A series of small, low-cost, short-duration NASA payloads launched from the space shuttle. The payloads were retrievable and reusable, and operated as autonomous sub-satellites. Spartan 201-1 flew in April 1993, Spartan 201-2 in September 1994, Spartan 201-3 in September 1995, and Spartan 201-5 in November 1998. In November 1997, Spartan 201-4 failed to deploy. Spartan 201-1 and Spartan 201-2 used telescopes to study the Sun's extremely hot corona and its expansion into the solar wind. The data provided new understanding about the source of energy that heats the corona and accelerates the solar wind particles. Spartan 201-3 carried X-ray, far-ultraviolet, and visible-light instruments to study the Sun’s corona, as well as galactic structures. Spartan 201-5’s data were used to re-calibrate NASA’s Solar and Heliospheric Observatory.