The workhorse satellite of U.S. remote sensing, Landsat 5, is heading into its 21st year of service. This is far beyond its expected lifetime, yet its continued functioning is key because Landsat 7 has had a significant (and unrepairable) malfunction since the spring of 2003. On March 1, 1984, NASA launched Landsat 5 somewhat earlier than planned since Landsat 4, which has a similar design, was experiencing some problems.
Landsat 6 was to have been the successor system, but it was lost during launch in 1993. Landsat 7 was launched in 1999 and developed a corrector mirror problem in 2003; it is still operating although only that part of the imagery it collects from directly below the orbit path is free from geometric anomalies.
Landsat 5 was originally expected to have a minimum operational life of three years. Instead, after over 100,000 orbits of the earth, it has collected over 29 million images!! It is projected that enough fuel remains on the satellite to support image collection into 2009.
More information: Historical overview of the entire Landsat program