STCAEM NTR Credit - © Mark Wade

On 11 May 1991 President Bush declared that he would support a Space Exploration Initiative program leading to a Mars Landing by 2014. To coincide with the announcement, the SEI Synthesis Group issued a report America on the Threshold that foresaw Mars expeditions based once again on nuclear thermal propulsion.

This was seen as a concession to Los Alamos National Laboratory, which was looking for a major project to offset the ending of further development of nuclear weapons in the post-Cold War period. There was no space station in the plan. Four architectures were studied, essentially a cut-price version of the 90 Day Study. There was no interest in the plan. Congress cancelled further funding for Mars studies in FY 1991. NASA's Exploration Office was dissolved. The Case for Mars V conference in 1993 was poorly attended.

• Summary: Chartered by National Space Council - "major study"; Explore Mars and provide significant science return
• Propulsion: Nuclear thermal
• Braking at Mars: propulsive
• Mission Type: conjunction
• Split or All-Up: split
• ISRU: ISRU
• Launch Year: 2014
• Crew: 6
• Outbound time-days: 210
• Mars Stay Time-days: 460
• Return Time-days: 230
• Total Mission Time-days: 900
• Total Payload Required in Low Earth Orbit-metric tons: 1080
• Total Propellant Required-metric tons: 600
• Propellant Fraction: 0.55
• Mass per crew-metric tons: 180
• Launch Vehicle Payload to LEO-metric tons: 150
• Number of Launches Required to Assemble Payload in Low Earth Orbit: 4
• Launch Vehicle: Shuttle Z