exceededambient.However,apotentialfailuremodewasdiscoveredwhenitwasdetermined thatwhen thespacecraftvoltagewasrampedupslowly,theACISmechanism controllercouldpowerupwithout receivingacommandtodoso.TheteamwasabletoduplicatethisconditionontheACIStesthardware andwasabletoshowthatthemechanism couldhavebeenpoweredupinadvertently. However,the temperature profileandlimitswitchactuationmeasuredatTRWduringthedooropenfailuredidnot matchthecharacteristics thatwouldbeobtainedforapre-ruptured sheardisk.Thisconclusionisbased onextensivetestingwithnon-flighthardwareandalsotheactualflightactuatorusedinthefailedopening attempt. IceonMechanism (Item8AandGofFaultTree) Icingofthemechanism wasthought tobeapromising failuremodesincethefailureoccurred whenthe doorwasopened coldandfrostwasseenontheObservatory opticalbenchpriortotheopening failure. However, analysis showed thaticecouldnothavecaused theproblem withthedoormechanism. Atthe timeofthefailure,several different pressure monitors insidethechamber showed thepressure tobein the10-6Torrrange. Atthistemperature (-60°C) andpressure, thesublimation rateforwater/ice isgreater thanthecollection rateanditwasunlikely thatenough icecouldexistonsurfaces at-60°Ctocausea problem. Coldersurfaces at,-90°Corless,didhavevisibleicebecause thecollection ratewashigher thanthesublimation rate.Somewater/ice wasprobably present onmechanism components sincethere wereincreases inpressure whentheheaterwasturnedontowarmthehousing to+25°C. However, the thickness ofwater/ice wouldhavebeenangstroms andnotthemillimeters whichwouldhavebeen required toblockthemechanism. Apressure increase wasalsoobserved withtheon-board Chandra vacuum monitor whentheheaterwasturnedon.However, evensmallamounts ofwater/ice couldresult inincreased pressure atvacuum levelsof10-5to10-6Torr.Thelocalpressure aroundthemechanism wouldhavehadtobeenanunlikely t0-3Torrinorderforenough icetocollecttocauseaproblem. Analytically, thiswasshowntobenon-credible. Tofurtherinvestigate thewater/ice failuremode,anengineering unitdoormechanism waspurposefully icedupbycooling itwithliquidnitrogen inambient pressure. Themechanism andSTARSYS actuator werecovered withicefromrelative humidity condensation andtherunning torqueincrease wasbarely measurable. Othertestsandstudies wereperformed anddiscounted icingasafailuremechanism. O-rinqAdhesion Mechanisms Duringthecourseofthisinvestigation various O-ringadhesion mechanisms havebeenproposed as possible causes forthefailure. Although nonehavebeenshownconclusively tooccurunderthefailure conditions oftheflighthardware, several remained viablepossibilities. Thisisespecially trueiftwoor moreadhesion mechanisms actedtogether synergistically. Allpossibilities focused ontheproperties of Vitonadhering tothegoldsurface ofthedoorsealundervarious conditions. Although thisfailuremodeis believed tobethemostprobable failuremode,notestoranalysis couldprovethatenough adhesion couldhaveexisted atTRWtoprevent thedoorfromopening. Lockheed Martinconducted manytests withengineering hardware thatduplicated theenvironmental conditions, butnotduringthe7-month periodthatexisted beforeandduringthefailure. Theteamwasunabletoreproduce, underany simulated circumstance, thefailure. PlansforFlightOpening Animmense effortwasexpended toidentify thefailuremechanism priortolaunch, butdespite thebest effortsofmanyablepeople, wewerenotabletoclearlyidentify thefailuremechanism. Infact,it sometimes seemed asifthefailurecouldnothaveoccurred, butithad.Astimewentonandwe approached thelaunchdatewithout anidentified cause,theteam's(andprogram management's) approach waseliminate or"mitigate" allpossible failuremodes. Thedecision toopenthedoorwarm, whichhadalways beentheplan,wouldeliminate anypotential issueswithicing,however unlikely. SinceO-ringstiction seemed tobethemostlikelyfailuremechanism, eventhoughitcouldnotbeproven, plansweredeveloped todealwithitintheflightsituation. Firstofall,itappeared thatthestiction level 26