NASA

NASA (the National Aeronautics and Space Administration) is the USA government organization for space technology and research. Among its astronomy and planetary science programs:

• Explorers Program - science missions, both astronomy and Earth science, including Small Explorer Program (SMEX) for Small Explorers, and Medium-class Explorer Program (MIDEX) for Medium Explorers, and for putting scientific payloads on other NASA missions that have the extra capacity, MO for Mission of Opportunity.
• Large Strategic Science Missions aka Flagship Missions - costly programs such as Cassini and JWST.
• New Frontiers Program (NF) - "medium-scale" planetary exploration missions: NF1, NF2, etc.
• Discovery Program - low cost missions, particularly exploring the solar system.

See list of NASA astronomy projects. NASA was formed in 1958, incorporating the older US agency, NACA, the National Advisory Committee on Aeronautics, which dated from 1915.