How long does it take humans to travel to the moon? Currently, Constellation is planning for the trans-lunar coast to take no longer than 4 days, or 96 hours. Apollo’s design requirement was for the coast time to range between 60 hours and 100 hours. The actual missions (Apollo 10-17) varied from 72 hours to 83 hours.
So why would it take longer on the future missions? It may not actually. At this point, Constellation is in the requirements definition and preliminary design phase for the lunar exploration portion of the program therefore requirements are set for the most stressing – maximum and minimum – types of conditions.
The trans-lunar cruise duration is a function of the energy or change in velocity (delta-V) applied at the trans-lunar injection, or TLI, burn. The energy requirements for the TLI burn will vary depending on where the planned landing site is located on the moon and when the mission is launched, among other factors. So, if a mission is launched on a more favorable opportunity, less energy will be required for the TLI burn and the trip would be quicker.
Since Constellation is planning for worst-case conditions at this point, the transfer time in the current plan minimizes the amount of propellant, and therefore the mass, required for trans-lunar injection. When Constellation flies actual missions to the moon, there will likely be the same flexibility as Apollo to shorten the duration of the flight toward the moon if it is desirable to do so.
Artist’s concept of NASA’s Orion crew exploration vehicle and Altair Lunar Lander while the Earth departure stage performs the trans-lunar injection burn (JSC2009-E-031248).