CAPE CANAVERAL, Fla. -- A homebound crew started packing up aboard the International Space Station Wednesday as astronauts aboard visiting shuttle Discovery geared up for a spacewalking excursion outside the outpost.

Loading Leonardo

A formal change-of-command ceremony will take place on the International Space Station at 4:50 p.m. EDT (2050 GMT) Wednesday as outgoing skipper Yuri Usachev hands the helm of the outpost over to U.S. astronaut Frank Culbertson. Click here for live coverage. Click here for live coverage.

With the linked shuttle-station complex circling 244 miles (390 kilometers) above Earth, Discovery mission specialists Daniel Barry and Patrick Forrester set out to test the spacesuits they'll wear during a five-hour sortie Thursday.

"You definitely have some big boy toys up there with you today," astronaut Robert Curbeam told the shuttle crew from NASA's Mission Control Center in Houston.

Curbeam's call came just after Mission Control beamed up appropriate wake-up music for the day: "Big Boy Toys," a country-and-western tune by Aaron Tippin.

Working inside Discovery's dual-level crew cabin, Barry and Forrester checked out an array of power tools that the pair will use during the first of two spacewalks planned during the shuttle's stay at the station.

Barry and Forrester also planned to check out their $12 million spacesuits as well as a pair of emergency jet backpacks they would use to fly back to the shuttle if their braided steel safety tethers were to snap during the sortie.

Shuttle pilot Rick Sturckow, who will direct the spacewalking work from Discovery's flight deck, provided the two astronauts with an assist during a daylong effort to prepare for the excursion.

Set to begin at 10:15 a.m. EDT (1415 GMT) Thursday, the spacewalk primarily will involve mounting a pallet of coolant tanks on the station's $600 million U.S. electric power tower.

Weighing in at 1,200 pounds (540 kilograms), the tanks hold a reserve supply of ammonia coolant for the air conditioning system housed within the station's $1.4 billion U.S. Destiny science laboratory.

A second spacewalk to route power cables outside the lab is scheduled for Saturday.

Seven other astronauts and cosmonauts, meanwhile, started loading up an Italian moving van with 2,000 pounds (900 kilograms) of personal luggage, surplus station equipment and garbage that will be returned to Earth with an outgoing station crew.

In space since March 8, Russian cosmonaut Yuri Usachev and two American astronauts - Susan Helms and Jim Voss - will taxi back to terra firma aboard Discovery to cap a five-and-a-half-month tour on the outpost.

The packing work all was taking place within a shuttle-borne shipping container now attached to an Earth-facing berthing port on the station's U.S. Unity module. Dubbed Leonardo, the cylindrical cargo carrier will be stowed back in Discovery's payload bay for the return trip to Earth.

Usachev, Helms and Voss will formally hand over the helm of the station to incoming skipper Frank Culbertson and his two Russian colleagues - Vladimir Dezhurov and Mikhail Turin - during a change-of-command ceremony at 4:50 p.m. (2050 GMT) Wednesday.

The ceremony had been slated for early next week.

Mission managers, however, moved it up just in case an emergency forces the shuttle to depart the station after hatches between the ships are closed as part of spacewalk preparations.

In that unlikely event, Culbertson and his crewmates would remain on the station while Usachev, Helms and Voss flew off on the shuttle.

Discovery remains scheduled to depart the station next Monday, heading toward a 12:48 p.m. EDT (1648 GMT) Aug. 22 landing here at Kennedy Space Center.

Culbertson and his crew are scheduled to remain in space until Dec. 9.