Derek Breit wrote: > Ummmm... Don't they mean the Uranium will decay in 8 million > years, and are not referring to the satellite?? Eight million years refers to the time to decay: http://msl.jpl.nasa.gov/QuickLooks/lageosQL.html Also, the LAGEOS satellites (76039A / 08820, 92070B / 22195) do not contain uranium; it is the conceptually similar satellites, Starlette (75010A / 07646) and Stella (93061B / 22824), that contain depleted uranium (at their core). LAGEOS and Starlette/Stella were required to achieve highly stable orbits, which in turn required minimizing the effects of non-gravitational forces, mainly atmospheric drag and solar radiation pressure. Since those forces are proportional to cross-sectional area divided by mass, both satellites were made small and dense. The LAGEOS satellites were placed in high orbits (~5900 km), far above the effects of the atmosphere; Starlette and Stella are in much lower orbits (800-1000 km). It might be supposed that the use of uranium in the latter was necessitated by the greater drag they experience, but I suspect it is more complicated than that. Data on shape, size and mass is available here: http://ilrs.gsfc.nasa.gov/satellite_missions/slr_sats.html#size The relevant values, with calculated A/m are: Dia m A/m m kg m^2/kg Lageos sphere 0.60 411 0.00069 Starlette sphere 0.24 47 0.00096 So the areal density of the two types of satellites are the same order of magnitude. Had Starlette been made of solid uranium, its areal density would have been less than half that of Lageos, so, altitude may not have been the primary factor in deciding on a uranium core. At the much greater altitude of Lageos, perturbations due to solar radiation pressure are similar in magnitude to those due to atmospheric drag in LEO, which may explain the similarity in areal density of the two satellites. For a sphere, A/m = 1.5 / (D * d); where D is volumetric density, and d is diameter. That formula may be convenient for assessing trade offs between size and density. Ted Molczan _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l
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