STS-105 Commander Scott Horowitz puts on a gas mask as part of Terminal Countdown Demonstration Test activities.

At Launch Pad 39A, STS-105 Commander Scott Horowitz puts on a gas mask as part of Terminal Countdown Demonstration Test activities, which also include emergency egress, a simulated launch countdown and familiarization with the payload. Mission STS-105 will be transporting the Expedition Three crew, several payloads and scientific experiments to the International Space Station aboard Space Shuttle Discovery. The current Expedition Two crew members on the Station will return to Earth on Discovery. Launch is scheduled no earlier than Aug. 9, 2001