From The Landsat Project Update (a USGS publication):

“The Landsat archive at USGS EROS is a continuous record that stores data from the very first Landsat mission up to the present day acquisitions of Landsat 5 and Landsat 7. Not all of these data are available to purchase (particularly the oldest), because of issues that are a consequence of their age. Every day, the staff at USGS EROS seek ways to make these older scenes available to the user community. In addition, the Landsat Ground Station downlinks about 300 new scenes daily. Between these two efforts, we have reached an exciting milestone – 2 million scenes! The effort is still underway to make all of the oldest and rarest Landsat data available.”