NASA and contractor managers and engineers are reviewing plans to make a second attempt to launch the shuttle Atlantis Sunday despite problems with troublesome low-level fuel tank sensors that derailed a launch try Thursday.

Three of four engine cutoff - ECO - sensors in the hydrogen section of the shuttle's external tank failed to respond properly during tests to verify their health shortly after fueling began. They later returned to normal operation and NASA managers tentatively decided late Friday to set up for another launch attempt Sunday.

At that time, space shuttle Program Manager Wayne Hale said if one or more ECO sensors failed to operate properly at any point in the countdown Sunday, the launch would be called off. But that plan still requires final approval and some senior managers favor standing down for repairs, work that would push launch into January.

NASA's Mission Management Team began meeting at 1 p.m. today and a final decision on how to proceed is expected later this afternoon.

The ECO sensors are part of a backup system intended to detect low fuel levels in time to order a main engine shutdown if some other problem caused the powerplants to use up hydrogen faster than expected. At least two sensors must be operational for the system to work.

In the wake of earlier problems with the cutoff sensors, NASA implemented a flight rule exception that would permit a countdown to proceed if three of four sensors were operating normally and certain other conditions were met.

But engineers do not yet know what caused two sensors to "fail wet" during fueling Thursday or why a third sensor did the same during de-tanking operations. Because the system is suspect, the flight crew office proposed proceeding with launch Sunday if, and only if, all four sensors are operating normally.

Other proposed safeguards include shortening the launch window to one minute to reduce the amount of fuel need to catch up with the station and thus provide a cushion to protect against any possible low-level engine shut downs. In addition, flight controllers would incorporate data from new instrumentation for the first time to monitor the status of the sensors during ascent.

There is some reason to believe the sensors will work normally the second time around. Similar problems have gone away during past launch campaigns after initial exposure to minus 423-degree hydrogen fuel. Whether all four sensors will cooperate Sunday remains to be seen.

Assuming engineers get the green light, fueling will begin at 5:55 a.m. Sunday for a launch attempt at 3:21 p.m. Engineers plan to test the ECO sensors throughout the countdown, beginning shortly after they are submerged in liquid hydrogen around 6:41 a.m.

The goal of Atlantis' mission is to deliver the European Space Agency's Columbus research lab to the international space station along with French astronaut Leopold Eyharts, who will replace Expedition 16 flight engineer Dan Tani.

Tani, Expedition 16 commander Peggy Whitson and Russian flight engineer Yuri Malenchenko ran into problems earlier today with circuit breaker trips that briefly prevented flight controllers from changing the orientation of a right-side solar panel as required for the shuttle's docking next week.

The trouble involved one of the two wings making up the starboard 4, or S4, array on the right side of the station's main power truss. Those arrays currently are prevented from rotating to track the sun by earlier trouble with a massive rotary joint.

Concern about the solar alpha rotary joint, or SARJ, is behind a push to add a fourth spacewalk to Atlantis' mission. Engineers want a detailed inspection to help them figure out what might be wrong and what might be needed to fix it.

While unable to rotate paddle-wheel style, the two S4 solar wings also can be turned about their long axis, much like changing the pitch of an airplane propeller, to maximize electrical output. Early today, however, one or more remote power control modules, which act like circuit breakers, tripped open and stalled the so-called beta gimbal joint used to turn the S4-1A array.

It's not yet clear what might have caused the circuit breaker trips, but engineers were able to develop a work-around that enabled them to turn the array as required. They also sent commands to reposition the contaminated right-side SARJ to improve electricity generation.

The circuit breaker trips are not expected to have any impact on the Atlantis mission.

Here is a revised countdown timeline for Sunday's proposed shuttle launch attempt (in EST):

EST...........EVENT

12:06 AM......Fuel cell activation
12:56 AM......Booster joint heater activation
01:26 AM......MEC pre-flight bite test
01:41 AM......Tanking weather update
01:56 AM......Final fueling preps; launch area clear
02:56 AM......Red crew assembled
03:41 AM......Fuel cell integrity checks complete

03:55 AM......Begin 2-hour built-in hold (T-minus 6 hours)
04:06 AM......Safe-and-arm PIC test
05:11 AM......Mission management team tanking meeting
05:00 AM......Crew wakeup
05:26 AM......Test team ready for ET loading
05:55 AM......Resume countdown (T-minus 6 hours)

05:55 AM......LO2, LH2 transfer line chilldown
06:00 AM......NASA TV coverage of fueling begins
06:06 AM......Main propulsion system chill down
06:06 AM......LH2 slow fill
06:36 AM......LO2 slow fill
06:41 AM......Hydrogen ECO sensors go wet
06:46 AM......LO2 fast fill
06:56 AM......LH2 fast fill
08:51 AM......LH2 topping
08:56 AM......LH2 replenish
08:56 AM......LO2 replenish

08:55 AM......Begin 2-hour 30-minute built-in hold (T-minus 3 hours)
08:55 AM......Closeout crew to white room
08:55 AM......External tank in stable replenish mode
09:26 AM......Astronaut support personnel comm checks
09:55 AM......Crew breakfast/photo op (recorded)
10:00 AM......NASA television coverage begins
10:01 AM......Pre-ingress switch reconfig
11:01 AM......Final crew weather briefing
11:11 AM......Crew suit up begins
11:25 AM......Resume countdown (T-minus 3 hours)

11:31 AM......Crew departs O&C building
12:01 PM......Crew ingress
12:51 PM......Astronaut comm checks
01:16 PM......Hatch closure
02:01 PM......White room closeout

02:05 PM......Begin 10-minute built-in hold (T-minus 20m)
02:16 PM......NASA test director countdown briefing
02:15 PM......Resume countdown (T-minus 20m)

02:17 PM......Backup flight computer to OPS 1
02:21 PM......KSC area clear to launch

02:26 PM......Begin final built-in hold (T-minus 9m)
02:57 PM......NTD launch status verification
03:12:00 PM...Resume countdown (T-minus 9m)

03:13:30 PM...Orbiter access arm retraction
03:16:00 PM...Launch window opens
03:16:00 PM...Hydraulic power system (APU) start
03:16:05 PM...Terminate LO2 replenish
03:17:00 PM...Purge sequence 4 hydraulic test
03:17:00 PM...IMUs to inertial
03:17:05 PM...Aerosurface profile
03:17:30 PM...Main engine steering test
03:18:05 PM...LO2 tank pressurization
03:18:25 PM...Fuel cells to internal reactants
03:18:30 PM...Clear caution-and-warning memory
03:19:00 PM...Crew closes visors
03:19:03 PM...LH2 tank pressurization
03:20:10 PM...SRB joint heater deactivation
03:20:29 PM...Shuttle GPCs take control of countdown
03:20:39 PM...SRB steering test
03:20:53 PM...Main engine start (T-6.6 seconds)
03:21:00 PM...SRB ignition (LAUNCH)