1. Introduction

The United States has been sending humans into space for approximately 50 years. The accomplishments of the human spaceflight program have been transformational technically, scientifically, culturally and politically. Less than 10 years after the first manned sub-orbital flight, the United States had sent astronauts to another world. With the development of the Space Shuttle, human operations in space has matured to the point where tasks such as servicing the Hubble Space Telescope (HST) and constructing the International Space Station (ISS) are not only accomplished, but are perceived as almost routine (Hawley, 1997, 2003). However, after 50 years of human spaceflight, the technology and experience necessary to undertake a human mission to Mars are arguably still to be acquired. A human mission to Mars would present challenges and risks far beyond what we have dealt with in human spaceflight so far, notwithstanding our impressive accomplishments.

During five decades of human spaceflight, American astronauts have accumulated an impressive amount of time in space. The vast majority of the on-orbit time has accrued from hundreds of individuals flying missions of one to two weeks duration. In my case, I flew five missions for a total of slightly more than 32 days. However, we do have the experience of several astronauts who have been in space for roughly six months at a time. The longest single flight by an American astronaut is 215 days by Mike Lopez-Alegria while the longest by any human is 438 days by Russian Cosmonaut Valeri Polyakov. Based on currently available technology, a one-way trip to Mars will take at least 220 days.

A number of technologies are important to a human mission to Mars, including the development of a heavy lift launch vehicle, closed-loop life support, advanced in-space propulsion, advanced spacesuits, and in-situ resource utilization (ISRU). These and other technical issues were the subject of the report "America at the Threshold" (Stafford, et al., 1991, hereafter The Stafford Report) produced by a committee of space experts, designated The Synthesis Group, and charged by President George H. W. Bush in 1990 with developing architectures and options for a human mission to Mars. A more current assessment of many of those same issues is provided elsewhere in Volume 12 of this Journal. Although I was a technical advisor to the Synthesis Group and many of the technical observations in this article come from the Stafford Report, the focus for the following discussion will be on some of the risks and challenges that I would think about based on my spaceflight and flight operations management experience were I selected to be one of the astronauts for a mission to Mars or responsible for choosing them.

2. Risks

Chief among the risks is the threat posed by radiation. Astronauts in low Earth orbit are protected to some extent by the Earth’s magnetic field. Still, I remember the flashes of light that I would occasionally see in flight when I was trying to sleep. I had learned about that phenomenon from the Apollo and Skylab astronauts. If a strong solar flare presented a threat to the crew, the Shuttle has the ability to return to Earth and the ISS crew could seek shelter in the best protected portion of the structure. If the solar event were sufficiently energetic to present an extreme hazard, the Station crew could evacuate in the Soyuz vehicles that are left docked to the Station. However, during interplanetary flight, astronauts would be exposed to high energy particles from the Sun and also to galactic cosmic rays. Galactic cosmic radiation presents the greatest threat to astronaut health (Johnson, et al., 2011). The Apollo astronauts were vulnerable and it was fortunate that there were no large radiation events while they were in transit or on the lunar surface. Even today, our ability to forecast solar activity is inadequate for use in long-term mission planning, although NOAA and NASA work together in support of Shuttle and Station to provide daily forecasts and associated mission impacts (Johnson, et al., 2011). Most likely, mission design will need to provide for shielding in flight. Shielding options potentially impose a large weight penalty on the spacecraft, although there may be options to use water as both a shield and as a supplement to a closed-loop life support system or even a source of hydrogen and oxygen. The risk of exposure to radiation and the difficulties of shielding argue for reducing the travel time which, in turn, argues for development of advanced space propulsion. Once on the surface of Mars the atmosphere and habitats will likely provide adequate protection (The Stafford Report).

The radiation risk to astronauts is currently managed by limiting both the annual and lifetime dosage and each astronaut’s exposure is measured in flight. Astronauts chosen for Mars missions may be required to accept the (hopefully small) risk of a fatal exposure. My experience says that many astronauts would be willing to accept higher risks for themselves than the physicians would allow. We have up to now not permitted that and have managed the lifetime exposure risk for all astronauts by policy at the program or Agency level. Assuming that it is unlikely that astronauts on a trip to Mars would make a second trip, managing the risk of radiation is more a design issue than a policy one once a decision is made as to what potential radiation dosage is acceptable. However, policy regarding lifetime exposure might still apply if the astronauts for the Mars mission had also flown one or more precursor missions in the lunar environment. Perhaps resistance to radiation exposure could be selected for through genetic testing or even genetically engineered into prospective astronauts (Vernikos, 2009). This represents one of several new and highly restrictive selection criteria that could potentially be imposed on applicants to be Mars astronauts.

De-conditioning will also be a risk. Long-duration spaceflight affects the human body in a variety of ways. Of significant concern is the loss of both bone density and muscle strength. Research has shown that exercise, particularly exercise that loads the bones, mitigates de-conditioning to some extent (Lane, et al., 2011). Exercise is normally required on a daily basis aboard the International Space Station and a substantial amount of crew time is devoted to it. Even so, crew members returning from long-duration flights are not able to walk un-aided and may spend several days or weeks in rehabilitation. A recent study even suggests that de-conditioning is a significant issue, perhaps more so than had been previously understood, despite the regular exercise that International Space Station astronauts routinely perform (Fitts, et al., 2010).

What will we expect in terms of performance from astronauts landing on Mars for the first time? Certainly a great deal of dedication will be required by the crew to adhere to exercise protocols in flight. I found that exercise in flight was beneficial physically as well as mentally and tended to help some symptoms of adaptation to weightlessness, such as back pain, but I never had to do it every day for nine months. Crew discipline will be important after landing in order to accomplish the required tasks while not overexerting and risking injury. One data point on performance by de-conditioned astronauts was obtained serendipitously when the returning crew of ISS Expedition 6 missed their landing site by several hundred miles and spent time on the ground awaiting the recovery crew. Before help arrived, the crew extricated themselves from the Soyuz and made some basic assessments of their ability to perform tasks after more than 5 months in space (see the article by Don Pettit in this issue). Even if crewmembers are functional after landing, life science research suggests that for both Shuttle and Station crewmembers, the feeling of having returned to normal occurs well in advance of actually returning to a clinical normal (Lane, et al., 2011). Spaceflight de-conditioning also argues for reducing the travel time as much as possible. Advancements in space propulsion, such as the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) could potentially reduce the transit time from months to weeks (Chang-Diaz, 2000). Nuclear propulsion options may have the potential to reduce the travel time to Mars to ~ 160 days (The Stafford Report). Artificial gravity has been considered in previous Mars mission concepts although that often comes with a habitable volume penalty. Human-powered centrifuges were being investigated for use as a de-conditioning countermeasure at Ames Research Center in the early 1990’s (Vernikos, 2009).

3. Psychological Challenges

A variety of psychological issues will need to be considered. Astronaut candidate selection has always included a psychological evaluation, but we have historically used the results more as a "select-out" than a "select-in" criterion. For Shuttle flights this was adequate because astronauts are being constantly assessed prior to mission assignment and the flight crew trains together for more than a year before a one to two week mission allowing for evaluation of any issues and time to address them prior to launch. Importantly, the missions were short enough that problems very rarely occurred in-flight. As long-duration flights with crew members from other countries became more common, the Astronaut Candidate Selection Board sought a way to select for traits specifically applicable to International Space Station crew members participating in the multi-national program. Rigorous criteria were elusive, however. To find astronaut candidates best suited to the challenges of missions to Mars it could very well be crucial to establish validated "select-in" requirements. These criteria would be another of the potentially new requirements that could significantly limit the pool of highly-qualified applicants.

Some astronauts have reported a feeling of isolation while flying in space, more commonly on long-duration missions. One can easily imagine the potential for feeling isolated on a trip to Mars. Early in the Shuttle program direct contact with friends and family was not possible while on-orbit. However, later in the Shuttle and Station programs, technology allowed crew members to be able to stay in touch with family and friends through e-mail, ham radio, and eventually internet telephone. Increased interaction with family was undeniably psychologically beneficial, but it raises other issues about exchanging information. We were always asked before flight if we wanted to be informed if something bad were to happen, such as the death of a family member. Crew members answered individually as to their preferences. Furthermore, current policy stipulates that operational information is exchanged only in formal channels, for example, on the open air-to-ground frequency. Family members were not authorized to share operational information with the crew, for example, that Mission Control was considering asking the crew to do an unscheduled spacewalk to address a developing problem. It may be neither practical nor desirable to withhold personal information from a crew member for the years it would take to complete a mission to Mars. After the recent accident that trapped 33 miners underground, Chilean officials contacted NASA for advice on how to maintain the miners’ mental health while they wait potentially months for rescue. Our experience with astronauts in flight, particularly on long-duration missions, suggests that being able to talk with loved ones, being informed completely and honestly as to current status of mission-related items, and having an advocate on the ground for their issues and concerns is a large part of maintaining a healthy outlook among the flight crew.

I found personal space to be an important component of psychological well-being on my missions. On the Shuttle, there is insufficient volume to allow for even a small assigned space for every crewmember, with the exception of sleep compartments that were shared by crewmembers on a small number of Spacelab missions where astronauts worked in shifts around the clock. Nevertheless, I found it comforting to have at a minimum my own sleeping bag hanging in a unique location on the wall and my music player and some personal mementos "velcro-ed" to the bulkhead nearby. On the International Space Station crewmembers have small sleep compartments. For a mission to Mars, even though the accommodations will need to be austere, it will be important to include an individual space for each crew member in the design of both the spacecraft and the habitat. It will also be important for the crew to have useful work to do while in transit. As an astronomer, I could imagine having a telescope and making observations that take advantage of my location in deep-space.

4. Operations Planning

Early in the Shuttle Program NASA developed a concept referred to as In-Flight Maintenance or IFM. The basic design of the Shuttle presumed that reliability and redundancy would be adequate to see the crew through to a successful mission conclusion. Additionally, flight rules were in place to protect crew safety by limiting mission duration in the event of significant systems failures. However, crews felt that it would be possible to increase the probability of mission success by manifesting the tools and spares necessary to allow the performance of maintenance on the vehicle in orbit. Initially, the maintenance tasks were limited to replacing key components, such as CRT’s or computers of various types. We replaced a failed CRT on-board Discovery in 1984 to allow a full complement for the flight deck crew to use for entry. The task was very difficult, however, since in weightlessness it was almost impossible to break the torque on the bolts holding the replacement in place due to lack of anchor points for the crewmembers performing the task. As confidence developed in the ability of the ground team to plan and the flight crew to execute IFM tasks, more and more sophisticated repairs were accomplished and additional accommodations for maintenance were provided. Maintenance is a standard activity today aboard the International Space Station and it will be important to design a Mars transfer vehicle with some number of critical spares and the ability of the crew to repair their spacecraft in flight. Assembly of the spacecraft will need to allow for the potential for maintenance in terms of access, appropriate anchor points, and other considerations.

Another challenge will be maintaining astronaut proficiency. For most of the history of the space program, astronauts have trained for months or years for a mission that lasted days or weeks. This allowed for the luxury of being able to practice the specific tasks over and over again until the crew was proficient even up to the last few days before launch. A crew member was never better prepared for a task than when he or she needed to perform it in flight. With the development of the International Space Station, it became necessary to evaluate how a crew member’s ability to perform a task would be affected if the task were executed months after the last task-specific training. One solution was to assign some detailed, time-critical operations, such as a spacewalk (extravehicular activity or EVA in NASA terminology) or robotics, to a visiting Shuttle crewmember who would have been recently trained on the specific task. While successful, this approach limited operational flexibility and was somewhat demoralizing to the ISS-assigned crew who often felt they were not being trusted with the more challenging assignments. Even during the Shuttle era, NASA managers were concerned about pilot proficiency when it came to landing the Shuttle after two weeks of de-conditioning and a lack of recent proficiency flying. To address the concern, an in-flight simulator was developed to run on a laptop computer replicating the flying qualities of the Shuttle. The pilot would attach a flight-like hand controller to the computer and then could practice approach and landing techniques in a manner very similar to the ground-based simulators and the Shuttle Training Aircraft.

A sign of operational maturity, as human spaceflight transitioned to more long-duration missions, was the migration from the "task training" approach characteristic of Shuttle missions to "skills training" more applicable for ISS increments. As the terminology suggests, crews are trained in a specific skills set applicable to a number of anticipated but as yet undefined tasks they could face during the increment. Long-duration flight also offers something of an advantage over Shuttle missions in the sense that there can be less pressure to get everything done on a schedule. For instance, if some tasks on a spacewalk take longer than planned, others can be deferred to a later spacewalk. That option rarely existed on Shuttle flights which were launched with little or no timeline margin. For human missions to Mars, some combination of skills training and in-flight training will be required so that the crew will be adequately proficient prior to and after landing. The level of proficiency required will be dependent to some extent on the planned length of stay. Longer stays can accommodate time for crew re-familiarization training. It will also depend on the extent to which automation is employed and how the division of labor between humans and computers or robots is allocated.

Human factors will be important in spacecraft design. Lack of attention to human factors can lead to increased crew training time, increased possibility of crew error and some level of crew frustration. These considerations would be even more important on a round trip to Mars lasting several years. Human factors considerations should include aids for crew situational awareness and sophisticated fault detection and identification systems to inform the crew of failure details and resulting options. These will be important as resource considerations and time-delay will limit the ability of Mission Control to be of real-time assistance. The Shuttle is a remarkable flying machine, but human factor considerations were often lacking in the design of crew interfaces.

A vehicle to send humans to Mars would likely be large enough that some amount of on-orbit assembly would be sensible or necessary. Fortunately, the building of the International Space Station has given us good experience at doing in-space construction, although assembly that can be done robotically would be desirable to that which would require EVA to reduce the timeline overhead and risk to the crew. To make human missions to Mars more affordable in terms of up-mass required, mission architecture could be designed so that the crew doesn’t have to take everything needed with them. Options include sending supplies and logistics ahead of the crew so they will be on Mars when the crew arrives. For example, growing food during the flight could offer a pleasant alternative to whatever pre-packaged food is manifested and also provide an interesting task to occupy time while en-route. Leaving home without the fuel necessary to return would be unprecedented, but some thought has been given to the production of fuel for the return trip from resources potentially available on Mars. It may be that this level of sophistication would evolve after the first few Martian missions characterized the environment and conducted proof-of-concept experiments. Water ice is present in some locations. In-situ resource utilization (ISRU) techniques could potentially produce methane from Martian atmosphere. Recently, the atmosphere has been found to have trace amounts of methane in the northern hemisphere summer (Mumma, et al. 2009). Whether methane could be collected at Mars for use as a fuel is not currently known. Ideally, the techniques would be verified and the fuel depot stocked prior to launching the crew.

Presumably, we will have had experience with the launch vehicle, the in-space propulsion system and the crew module to the point where we will have a reasonable understanding of those risks before we commit to a human mission to Mars. In the Shuttle era, astronauts spent a great deal of time considering, developing and verifying a variety of ascent and on-orbit abort options. However, the opportunity to abort a mission to Mars after leaving low Earth orbit will be limited. Once on orbit, the Space Shuttle could always exercise the option to attempt a landing in the event that a meteorite or piece of debris punctured the pressure vessel or some critical systems failure occurred. Crews aboard the International Space Station have the capability to leave in a Soyuz vehicle which is always available. (During the Space Station Freedom development there was a concept known as safe haven where a crew would go to await rescue in the event of a catastrophic failure. Astronauts argued that a lifeboat vehicle was necessary and at one time a Crew Escape Vehicle was in development. With the transition from Freedom to International Space Station, the CEV concept was implemented with Soyuz spacecraft supplied by the Russian partners). Apollo spacecraft were sent to the Moon on a free-return trajectory meaning that, in principle, if something happened, including a propulsion system failure, the spacecraft would circle around the Moon on a trajectory that would return it to Earth. The trip would still take several days which would challenge the combined resources of the ground and the crew as it did on Apollo 13. Abort options on a trip to Mars would be at best limited and, most likely, not exist. Conceivably one option might be to head expeditiously for the Martian surface whether or not a return capability exists. Perhaps advancements in space propulsion, such as VASIMR or other designs, could allow for a powered return to Earth abort option. Even that option could take several months (Chang-Diaz, 2000). Architecture options that include testing the propulsion system, crew module, and other hardware and techniques in or around the Moon would provide a relatively near-by but still challenging environment before committing to Mars. I would anticipate that astronauts selected for a Mars mission program would advocate strongly for a flight test program in the lunar environment.

The first human mission to Mars might be an orbital rather than a landing mission. Would that incremental approach make sense (technical, economic, political) as it did for the Apollo program to the Moon? Would the first landing mission be designed for a surface stay of a few weeks or for more than a year? Assuming that the transfer orbit would be a minimum energy trajectory, known as a Hohmann transfer, the trip to Mars would take on the order of 220 days using today’s chemical propulsion technology. To take advantage of the minimum energy return, a short duration stay (~ 60 days) and a long duration (~ 500 days) stay are both options (The Stafford Report). A surface stay of a few weeks would allow for proof-of-concept activities such as verification of advanced spacesuit function; demonstration of an ability to traverse presumably with some sort of vehicle; characterization of the environment and, in particular, of the soil to determine its suitability for ISRU purposes, its threat to equipment or potential toxicity. Limited scientific investigations could be planned. However, de-conditioning might restrict the ability of the crew to perform efficiently for a significant portion of a several-weeks long stay. The longer surface mission would enable significant science, but also expose the crew to greater risk if systems don’t function as planned. Potential science objectives have been documented in several different studies (e.g., Mars Science Program Synthesis Group, 2003). There are rational arguments in support of both the short and the long duration surface stay on the first landing. I could envision the crew advocating for the development of a set of mission extension "go, no-go criteria" which, if met at the end of a short stay, would authorize the continuation of the surface mission for the long duration.

For any length of stay, the time-delay in communications with Earth will be a consideration. Depending on where the planets are in their orbits, the round-trip travel time for a radio signal could be as much as ~ 40 minutes prohibiting routine air-to-ground communications and providing a potential source of frustration for the crew and contributing to the feeling of isolation. The crew will generally be unable to rely on the Mission Control team for real-time support. The systems, tasks, training and flight planning will have to be developed under the assumption that the crew will be self-reliant. I could imagine the team on the ground developing a plan for the upcoming week and a more detailed plan for the next day or two, perhaps with some suggestions as to how the crew should proceed in the event of different potential outcomes. The long-range plan could be updated as necessary given the events of each day. Even though there should be the appropriate infrastructure in place to allow the ground to monitor the crew’s activities, the crew would most likely spend some time at the end of each day developing a summary of the day’s activities and results, presumably emphasizing their perspectives on what took place, that the ground could then receive and consider while the crew sleeps. Much of the decision making would necessarily be the responsibility of the crew based on their expertise, training, and an understanding of what authorities they have been delegated. Excursions on the surface would not be scripted as EVA’s are today and the crew would have a great deal of flexibility as to how the activities would be conducted. However, they would need to have a good understanding of any constraints and the discipline to honor them. For instance, during Shuttle or ISS missions it has been the ground team’s decision when to end an EVA based on consumables, crew fatigue or other factors even when an astronaut might express the desire to continue. The skills the crew should posses would include not only the appropriate scientific and engineering skills but also demonstrated good judgment and decision making. An immense amount of scientific and engineering data could be made available electronically to the crew for reference while they planned for or performed specific tasks. I would expect the crew to operate on Mars time ("sols" in the terminology of the Mars Exploration Rover team).

5. Program Management Considerations

The Shuttle made space travel accessible to a wide variety of individuals. The Shuttle’s design, developed to allow for large payloads to be delivered to low Earth orbit, also allowed for a large number of crew members and a wide range of astronaut size and expertise. In fact, the height requirement for Shuttle astronauts when I was selected was very liberal (5’ 0" – 6’ 4"), although due to the constraints of the Soyuz spacecraft, size requirements for Station astronauts are somewhat more restrictive. The Shuttle can be flown with a crew of two as it was during the Orbital Flight Test program. Crews of four flew the first operational flights. Over time, larger crews were a beneficial side-effect of the Shuttle’s design and allowed for a more ambitious set of mission objectives and a diverse set of space flyers, including non-professionals and representatives from other countries. A spacecraft designed for a mission to Mars would have a far more specialized purpose and it would be costly to specify a requirement to accommodate the range of sizes that characterized the Shuttle astronauts. Restrictive anthropometric requirements would most likely be another significant constraint on the number in the astronaut candidate pool compared to what was typical during the Shuttle era.

The Space Shuttle operated with a "fail-operational, fail-safe" philosophy, which means that the first failure will still allow the completion of the mission and a second failure will allow a safe return, although some or all of the mission objectives might have to be sacrificed. This simple and reasonable sounding approach was challenging to implement because the second failure could be in a different system from the first and we were constantly learning the complex ways in which the various systems interacted. Even years after the first Shuttle flight, analysis or testing would reveal a violation of the fail-operational, fail-safe approach that would require a modification to hardware, procedures or both. What will be the risk management approach for a human mission to Mars? How much redundancy will be designed in? What is the role of the astronaut in helping to make those decisions? In my years at NASA we had astronauts involved in virtually all aspects of the programs – developing and monitoring training, reviewing requirements, testing hardware and software, developing flight rules and procedures, etc. In this way, the interests of the flight crew were represented, whoever the specific individuals for a specific mission would turn out to be. We were part of the decision making process that adjudicated risk. I always felt I could find someone who would be willing to fly in the face of arbitrarily large risks and there were also some astronauts who made what I considered to be unreasonable demands on the programs in the name of crew safety. One of our jobs was to establish some level of appropriate "community" risk acceptance. I would anticipate that would continue, although a smaller corps of astronauts would require optimizing the ways in which they participate.

Arguably the technology exists today to send humans to Mars, although the current implementation of that technology does not. For example, a new heavy lift launcher will have to be developed. Also lacking is the level of technological sophistication to allow a human mission to Mars to be done in an "affordable" way and with a level of risk that the nation would accept. What level of risk will be acceptable? Time and money can be invested in understanding and mitigating identified risks, but there always remain what we referred to as the "unknown unknowns". The management challenge is to optimize risk mitigation and risk acceptance. After the Challenger accident, I had felt that a future accident would result in the termination of the Shuttle program. Therefore, my motivation in helping to manage risk was not only my personal safety and the safety of the others assigned to fly, but also the responsibility I felt for the preservation of human spaceflight. The Apollo program still met the goal of landing astronauts on the Moon by the end of the 1960’s despite the loss of a crew on the pad. The program continued despite the explosion on Apollo 13. I am not optimistic that a Mars program would be allowed to continue in the event of significant early failures.

6. Some Personal Perspectives

As a non-minority male who never went to the Moon, I was seldom the focus of media attention during my spaceflight career. However, I was married to Sally Ride when she became the first American woman to fly in space (1983, STS-7), I flew with Judy Resnik when she became the second American woman to fly in space (1984, STS-41D) and I was assigned to Eileen Collins’ crew when she became the first American woman to command a space mission (1999, STS-93). I have seen and experienced some of the pressures that the demands from the public, press, managers and politicians exert. Those pressures are somewhat manageable, but can be a serious distraction while trying to focus on preparing for or executing a space mission. Ironically, during the Mars mission itself, it may be difficult to keep the public engaged, at least during the trip from Earth. I could anticipate a significant portion of the timeline in-transit being devoted to public affairs events. Perhaps, unlike on an activity-packed Shuttle flight where I generally saw public affairs events as a necessary evil, participation in interviews or "VIP phone calls" would be welcome and provide a connection with activities back on Earth. Being able to represent NASA and our programs to the public was always part of the job. Some astronauts do it better than others and those skills will be important in choosing the crew for a Mars mission. However, I would resist making it a high priority compared to other qualifications. I could also imagine identifying a small cadre of astronauts for training for the first human mission to Mars, with the prime crew named relatively close to launch in order to attempt to minimize the distraction on crew training or at least to enable a greater sharing of the burden.

I have seen the Earth from space, but it would be a unique experience to see the Earth from the perspective of a Mars transfer orbit or from the Martian surface. I recall on a mission in 1997 we overflew the United States one cloud free night pass at an altitude of ~ 340 nautical miles. I was able to identify several major cities and follow the lights to one faint, seemingly insignificant light in the middle of Kansas. That was my home town. It seemed so big to me when I was in high school, but from my perspective in orbit it was barely visible. I could imagine having the same feeling as I think about Earth becoming smaller and less significant in the sky during the trip to Mars. On the other hand, the view of the stars would be impressive. From low Earth orbit when moonlight doesn’t interfere, the prominent constellations are almost obscured by the great number of stars visible. Apollo astronauts experienced what it is like to stand on another world and see the Earth. From the surface of Mars the Earth would appear to be merely a bright star rather than the familiar blue globe. The first photographs of the Earth from the Moon arguably promoted (at least for a time) a sense of community among the people who occupy our planet. Seeing the Earth as one of many "stars" in the heavens would tend to focus on our insignificance in the vastness of the cosmos.

How might an astronaut approach considering the risks of a mission to Mars? From the perspective of the crew, the risk, whatever it is, needs to be worth accepting in the context of the mission objectives. I was privileged to be part of two missions involving the Hubble Space Telescope, the initial deployment in 1990 and the second HST servicing mission in 1997. One of the noteworthy features of a mission to HST is that the crew flies in the highest orbit achievable by the Shuttle. A standard failure for which we trained is the loss of propellant due to a leak caused by mechanical failure or a debris or micrometeorite strike. From HST’s orbit, the crew needs all the propellant the have to safely return. The procedure in the event of a propellant leak is to immediately lower the orbit to an altitude that will allow a subsequent de-orbit with the propellant projected to remain. Were that to actually happen while we were attached to HST (or worse, while our EVA crew members were actually working on HST), we would have attempted to secure and then jettison HST prior to executing the maneuver in a low-probability attempt to save the telescope. That would almost certainly have resulted in the loss of HST, but perhaps not in the salvation of the Shuttle and crew. It was interesting to discuss preflight whether to even attempt such a maneuver with knowledge that the Shuttle could very well be stranded in orbit and almost certainly HST would be lost. Specifically, if the leak were large enough, would we accept our fate in order to continue to work on and preserve HST? We recognized that we also had a responsibility to preserve the Shuttle if at all possible, so we contemplated many hypothetical combinations of failures and outcomes. Regardless, we all knew before launch that there were potential failures, some unique to that mission and beyond what is normally accepted for launch and re-entry, that could cost us our lives. Certainly the experience of flying in space has a certain risk/reward balance at the individual level, but we had to be comfortable that the return for the nation for a successful mission was worth the risk as well. Accidents have not only taken the lives of astronauts but have also have cost time, money, and confidence in the human spaceflight program. In this specific case, we felt that the potential contribution of HST to mankind’s understanding of the universe was worth the risk we were taking for both ourselves and the program.

7. Conclusion

Could I imagine volunteering for or accepting an assignment for a trip to Mars? There is an appeal in attempting to do things and go places on behalf of the human race in the name of exploration and science that few or none have done before. Future generations of launch vehicles will presumably be safer than the Shuttle at transporting people to low-Earth orbit. However, after 50 years of human spaceflight, we have relatively little experience at dealing with the challenges of deep space or operations on a planetary surface. After 30 years of operating the Shuttle, we have progressively improved at reducing the number of unknowns. Now, as the program comes to an end, we understand the risks fairly well and can manage them better than ever before. It is not so clear that we yet know where to draw the line between unwise risk-taking and necessary risk-taking as it would apply to a human mission to Mars. Presumably, advances in technology and precursor missions will reduce the number of unknowns to something that can be responsibly managed.

There are other sacrifices that astronauts make that come with the job – separation from family for extended periods of time, long hours and lots of travel, sacrificing a certain amount of privacy, the pressure of having to perform complex and risky tasks with a great deal at stake in view of the public. Certainly military personnel and their families experience many of these stresses, including separation, performing hazardous operations and concern on the part of their loved ones. Would we ask even more of a potential crewmember on a trip to Mars? The Mars crew would experience separation from family longer than military personnel or previous astronauts have endured. Perhaps that argues that only single people should be considered. Presumably the medical requirements would be far stricter for astronauts intended to crew a spacecraft to Mars than have been imposed on Shuttle or even Station astronauts. Would medical experts and managers additionally want to perform prophylactic procedures on potential crew members, such as removing appendices, to reduce the chance of an in-flight medical emergency? Perhaps potential crew members would be the subject of genetic engineering to improve resistance to radiation as described above or for some other mission-success purpose. No doubt there would be a high price to pay for the privilege of representing humanity on the first trip to Mars.

So, I could envision a small cadre of astronaut candidates that were selected to a highly restrictive set of criteria – certainly more like the selection of the Mercury astronauts than the Shuttle astronauts. Development and operations costs would argue for a limited number of crewmembers to fly the mission. How large a crew would we send to Mars? More astronauts requires more resources, however, a larger crew mitigates the feeling of isolation and enables more activities to be planned. A crew size of 3 or 4 might be an appropriate compromise.

All of this presumes some sort of program of missions to Mars with identified scientific and technical objectives, with the crew selected to have the required skills, and with the appropriate amount of robotic and human precursor missions. Undertaking an exploration program that includes a human mission to Mars would also have benefits in terms of technology development and the associated jobs, new scientific discoveries, and inspiring today’s students to become tomorrow’s scientists and engineers. However, at least one somewhat provocative suggestion has been made by a former NASA employee to send one person on a one-way trip to Mars (McLane, 2006). The premise is that a one-person, one-way journey would be affordable and would capture public interest and provide incentive to continue, at least to send provisions to Mars to keep the hero alive. Certainly, explorers throughout history have not necessarily been expected to return home. I think, however, that today’s culture would view this as a stunt and it would not be in the best interests of our nation nor worth the risk except perhaps in the mind of the one traveler. I have no doubt that there would be an abundance of volunteers. I would not be one. However, humanity’s future is beyond low-Earth orbit and to be one of the individuals to take humanity’s first steps on another planet as part of a well-considered program of exploration is worth accepting an amount of risk appropriate to the historical nature of the quest.

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