1. Mars Expedition Overview

Mars is a rocky planet, like Earth. It is formed around the same time, yet with only half the diameter of Earth. The levels of mean surface temperature that exist on Mars are also present on Earth in some extreme places where humans have successfully ventured. The corresponding lengths of a day and the obliquities of Earth and Mars are also similar. Both planets experience seasons owing to their eccentricities and obliquities (see Levine et al., 2010a,b,c,d). Among the planets and moons in the solar system, where man can colonize, Mars will be the first. As the resources of Earth are finite, it is likely, that the long-term future of the human race is in space (Mitchell and Staretz 2010; Zubrin 2010). Humans, as a species, should start thinking towards freeing themselves from the constraint of a singularity called Earth.

Within a decade of the first launch of artificial satellite in 1957, Mariner- 4 flew past Mars in 1965, providing the first close-up photographs of Mars. In the early 1970, Mars- 2 and Mars- 3 from the erstwhile USSR were the first two space probes to enter into orbits around Mars. In 1976, the two American Viking probes entered Mars orbit and released static lander modules that made a successful soft landing on the planet's surface. The next leap in Mars exploration came, in proving the feasibility of autonomous control over a robot, the rover, Sojourner, as part of the Mars Pathfinder mission in 1997. In the second decade of the 21st century, plans are already afoot for a Mars Sample Return mission that would use robotic systems and a Mars ascent rocket to collect and send samples of Martian soils to Earth for detailed physical and chemical analysis. These robotic explorations will pave the way for manned mission to Mars (Gage 2010a; Podnar et al., 2010). In the spirit of human endeavor and exploration, a human mission to Mars could be likened to the expedition by Vasco da Gama and Christopher Columbus, to discover the sea route between Europe and India. Of course, technical challenges for human mission to Mars are several orders more complex.

A single human expedition to Mars with a crew of six, would require at least nine launch vehicles with a Low Earth Orbit (LEO) payload capacity of about 100 metric tonne. More complex is the scenario, in the context of human settlement on Mars. To establish a vibrant colony, we estimate that an initial population of 150-180 would be required to allow normal propagation for 60-80 generations, which is equivalent to 2000 years. To promote the variability in gene pool, it is advantageous to choose settlers from diverse backgrounds and with different skill. As a consequence, Mars expedition and colonization would entail international cooperation on an unprecedented scale (Joseph 2010).

2. Mission Opportunities

The dates for a Mars expedition are chosen through assessment of the variability of mission opportunities across Earth-Mars synodic cycles. Mission opportunities occur approximately every 2.1 years in a cycle that repeats roughly every 15 to 17 years (the synodic cycle). Within this time span mission opportunity characteristics are similar but not the same. Along with human missions, one-way cargo delivery trajectories would also be required and which would depart during each opportunity preceding each crewed mission.

Two types of mission trajectories, namely, conjunction-class and opposition-class, have been discussed in the context of human expedition to Mars (Rapp, 2008) and are depicted in Figure 1. Crew in conjunction class trajectory from Earth would spend about 6-9 months in space before arriving at Mars to spend another 14-20 months before the initiation of return trip, again through conjunction class trajectory. However, there can be unforeseen circumstances such as severe dust storms, radiation hazards, alien pathogens (Levine et al., 2010a,b,c,d; Rummel et al., 2010; Straume et al., 2010; Stuster 2010), which may compel them to leave Mars early for safe return. Under these emergency contingencies, they can lift-off from Mars after spending 1-3 months on the surface and follow the opposition class of trajectory to Earth through gravity assist by Venus.

The return to Earth through opposition-class trajectory will take much longer (10-15 months). Conjunction-class trajectories are the main focus for expedition to Mars. However, opposition-class trajectories, notwithstanding having higher propulsive, shielding and life support requirements, can serve as a mission salvage option. But even before mission salvage, there must be requirements for mission abort.

A planned option to provide an abort-return trajectory may bolster the capability to handle any unforeseen eventualities. In such a transfer, the spacecraft is placed on a trajectory from Earth to Mars, which also returns to Earth at a specified point in the future, without the need for any major additional action except for small mid-course corrections. Typically such free-return trajectories employ a resonance in the orbital periods of the transfer trajectory and Earth about the Sun. Abort options clearly place a requirement on the outbound spacecraft to support the crew during the lengthy return to Earth.

3. Preparation for Mars Expedition

The Mars expedition should be undertaken through execution of split mission architecture. A portion of assets required for Mars expedition (e.g. food, supplies, fuel, robots) would be sent to Mars prior to the arrival of the crew (Gage 2010a,b; Podnar, et al., 2010). This mission architecture allows a lower-energy/longer-duration trajectory to be utilized for these pre-deployed assets. The flexibility to pre-position some of the mission infrastructure also allows for the preparation of the ascent propellant, while on Mars, using the Martian environment as the source for raw materials (Zubrin 2010). The pre-deployment of a propellant processing facility and implementation of in-situ resource utilization results in a net decrease in the total mass that is needed to complete a mission. It also results in a significant reduction in the size of landers.

It is envisaged that at least two major components are required: propellant and consumables. Fortunately, abundant resource of water on Mars has the potential to provide propellants and consumables. Water can be electrolyzed to produce hydrogen, which can be used as propellant. A surface nuclear power reactor can provide energy required to carry out electrolysis. Furthermore, this power system would be adequate to meet the needs of the human crew when they arrive.

The separation of the mission elements into pre-deployed cargo and crew vehicles allows the crew to fly on a higher-energy/shorter-duration trajectory, thus minimizing their exposure to the hazards associated with inter-planetary travel (Hawley 2010; Straume et al., 2010; Stuster 2010).

Owing to the significant amount of mass required for a human mission to Mars, numerous heavy-lift launches which can put a LEO payload of the order of 100 metric ton, would be required. The mission architecture conceived for the expedition to Mars for human settlement is described in Figure 2.

Of the ten launches required for a six member crew mission to Mars, the first five launches would ensue two years prior to the actual crew launch. These are represented in the left side of the Figure 2. The pre-deployment of the first two cargo modules, namely, the descent/ascent vehicle and the surface habitat, takes place through the first five launches. These two vehicle sets to Mars are first assembled (via rendezvous and docking), and checked out in LEO. After all of the systems have been verified and are operational, transfer phase is initiated and they are injected into a minimum energy transfer from geocentric orbit to Mars just over 2 years prior to the launch of the crew. The descent/ascent vehicle module lands on Mars at a pre-selected location and its nuclear reactor starts transforming water from Mars environment into propellant to be used for Mars ascent as well as surface operations. The surface habitat module enters into an eccentric areocentric orbit and waits for the arrival of the crew.

The second batch of launches for Mars expedition begins during the next injection opportunity. These include the launch, assembly, and checkout of the Mars Transfer Vehicle which contains Transit Habitat Module, Nuclear Rocket and Hydrogen Tanks. Mars Transfer Vehicle serves as the interplanetary support vehicle for the crew for a round-trip mission to Mars, though it must be recognized that this may be a one way journey (Schulze-Makuch and Davies 2010). Assembly of these modules would require four launches from Earth.

After the Mars Transfer Vehicle is operational, the flight crew embarks on the journey to Mars along an appropriate fast-transit trajectory towards Mars. These are depicted on the right side of the Figure 2. The length of this outbound transfer to Mars is dependent on the mission date, and ranges from 175 days to 225 days. Upon arrival at Mars, the crew vehicle enters into an elliptic parking orbit before performing a rendezvous with the Surface Habitat Module, which is already in Mars orbit.

There are many who envision a human mission to Mars before the end of the next decade (Joseph 2010; Mitchell and Staretz 2010; Zubrin 2010). However, if we were to wait until 2035, this window of opportunity would shorten the trip considerably, to a little over 200 days, which makes it more practical for settlement and resupply. Thus, the the initial plans for Mars settlement may take place 25 years into the future, beginning in 2035. This 2035 opportunity offers a trajectory to Mars, from a circular Earth orbit of 400 km or so, and is achieved through a velocity addition of around 3.7 km/s. Upon reaching Mars, another velocity reduction of a little less than 1 km/s is employed in conjunction with aero-capture to enter into an elliptic Mars Orbit that is an eccentric areocentric orbit of 250 km • 34000 km.

The 2035 opportunity can be utilized to send necessary infrastructure and other materials to Mars for eventual human settlement. During the very next opportunity in 2037, the crew can experience a typical transit time of a little less than 180 days to Mars. Crew can stay on Mars for about 540 days and can return to Earth in 2040. During any of these opportunities, it is also possible to achieve Trans Mars Insertion (TMI) using Moon and Earth gravity assists (Penzo, 1998).

Nuclear thermal rocket propulsion is preferred as the interplanetary propulsion system for both cargo and crew vehicles. In nuclear thermal rockets, a fluid with low molecular weight like liquid hydrogen is heated using energy liberated by fissile material and expelled through a nozzle to create thrust. Propellants, other than liquid hydrogen, such as ammonia or water, would provide reduced exhaust velocity but their greater availability in places like Mars can increase operational flexibility. Nuclear power systems are also best suited for surface operations where sunlight is limited and reliability is paramount. On Mars, though the night period is only 12 hours, sunlight at the surface is reduced to about 20% that of Earth. Martian dust storms and missions to the higher latitudes, further decrease the availability of sunlight for solar power (see Levine et al., 2010a,b,c,d).

A key advantage of surface fission reactor systems is that they produce constant power to allow continuous day and night surface operations. Relative to comparable solar power systems with energy storage, a fission reactor offers significant mass and volume savings. This large number of launches necessitates a significant launch campaign with international participation that must begin several months prior to the opening of Mars departure window. The reference strategy that is adopted eliminates on-orbit component-level assembly of the mission elements by segmenting the systems into discrete modules and using automated rendezvous and docking of the major modules in LEO. Launches occur 30 days apart and are completed several months before the opening of Mars departure window to provide a margin for technical delays and other unforeseen problems. This strategy requires that the in-space transportation systems and payloads stay in LEO for several months prior to departure for Mars. In this regard, the risk due to derelict space objects must be mitigated through Space Object Proximity Analysis (SOPA).

4. Mars Transfer Trajectory

A Mars transfer trajectory is of the order of 200 days. During this time, the crew prepare for the tasks ahead. Prior to and during the journey precautions must be taken to ensure the physical and psychological health of astronauts as the perils are many (Bishop 2010; Fiedler and Harrison 2010; Harrison, and Fiedler 2010; Hawley 2010; Suedfeld 2010). Creating a home-like environment that mirrors life on Earth is important for Mars expedition. Family members can contribute in engaging the crew in two-way communications. With such support the crew in space could still be an integral part of the family back home (Pletser 2010; Johnson, 2010). However, in this context, it may be recalled that the maximum distance between Earth and Mars is roughly 400,000,000 km, 1,000 times the distance between Earth and Moon. Because of the loss of signal strength due to the increased distance, communications from Mars are more challenging than communications from the Moon. The large distance to Mars also implies long signal transit times, with a round-trip time of up to 44 minutes. An interplanetary electronic mail service can be the choicest mode of communication in the future.

Radiation in space is a major issue to contend with (Straume et al., 2010). From the standpoint of humans in interplanetary space, the two important sources of radiation for Mars expedition are, the heavy ions (atomic nuclei with all electrons removed) of galactic cosmic ray and sporadic production of energetic protons from large solar particle events (Straume et al., 2010). The constant bombardment of high-energy galactic cosmic ray particles delivers a lower steady dose rate compared with large solar proton flares which on occasion deliver a very high dose in a short period of time (of the order of hours to days). Various active and passive shielding options to protect astronauts from space radiation are described by Seedhouse (2009) and Straume et al., (2010).

Another important factor for Mars expedition is absence of gravity during Mars transfer trajectory (Moore, et al. 2010). One of the major effects of prolonged weightlessness seen in long-duration space flights has been a reduction in bone mineral density (Harrison and Fiedler 2010; Moore, et al. 2010). In this context, alternate medicinal systems like ayurveda hold promise in alleviating ailments resulting from weightlessness. Formulations comprising Terminalia arjuna, Withania somnifera and Commiphora mukul are well known for their bone remineralization (Mitra et al., 2001). At the same time, the efficacy of ayurvedic formulations in zero gravity conditions needs to be ascertained. Similarly, physical exercise and yoga can help maintain physiological and psychological health of the crew. Some aspects of yoga exercises are the theme of experiments conducted during the joint Indo-Soviet manned space program in Salyut-7 (Wadhawan et al. 1985: Harland, 2005). The results suggest that yoga exercises have definite beneficial effects in preventing muscular atrophy and on the psycho-physiological well being during space flight.

5. Entry, Descent and Landing (EDL)

One of the most important strategies needed to enable human mission to Mars is efficient aero-assist technology for descent. Mars entry, descent and landing bring forth many engineering challenges (Drake 2010). Challenges emanate from an atmosphere, which is thick enough for substantial heating, but not sufficiently dense for low terminal descent velocity. Also the surface environment is made of complex terrain patterns with rocks, craters, and dust. Relative to Earth, Mars atmosphere is thin, less than 1% in surface pressure (see Levine et al., 2010a,b,c,d). As a result, Mars entry vehicles tend to decelerate at much lower altitudes and, depending upon their mass, may never reach the subsonic terminal descent velocity of Earth aerodynamic vehicles. Because hypersonic deceleration occurs at much lower altitudes on Mars than on Earth, the time remaining for subsequent entry-descent-landing events is often a concern. On Mars, by the time the velocity is low enough to deploy decelerators, the vehicle may be near the ground with insufficient time to prepare for landing.

As the spacecraft approaches the planet Mars it will enter into a hyperbolic path. Mars can be reached and a successful landing ensured entirely through expending propulsive energy or through direct entry, aero-capture or aero-braking, which utilize the Martian atmosphere to a substantial advantage (see Figure 3). In direct entry mode, the module soft lands on the Martian surface after reducing the velocity using on board propulsive energy and by continuously maneuver the module in such way that it lands on the surface vertically with velocity not larger that a few meters per second. Direct entry is associated with intense heating and high deceleration load (Drake 2010).

The Viking 1 and 2 missions employed direct entry to place the landers on the Martian surface. Aero-capture is a method that is employed to directly capture into a planet’s orbit from a hyperbolic arrival trajectory using a single, atmospheric aerodynamic drag pass, thereby reducing the propellant required for orbit insertion. In aero-capture a spacecraft dips into the mid-atmosphere with hyperbolic entry velocity so as to achieve the capture velocity and orbital insertion in a single drag pass. Subsequently, a propulsive maneuver is performed to raise the periapsis. In contrast, when aero-braking is employed, a spacecraft is first captured into an elliptic orbit around the celestial body through propulsive maneuver. Then the orbit size is reduced gradually over a period of time through successive upper-atmospheric drag passes. Though aero-capture uses less propellant as compared to aero-braking, additionally an aeroshell is a required to protect the payload from aero-thermal load during the atmospheric passage of spacecraft. The landing site is chosen at a relatively flat area where cargo elements can land safely. Also the landing site must be suitable to allow for in situ resource utilization (Gage 2010b).

6. The New Beginning On Mars

Mars is a unique and universal place, not for one group, not for one creed, not for one nation but for the entire humanity (Mitchell and Staretz 2010). The human mission to Mars will be a major step towards fulfilling the age old dream and human desire to explore the planets. It may take a large number of missions to Mars to perfect the technology associated with human travel to deep space. It may take the crew a few weeks or months to acclimatize to the gravity of Mars (about 0.38 g). After the crew has acclimated, the initial surface activities would focus on transitioning from a lander mode to a fully functional surface habitat mode (Boston 2010; Gage 2010b; Schulze-Makuch and Davies, 2010; Zubrin 2010). This would include performing all remaining setup and checkout operations that could not be performed prior to landing, as well as transfer of hardware and critical items from the pre-deployed Mars-Ascent-Descent Vehicle System. Thus, a new beginning for the humanity is ushered in. The journey to Mars will be a first step toward colonizing the entire cosmos.

Acknowledgements: Authors would like to acknowledge S. Swaminathan, Ph.D. for his valuable contributions in the preparation of this paper.