As NASA’s priorities change over time, missions are phased out or cancelled. NASA may choose to end support of successful programs after they have completed their original mission in order to support new scientific and exploratory goals. Some programs end due to technology failures or unjustifiable costs. Each past program, however, has led to lessons learned. The past programs presented below are important due to their contributions to not only NASA’s future space goals, but also advances here on earth.
SPACE SHUTTLE PROGRAM
The Space Shuttle, as humanity's first reusable spacecraft, demonstrated breakthrough manned flight technology and was essential to the construction of the International Space Station (ISS). The space shuttle fleet - Columbia, Challenger, Discovery, Atlantis, and Endeavour - flew 135 missions, in order to repair satellites, conduct cutting-edge research, and build the International Space Station. The Space Shuttle program flew its last flight in July 2011, after more than thirty years of operations.  The program not only advanced robotics and how we build and fly airplanes, but also set the groundwork for the development of commercial space vehicles - such as the SpaceX Falcon 9, with its Crew Dragon capsule - that carries astronauts to ISS, and possibly beyond. Developed under the NASA Commercial Crew Program, this NASA/industry partnership provides transport services to and from low-Earth orbit and the ISS - allowing NASA to focus on building spacecraft and rockets for deep space missions.
With an ultimate plan to use the moon to prepare for future human and robotic missions to Mars and other destinations, Constellation was a program to develop a spacecraft (Orion) and booster vehicles to replace the Space Shuttle – and eventually send astronauts back to the Moon for the first time in 50 years.  After determining that the program was over budget, behind schedule, and lacking critical technologies, funding was cut for Constellation in 2010, culminating in the eventual cancellation of the program. While the Constellation program to service the ISS is being replaced with the Commercial Crew Program, NASA’s work was allowed to proceed on the Orion capsule and Orion had its first flight test in 2014. In 2017 Orion became part of the current crewed lunar exploration program, Artemis. 
ASTEROID REDIRECT MISSION (ARM)
Introduced in 2013, NASA’s Asteroid Redirect Mission (ARM) called for sending a robotic spacecraft to a near Earth asteroid to grab a boulder a few meters across and return it to cislunar space. Astronauts flying on an Orion spacecraft would then visit the boulder, perform studies and collect samples for return to Earth. ARM struggled to win support, particularly in Congress, where some members felt the mission was not relevant to NASA’s long-term goal of sending humans to Mars in the 2030s. While ARM was phased out in 2017, NASA emphasized that many of the central technologies in development for that mission, such as solar electric propulsion, will continue, as they constitute vital capabilities needed to advance NASA’s human path to Mars. Work on ARM’s solar electric propulsion technology, as well as other ARM elements, will continue, as NASA notes that “the capabilities that we were developing in ARM, were not mission-specific,” [but are] “applicable to a wide variety of missions.” 
Updated October 2020 by Tina Allen