Natural and Induced Environment in Low Earth Orbit

Abstract Estimating the long-term exposure of astronauts on the developingInternational Space Station (ISS) requires an accurate knowledge ofthe internal exposure environment for human risk assessment and otheronboard processes. The natural environment is moderated by the solarwind, which varies over the solar cycle. The neutron environment withinthe Shuttle in low Earth orbit has two sources. A time dependent modelfor the ambient environment is used to evaluate the natural and inducedenvironment. The induced neutron environment is evaluated usingmeasurements on STS-31 and STS-36 near the 1990 solar maximum. Introduction The commitment of astronauts to long-term exposure to the space environment on the InternationalSpace Station (ISS) requires resolution of issues concerning ionizing radiation. For the high inclinationof the ISS (51.6°), computational models indicate that about half of the ionizing radiation exposure nearsolar minimum results from Galactic Cosmic Rays (GCR, 233 µSv/d), and the bulk of the remainder isfrom trapped particles (166 µSv/d, Wu et al. 1996). There are, of course, contributions from the neutronalbedo of 25 to 54 µSv/d (varies with solar cycle) excluding effects of intervening material (Wilson et al.1989). Within the craft, the environment is a complex mixture of surviving primary particles and secon-dary radiations produced in the spacecraft structure. Various arrangements of detectors have been used tostudy the composition of the internal radiation fields within the spacecraft which need to be understood interms of computational models to allow a better understanding of the local environment of the astronauts’critical tissues. As a result, a number of studies of the low Earth orbit (LEO) environment have beenmade to better understand the nature of the radiations within a spacecraft (Dudkin et al. 1992; Keith et al.1992; Badhwar et al. 1995a; and Dudkin et al. 1995) and to understand these results in terms of computa-tional models (Badhwar et al. 1995b; Shinn et al. 1995; and Shinn et al. 1998).Measurements of neutrons on Cosmos-2044, flown at an 82° inclination between 216–296 km, re-sulted in 35 µSv/d using nuclear emulsion (Dudkin et al. 1992) and compares favorably with the neutronalbedo model of 25 µSv/d estimated for near polar orbits at the cycle 20 solar minimum (Wilson et al.1989). Similar measurements within the Spacehab on STS-57 in a 28.5° inclination orbit at 462 km yield174 µSv/d compared to 12.5 µSv/d from the albedo neutrons near solar maximum. Unlike the Cosmos-2044 spacecraft, the Shuttle is itself a strong source of neutrons, especially within the massive Spacehabmodule in the Shuttle bay. Indeed, time-resolved neutron measurements on the Mir and Salyut stations(Lobakov et al. 1992) reveal strong neutron levels, mainly within the South Atlantic Anomaly (SAA)passage through the trapped proton belt against a lower background of neutrons in the remainder of timeoutside the SAA.Neutron measurements using Bonner spheres and gold foils were made (Keith et al. 1992) near solarmaximum in the low inclination (28.5°) with high altitude (617 km) flight STS-31 in April 1990 and inthe high inclination (62°) with low altitude (246 km) flight STS-36 in February 1990. The neutron doseequivalent on STS-36 was found to be 45 µSv/d compared to 25 µSv/d from the albedo model, and onSTS-31 the measurements were 345 µSv/d compared to 12.5 µSv/d from the albedo model, again show-ing the Shuttle to be a strong source of neutrons. Small spacecraft have relatively few locally producedneutrons as seen on Cosmos-2044 and also on the Orbiting Geophysical Observatory (OGO-6) satellitewhere only 3 to 4 percent corrections of the albedo neutron measurements resulted from neutrons pro-duced locally in the spacecraft materials (Jenkins et al. 1971; see also Dudkin et al. 1992).