Astronaut <a href="https://en.wikipedia.org/wiki/David_Wolf_(astronaut)" class="extiw" title="en:David Wolf (astronaut)">David Wolf</a> during <a href="https://en.wikipedia.org/wiki/Mir_EO-24" class="extiw" title="en:Mir EO-24">Mir Expedition 24</a>.
NASA
<a rel="nofollow" class="external free" href="https://spaceflight.nasa.gov/history/shuttle-mir/history/h-f-wolf-letter.htm">https://spaceflight.nasa.gov/history/shuttle-mir/history/h-f-wolf-letter.htm</a>