Hyperfine transition of <a href="https://en.wikipedia.org/wiki/neutral_hydrogen" class="extiw" title="w:neutral hydrogen">neutral hydrogen</a>.
<div style="display:inline-block;padding:0 3px;background-color:#BDC"><a href="//commons.wikimedia.org/wiki/File:Pioneer_plaque.svg" title="File:Pioneer plaque.svg"><b>Original: </b></a></div> <div style="display:inline-block;padding:3px 0;"><a href="https://en.wikipedia.org/wiki/NASA" class="extiw" title="w:NASA">NASA</a></div> <div style="display:inline-block;padding:0 3px;background-color:#CCF">Vector: </div> <div style="display:inline-block;"><a href="//commons.wikimedia.org/wiki/User:Mysid" title="User:Mysid">Mysid</a></div>
1972-02-25
Extracted from the <a href="//commons.wikimedia.org/wiki/File:Pioneer_plaque.svg" title="File:Pioneer plaque.svg">Pioneer plaque.svg</a>.