Columbia commander Rick Husband, pilot William McCool and flight engineer Kalpana Chawla tested the shuttle's re-entry systems today, setting the stage for landing Saturday to close out a 16-day science mission. Touchdown on runway 33 at the Kennedy Space Center currently is targeted for 9:15:50 a.m. EST.

Good weather is expected, with scattered clouds at 3,500 feet and 25,000 feet, winds out of 300 degrees at 10 knots gusting to 15 and visibility of seven miles. There are no predicted flight rule violations.

NASA does not plan to staff the shuttle's backup landing site at Edwards Air Force Base, Calif., for Saturday's landing attempt. Instead, Husband and company will have two opportunities on back-to-back orbits - 255 and 256 - to land in Florida. If they can't make it for any reason, they will remain in orbit an extra day and try again Sunday.

Entry flight director Leroy Cain says the shuttle Columbia sailed through morning entry tests with flying colors and that with good weather expected, the crew should have no problems returning to the Kennedy Space Center early Saturday.

"This has been a very successful mission," Cain said. "Fair to say it's far exceeded folks' expectations from a science standpoint. So we're very pleased.

"The vehicle Columbia and the crew are doing very well today," he continued. "It's been a quiet day with respect to science as we start to ramp down on most of those experiments and put them away. We spent most of our day today getting set up for entry.

"We did our normal flight control systems checkout. ... All the systems we use for entry, we had no problems, the vehicle performed flawlessly today as it has the entire mission."

The only issue - and Cain said it was not significant - is a bit of possible tile damage on Columbia's left wing. Video of launch shows what appears to be a piece of foam insulation from the shuttle's external tank falling away during ascent and hitting the left wing near its leading edge.

But Cain said engineers "took a very thorough look at the situation with the tile on the left wing and we have no concerns whatsoever. We haven't changed anything with respect to our trajectory design. It will be a nominal, standard trajectory."

Commander Rick Husband and pilot William McCool plan to fire Columbia's twin braking rockets at 8:15 a.m. EST Saturday to begin the hourlong glide to Earth. Touchdown on runway 33 at the Kennedy Space Center is expected at 9:16 a.m. EST.

For early risers in the San Francisco area, Columbia should put on quite a show as it streaks across the sky in pre-dawn darkness. Assuming an on-time deorbit "burn," Columbia will make landfall around 5:55 a.m. PST.

"This will be a very good visual sighting for folks on the West Coast as well as mid Arizona, the New Mexico area," Cain said. "It should be a pretty spectacular event for folks (in the San Francisco area) who have never seen a shuttle sighting at night. It's a sight to see and this one should be a very good one."

Following touchdown, four of Columbia's seven crew members - Laurel Clark, Michael Anderson, David Brown and Ilan Ramon, the first Israeli in space, will be carried off the shuttle on stretchers as part of ongoing medical research charting the body's adaptation to weightlessness and re-adaptation to gravity.

"Post flight, we have data collection for approximately a month," Clark said during a crew news conference earlier this week. "For several days we'll be very busy and then we have data collections every couple of days and that goes out for about a month or 45 days afterward."

Cain said Columbia has enough on-board supplies to remain in orbit until Wednesday in a worst-case scenario. But no such problems are expected and the forecast for Florida calls for good weather Saturday with scattered clouds and light winds.