 On 22 July 2009 Cassidy stepped outside the ISS alongside NASA astronaut David Wolf for his first spacewalk, the two began work replacing batteries on the P6 Truss and continued work on installing the JEM-EF, the EVA was called off after 5 hours and 59 minutes due to excess amounts of Carbon dioxide in Cassidy's suit.Over the course of STS-127 Cassidy participated in two more spacewalks alongside NASA astronaut Thomas Marshburn, the two completed replacing the batteries on the P6 truss and completed the installation of the JEM-EF over the two excursions lasting 7 hours and 12 minutes and 4 hours and 57 minuites respectively.Cassidy's third spacewalk marked the completion of the Japanese Experiment Module, a feat that had taken three space shuttle missions starting with STS-123.