Because Space Mountain has a tubular track, the easiest way to measure it's worthiness is to keep it pumped up with 100psi of air at all times. If the track gets a stress fracture in it, the pressure drops, an alarm sounds and the attraction automatically gets shut down. The track is then inspected and the stress fracture is welded. The track would then get pumped back up with 100psi and if the weld worked the ride would go back online.
Unfortunatley, after years of repairs, the damn thing had so many fixes on it that it rode like crap, and eventually it got to the point (April 2003) that TDA (and Imagineering) realized that the tracks useful lifespan had reached an end, and so it was decided that they would close it down and do a complete track change.
The refurb lasted a little over 2 years, and with Matt Ouimet at the helm, Disneyland poured on the steam (and the overtime hours) in order to reopen Space Mountain 2 days before Disneylands 50th anniversary.