<div><p>Sorry for the late response -- I shouldn't have posted my request just before I was to be away for awhile.</p>
<p>The observatory codes are authoritative for the IAU in the sense that if an earth-based observatory wants to report any solar system observation (on asteroids, comets, satellites) to the IAU, it needs to be assigned an observatory code by the MPC. The code also fixes a reporting observatory to a particular geo-location. (moving the observatory requires an assignment of a different code.) The code is indeed an "MPC observatory code," but one will sometimes see it referred to as "IAU observatory code" (see for example, the <a href="" rel="nofollow">Observatoire de Paris Data Centre skybot service</a> and <a href="" rel="nofollow">IAU Observatory Code C14</a> and <a href="" rel="nofollow">Private Observatory Meyer/Obermair, Austria</a> ).  Wolfram-Alpha refers to the code as "Minor Planet Center observatory code" (see, for example, <a href="" rel="nofollow"></a> ).</p>
<p>The MPC does not classify observatories.  An observatory can be one operated by a huge research institution or something amateur astronomers have built in their backyard. An observatory-organization may operate more than one observatory station.  Moreover, an observatory site may be re-engineered (with better equipment) for a different astrometry project.  In which case, each observatory station / observatory-project can be separately assigned its own code.  Thus, the MPC code 005 is assigned to Paris Observatory-Medoun; 007 for Paris Observatory-Paris (Meridian); 675 for Palomar Mountain; 261 for Palomar Mountain-DSS; and 644 for Palomar Mountain/NEAT.</p>
<p>A namespace solution (instead of a property) is fine. As to which namespace to use, either one works for me. I'm not really sure I understand the difference between the one favored by Gordon vs. the one favored by Philip.</p>
<p>Besides the observatory code, the MPC also assigns the provisional designation and the sequential numbering of minor planet / comet discoveries. The provisional designation and sequential numbering are "authoritative" and are universally recognized by the IAU.  The minor planet and comet number (also referred to as "IAU (object) number" -- see <a href="" rel="nofollow"></a> ) may be a good candidate for creating keys and other weblinks:</p>
<p>asteroid 4 Vesta:<br><a href=""></a> <br><a href=""></a> <br><a href=""></a> <br><a href=""></a>+4 <br><a href=""></a>+planet+4 <br></p>
<p>comet 190P/Mueller:<br><a href=""></a><br></p>
<p>comet C/1993 A1:  <br><a href=""></a>+A1 <br></p></div>