Two trajectories to Mars by the 2030s

In a Nov. 8, 2021 op-ed, “When will we finally set foot on Mars?” we argued that NASA should provide greater clarity and definition concerning how humanity will return to the Moon by the mid-2020s and then go to Mars by the mid-2030s. Since then, NASA Administrator Bill NelsonClarence (Bill) William NelsonOvernight Energy & Environment — Earth records its hottest years ever Global temperatures in past seven years hottest ever observed, new data show NASA welcomes chief scientist, senior climate adviser in new dual role MORE has articulated NASA’s revised timeline for returning to the Moon, now no sooner than 2025. We now call on NASA and its partners to also clarify the timeline for initial human missions to Mars as well.

Two options under consideration are a long-stay (or conjunction class) mission, with approximately one and a half years on the Martian surface, during which considerable science and preparation for future missions can be performed by the astronauts; and a short-stay (or opposition class) mission, which enables significantly less overall time that the astronauts will be away from Earth but which allows for only approximately 30 days on the surface. Short-stay missions will last nearly two years in total, whereas long-stay missions will last about two and a half to three years. One of the justifications for the short-stay (opposition class) mission trajectory option is crew safety

Regardless of which mission profile is used, crews on missions to Mars will face significant challenges and potential dangers. Crews will be traveling in an isolated vehicle in the vacuum of space, and they will also face extended time during transit to and from Mars in a microgravity environment (and with one-third Earth’s gravity on the surface of Mars).


During transit, they will face greater exposure to solar and galactic cosmic radiation, as well as the risk of potential serious or even catastrophic accidents resulting from mechanical failure or crew medical emergencies. As such, at first glance, the short stay option would seem to be preferable. The less time in space, the argument goes, the less potential exposure to danger.

But that is not necessarily the case, and the decision as to which trajectory to use will involve a balancing of many factors. While there are some advantages to shortening the time of the overall mission, there are also significant challenges and dangers that come with a short-stay approach.

It is true that astronauts on short-stay missions will be away from Earth for a shorter period of time. But due to the trajectory required for such missions, they will be subject to a significantly greater period in transit (to and from Mars) in deep space; that is, approximately 650 days as opposed to about 450 days for the long-stay mission. The periods in which the crew are traveling to and from Mars are considered potentially the most hazardous parts of the trip, being exposed to the deep-space environment. In fact, in the short-stay mission scenario, the crew will have to travel inside the orbit of Venus, far closer to the Sun than is the Earth, during their return trip, greatly increasing the potential dangers from solar radiation and making thermal control more difficult. 

In addition, favorable launch windows for short-stay (opposition class) missions occur only once every 12-15 years, whereas launch windows occur every two years for long-stay (conjunction class) missions. One of the launch windows being considered for the first human landing (earlier Mars orbital precursor missions are also under consideration) is a short-stay mission in 2039. But 2039 is not an optimal year for an opposition class mission and would be marginal in performance even for advanced propulsion systems.

Short-stay (opposition class) missions are possible using chemical propulsion, but would require an enormous amount of propellant in comparison to that required for long-stay/conjunction class missions. Nuclear propulsion could offer significant performance advantages. But it has not yet been developed and proven, and in fact that will take many years even under the most optimistic predictions. It may not be ready by 2039 or even later, potentially delaying the first mission to long after what could be possible with conventional propulsion methods that are available today.


The development of advanced propulsion such as nuclear propulsion could be advantageous to long-term human activities on Mars, and because of the relatively long period of time that will be required to come online, its development should begin as soon as possible. But it should not be placed in the critical path to achieve having humans on Mars. It is a technology that should be integrated when it is ready. Initial Mars missions can and should be planned to utilize near-term conventional propulsion technology.

As outlined in a recent Explore Mars white paper, no matter which trajectory or mission design we chose to utilize, going to Mars will be extremely challenging and difficult. It is also possible that a short-stay option might be most practical to utilize on the very first voyage to the surface to Mars with subsequent missions utilizing the long-stay option.

Ultimately, we will need to find the right balance to best achieve mission success, taking into consideration all aspects of the journey, including crew safely, productivity and political and programmatic momentum (the importance of which should not be understated).

But we also must not be unreasonably inhibited by the risk. If we are not willing to accept some level of risk, we will never send humans to Mars or anywhere else in the cosmos. As a nation and as international partners, it is time for the United States to fully commit not only to surface activities by humans on the lunar surface starting in 2025, but also to a fully integrated program that leads to women and men walking on the surface of the red planet by the mid-2030s.

Chris Carberry is CEO of Explore Mars, Inc. and author of the book “Alcohol in Space: Past, Present, Future.” Rick Zucker is vice president, policy for Explore Mars, Inc.