NASA intends to make Tuesday's test of its hydrogen fuel sensor system as realistic as possible by pumping both liquid hydrogen and liquid oxygen into space shuttle Atlantis' external fuel tank just as would be done on launch day.

That is because launch controllers at NASA's Kennedy Space Center want to find out exactly what caused the sensor system to send false readings during two launch attempts for Atlantis' STS-122 mission earlier this month.

The launch team will begin pumping the liquid fuels, informally known as "cryos", into the tank at 7 a.m. Then a team of trained technicians will go to Launch Pad 39A and turn on a set of special instruments to begin a series of carefully orchestrated measures meant to pinpoint the location of the issue.

While the technicians work the instruments at the launch pad, engineers and launch controllers will be running the system through a series of evaluations from the Launch Control Center.

The entire test, from filling the tank to draining its half-million gallons of propellant and a second series of tests, is expected to last about 13 hours.

Atlantis' next launch opportunity is targeted for no earlier than Jan. 10, 2008.

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