Live in Orbit: Astronauts Test Landing Systems

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Discovery's astronauts are checking out systems that will play crucial roles in a planned landing Saturday while mission managers keep close tabs on the weather at Kennedy Space Center.

Circling high above the planet, the astronauts are testing the flight control aerosurfaces that will serve to help slow the shuttle from 17,000 mpg at the start of an hour-long reentry to 225 mph at touchdown.

The astronauts also will be hot-firing the shuttle's 44 nose-and-tail steering thrusters, which are used during reentry to maneuver the ship on sweeping S-turns -- moves made to dissipate speed as the orbiter carves its way through the atmosphere.

The weather forecast for landing looks good, but not perfect.

The shuttle and its crew are scheduled to land at KSC's three-mile runway at 1:39 p.m. Saturday; the crew would have a second opportunity at 3:14 p.m.

The forecast calls for scattered clouds at 5,000 and 20,000 feet.

However, forecasters at the Spaceflight Meteorology Group at NASA's Johnson Space Center in Houston say there is a slight chance the scattered clouds at 5,000 feet might build in to a cloud deck that could prevent mission commander Lee Archambault from seeing the runway on final approach. That would be a flight rule violation.

Stiff winds are expected, but coming out of the south, they would present Discovery with either a 20 knot headwind on Runway 15 on the north end of the landing strip, which would be within limits.

Sustained winds of 13 knots with gusts to 21 knots are expected, but due to the direction, they would be within limits.

The visibility is expected to be seven miles.

The weather on Sunday is expected to deteriorate significantly.

Forecasters say there would be a chance of thick electrically charged clouds, thunderstorms within 30 miles of the runway and stiff crosswinds.

ABOUT THE IMAGES: Click to enlarge the top image, which shows the shuttle's left and right elevons as Discovery's astronauts exercised them as part of a flight control system checkout -- a standard day-before-landing test. You can also click to enlarge (top to bottom) the long-range, mid-range and short-range ground tracks for an atmospheric reentry and landing on the shuttle's 201st orbit of Earth.