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Managing Space Radiation Risk in the New Era of Space Exploration
The return to the Moon plan for the Constellation program is quite similar to that of the Apollo program. However, Constellation will use a two-launch method. The Ares V Heavy Lift Launch Vehicle will launch the Earth Departure Stage (EDS) containing the propulsion system and the Lunar Lander. The four-member crew will be launched in the Orion Crew Exploration Vehicle to low Earth orbit (LEO) using the Ares 1 Crew Launch Vehicle, which will then dock with the EDS before heading to the Moon. Once the combined vehicle is in lunar orbit and systems checks have ensured a “Go” for landing, the Lunar Lander will separate from the Orion and initiate the lunar landing.
The Orion Block-2 design requirements respond to two major mission scenarios:
Supporting crew and cargo transportation to the International Space Station (ISS) in LEO and returning them to Earth, and
Transporting crew to low lunar orbit (LLO) and returning them safely to Earth. As part of the lunar missions, the Orion will rendezvous and dock with the Lunar Lander (already mated to the EDS) in LEO, and will provide piloting, guidance, and navigation to the combined cislunar spacecraft.
On both of these missions, the Orion capsule would be the principal shielded volume; that is, it would be either the only volume, or it would be the most likely shielded module from which the crew could fly the mission and still benefit from a level of protection. The requirements provided by NASA to Lockheed Martin are as follows (T. Shelfer, Lockheed Martin, “Crew Exploration Vehicle Requirements—Contractor’s Perspective,” presented to the committee on February 21, 2007):
Orion shall provide radiation protection, consistent with the principles of ALARA [As Low As Reasonably Achievable], to ensure that the tissue-averaged effective dose to any crew member does not exceed 15 cSv for the worst-case SPE, defined as the King parameterization of the August 1972 event.
The vehicle shall continuously measure and record the external fluence of particles of Z < 3, in the energy range 30 to 300 MeV per nucleon and particles of 3 ≤ Z ≤ 26, in the energy range 100 to 400 MeV per nucleon and integral fluence measurement at higher energies, as a function of energy and time, from a monitoring location that ensures an unobstructed free space full-angle field of view 65 degrees or greater.
The vehicle shall provide an omnidirectional, portable system that can continuously measure and record the dose equivalent from charged particles with linear energy transfer 0.2 to 1,000 keV per micrometer, as a function of time, at an average tissue depth of at least 2 mm.
Preliminary analyses by NASA and Lockheed Martin indicate that the Orion capsule provides adequate shielding from its structure, avionics, life support, other hardware, consumables, and waste storage such that lower-energy SPEs would not be a threat. However, for the rarer, higher-energy events, doses could accumulate beyond the acceptable limit. For this reason, the Orion capsule itself must either incorporate sufficient shielding or else have the capability to reconfigure shielding and functional hardware to provide a radiation storm shelter for the astronauts. The duration of the most hazardous portion of an SPE or a close series of SPEs can be hours to a few days. Thus, the Orion capsule must be capable of providing the storm-shelter capability for a somewhat extended period of time, and astronauts must have access to food, water, and minimum hygiene facilities. Although it will be permissible to leave the radiation storm shelter for short periods (minutes to fractional hours) to meet personal needs or perform a task required for mission success, the astronauts should spend the duration of an SPE inside the shelter. As stated above, protection is accomplished by time, distance, and shielding. In this case, the astronauts can safely leave the shielded area if they return quickly. Lockheed Martin designers considered several solutions: hull shielding, deployable water shielding, shielding integrated into seats, and a deployable, high-density polyethylene (HDPE; defined by a density of greater than 0.94 g/cm3). At the time of this writing (summer 2007), the Orion project plans to provide 2.5-cm-thick slabs of HDPE for use by the astronauts to configure an in-space shelter inside the Orion capsule itself. The HDPE shield was the only one that could feasibly provide the necessary amount of shielding. The shielding would be 2.5 cm thick and would be stowed on the floor of the Environmental Control and Life Support System when not in use. At the time of this analysis, Orion was still in its first design cycle. More detailed radiation analyses and shielding configurations are planned in future iterations (T. Shelfer,