Tracy Caldwell Dyson (born Tracy Ellen Caldwell; August 14, 1969) is an American chemist and NASA astronaut. Caldwell Dyson was a Mission Specialist on Space Shuttle Endeavour flight STS-118 in August 2007. She was part of the Expedition 24 crew on the International Space Station between April 4, 2010 and September 25, 2010. She has completed three spacewalks, logging more than 22 hrs of EVA including work to replace a malfunctioning coolant pump.