. The results showed that astronauts experience osteoporosis (bone density collapse due to the loss of bone minerals). Besides these concerns, inadequate ingestion of significant consumables and irregular exercise might result in loss of muscle strength and endurance.
Scientific Observation of Health caused by Zero-gravity: An analysis from the Space Shuttle, Mir Program, and ISS Expedition mission showed that the crew experienced serious effects on their muscle system, bones, and cardiovascular activity. After returning from the Space station crew had additional issue of improper blood pressure and blood circulation to the brain that displays the challenge in rehabilitation. But for the astronaut on a mission to Mars will remain in microgravity and zero-gravity environment over transit duration 180-270 days and may cause serious health issues. Hence sufficient countermeasures should be taken into consideration and the best way to confront this challenge is simulation or generation of artificial gravity on the spaceship. Further adequate food habitation and regular exercise will help make the astronauts remain fit and healthy throughout the mission.
E. Solar Irradiance and Temperature
Solar irradiance plays a crucial role in power generation and temperature regulation during interplanetary transit to Mars. Lower solar irradiance would reduce the solar array output required for powering the spaceship and its operating devices onboard modules. It is because the availability of solar irradiance or intensity of sunlight steadily decreases as we move far away from the sun shown in Fig 13. In general assumption, the solar array output reduces from 3000 watts at 1366 W/m² intensity (at Earth) to 1000 watts at 588 W/m² (at Mars) and the record is assumed as per the solar cell configuration of NASA’s Mars Reconnaissance Orbiter.
Corresponding to solar irradiance, the temperature of the interplanetary medium decreases by the function of inverse square law. Biswal and Annavarapu have shown the mean temperature variance of planets in Fig 14 based on the data provided by Williams et al. You can see, the temperature variance directly affects the thermal control systems of the space vehicle and proper thermal insulation is very much essential to keep astronaut warm and healthy in a hard environment. In addition to this, temperature variance also affects the fuel storage (zero boil-off and freezing), electronic components, and life support systems (for the growing plants or life aboard the spaceship).
F. Effect of Nuclear Hazards
Nuclear energy is considered as the source of power for the future of deep space exploration and transportation systems. Mars scientists and engineers have proposed to manoeuvre nuclear electric propulsion and nuclear thermal propulsion rockets (NEP, NTR) to minimize the transit duration from Earth to Mars and to enhance space vehicle for a faster mission. Since the NEP and NTR are the emerging technologies for the advanced propulsion system and a progressive stride towards interstellar transit. Furthermore, it is the only choice that can meet our strategy of stellar and deep space exploration including interplanetary transportation systems. Because the potential for solar irradiance and solar power is limited beyond Mars orbit to employ solar electric propulsion system (SEP). Nevertheless, the NEP, NTR has direct effects on crew health affecting cardiovascular tissue, increasing the risk of cancer throughout the lifetime, and hereditary diseases.
“Hence, during the construction and assembly of the space vehicle, we endorse to mount the crewed module or base far away from the nuclear reactor with adequate shielding to overcome this challenge.”— told Biswal, lead author of the study
G. Isolation and Psychological Effects
A manned mission to Mars takes an average of 2.5 to 3 years that leads to the complete state of isolation/confinement of astronauts eventually affecting the behavioural and psychological patterns. Astronauts may encounter mood and cognition issue, the risk of anxiety, depression, digestive problems, loneliness, hypo or hypertension as part of behavioural changes. Psychological changes include positive moods and relationships patterns with other crew members. A collaborative study performed by Russians and ESA’s project members entitled “Mars-500” showed an increase in positive emotions among crew members. But, according to Biswal and Annavarapu, these results may remain undesirable because the actual environment of a space vehicle during interplanetary transit cannot be simulated on Earth. So, the experiences from Mars analog research stations and Arctic research stations can be treated for planning a mission to some extent.
The experiences from the crew of International Space Station (ISS) is inconsistent because the crew on transit to Mars encounter interrupted communication with their ground and family’s relations than crew aboard ISS. But instead, they have to hold on for about 40 minutes for both transmitting and receiving a single message and hence this may lead to stressful situations and social concerns. These changes are unanticipated and are far beyond how well trained and experienced they are. Therefore, it is substantial to sort out the astronauts who are physically and psychologically fit with multiple interdisciplinary skills. In addition to this, they must be capable of managing themselves during confined and stressful situations.
H. Communication and Interplanetary Internet
Communication: Communication poses a crucial role in mission engagement and keeps the astronauts updated about the mission strategies from the ground. Identically it enhances the psychological personality of the crew and increases the success probability of mission accomplishment. The scientific demand for the human and robotic exploration missions to Mars and beyond is expanding, so high bandwidth and uninterrupted advanced communication relays are highly required. Since at a distance of 1.5 to 2.5 AU (Mars encounter and beyond) the communication interlink is limited to 24 to 40 minutes and it may not be an efficient approach to stay tethered with operating science mission orbiters and crewed space vehicles. Because human mission beyond Moon and LEO is completely new and the crew is exposed to the inexperienced environment. So it is very significant to keep the crew updated about the safety measures and their next move. Additionally, the technological unavailability of persistent communication coverage for manned vehicles (other space probes in Mars orbit or on the surface), proper interlink during superior solar conjunction, the need for simultaneous control over multiple proximity operations on Mars vehicles (Spaceships, orbiters, landers, and rovers), and the exigency to access the Deep Space Network (DSN) for current mission trends are the some of the significant challenges on the way to communication systems. Detailed Review on Advanced Communication Technologies for Human Mars Exploration and their countermeasures are explained in Bhasin et al.
Interplanetary Internet: Interplanetary Internet provides easy access to reliable scientific resources for the crew and enables the public to get updated with live coverage from interplanetary space. This mode of consistent internet and communication from the public directly to the crew may minimize the mental stress (due to good appreciation and encouragement from public interactions) thereby increasing their interdisciplinary activities. Further, interplanetary internet enables the onboard crew to perform various scientific studies or research works as part of their free time with massive access to research papers (optional assignment). The current trend of networking technology can able to accomplish an interlink transfer of ~100-2015 Mbps for downlink and ~10-25 Mbps for uplink via EarthMars trunk line and need to be upgraded for advanced communication relay. In addition to this, NASA is currently planning either to park two-three telecommunication relay orbiters in HMO or MSO to form an integrated constellation architecture for providing increased bandwidth and data transfer rate or to park in Earth-Sun Lagrangian point for deep space communication access with optical fibre and laser-guided communication system.
I. Cryogenic Fuel Management during Interplanetary Transit
Alike on-orbit or in-space refueling I discussed in last part of this episodic series post, cryogenic fuel management (CFM) is necessary during the extended presence of humans on an exploration mission to Mars, and beyond. Because propellant plays a vital role in transiting space vehicles from a point to the destination and assists in performing sturdy mid-course and trajectory correction maneuver. Nevertheless, fuel depletion may prompt in the decline of the mission and it is necessary to effectuate proper CFM procedure. Further, the future exploration mission greatly relies on three factors of the extraterrestrial based cryogenic fuel management system, they are management of cryogenic propellant (liquid hydrogen, liquid methane, liquid oxygen), storage, and distribution. As I discussed in last part, active thermal control, demonstration of cryogenic propellant management, fuel transfer, instrumentation leak detection, liquid acquisition devices, low-gravity mass gauging, passive thermal control, pressure control, refrigeration, storage, system feed testing, and zero boil-off are the key technology for fuel management.
These elements of CFM pose a substantial challenge for fuel management on the way to interplanetary transit to Mars. Efforts to address this challenges were developed in NASA’s ground-based laboratories include Cryogenic Propellant Operations Demonstrator (CPOD), Experimentation for the Maturation of Deep Space Cryogenic Refueling Technology (MDSCR), In-Space Cryogenic Propellant Depot (ISCPD), Zero Boil-Off Tank (ZBOT). Till date, accomplished CFM technology and the demand for future were clearly reviewed by Chato in his paper. Now, Biswal and Annavarapu have shown the approximate time duration required for propellant storage and technology readiness level for CFM in Fig 15 and Fig 16.
J. Waste Recycling and Management
Long duration transit to Mars may result in the generation of large amount to trash that ultimately plunges the crew to biological and physical hazard. The entire trash generated includes exhaled CO2 gases from astronauts, used waters, human wastes, biological wastes, solid wastes, nuclear wastes, and medical wastes. Amidst these, biological, nuclear and medical waste poses a serious threat to crew and may increase the probability of crew sickness and a prolonged threat to cancer. So, the disposal of these trashes requires adequate attention and disposal procedure (anaerobic digester) with consideration of crew safety and risk. According to Lockhart, a crew of four can generate a waste of about 2.5 tons per year. In consequence manned mission to Mars for 2-3 years may result in the generation of 7.5-8 tons. Hence, the measure for recycling and reusing is an ideal approach for a sustainable interplanetary transit rather than conventional trash disposal and venting, because the cargo re-supply and availability is limited from the Earth. The current trend of recycling method followed aboard International Space Station can be further modified and enhanced to accommodate adequate waste management, water recycling, and oxygen generation during interplanetary transit to Mars.
K. Extra-Vehicular Activity
Space colonization highly necessitates the potential capabilities of hours of extravehicular activity performed by the astronaut outside their shielded spacecraft with sophisticated skills and mature technology. EVA plays a great role in accomplishing space assembly, servicing, and space vehicle management and can be extended to provide ecstatic owing to the psychological effects experienced by the astronauts during interplanetary travel. EVA requires a bounded environment in terms of pressure, temperature, and the concentration of oxygen with the additional capability of supplying water and food, temperature regulation, pressure retention, and waste collection within the suit.
But the challenges that affect EVA are direct exposure of astronauts to the galactic cosmic radiation, elements of solar flares, difficulty in balancing their momentum in zero gravity, mechanical hazards and the risk of losing in space as a result of a defective tether. In addition to this galactic particles or micro asteroids travelling with a relative velocity of 10 km/s may hit and puncture the space suit pushing into a critical situation. Further, an analysis on these challenges by Pate-Cornell grouped the overall EVA challenges into eight categories namely failure of airlock and life support system, fire in the suit, mechanical and radiation accident, separation, spacesuit failure, and de novo events. Hence, astronaut equipped with good EMU (Extravehicular Maneuvering Unit) suit may reduce the risk of losing in space. Moreover, healthy space suit with high tolerance (when exposed to vacuum environment) and multiple capabilities can increase the robustness and longevity of EVA. The suit should be developed concerning the experiences encountered by EVA astronauts aboard ISS and other human spaceflight program. Furthermore, NASA’s Johnson Space Center is currently involved in the development of Robonaut (Robotic astronaut’s assistant) that can enhance the efficiency of EVA hours with improved capabilities of strength, dexterity, and mobility.
L. Mars Approach and Orbit Capture
Once we vast-off from the Earth, the next destination is approaching Mars. Accurate targeting and successful Mars Orbital Insertion (MOI) thereby diminishing the delta velocity is very significant for a successful mission. Because, once the space vehicle remains ineffective in capturing orbit, it is very difficult to maneuver back the spaceship to its destination point. As I discussed earlier in last part, the factors such as the delta velocity, fuel management, proper functioning of navigation, maneuvering, and ignition system determines the success probability of Mars Orbital Insertion. However, many inexperienced robotic spacecraft like ESA’s Mars Express and ISRO’s Mars Orbiter Mission (MOM) has successfully demonstrated excellent Mars Orbital Capture in their first attempt. But, in the case of the crewed mission, it is extremely recommended that experiences and lesson learned from past successful Mars probes can be doubtlessly applied for orbital capture. Since inaccurate targeting, timing, and malfunction of breaking engine could pose the space vehicle stranded in interplanetary space or could cause the vehicle destroyed in Mars atmosphere and mission tragedy.
So that’s all about interplanetary challenges.. We will discuss final challenge associated with Mars on last 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
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