Twenty-four spacecraft were formally given the Luna designation, although more were launched. Those which failed to reach orbit were not publicly acknowledged at the time and not assigned a Luna number and ones which failed in low Earth orbit were usually given Cosmos designations

From a physical electronic viewpoint, thermionic energy conversion is the direct production of electric power from heat by thermionic electron emission. From a thermodynamic viewpoint (1)it is the use of electron vapor as the working fluid in a power-producing cycle. A thermionic converter consists of a hot emitter electrode from which electrons are vaporized by thermionic emission and a colder collector electrode into which they are condensed after conduction through the interelectrode plasma. The resulting current, typically several amperes per square centimeter of emitter surface, delivers electrical power to a load at a typical potential difference of 0.5–1 volt and thermal efficiency of 5–20%, depending on the emitter temperature (1500–2000 K) and mode of operation. Details of the history, science and technology of thermionic energy conversion can be found in books on the subject (2, 3).The summary here is brief but more current.

During the period 1973-1983, however, significant research on advanced low-temperature thermionic converter technology for fossil-fueled industrial and commercial electric power production was conducted in the US, and continued until 1995 for possible space reactor and naval reactor applications. That research has shown that substantial improvements in converter performance can be obtained now at lower operating temperatures by addition of oxygen to the cesium vapor (9, 10), by suppression of electron reflection at the electrode surfaces (11), and by hybrid mode operation. Similarly, improvements via use of oxygen-containing electrodes have been demonstrated in Russia along with design studies of systems employing the advanced thermionic converter performance (12).