Space Shuttle missions are currently limited to 11 days*, primarily due to depletion of the power reactants (hydrogen and oxygen). A power system Mission Extension Kit (MEK) is described which could provide the capability to stay on orbit 10 additional days**. These extra days would benefit Space Station construction and missions such as materials processing, Earth and celestial observation, and life science studies (Spacelab). Other constraints to longer missions which may dictate minor Orbiter modifications will be discussed. The power system MEK is particularly desirable because of its existing flight qualified hardware which can be delivered within 3 to 4 years.