A step closer to mars: it's tall order, but within a generation, technology will take humankind farther from home than we have ever gone before.
Activities focused on space will continue to accelerate advances in robotics, power technologies, and life-support systems. They will enhance our creativity in finding new solutions for Earth-related problems, and can have significant impact on society and the economy in unforeseeable ways.
The study of human physiology in space environments can significantly enhance the understanding of various terrestrial diseases and provide stimulus for development of innovative medical technology. Smart, integrated medical systems being developed for space may one day deliver high-level care to rural clinics, nursing homes, and isolated accident sites.
In January, 2004, President Bush announced a vision of space exploration that would return human beings to the moon and eventually send them to Mars.
The engineering of highly reliable, robust systems for those human space missions, along with the creation of a livable, artificial environment on Mars, will provide a new arena for the innovation of future technologies, allowing scientific progress and creating economic growth. Future decades may witness industrial commercialization of extraterrestrial resources, combined with strong international cooperation for the benefit of humankind.
While some of today's technologies might be adequate for human Mars missions, more advanced technologies could lower the cost, and make exploration safer and more productive. Finding the resources for these technologies and deciding the best way to invest them are among the challenges that engineers and administrators face.
But these and other questions surrounding Mars exploration had been under study long before the president discussed his vision with the American people.
Studies of human Mars missions have been conducted in the last two decades by NASA, other space agencies, and non-government groups, including the Mars Society. NASA has developed a series of design reference missions to serve as guideposts toward sending a human crew to Mars and to provide a basis for comparing different approaches and criteria. NASA'S vision is to combine the knowledge gained from robotic Mars missions and the experience of human lunar missions to develop a plan to send people to Mars in the 2025-2030 time frame. A formal assessment of potential approaches for a human Mars mission will be made by NASA in the coming year.
There are engineering challenges to be met at each stage of the way. Technology must be developed to transport the infrastructure, facilities, and crew from Earth to Mars. The infrastructure and facilities themselves need to be developed. Advanced technology is needed to provide life support (particularly consumables) to the crew during all phases of the mission, and to lift the crew from the surface of Mars and transport them back to Earth.
If space exploration is about venturing to new worlds and understanding the universe in ever-increasing detail, then both robots and humans are needed, as key components of an integrated, networked human-robotic exploration system. The strength of each partner can make up for the other's weaknesses. This applies specifically to the intelligence and flexibility of human participation, on the one hand, and the beneficial use of robotic assistance to amplify human performance, on the other.
There are different strategies for getting to Mars, a few of which require some assembly of spacecraft in low Earth orbit. Depending upon the mission architecture adopted, a human Mars mission will require 250 to 500 metric tons of mass to be delivered to low Earth orbit, which is to say two to four times the amount required to support a human lunar expedition. Thus, if a Saturn V class launch vehicle is developed capable of boosting a lunar mission with a single launch, two to four such missions will be required to send a crew to Mars. Alternatively, if a decision is made to launch lunar missions with several medium-lift vehicles, then dozens of launches might be required to send humans to Mars.
Every launch, however, increases mission risk, because if a single delivery of critical hardware should fail, the entire mission would be lost. Also, if the strategy requires extensive assembly in orbit, mission complexity and risk increase even more. Furthermore, all launches must not only be successful; they also must be on time. If propulsion stages, for instance, were held too long in orbit by excessive delays, they could lose cryogenic propellant, and thus become useless.
The reliability of successful launches achieved historically for American expendable systems is about 0.9, and the success rate for on-time launch has been closer to 0.5. In the face of this reality, any mission design that requires four successful on-time launches, let alone a dozen, is an open invitation to failure.
The development of heavy-lift launch vehicles in the Saturn V class or more powerful is thus an essential requirement for a successful moon-Mars program. Such a vehicle would allow a moon mission to be accomplished in a single launch, and Mars missions to be done in two to four launches. The on-time requirement for the Mars launches can be greatly mitigated by adopting mission strategies in which each booster sends its own payload directly to Mars independently. The crew leaves Earth only after it has been confirmed that all the other payloads have arrived on Mars safely. Such direct injection mission designs also eliminate the need for on-orbit assembly, and the costly orbital infrastructure required to support it.
Getting There on Less
While a host of space propulsion systems, ranging from solar reflective light sails to magnetic fusion or even anti-matter, have been proposed for human missions to Mars, there are really only three viable candidates for the foreseeable future.
Chemical propulsion has already supported human lunar missions. However, the exhaust velocity obtainable by such systems is limited. To date, the best exhaust velocity achieved with chemical engines is 4,440 meters per second using hydrogen and oxygen propellant. That is close to the theoretical limit of about 4,700 m/s. Thus, significant further improvement of this technology appears to be impossible.
Greater exhaust velocity is a desirable goal. A doubling of exhaust velocity means that an engine would need only half as much propellant to exert a given amount of impulse, thereby reducing the mass, and thus the cost, of a mission.
Nuclear thermal rocket engines work by using a solid-core fission reactor to heat hydrogen propellant, which is passed through the engine block as a coolant and then ejected from the nozzle to produce thrust. Because such devices decouple the energy source from the motive mass, they can achieve significantly higher exhaust velocities than chemical engines. In a ground test program conducted jointly by NASA and the Atomic Energy Commission during the 1960s, nuclear thermal rocket engines were fired with thrust levels ranging from 15,000 to 250,000 pounds, and exhaust velocities of 8,500 m/s. Limited only by the temperature tolerance of reactor materials, exhaust velocities for this technology approaching 10,000 m/s appear achievable.
However, as a result of the post-Apollo decisions to terminate research, space-rated nuclear rocket engines do not exist today. There is little doubt that this technology could be made to work, but the need to accommodate a renewed ground test program with today's more radio-logically sensitive political environment poses issues that have yet to be resolved.
There are also electric propulsion systems, which accelerate a charged propellant via either electrostatic or magnetohydrodynamic means. There is thus almost no limit to the theoretical exhaust velocity of such technology and, in fact, exhaust velocities of 50,000 to 100,000 m/s--10 to 20 times those of chemical engines--have been demonstrated. The problem, however, is that electric power must be supplied to drive such units. This could be done in space using either photovoltaic or nuclear sources, but the mass of such systems would be considerable, and would greatly affect performance.
If an electric power source were large enough to reach Mars in six months, a trip time obtainable with chemical systems, the mass of the mission would become excessive. If the power system were kept small, the transit times would grow to years.
If electric propulsion represents an inferior technology for use on piloted Mars missions, it does, however, offer some interesting potential for cargo delivery or as a support system. For instance, it could drive a kind of transport craft to raise cargo payloads to high orbits loosely bound to the Earth. Once a payload has been so positioned, it can be driven rapidly to Mars with a relatively small amount of chemical propulsion, allowing the electric tug to return to low Earth orbit for another load.
In order to achieve Mars orbit insertion and descent to the surface, rather large accelerations are required, which if implemented via chemical propulsion, require very large propellant masses. Until now, all Mars orbiting spacecraft have been captured into orbit using rocket propulsion. It would be highly advantageous from the point of view of reducing mission mass to accomplish this orbital capture maneuver using aero braking (and aerocapture friction against the planet's atmosphere) in place of propellant.
The direct entry technique, which was performed successfully by Mars Pathfinder and the two Mars exploration rovers, might be acceptable for delivering cargo to Mars, but is undesirable for human landings, since it precludes a pause in orbit that might be advisable if bad weather prevails at the time of arrival.
Aerocapture is a maneuver designed to take advantage of Mars' atmosphere to slow a spacecraft to orbital capture velocities. A vehicle that enters orbit in this manner would need to carry less mass in the form of propellant than a craft relying solely on engines. Aerocapture begins with a shallow approach angle to the planet. Descent into the relatively dense atmosphere causes not only sufficient deceleration to enter an orbit, but also sufficient heating to require a heat shield.
Two strategies have been proposed for returning from Mars. In one, an Earth return vehicle is placed in Mars orbit, and an ascent vehicle carries the crew from the Martian surface to a rendezvous with the ERV. The crew members transfer to the ERV, which returns them to Earth. This strategy requires minimum lift-off mass from Mars, but it requires two vehicles.
In the other strategy, there is one vehicle, which ascends from the surface of Mars and returns all the way to Earth. The vehicle must lift the propellants required for the trip from Mars orbit back to Earth, as well as the capsule and all consumables and systems for a 180-day journey. Such an approach, however, may be advantageous if the return propellant can be made on Mars out of local materials.
Living off the Land
Regardless of the method proposed, technology development is required in a number of areas. Increasingly lighter-weight structures and materials must combine with advanced propulsion and power systems, thermal and environmental control systems, and avionics and communications systems. Equally important will be technologies that will let the crew make use of resources they find on Mars.
Throughout history it has been invariably shown that those explorers have succeeded best who were prepared to make intelligent use of the resources available in their environment. The Lewis and Clark expedition would have been impossible had they tried to bring with them all the food and water necessary for themselves and their horses for a three-year transcontinental expedition.
When we consider human Mars missions, the case for local resource utilization becomes still stronger, as the costs and difficulty involved in transporting necessary materials from Earth are enormous. Not having to deliver fuel, for instance, could reduce mission mass significantly.
The Martian environment is believed to be a rich source of materials from which propellants can be made. Large regions of the Martian surface have been identified from orbit as containing more than 60 percent water by weight. Such water, now in frozen mud, could be accessed and electrolyzed to produce both oxygen and hydrogen rocket propellants. Hydrogen so obtained could also be reacted with the carbon dioxide that makes up the Martian atmosphere to produce methane and oxygen to fuel a rocket, or alternatively, methanol and oxygen for a fuel cell.
Methane or methanol and oxygen can also be produced on Mars by reacting local C[O.sub.2] with hydrogen (which constitutes 5 percent of the propellant mass) transported from Earth. Martian soil contains virtually every element known on Earth, and many materials have been concentrated into ore by the planet's complex hydrological and volcanic history, making use of mineral resources feasible, too.
In the longer term, the inhabitants of a permanent planetary base may not only make their propellants, but other life support consumables, such as air, water, and food, out of local resources. In this respect, Mars is much more promising than the moon for long-term occupation. Carbon dioxide, nitrogen, and water required for plant growth are plentiful on the Red Planet's surface, but extremely rare in the lunar environment. The rich carbon supply on Mars also suggests a possibility of local production of such essentials as plastics, lubricants, and synthetic fabrics. The Martian regolith can also be used as a shielding material for habitats.
For the coming age of space exploration, Mars compares to the moon as North America compared to Greenland during the era of European maritime exploration. We will reach the moon first, but Mars has the promise of a place where we can settle.
Making Space Safe for Humans
The challenges of time, distance, and hostile environment necessitate special demands to protect and support a crew on a mission to Mars.
Space vehicle launch, docking, navigation, and landing will require complex interactions between automated systems and human operators. NASA is developing technologies to improve the reliability of automated rendezvous and docking systems for in-space vehicles. It is working on intelligent software agents that can help the crew in performing fault detection and repairs.
Physical tasks on the Martian surface will require protective suits that allow explorers to function effectively without the risk of excessive fatigue. Mars travelers may be subjected to long periods of microgravity during interplanetary travel and to an extended period of reduced gravitation on the surface of Mars. The acceleration of Mars, roughly 3.7 meters per second, is about one-third that of Earth.
Travelers will face psychological and physical challenges induced by the long-term confinement of the voyage. They will also need protection from changing levels and various types of radiation in the environment.
Prolonged exposure to reduced gravity, confinement, and, of course, radiation pose risks to human health and performance. NASA is working to develop smart, integrated medical systems to assist in delivering quality health care to space travelers. These systems include minimally invasive and noninvasive methods of gathering health data, automated devices to aid in diagnosis, and even surgical techniques for use in space.
Robotic exploration of Mars has been an important part of the space exploration missions of the United States, Russia, Europe, and Japan. Between 1960 and 2005, a total of 40 orbiters, landers, and rovers have been launched toward Mars to gather data about the planet. The U.S. alone has sent 19 robotic missions to Mars, of which 14--three fly-bys, six orbiters, three stationary landers, and two rovers--have been successful.
As a result of this activity, a great deal of advanced data has been obtained about Mars, and it is unclear how much more is truly necessary before a human Mars mission can be attempted. In particular, the recently arrived NASA Mars Reconnaissance Orbiter will soon be mapping the planet with photographic resolutions better than 20 cm per pixel, which will allow us to see objects as small as 60 cm across. This is more than adequate to identify the landing hazards of interest to a human landing craft. It is also sufficient to create, in advance, the kind of maps that human explorers might need.
Current and future robotic missions can complement human exploration. Robots can enhance knowledge about the Martian environment, scout landing sites, deliver elements of infrastructure, assess resources, test and validate new technologies, and alert the crew to impending failures or dangers. If human explorers will need to drill to find ground water, orbiters could be sent ahead with powerful ground penetrating radar. (There is already some GPR on both the European Mars Express and the Reconnaissance Orbiter, but in neither case is it the primary instrument.) A robotic sample-return mission to likely landing sites could provide detailed information before a human landing, and test the operational aspects of a round trip.
Down to Earth Practice
The success of a human mission to Mars will require that crews be trained to solve problems that can occur. It is to identify those potential problems that a number of Mars analog experiments and simulation studies have been conducted by NASA, the Mars Society, and other organizations. These studies, which address some of the human factors of Mars exploration, started with short-term simulations and are moving toward simulating full-term Mars missions.
The terrestrial analogs for Mars are setting on Earth where some of the environmental conditions, geological features, and biological attributes are reminiscent of those to be encountered on Mars. The environments offer opportunities for partial simulations of Martian conditions.
The Mars Society has built two analog Mars stations, one in the desert in southern Utah, and the other on Canada's Devon Island in the high Arctic. Since 2001, some 60 crews of six people each have conducted simulated Mars exploration missions at these stations. For two to four weeks at a time, these crews attempt to conduct a sustained program of field exploration while operating under some of the same constraints that will be faced by explorers on an actual human Mars mission.
For example, no one is allowed to go outside without wearing a spacesuit simulator. The crew is responsible for all of its own field work, lab work, reportage, repair of equipment, and chores of daily life. They work in telescience collaboration with a remote science team, a mission support group, and an engineering team located in the continental United States. Conducting operations in this way yields many insights into technologies, techniques, field exploration tactics, crew skills, character types, and operational procedures that will be most useful in carrying out actual expeditions to the Red Planet.
The Mars Society plans to send a single crew in 2007 to conduct a four-month simulated Mars mission on Devon Island, 900 miles from the North Pole. An open call for crew volunteers has been posted on the Mars Society's Web site, www.marssociety.org.
The Institute of Biomedical Problems of the Russian Academy of Science is preparing a ground-based experiment that will confine six volunteers for more than 500 days, beginning in the fourth quarter of 2007, with the overall goal of identifying the needed biomedical support for a human Mars mission.
The experiment will be conducted in a complex of five modules, with a total volume of 550 cubic meters. The modules contain individual cabins for the crew and working quarters for carrying out biomedical research. During the experiment, the crew will be subjected to stress and various emergencies, including malfunctioning of equipment and systems.
For More Information
readers interested in pursuing the subject covered in this article will find directions to more information at http://www.aee.odu.edu/humanmarsportal.
The Web site, created as a companion to Mechanical Engineering magazine's November Feature Focus, contains links to material on human Mars mission strategies, robotic Mars missions, and a media gallery. There are also links to other online services and features of the Center for Advanced Engineering Environments at Old Dominion University. Further information about the Mars Society and its activities can be found at www.marssociety.org.
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|Comment:||A step closer to mars: it's tall order, but within a generation, technology will take humankind farther from home than we have ever gone before.|
|Author:||Noor, Ahmed K.; Zubrin, Robert; Stanley, Douglas|
|Article Type:||Cover story|
|Date:||Nov 1, 2006|
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