The cost and danger of human spaceflight programmes can only be justified if they are part of a collaborative, international pathway plan aimed at putting boots on “other worlds” – ultimately Mars. And, according to a report by the US National Research Council, since progress “beyond low Earth orbit will be measured in decades and hundreds of billions of dollars”, there is no hope of sustaining such a pathway without annual, above-inflation rises in NASA’s human spaceflight budget.

Those budget hikes, the report concludes, would have to be about 5% per year in order to “enable pathways with viable mission frequency and greatly reduce technical, cost and schedule risks”.

Without recommending any particular pathway, the NASA-sponsored report notes: “All the pathways culminate in landing on the surface of Mars – which is the most challenging, yet technically feasible destination – and have anywhere between three and six steps, that include some combination of missions to asteroids, the moon and Martian moons.”

Cornell University space research centre director and committee co-chair Jonathan Lunine stresses that “for the foreseeable future”, these are the only feasible destinations. And, he adds: “The United States has been a leader in human space exploration for more than five decades, and our efforts in low Earth orbit with our partners are approaching maturity, with the completion of the International Space Station.

“We as a nation must decide now how to embark on human space exploration beyond low Earth orbit in a sustainable fashion.”

Echoing the so-called “global exploration roadmap” – a set of internationally agreed guidelines for human exploration beyond low Earth orbit – the report identifies ten high-priority capabilities, including Mars entry, descent and landing, radiation safety and in-space propulsion and power.

The National Research Council is potentially at odds with NASA in one important respect, however. While the Council found “a return to extended surface operations on the Moon would make significant contributions to a strategy ultimately aimed at landing people on Mars”, NASA itself has no plans to land on the Moon. Indeed, according to the agency’s chief scientist Ellen Stofan, the technical requirements of a Moon landing are so far removed from the vastly greater challenge of landing on Mars – a larger body with an atmosphere – as to be nothing more than a budget-sapping distraction.

Both parties see Moon operations as an opportunity for NASA’s international partners to participate in the research effort needed to realise a Mars mission. However, budgets are sure to be a huge sticking point, and while both NASA and the National Research Council stress that Mars must be an international effort, Purdue University president and committee co-chair Mitchell Daniels hints at the ultimate requirement for US leadership. Daniels says: “Human space exploration remains vital to the national interest for inspirational and aspirational reasons that appeal to a broad range of US citizens.”

Daniels also calls on America’s elected leaders to “assure that the leadership, personnel, governance and resources are in place in our human exploration programme”.

Whether US political support for large NASA budget increases is achievable – let alone sustainable – remains to be seen. The report also found that while US “public opinion of the space programme since its inception has been generally positive”, among the general public and NASA stakeholders there is “no majority agreement on a single rationale for human spaceflight”.