Dose equivalents received by the Skylab 4 crew were the highest received in any previous NASA mission, but remained well within limits set for the mission. Due to the low rates involved, dose equivalents for each crewman were well below significant clinical effects (long-term effects such as generalized life shortening, increased neoplasms, and cataract production). NASA career limits were 400 rem for blood forming organs, 1200 rem for skin and 600 rem for the lens of the eye. The Skylab 4 crewmen could have flown a mission comparable to Skylab 4's duration every year for 50 years before exceeding their career limits.
As previously stated, NASA was most concerned about the unexpected radiation events that might occur during Skylab. The results of several unexpected radiation events are listed below along with the results obtained from the radiation monitoring hardware.
Solar Particle Events
The Skylab missions were flown during a period when solar activity was approaching a minimum in the Sun's cycle. Although particle emissions from these events were of low energy and intensity, several events of scientific interest nevertheless occurred. However, when coupled with the shielding effects of the Earth's magnetic field, doses from solar particle events were below the limits of the onboard instrumentation detection capabilities (less than 10 millirad per event).
High Altitude Nuclear Tests
During Skylab 3, four nuclear devices were detonated by France at the Murona test site; these tests produced no ionizing radiation problems for Skylab. The only concern was the possibility of eye damage if the crew accidentally observed a test. Therefore, visual observation of the Earth in the vicinity of the test area was strictly avoided.
Onboard Radiation Sources
Various experiments conducted aboard Skylab involved radiation sources (Pm-147, H-3, Ag-110m, Zn-65, Am-241 and Th-232). One of the larger sources originated from radioluminescent markings for one of the experiments; approximately 200 mCi of promethium 147 were involved. Small amounts of this material became detached from the experiment, when a belt in the experiment jammed. The material remained within a sealed portion of the experiment, and thus did not escape into the spacecraft atmosphere. After determining that the belt could not be fixed without releasing the Pm-147 into the spacecraft atmosphere, an alternative method for belt alignment was devised and used for the remainder of the mission, thus ensuring both crew safety and experiment success.
Detailed Dosimetry Results
Mean dose rates from passive dosimeters show a trend toward increased values as use of food, water, propellants, and other expendables reduced the overall spacecraft shielding. The galactic cosmic ray component measured by the passive dosimeters accounted for 30-50% of the film vault doses, and 20-30% of the crew dose means. The majority of the remaining dose originated from protons in the Van Allen Belts and softer secondary radiations generated by passage of primary particles through spacecraft materials.
Van Allen Belt electrons did not penetrate into the spacecraft, nor did they penetrate deeply enough (3 mm tissue equivalent) during EVAs to register on the dosimeters. Therefore, electron doses to the skin were calculated from the electron-proton spectrometer data.
Doses to the blood forming organs averaged 0.66 of the doses to the skin. This value was derived from the dual sensors of the Van Allen Belt Dosimeter and was in agreement with the value obtained from the crew-worn and film vault dosimeters.
In terms of neutron dosimetry, thermal (0.02 to 2.0 electronvolts) and intermediate (2.0 to 2 x 103 electronvolts) neutrons both contributed to the crew dose equivalent at a combined rate of approximately 0.1 millirem/day. Direct measurement of fast neutron fluence by suspended track analysis of crew-worn nuclear emulsions was not possible due to the high track densities obtained on the Skylab missions. Therefore, upper limit dose calculations were made based on nuclear emulsion disintegration star analysis (to determine neutron production rates) and iridium/tantalum evaluation. Both methods showed excellent agreement with upper limit rates of 12.5 millirem/day (for fast neutrons with mean energy of approximately 1 megaelectronvolt.)