is outfitted with an explosive-based mechanism to allow the arm to be jettisoned. This safety system would have allowed the Orbiter's payload bay doors to be closed in the event that the arm failed in an extended position and was not able to be retracted. The Canadarm is long and diameter with six degrees of freedom. It weighs by itself, and as part of the total system. The Canadarm has six joints that correspond roughly to the joints of the human arm, with shoulder yaw and pitch joints, an elbow pitch joint, and wrist pitch, yaw, and roll joints. The end effector is the unit at the end of the wrist that grapples the payload's grapple fixture. The two lightweight boom segments are called the upper and lower arms. The upper boom connects the shoulder and elbow joints, and the lower boom connects the elbow and wrist joints. Service history A simulated Canadarm installed on the was seen when the prototype orbiter's payload bay doors were open to test hangar facilities early in the Space Shuttle program. The Canadarm was first tested in orbit in 1981, on 's STS-2 mission. Its first operational use was on STS-3 to deploy and manoeuvre the Plasma Diagnostics Package. Canadarm subsequently flew on more than 90 missions with all five orbiters. Since the installation of the Canadarm2 on the International Space Station (ISS), the two arms have been used to hand over segments of the station for assembly from the orbiter's Canadarm to the Canadarm2; the use of both elements in tandem has earned the nickname of "Canadian Handshake" in the media. Retirement The Canadarm's 90th and final Shuttle mission was in July 2011 on STS-135, delivering the Raffaello MPLM to the ISS and back. It is on display with Atlantis at the Kennedy Space Center Visitor Complex. Discovery'''s Canadarm is displayed next to it in the National Air and Space Museum's Udvar-Hazy Center. Endeavour left its OBSS at the International Space Station as part of its final mission, while its Canadarm was originally going to be displayed in the headquarters of the Canadian Space Agency (CSA). However, Endeavours Canadarm is now on permanent display at the Canada Aviation and Space Museum in Ottawa. The last of the Canadarms to fly in space, the SRMS flown aboard Atlantis'' on STS-135 in July 2011, was shipped to NASA's Johnson Space Center in Houston for engineering