However, less than one hour later, at around 6:52 a.m. EST, one of the four sensors, the number three ECO sensor, began to act up. It failed to sense that a “wet” condition occurred inside the hydrogen tank; that is, liquid hydrogen was present inside the tank.

The new rule for STS-122, only, is that all four engine cutoff sensors must be working properly before a launch can be attempted. The four ECO sensors are part of the emergency engine cutoff system. The fuel cutoff sensor system is a series of redundant systems that protect the Space Shuttle Main Engines (SSMEs).

The SSMEs burn liquid hydrogen and liquid oxygen to produce a thrust of about 375,000 pounds (1.7 million newtons) at sea level. The three SSMEs burn for only about 8.0 to 8.5 minutes each ascent into space. The liquid oxygen tank is positioned at the top portion of the external tank (what is called the forward tank), while the liquid hydrogen tank is below it in the lower portion of the ET (what is called the aft tank).

The oxygen tank volume is approximately 143,000 gallons (540,000 liters) and it holds about 1.36 million pounds (616,500 kilograms) of liquid oxygen. The hydrogen tank volume is about 383,000 gallons (1.5 million liters) and holds about 226,000 pounds (102,000 kilograms) of liquid hydrogen.

The mission was scrubbed for Sunday around 7:25 a.m. EST (1225 GMT). This postponement is a continuation of sensor problems that began on Thursday, December 6th for STS-122 mission, all caused by the engine cutoff sensors.

NASA will make a later decision as to whether to attempt further launches on or before Thursday, December 13th, the end of the current launch window for the STS-122 mission. However, it seems unlikely the launch will occur during this time frame.

With the faulty sensors NASA will most likely have to roll-back the orbiter to the Vehicle Assembly Building (VAB) and make extensive examinations of the sensors and circuitry.

However, a decision has not been made yet.

Another attempt to launch could be made on Monday at approximately 2:55 p.m. EST. NASA can try on last time on Thursday, or even possibly Friday. If unable to launch at that time, they will have to hold off until early in January 2008.

UPDATE by AUTHOR: Just after the writing of this article, NASA officials stated that STS-122 will be postponed until January 2008 in order for engineers to investigate more completely the engine cutoff (ECO) sensor problem in the hydrogen tank. January 2, 2008 is now considered the earliest date in which to launch space shuttle Atlantis and its STS-122 astronaut crew.

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