Project Mercury proved that people were able to survive and function in space. The American Alan Shepard made his suborbital trip with the first crew to see the space. John Glenn performed the pioneer orbit voyage of the U.S. This initiative had laid down basic operation protocol in regards to human spaceflight.
Project Gemini came up with methods that were needed to reach the Moon. Astronauts perfected important maneuvers in spacecraft such as rendezvous and docking. They were the first American Extravehicular Activities (EVAs). The initiative demonstrated that human beings could spend a long time in space.
Project Apollo was the first lunar landing by humans. Apollo 11 made a landing of astronauts at the moon in 1969. Later Apollo missions carried scientific exploration to the Moon. This is the only initiative to put humans on a different planet.
The Space Shuttle program was able to offer low Earth orbit access in a reusable manner. Satellites and scientific platforms were launched using shuttle missions. They carried out elaborate work in satellite services and recovery. Most importantly, the Shuttle fleet allowed creation of the International Space Station.
International Space Station (ISS) is a continuous human station in space. It has been continuously crewed since November 2000 and is also a special microgravity research laboratory. International collaboration maintains its activities and research. The ISS helps to conduct the research important to further missions into deep space.
Conclusion:
Human spaceflight achievements of NASA began with early orbital flights keeping one astronaut aboard Mercury, vital skills learned with Gemini, and the landing of astronauts on the Moon with Apollo. The Space Shuttle allowed long-term operations and construction of the space stations in the orbit of the planet. The International Space Station has given a long-term facility on which the research can be conducted and in future the exploration can be made.