Now veterans of over three days in space, the two astronauts aboard China's Shenzhou 6 capsule are more than halfway through their anticipated five-day mission after already completing several experiments and key tests of the performance of their spacecraft, which continues to function well.

The crew launched early Wednesday from the remote Jiuquan launch center in northwest China's Gobi desert, and its Long March 2F rocket delivered astronauts Fei Junlong and Nie Haisheng to orbit just minutes later to begin their shakedown cruise designed to check the Shenzhou's performance. The spacecraft used its on-board propulsion system to fine-tune its path around Earth about seven hours later, achieving a circular orbit just over 200 miles in altitude.

Within nine hours after liftoff, Fei became the first man to enter the orbital module of a Shenzhou craft. During China's historic first manned flight in 2003, pilot Yang Liwei was confined to his seat in the entry capsule throughout the 21-hour trip.

The Shenzhou 6 astronauts also removed their bulky launch and entry spacesuits during the first day of their mission, and donned standard work clothes to allow them to live more comfortably in a shirtsleeve environment, said the Xinhua news agency.

The families of the two new national heroes visited the mission's Beijing flight control room to participate in a radio communications link with the crew late Wednesday, a day before astronaut Nie Haisheng's 41st birthday.

On Thursday, the pair carried out the planned schedule of experiments and tests to further examine the operating characteristics of the Shenzhou design, which features an entry module situated between orbital and service modules, each fitted with a pair of solar arrays for electricity production.

Included in the checkout regime were disturbance tests to gauge the response of the craft to a variety movements and vibrations. "These experiments were conducted with exaggerating movements in a bid to test the effects of their actions to the spacecraft," a Xinhua report said.

"The results proved that the spacecraft was fully capable of enduring all the disturbances, and then (the) astronauts (will) be allowed to move in a relatively free way."

Shenzhou 6 reportedly passed the tests with no ill effects, putting both designers and ground controllers at ease with the excellent performance of the craft thus far. Other work completed on the second day of the test flight included a check of the seal on the hatch between the entry and orbital modules to ensure it is airtight for the separation of the two components prior to the astronauts' return to Earth.

An orbital adjust burn was conducted during the spacecraft's 30th trip around Earth early Friday, Beijing time, to boost its trajectory after the natural effects of slight atmospheric drag had begun to decay its orbit to lower altitudes.

Other experiments carried out on the mission have focused on life and material sciences, Earth observation, and environmental studies, Xinhua reported.

This mission carries many more supplies and comforts than afforded to Yang Liwei in his 2003 flight, and the astronauts have taken advantage by using sleeping bags for rest, an improved sanitation system, heated food, and purified drinks.

The astronauts typically consume three meals a day, with each consisting of five to six separate dishes. Included in the rations are rice, beef balls, and dehydrated vegetables, Xinhua said.

Pre-flight reports said the Shenzhou 6 mission would last around 119 hours, or about five days in duration. If true, this would result in a touchdown of the capsule early on Monday in the grasslands of the northern Chinese province of Inner Mongolia -- the landing site of the previous five Shenzhou flights.

Recovery teams are already stationed in the nearby Hongger Township in case management officials in Beijing order an early return of the astronauts. Such a decision is possible and would likely be based on expected weather conditions at the landing site.

China has fielded nine ground stations worldwide in support of the mission, in addition to a fleet of four tracking ships at sea in the Atlantic, Pacific, and Indian Oceans to allow more communications opportunities with the crew in space.

According to Xinhua, thirteen landing sites have been selected by program officials, including the primary locale in Inner Mongolia, a backup at the Jiuquan launch center, and eleven other emergency sites scattered across the globe in Australia, northern Africa, Europe, and the United States.

When Shenzhou 6 returns to Earth, controllers will command the orbital module to first separate from the return capsule and attached service module, allowing the former to continue flying in space for other unmanned experiments. The service module will then fire braking thrusters to slow the craft enough to nudge it out of orbit and on a path to return to Earth.

Before encountering the upper reaches of the atmosphere, the return capsule will jettison the service module, which is designed to burn up during the fiery re-entry. Meanwhile, the return capsule -- protected by an ablative heat shield -- will plummet through the upper atmosphere and intense heat before deploying parachutes to slow the craft on its way to a soft touchdown in the plains of northern China.

The crew will be pulled from the capsule shortly after recovery forces arrive via helicopter just minutes after landing.