and surface stay times, but transit times to and from Mars can be relatively short and no VSB is required on either mission leg. The long surface stay mission also has a lower total V requirement than the short stay option, and therefore requires less propellant and less mass delivered to LEO. Because of its attractive features—lower total V, a long surface stay, short transit times, and no VSB—the fast conjunction/long stay mission option was selected as the baseline for DRA 5.0.

[Figure: Figure 1.—Features and characteristics associated with opposition and fast conjunction Mars missions.]

Recent measurements of the energetic particle radiation environment inside the Mars Science Laboratory (MSL) spacecraft during its 253-day journey to Mars were made by the RAD attached to the Curiosity rover located inside MSL (Ref. 6). The detector measured an average of ~1.8×10–3 Sv per day indicating that astronauts could receive a radiation dose of ~0.66 Sv (~66 rem)—the limiting value established by NASA—during a fast conjunction Mars mission involving a 1-year journey through deep space. This dose rate could also be higher depending on the time in solar cycle that the mission occurs. When coupled with the potential for additional crew dose received during the lengthy exploration phase on Mars, it is of interest to see what proven NTP can do to reduce transit times further. Can NTP cut transit times in half and if so what are the engine requirements and vehicle configuration impacts? The NTR was selected as the propulsion system of choice in DRA 5.0 because of its high thrust (10’s of klbf) and high specific impulse (Isp ~900 to 940 s), its increased tolerance to payload mass growth
and architecture changes, and its lower IMLEO which is important for reducing the HLV launch count, overall mission cost and risk. With a 100 percent higher Isp than today’s best chemical rockets, the use of NTP reduced the required launch mass for the baseline long stay mission by over 400 t—the equivalent mass of the International Space Station. For the higher energy, short stay opposition missions examined during Phase I, NTP provided an even greater mass savings—over 530 t compared to chemical propulsion. This is important to note since short stay missions will likely be selected by NASA when it decides to send humans to orbit Mars in the early 2030s as outlined in the United States’ National Space Policy (Ref. 7). Lastly, it should not be overlooked that NTP is a proven technology successfully ground tested at the performance levels required for a human mission to Mars. The smallest and highest performing engine tested during the Rover/NERVA programs (Ref. 8)—the 25 klbf “Pewee” engine is sufficient for a wide variety of human mission to Mars when used in a clustered engine arrangement. This means there are no large scale-ups in size or performance needed for NTP as there are with other proposed non-chemical propulsion options. DRA 5.0 featured separate cargo and crewed Mars transfer vehicles each using a common “core” NTPS with three 25 klbf Pewee-class engines. Two cargo vehicles were used to pre-deploy surface and orbital assets to Mars ahead of the crew who arrived during the next mission opportunity (~26 months later). The “Copernicus” crewed MTV (Fig. 2) is a 0-gE vehicle (Refs. 9 and 10) consisting of three basic components: (1) the NTPS; (2) the crewed payload element; and (3) an integrated “saddle truss” and LH2 propellant drop tank assembly that connects the two elements. With a propellant capacity of ~190 t in its two tanks, Copernicus can achieve 1-way transit times to Mars and back ranging from ~150 to 220 days over a 15-year synodic cycle starting in 2033.

[Figure: Figure 2.—“Copernicus” crewed MTV in LEO prior to departure.]