Commander Vinogradov and flight engineer Williams also set aside time each day to continue to familiarize themselves with the orbiting laboratory.

Williams operated the Capillary Flow Experiment, which uses liquid silicone to study how fluids move in a microgravity environment. This portion of the experiment examined the interface between the liquid and the solid surface of the container. The results could be used by designers of systems for future spacecraft.

Williams also set up and activated cameras that will be remotely operated by middle school students to take photos of Earth through the station window. Called the Earth Knowledge Acquired by Middle School Students, or EarthKAM, experiment, it allows students to study the Earth and control a special digital camera mounted on the station.

Participants can photograph coastlines, mountain ranges and other geographic items of interest from a vantage point of space. More than 112 schools from eight countries signed up for this session of the experiment, the 22nd time it has been performed aboard the station.

Williams and Vinogradov completed the first of three sessions with the Renal Stone experiment, a study of whether potassium citrate can be used to reduce the risk of kidney stone formation. Astronauts have an increased risk of developing kidney stones because urine calcium levels are typically much higher in space.

The crew recorded all consumed food and drinks and collected urine samples for later return to Earth. An understanding of the crew's diet during the urine collection timeframes will help researchers determine if the excess calcium in the urine is due to diet or a response to the microgravity environment.

The two astronauts also spent several hours practicing the use of a manual docking system for this week's arrival of the Russian Progress 21 cargo vehicle. The computer-based training will ensure they will be able to take control of the Progress if the automated system does not work properly.

The 21st Progress to visit the station is scheduled to launch from the Baikonur Cosmodrome in Kazakhstan at 11:03 a.m. Central U.S. Time Monday, and dock with the space station at 12:40 p.m. Central Time Wednesday.

Last week, Russian controllers aborted a planned adjustment of the station's orbit before any engines were fired when downlink telemetry showed one of two sunshade covers on the Zvezda Service Module thrusters was not fully open. The station's onboard software detected the cover was not properly opened and did not ignite the thrusters.

The firing was designed to test two thrusters that have not been used since Zvezda docked to the station in July 2000. Zvezda has several other thrusters that could be used if needed. Engineers at the Russian Mission Control Center in Korolev are reviewing data and considering whether additional tests are required.

Last Friday, Vinogradov and Williams talked with controllers about an electrical repair procedure planned for Monday. The pair will replace a failed type of circuit breaker called a Remote Power Control Module in the Destiny Laboratory. The RPCM failed during the last crew's stay aboard the station, and power for several systems has been routed by an alternate path until it is replaced.

Both astronauts are scheduled to remain in orbit until next October. During that time, they expect to welcome two space shuttles and perform two spacewalks. European Space Agency astronaut Thomas Reiter will join them when space shuttle Discovery arrives on the STS-121 mission, still targeted for launch no earlier than July 1.

Reiter will increase the station crew size to three for the first time since May 2003 when it was reduced to conserve supplies following the Columbia accident.

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