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Detailed modeling of engines using a homogeneous charge of various fuels has shown that by knowing the precise conditions ( fuel species , temperature , and density ) at the start of compression , the beginning of combustion can be accurately predicted . Although the spectrometer and magnetometer can collect data without being pointed at the planet , they also utilized this excellent opportunity due to Surveyor 's low altitude of 163 miles ( 263 km ) at the start of the third orbit .