caused by faulty temperature sensors. The moment at which an ATO became possible was referred to as the "press to ATO" moment. In an ATO situation, the spacecraft commander rotated the cockpit abort mode switch to the ATO position and depressed the abort push button. This initiated the flight-control software routines that handled the abort. In the event of a loss of communication, the spacecraft commander could have made the abort decision and taken action independently. A hydrogen fuel leak in one of the SSMEs during the STS-93 mission resulted in a slight underspeed at main engine cutoff (MECO) but did not necessitate an ATO, and Columbia achieved its planned orbit; if the leak had been more severe, it might have necessitated an ATO, RTLS, or TAL abort. Preferences There was an order of preference for abort modes: ATO was the preferred abort option whenever possible. TAL was the preferred abort option if the vehicle had not yet reached a speed permitting the ATO option. AOA would have been only used in the brief window between TAL and ATO options, or if a time-critical emergency (such as a medical emergency on board) developed after the end of the TAL window. RTLS resulted in the quickest landing of all abort options, but was considered the riskiest abort. Therefore, it would have been selected only in cases in which the developing emergency was so time-critical that the other aborts were not feasible, or in cases in which the vehicle had insufficient energy to perform the other aborts. Unlike with all other United States orbit-capable crewed vehicles (both previous and subsequent, as of 2021), the shuttle was never flown without astronauts aboard. To provide an incremental non-orbital test, NASA considered making the first mission an RTLS abort. However, STS-1 commander John Young declined, saying, "let's not practice Russian roulette" and "RTLS requires continuous miracles interspersed with acts of God to be successful." Contingency aborts Contingency aborts involved failure of more than one SSME and would generally have left the orbiter unable to reach a runway. These aborts were intended to ensure the survival of the orbiter long enough for the crew to bail out. Loss of two engines would have generally been survivable by using the remaining engine to optimize the orbiter's trajectory so as to not exceed structural limits during reentry. Loss of three engines could have been survivable outside of certain