The shuttle Discovery's countdown is on track today for a launch attempt Saturday at 3:49 p.m., weather permitting. Forecasters continue to predict a 60 percent chance of unacceptable weather Saturday, Sunday and Monday due to electrically charged anvil clouds within 23 miles of the launch area and a possibility of showers.

Thunderstorms Thursday afternoon delayed work to load Discovery's internal tanks with liquid oxygen and hydrogen for the ship's electricity producing fuel cells. But the launch team had eight hours of built-in hold time Thursday to make up for any delays and by this morning, the countdown was back on track.

Based on the actual load, flight controllers now believe Discovery may have enough power generation capability to permit a one-day mission extension - and a third spacewalk to test wing leading edge repair techniques - even if launch is delayed to July 4. Based on earlier predictions, July 3 appeared to be the cutoff for a mission extension, barring extensive crew conservation procedures. In any case, no decision will be made on a possible mission extension until well into Discovery's mission.

NASA Test Director Jeff Spaulding said today the launch strategy calls for making back-to-back attemps Saturday and Sunday, if necessary, before standing down a day to give the team a break. Two more attempts could be made Tuesday and Wednesday before a two-day stand down to top off the internal hydrogen and oxygen tanks. Discovery's launch window closes July 19.

Spaulding said the possibility of making three launch attempts in a row Saturday, Sunday and Monday has not been ruled out. But if NASA went down that road and didn't make it, launch would be delayed another four days to refill launch pad fuel tanks and to top off the fuel cell system.

Here is the remainder of Discovery's countdown in text format. Note: NASA times the countdown to the opening of the shuttle's 10-minute launch window, not the actual launch time, and rounds down to the nearest minute. The latest estimate shows the launch window opens at 3:43:38 p.m. All events in the countdown prior to the release of a final hold at the T-minus nine-minute mark are based on the window open time of 3:43 p.m. The countdown will resume at the T-minus nine-minute mark based on the actual launch time, which is roughly the moment Earth's rotation carries the pad into the plane of the space station's orbit. As of today, the estimate is 3:48:38 p.m.

HH:MM......EVENT

06/30/06
01:10 PM...Communications system activation
01:40 PM...Crew module voice checks
02:50 PM...Flight crew equipment late stow
06:00 PM...Rotating service structure to park position
07:50 PM...Ascent switch list configuration
10:53 PM...Resume countdowns
10:53 PM...Terminate pad tours

07/01/06
12:03 AM...Fuel cell activation
12:53 AM...Pad clear of non-essential personnel
12:53 AM...Mission control in launch comm configuration
01:23 AM...Solid rocket booster joint heater activation
03:08 AM...Final fueling preps; launch area clear
03:53 AM...Begin 2-hour built-in hold
04:03 AM...Safe-and-arm PIC test
04:28 AM...External tank ready for fueling
04:43 AM...Mission management team tanking meeting
05:00 AM...NASA television coverage begins
05:53 AM...Resume countdown
05:53 AM...Liquid oxygen (LO2), hydrogen (LH2) transfer line chilldown
06:03 AM...Main propulsion system chill down
06:03 AM...LH2 slow fill
06:33 AM...LO2 slow fill
06:38 AM...Hydrogen engine cutoff sensors go wet
06:43 AM...LO2 fast fill
06:53 AM...LH2 fast fill
08:08 AM...LH2 topping
08:48 AM...LH2 replenish
08:53 AM...LO2 replenish
08:53 AM...Begin 3-hour built-in hold
08:53 AM...Closeout crew to white room
08:58 AM...External tank in stable replenish mode
09:08 AM...Astronaut support personnel comm checks
09:38 AM...Pre-ingress switch reconfig
10:10 AM...Crew photo opportunity
11:19 AM...Crew weather briefing
11:19 AM...Astronauts begin donning pressure suits
11:53 AM...Resume countdown
11:59 AM...Crew departs O&C building
12:29 PM...Crew begins strapping in
01:18 PM...Astronaut communications checks
01:44 PM...Hatch closure
02:18 PM...White room closeout
02:33 PM...Begin 10-minute built-in hold (T-minus 20 minutes)
02:35 PM...NASA test director countdown briefing
02:43 PM...Resume countdown (T-minus 20 minutes)
02:44 PM...Backup flight computer (BFS) loads OPS 1 software
02:48 PM...Kennedy Space Center area clear to launch
02:54 PM...Begin final built-in hold (T-minus nine minutes)
02:59 PM...RTLS runway verification
03:04 PM...NASA test director launch status verification

HH:MM:SS

03:39:38 PM...Resume countdown (T-minus nine minutes)
03:41:08 PM...Orbiter access arm retraction
03:43:38 PM...Launch window opens
03:43:38 PM...Hydraulic power system (APU) start
03:43:43 PM...Terminate LO2 replenish
03:44:38 PM...Purge sequence 4 hydraulic system test
03:44:38 PM...Inertial measurement units (IMUs) to inertial
03:44:43 PM...Aerosurface profile (steering test)
03:45:08 PM...Main engine steering test
03:45:43 PM...LO2 tank pressurization
03:46:03 PM...Fuel cells to internal reactants
03:46:08 PM...Crew lears caution-and-warning memory
03:46:38 PM...Crew closes visors
03:46:41 PM...LH2 tank pressurization
03:47:48 PM...Soplid rocket booster joint heater deactivation
03:48:07 PM...Shuttle flight computers take control of countdown
03:48:17 PM...Booster steering test
03:48:31 PM...Main engine start (T-6.6 seconds)
03:48:38 PM...Booster ignition (LAUNCH)
03:53:38 PM...Window closes