A new publication by the US Academies of Sciences, Engineering, and Medicine, “Space Nuclear Propulsion for Human Mars Exploration (2021)” seeks to identify primary technical and programmatic challenges, merits, and risks for maturing space nuclear propulsion technologies of interest to a future human Mars exploration mission. The study, undertaken by the Space Nuclear Propulsion Technologies Committee, was sponsored by NASA.

The 104-page study, released on 12 February, assesses the present state of the art, potential development path, and key risks for a nuclear thermal propulsion (NTP) system and a nuclear electric propulsion (NEP). While NEP converts the thermal energy from a nuclear reactor into electrical energy to power electric thrusters, NTP uses the thermal energy from a nuclear reactor to heat a rocket propellant and create thrust. Each system has its own advantages and limitations for use in a crewed mission to Mars.

As requested by NASA, each system was assessed with regard to its ability to support a particular baseline mission — an opposition-class human exploration mission to Mars with a 2039 launch date. For both systems, efforts to mature the requisite technology and mitigate key technical risks were integrated into a development and demonstration roadmap. Infusion of technology results, expertise, and synergy with other government programmes and missions was also examined.

The report says that, using nuclear propulsion technologies to support a human mission to Mars in 2039 will require NASA to pursue an aggressive and urgent technology development programme. NASA should commit within the year to conducting an extensive and objective assessment of the merits and challenges of using different types of space nuclear propulsion systems and to making significant technology investments this decade. Such a programme must include subsystem development, prototype systems, ground testing, and cargo missions as a means of flight qualification prior to first crewed use, the report says.

“Safely transporting astronauts to and from Mars will require advances in propulsion systems to develop spacecraft that are up to the challenge,” said Roger Myers, owner of R. Myers Consulting and co-chair of the committee that wrote the report. “Nuclear propulsion systems have the potential to substantially reduce trip time compared to non-nuclear approaches. Synergy with other space mission applications and terrestrial power programs is also significant and will bring about added value.”

Studies comparing NEP and NTP systems are needed to assess the viability of each system for a crewed mission to Mars. Given the need to send multiple cargo missions to Mars prior to the first crewed mission, NASA should use those cargo missions as a means of flight qualification of the selected nuclear propulsion system before it is incorporated into the first crewed mission.

NEP and NTP each have challenges, which are identified in the report. The fundamental challenge for developing an NEP system is scaling up the operating power for each subsystem, something that requires power levels that are orders of magnitude greater than have ever been achieved to date. Another challenge is developing a compatible chemical propulsion system to provide the primary thrust when departing Earth’s orbit and when entering and departing Mars’ orbit. The fundamental challenge facing an NTP system is the ability to heat its propellant to the proper temperature, approximately 2,700 K. Other challenges include the long-term storage of liquid hydrogen in space with minimal loss; the need to rapidly bring an NTP system to full operating temperature, preferably in under one minute; and the need to develop full-scale ground test facilities that can safely capture the NTP exhaust.

“Space nuclear propulsion technology shows great potential to facilitate the human exploration of Mars,” said Bobby Braun, director for planetary science at the Jet Propulsion Laboratory and co-chair of the committee that wrote the report. “However, significant acceleration in the pace of technology maturation is required if NASA and its partners are to complete this mission within the stated timeline.”

In the near-term, NASA and the Department of Energy (DOE), with inputs from other key stakeholders, including commercial industry and academia, should conduct a comprehensive assessment of the relative merits and challenges of highly enriched uranium (HEU) and high assay, low-enriched uranium (HALEU) fuels for NTP and NEP systems the report says. It notes that a comprehensive assessment of HALEU versus HEU for NTP and NEP systems that evaluates a full set of critical parameters as applied to the baseline Mars mission has not been performed. Moreover, there is no recent trade study comparing NEP and NTP systems for crewed missions to Mars, in general, or the baseline mission in particular.