O-ring damage was accepted. The Columbia Accident Investigation Board called this tendency the "normalization of deviance" – a gradual acceptance of events outside the design tolerances of the craft simply because they had not been catastrophic to date. The subject of missing or damaged thermal tiles on the Shuttle fleet only became an issue following the loss of Columbia in 2003, as it broke up on re-entry. In fact, Shuttles had previously come back missing as many as 20 tiles without any problem. STS-1 and STS-41 had both flown with missing thermal tiles from the Orbital Maneuvering System pods (visible to the crew). The problem on Columbia was that the damage was sustained from a foam strike to the reinforced carbon-carbon leading edge panel of the wing, not the heat tiles. The first Shuttle mission, STS-1, had a protruding gap filler that diverted hot gas into the right wheel well on re-entry, resulting in a buckling of the right main landing gear door. Risk contributors An example of technical risk analysis for a STS mission is SPRA iteration 3.1 top risk contributors for STS-133:<ref>Hamlin, et al. [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100005659_2010007106.pdf 2009 Space Shuttle Probabilistic Risk Assessment Overview] (.pdf). NASA.</ref> Micro-Meteoroid Orbital Debris (MMOD) strikes RS-25-induced or RS-25 catastrophic failure Ascent debris strikes to TPS leading to LOCV on orbit or entry Crew error during entry RSRM-induced RSRM catastrophic failure (RSRM are the rocket motors of the SRBs) COPV failure (COPV are tanks inside the orbiter that hold gas at high pressure) John Young and Jerry L. Ross were among those astronauts who believed that the shuttle was always an experimental craft, not an operational vehicle for routine spaceflight as President Ronald Reagan declared after STS-4. Rick Hauck said in 2017 that before STS-1 he saw an analysis estimating the risk of loss of the vehicle as one in 280, but an internal NASA risk assessment study (conducted by the Shuttle Program Safety and Mission Assurance Office at Johnson Space Center) released in late 2010 or early 2011 concluded that the agency had seriously underestimated the level of risk involved in operating the Shuttle. The report assessed that there was a 1 in 9 chance of a catastrophic disaster during the first nine flights of the Shuttle but that safety improvements had later improved the risk ratio to 1 in 90. In 1984 Reagan signed a National Security Decision Directive stating that the shuttle