Aggressive Nuclear Propulsion R&D Effort Needed To Send Humans to Mars in 2039

A new report from the National Academies of Sciences, Engineering, and Medicine concludes that if the goal is to send humans to Mars in 2039, an aggressive nuclear propulsion research and development program is needed and there is no time to waste.

NASA gave the Academies’ study committee a baseline mission against which to evaluate the maturity of Nuclear Thermal Propulsion (NTP) and Nuclear Electric Propulsion (NEP) technologies to take a crew of four to and from Mars.

The basic parameters were launch in 2039 with a total trip duration of no more than 750 days; stay-time on the Mars surface of 30 days; crew of four, of whom two would land on Mars; separate cargo and crew vehicles using the same propulsion system, with cargo vehicles arriving at Mars prior to the first crew departure from Earth; and vehicle systems, cargo and propellant launched by multiple launch vehicles to an assembly point on low Earth orbit or cislunar space.

Co-chaired by Robert (Bobby) Braun of the Jet Propulsion Laboratory and Roger Myers of R. Myers Consulting, who retired from Aerojet Rocketdyne after heading its Advanced In-Space Programs, the committee concluded that a great deal of research and development remains to be done before making any decisions.

The study did not address the organization, funding, or other factors needed for a human mission to Mars, only an assessment of nuclear propulsion technologies that could make it happen.

Spacecraft can be launched to Mars only when the planets are correctly aligned.  Conjunction opportunities are 26 months apart and require the least performance (“delta V”) from the propulsion system.  All the robotic Mars missions launched to date have been during conjunctions.  However, spacecraft can be launched to Mars when the planets are at opposition if a swing-by of Venus is added. Opposition missions require more propulsion system performance, but reduce total trip time, an important consideration in terms of crew health and safety.

According to the report, a conjunction-class mission launched in 2039 would take 916 days roundtrip: 210 days getting there, 496 days on the surface until the planets are correctly aligned again, and 210 days home. A comparison opposition-class mission would launch in 2037 and last 650 days: 217 days enroute, 30 days on the surface, and 403 days home via a Venus swingby.

To achieve those trip times, an NEP system would need to be augmented by an in-space chemical propulsion system (liquid methane and liquid oxygen), while an NTP system would be sufficient by itself.

Considerable research was done on NTP in the 1960s in the Rover/NERVA program, but much of that expertise has been lost and design issues remain unresolved.

“NTP development faces four major challenges that, with adequate resources, can be overcome to execute the baseline mission in 2039. As noted above, these challenges are (1) heating propellant to approximately 2700 K at the reactor exit for the duration of each burn, (2) the long-term storage of liquid hydrogen in space with minimal loss, (3) the lack of adequate ground-based test facilities, and (4) rapidly bringing an NTP system to full operating temperature (preferably in 1 min or less).”

Concern about the possibility of increased nuclear proliferation here on Earth in the processing of Highly Enriched Uranium (HEU) has prompted calls to use High-Assay Low Enriched Uranium (HALEU) instead. HEU is enriched to 20 percent or more U-235, versus 1 percent in naturally occurring uranium. HALEU is enriched to 5-20 percent.  The Trump Administration’s December 2020 Space Policy Directive-6 (SPD-6) on Space Nuclear Power and Propulsion (SNPP) weighed heavily in favor of HALEU, saying HEU should be used only when a mission otherwise would not be viable.

The ASEB committee concluded, however, that not enough information is available to make an informed decision on HEU versus HALEU and called for NASA and the Department of Energy to “conduct a comprehensive and expeditious assessment” by the end of this year if the goal is to have a system ready for launch in 2039.

The committee was not optimistic that even an aggressive R&D program could make NEP ready for 2039.

For either approach, much work remains to be done and there is no time to waste.

NEP and NTP systems show great potential to facilitate the human exploration of Mars. Using either system to execute the baseline mission by 2039, however, will require an aggressive research and development program. Such a program would need to begin with NASA making a significant set of architecture and investments decisions in the coming year. In particular, NASA should develop consistent figures of merit and technical expertise to allow for an objective comparison of the ability of NEP and NTP systems to meet requirements for a 2039 launch of the baseline mission.

In a statement, Braun said “significant acceleration in the pace of technology maturation is required if NASA and its partners are to complete this mission within the stated timeline.”

Congress has set aside considerable sums of money in recent years for development of NTP.  The FY2021 appropriations bill, for example, allocates $110 million for NTP, of which no less than $80 million is for design of test articles to enable a flight demonstration.  That actually is a step back from FY2020 which also allocated $110 million, but said $80 million was for design of a flight demonstration by 2024, not just design of test articles to enable a demonstration.