Impact testing of general purpose heat sources (GPHSs) and their component GPHS clads is done to benchmark extensive safety calculations quantifying launch safety. Impact testing is done in the Isotope Fuels Impact Tester (IFIT), a large-bore gas gun at Los Alamos National Laboratory. Efforts to conduct an impact test at the extreme low end of the temperature range for launch have highlighted uncertainties in determining the GPHS clad temperature during impact tests. In IFIT impact tests, the GPHS clad temperature is inferred from the temperature of the radiological confinement. Heating tests have been done in the IFIT to determine the fueled clad surface temperature as a function of the surface temperature of the tantalum radiological confinement can. Direct measurement of clad temperatures in the impact configuration are described and the effect of emissivity of the various components indicated. The analytical model used to predict clad temperatures is seen to work well at temperatures above 625°C. Appropriate values of emissivity for use in the model were measured in the experiment. Calculation of the experimental clad impact temperature using the ANSYS thermal transport model is necessary at clad temperatures below 625°C. ANSYS modeling indicates that the clad temperature in a recent low-temperature impact was outside the relevant range for launch safety modeling of GPHS clad behavior.