While there's a chance of rain and even thundershowers near the Kennedy Space Center this afternoon, NASA Spaceflight Meteorology Group is predicting a 70 percent chance of acceptable weather for the shuttle Endeavour's planned landing. Touchdown on runway 15 is targeted for 3:48 p.m.

"We have two opportunities to land at the Kennedy Space Center," said lead flight director Wayne Hale. "The first opportunity may have slightly better weather than the second opportunity, is what our weatherman tells us."

"He's given us about a 70 percent chance of getting acceptable weather at the Cape. The major concerns we'll have are clouds and rain showers in the area."

Flying upside down and backward over the Indian Ocean, commander James Wetherbee plans to fire Endeavour's twin braking rockets for three minutes starting at 2:42 p.m., slowing the shuttle by 233 mph and dropping the far side of its orbit deep into Earth's atmosphere.

A half-hour later, Endeavour will slam into the discernible atmosphere 400,000 feet - 76 miles - above the southern Pacific Ocean. From there, the shuttle's flight computers will guide the spaceplane to a point about 50,000 feet above the Kennedy Space Center. A few moments later, Wetherbee will take over manual control and guide the shuttle to touchdown.

Resting on their backs in recumbent seats bolted to the floor of the shuttle's lower deck will be outgoing space station commander Valery Korzun, flight engineer Sergei Treschev and science officer Peggy Whitson. The Expedition 5 crew was launched to the space station June 5 and with a landing Wednesday, they will have logged 182 days off the planet.

Whitson said she plans to walk off the shuttle if she's physically able.

"Why not? If I can, I want to walk off," she told CBS News Tuesday. "Our station crew members have been coming back in better shape than the folks who came back from Mir and I think a lot of it has to do with the exercise training program that we're working on now, using resistive exercises in addition to the treadmill and ergometer."

In a news conference Friday, Whitson said she was looking forward to a steak dinner with a Caesar salad and "tons of garlic." Tuesday, she added one more item to her dream meal: "Drinking a soft drink, something carbonated. We don't have any carbonated drinks up here so I'm looking forward to that. And anything with ice in it would be very nice as well!"

Weather permitting, of course.

A front moving toward Florida is expected to rule out any landing attempts Thursday and stormy weather in front of the system may cause problems Wednesday. Conditions are expected to improve Friday, after the front moves through.

Unlike forecasters at the Cape Canaveral Air Force Station, the Spaceflight Meteorology Group in Houston does not normally issue launch or landing forecast percentages and it's not clear what prompted the 70 percent "go" forecast Wednesday.

"This is not a great scientific thing to give you a percentage on the weather," Hale said. "This is kind of a gut check, an approximate feel. But we feel pretty good we will have a chance at it (today). We're not coming in expecting that we're doing just a drill and we won't have any opportunity to land. We're feeling pretty good that it may really come into good enough standing that we can get on the ground."

Endeavour has enough on-board supplies to remain in orbit until Sunday in a worst-case scenario. Given the improving forecast for Friday, NASA has no plans to activate its alternate landing site at Edwards Air Force Base, Calif., before then. NASA wants to get Endeavour back on the ground in Florida if at all possible to avoid a processing delays readying the ship for its next flight.

If stormy weather prevents a Florida landing Wednesday, NASA managers could elect to bypass Thursday and to try again Friday, assuming the current forecast holds up. But Hale said he would not rule out a Thursday landing attempt before flight controllers had a chance to judge the actual weather conditions.

"Everybody thinks there's a great secret to making the deorbit decision with regard to the weather," Hale said. "There is no real secret here. The plan is you get ready, you get prepared and when the clock ticks down to the time of the opportunity, you look at the conditions and either they are good enough to land or they're not.

"We would not, as a matter of course, decide not to attempt Thursday on Wednesday," Hale said. "We would wait until Thursday morning and come in and look what the weather has shaped up. ... So you wait until its time to start the deorbit preparations and you take your very best look at what the weather is and the forecast for the landing time and you either start in deorbit prep or you don't."

Weather is an issue for two reasons. The shuttle cannot fly through rain because high-velocity impacts with rain drops would erode its protective heat-shield tiles. At best, major repairs would be required. At worse, the shuttle's aerodynamic stability could be threatened.

Lightning is another matter altogether.

"We're very concerned about lightning striking the orbiter," Hale said. "It's a digital fly-by-wire airplane, if the computers don't work you don't fly. So we try to avoid any chance of having lightning strike the orbiter during landing.

"If we get unlucky, we will not try to go to California or New Mexico (Wednesday). We will instead stay on orbit and try to continue to deorbit to the Kennedy Space Center.

But Thursday "is not very good. A major front will be coming through the central Florida area and it looks like that might be a poor day to consider landing at the Kennedy Space Center. But we have enough consummables on board - rocket propellant for attitude control, lithium hydroxide to scrub the carbon dioxide out of the air, certainly all the cryogenics we need to make electrical power and water for the crew - to continue on until Sunday afternoon if need be.

"So we will continue to try for the Kennedy Space Center. The forecast for plus two days, which would be Friday, is excellent at the Kennedy Space Center.

Here is a detailed timeline of entry events for Wednesday's two landing opportunities:

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

First Opportunity: Rev. 170 Deorbit to KSC

10:42 AM.....Begin deorbit timeline
10:57 AM.....Radiator stow
11:07 AM.....Mission specialists seat installation
11:13 AM.....Computers set for deorbit prep
11:17 AM.....Hydraulic system configuration
11:42 AM.....Flash evaporator cooling system
             checkout
11:48 AM.....Final payload deactivation
12:02 PM.....Payload bay doors closed
12:12 PM.....Mission control 'go' for transition
             to OPS-3 entry software
12:22 PM.....OPS-3 entry software loaded
12:47 PM.....Entry switchlist verification
12:57 PM.....Deorbit maneuver update
01:02 PM.....Crew entry review
01:17 PM.....Commander, pilot, don entry suits
01:34 PM.....Inertial measurement unit alignment
01:42 PM.....Commander, pilot strap in; others
             don suits
01:59 PM.....Shuttle steering check
02:02 PM.....Hydraulic system prestart
02:09 PM.....Toilet deactivation
02:17 PM.....Payload bay vent doors closed
02:22 PM.....Mission control 'go' for deorbit burn
02:28 PM.....Remaining crew members strap in
02:37 PM.....Single hydraulic power unit start
2:40:50 PM...TDRS-West acquisition of signal
2:42:14 PM...Deorbit ignition
2:45:14 PM...Deorbit burn complete
3:16:41 PM...The shuttle hits the discernible
             atmosphere 400,000 feet up
3:21:24 PM...4-degree right roll command
3:28:11 PM...63-degree roll reversal
3:41:39 PM...Velocity less than mach 2.5
3:43:51 PM...Velocity less than mach 1
3:44:42 PM...Shuttle banks to line up on runway
3:48:13 PM...Landing on runway 15

Second Opportunity: Rev. 170 Deorbit to KSC

04:00 PM.....Mission control 'go' for deorbit burn
04:06 PM.....Remaining crew members strap in
04:15 PM.....Single hydraulic power unit start
4:20:20 PM...Deorbit ignition
4:21:48 PM...AOS TDRS-West
4:23:21 PM...Deorbit ignition complete
4:54:05 PM...Shuttle hits discernible atmosphere
4:58:45 PM...14-degree right roll command
5:15:12 PM...45-degree roll reversal
5:19:12 PM...Velocity less than mach 2.5
5:21:24 PM...Velocity less than mach 1
5:21:37 PM...Shuttle banks to line up on runway
5:25:45 PM...Landing on runway 15