On this day in 1999, Landsat 7 was launched into orbit. Landsat 7 is the considered the most accurately calibrated Earth-observing satellite, i.e., its measurements are extremely accurate when compared to the same measurements made on the ground. Landsat 7’s excellent data quality, consistent global archiving scheme, and free data are hallmarks of its impressive tenure.

The Landsat 7 mission went flawlessly until May 2003 when a hardware component failure left wedge-shaped spaces of missing data on either side of its images. Despite these problems and the resulting data loss of 22 percent of each image, Landsat 7 data is still highly useful for many scientific applications.