This later date, from January 24, was announced in order to give NASA and contractor personnel sufficient time to fix the faulty fuel-cutoff sensors (ECOs) that have plagued this mission and earlier missions.
The defective ECOs have not accurately sensed when liquid hydrogen was being input into the hydrogen tank. The hydrogen tank is positioned in the bottom portion of the gigantic external fuel tank (ET)—filling about two-thirds of the ET (while the oxygen tank is located in the top portion of the ET—filling about one-third of the ET).
The ECOs for the hydrogen tank are located at the bottom of the ET. There are also identical ECOs for the oxygen tank. The two tanks supply liquid hydrogen and liquid oxygen—in a mixture ratio of 1 to 6 parts—to the three space shuttle main engines (SSMEs) of the orbiter.
It is very important that NASA mission controllers know exactly how much liquid oxygen and liquid hydrogen are left in each tank as the SSMEs burn for about 8.5 minutes during the liftoff of the shuttle. If the engines were to operate without being fed fuel, they would most likely eventually explode, destroying the orbiter and killing its crew.
A schematic of the ECOs are shown at the NASA website “ECO Sensors” and a more detailed schematic is provided at “Overall Schematic of Engine Cutoff (ECO) and Liquid Level (Point) Sensors.”
The main work of the NASA technicians is to install a replacement feed-through connector in the ECO sensor system. The pins within the feed-through connector have been soldered (rather than plugged in as is currently done) in such a way as to, as NASA stated: “create a connection that allows sensors inside the tank to send signals to the computers onboard Atlantis.” A technician is shown performing the soldering technique at “Connector Awaits Installation.”
The previous connectors were faulty and caused postponements of two launch attempts in December 2007.
At the time of the announcement, NASA officials emphasized that the February 7th date was "tentative" because a Russian Progress M-63/28P supply freighter is expected to be launched on February 7th and shortly afterwards, on February 9th, arrive at the International Space Station--at about the same time that Atlantis would arrive at the station, if this February 7th date is kept.
Different arrangement will have to be made by the Russians because the station can only support the docking of one space vehicle at a time.
However, SpaceFlightNow.com is reporting that the Russian Progress supply ship will be launched on February 5th with a docking on the 7th. This launch schedule, if confirmed, will allow Atlantis to dock on the 9th.
Another consideration for the "tentative" launch date is that NASA officials, also, still need to have engineers test the new ECOs to make sure they work properly before a launch can be performed.