Flying upside down and backward, Columbia will perform the deorbit burn early Tuesday to drop from orbit for the glide back to Earth. Photo: NASA animation/Spaceflight Now

The Columbia astronauts tested the shuttle's re-entry systems early today, firing up one of the ship's hydraulic units, test firing steering jets and calibrating cockpit instruments to make sure everything's ship-shape for landing Tuesday. There were no problems of any significance and the crew is pressing ahead with work to stow equipment and tidy up for re-entry.

"It's time to bring this incredible mission to a close," said entry flight director John Shannon. "And toward that end, the crew got up this morning, they're primary focus of the day was to prepare the orbiter, both inside and outside, for entry tomorrow.

"The commander and pilot have done their exercise for the day, they've worked through stowing equipment, getting the middeck lockers all closed out and about two hours ago, they ran through all the entry and landing systems checks that we perform on the day before entry.

"You may have heard we had one small problem during that, that was when we test fired all the thrusters we would use in the deorbit. One of the thrusters that's used for yaw steering control failed. It is not significant to us. There are four of those thrusters that fire in that direction. We require one of them, we'd like to have two of them. Right now we have three, so we're happy with that configuration."

He said the crew would work through the normal deorbit timeline and that no changes would be made because of the degraded flow of Freon coolant through one of Columbia's two coolant loops.

"There are no procedure changes at all related to that problem," Shannon said.

Forecasters are predicting generally good weather for Columbia's planned 4:32 a.m. touchdown at the Kennedy Space Center, with light winds, just a few clouds and only a slight chance for showers within 30 nautical miles. Conditions are expected to deteriorate somewhat Wednesday as central Florida falls under the influence of a lower pressure system.

Shannon said NASA will not staff Edwards Air Force Base, Calif., for Tuesday's landing attempt. But if Columbia is forced to remain in orbit an extra day, Edwards likely would be staffed Wednesday when the shuttle would land, one way or the other, at either Kennedy or Edwards. Here are all the landing opportunities for Tuesday and Wednesday at a glance (all in EST):

NASA plans to only call upon the prime landing site at Kennedy Space Center on Tuesday. If weather prevents a homecoming tomorrow in Florida, then both KSC and the backup landing site at Edwards Air Force Base in California will be available on Wednesday. Here is a look at the landing opportunities for Tuesday and Wednesday (all times are EST):

Tuesday
ORBIT...SITE....DEORBIT IGNITION...LANDING
165.....KSC.....03:22 a.m..........04:32 a.m.
166.....KSC.....05:05 a.m..........06:13 a.m.

Wednesday
ORBIT...SITE....DEORBIT IGNITION...LANDING
179.....KSC.....01:30 a.m..........02:40 a.m.
180.....EDW.....03:01 a.m..........04:12 a.m.
........KSC.....03:12 a.m..........04:21 a.m.
181.....EDW.....04:43 a.m..........05:52 a.m.

For a landing on the first opportunity Tuesday, Commander Scott Altman and pilot Duane Carey will fire Columbia's twin orbital maneuvering system rockets for four minutes and six seconds, slowing the ship by 284 mph. From that point, it will take the shuttle 33 minutes and 23 seconds to fall into the discernible atmosphere at an altitude of 400,000 feet. Range to touchdown at "entry interface" is 4,912 miles. Here's the crew's entry timeline for Monday night and Tuesday (in EST):

Monday
11:22 PM......Begin deorbit timeline
11:37 PM......Payload bay radiator stow
11:47 PM......Mission specialists seat
              installation
11:53 PM......Computers set for deorbit prep
11:57 PM......Hydraulic system configuration

Tuesday
12:22 AM......Flash evaporator checkout
12:28 AM......Final payload deactivation
12:42 AM......Payload bay doors closed
12:52 AM......Mission control 'go' for OPS-3
              entry software load
01:02 AM......OPS-3 transition
01:27 AM......Entry switchlist verification
01:37 AM......Deorbit maneuver data update
01:42 AM......Crew entry review
01:57 AM......Commander/pilot don entry
              suits
02:14 AM......inertial measurement unit
              alignment
02:22 AM......CDR/PLT strap in; mission
              specialists don suits
02:39 AM......Shuttle steering check
02:42 AM......Hydraulic system prestart
02:49 AM......Toilet deactivation
02:57 AM......Payload bay vent doors
              closed for entry
03:02 AM......MCC 'go' for deorbit burn
03:08 AM......MS seat ingress
03:17 AM......Single APU start
03:22:40 AM...Deorbit ignition
03:26:46 AM...Deorbit burn complete
03:27:28 AM...TDRS-West acquisition of signal
04:00:51 AM...Entry interface; shuttle hits
              discernible atmosphere
04:05:19 AM...83-degree left roll command
04:15:10 AM...57-degree roll reversal
04:25:39 AM...Velocity less than mach 2.5
04:27:39 AM...Velocity less than mach 1
04:27:58 AM...Shuttle on the heading alignment
              cylinder
04:31:56 AM...Landing

If landing is delayed one orbit, the deorbit burn would last four minutes and 10 seconds, slowing the shuttle by 288 mph. Time to entry interface would be 33 minutes and eight seconds and range to touchdown at that point would be 4,800 miles. Here's the one-orbit-late timeline:

04:45 AM......MCC 'go' for deorbit burn
04:51 AM......MS seat ingress
05:00 AM......Single APU start
05:05:32 AM...Deorbit ignition
05:09:02 AM...AOS TDRS
05:09:42 AM...Deorbit ignition complete
05:42:10 AM...Entry interface
05:46:38 AM...84-degree left roll command
06:02:50 AM...34-degree roll reversal
06:07:03 AM...Velocity less than mach 2.5
06:09:05 AM...Velocity less than mach 1
06:09:36 AM...Shuttle on the HAC
06:13:24 AM...Landing

During an overnight crew news conference, Altman said he had no concerns about the reduced flow of Freon in one of Columbia's two cooling systems, saying the shuttle could easily land with either system operational. As it stands, both loops are up and running.

"As I look back over this mission, we had kind of a rough start," Altman said. "Everybody came together, the team really responded and from that point on, from the time we had the Freon failure, it's been an uphill climb. We've worked incredibly hard, been very busy, but also, I think, incredibly successful.

"I just couldn't be prouder of the whole team, both all of us up here and the folks down there who worked so hard to make this a success," he said. "And yes, we are exhausted but we are also exhilarated. It's unbelievable we got everything we set to do accomplished. We're really thrilled about that and looking forward to coming home."

Lead spacewalker John Grunsfeld said he felt a bit sad watching Hubble recede in the distance Saturday after the rejuvenated observatory was released from Columbia. When asked if the crew might be experiencing an emotional letdown in the wake of the busy mission, Grunsfeld said the astronauts were still basking in the glow of success.

"I wouldn't exactly call it an emotional letdown, I'm still riding high on the fact that we did everything we came to do and that we didn't break the telescope," he said. "That's always a big fear going into a complex mission like this, that you arrive at a Hubble Space Telescope that's working very well in the hopes of making it much better and then something doesn't go well. But we put a lot of work into this. The team on the ground was excellent in their support, and we did everything we came up to do.

"I'm a little saddened," he added. "You know, seeing Hubble go, it's kind of like seeing a friend depart that you know you won't see for a long time, if at all. But as far as the work we did, there's really no letdown. I'm just thrilled that everything was done and I just can't wait to see the images from the advanced camera."

As for the grueling pace of the mission - a new single-mission spacewalk record was set in the five Hubble EVAs - Grunsfeld and fellow spacewalker James Newman said the workload was not unreasonable, given their training and support on the ground.

"It comes down to getting into a rhythm and knowing your job," Newman said. "What I found on this flight was that although five was fatiguing, we ended up in a good rhythm. The key to maintaining margin, though, would be to add a day of rest in between in order to maintain the pace. And then, if you did that occasionally, you could continue, perhaps, indefinitely.

"As far as in a row, five was a good number, I thought we still had margin on the last spacewalk. If we'd needed to, Mike and I could have gone out on a sixth spacewalk to accomplish mission success tasks, and there would have been good margin to do that safely."