With less than five days left before half the crew aboard the International Space Station departs, the six astronauts and cosmonauts of Expedition 38 performed maintenance on station systems, conducted research and packed items for the journey back to Earth.

Following the crew's daily planning conference with the flight control teams around the world, Flight Engineer Mike Hopkins went to work in the station's Tranquility node to begin the removal and replacement of a catalytic reactor inside the Water Recovery System.

Hopkins temporarily removed the COLBERT treadmill and rotated the large, refrigerator-sized rack that houses the recycling system to gain access to the connections at the front and back of the rack.

Flight Engineer Rick Mastracchio then joined Hopkins to help remove the old catalytic reactor and install a new one. The two NASA astronauts also installed a temporary filter between the reactor and the microbial check valve to support a system flush of the replacement unit. Part of the station's Environment Control and Life Support System, the Water Recovery System recycles condensation and urine into drinkable water.

Afterward, Hopkins rotated the Water Recovery System back into place, reinstalled the treadmill and removed the temporary filter. Mastracchio meanwhile inspected the failed catalytic reactor and packaged it up for return to Earth.

Mastracchio also spent some time installing video hardware for the ham radio located in the Columbus module. He performed a quick test of the hardware to verify basic functionality. A full test to commission the new system is scheduled for Saturday.

Flight Engineer Koichi Wakata spent much of the day working in the Kibo laboratory. The Japan Aerospace Exploration Agency astronaut first checked in on the Space Midge experiment, observing the progress of insects that he rehydrated two weeks ago.

Larvae of the sleeping chironomid, a kind of midge native to semiarid regions of Africa, are able to withstand nearly complete desiccation. Researchers are studying this midge to examine the impact of desiccation tolerance in a space environment as well as gene expression in response to microgravity.

Later, Wakata continued preparing the Fluid Physics Experiment Facility for the next phase of the Marangoni experiment. The Japanese astronaut carefully cleaned contamination from the facility's mirrors and replaced experiment hardware. Marangoni convection is the flow process that results from the difference in surface tensions where a liquid and a gas come together.

By studying this process in microgravity, researchers hope to uncover fundamental properties that could improve the production of semiconductors and optical crystals and contribute to various micro-fluid handling techniques, such as those used in DNA examination and clinical diagnostics.

On the Russian side of the complex, Commander Oleg Kotov and Flight Engineer Sergey Ryazanskiy focused their attention on their return to Earth Monday night along with Hopkins aboard the Soyuz TMA-10M spacecraft.

Kotov and Ryazanskiy spent much of the day packing science cargo, hardware and personal items for the trip home aboard the Soyuz. The third Russian cosmonaut aboard the station, Flight Engineer Mikhail Tyurin, assisted his crewmates as he transferred Biorisk experiment hardware and Matryoshka radiation detectors into the Soyuz.

To prepare themselves for the return to Earth's gravity after 166 days in space, Kotov and Ryazanskiy each participated in Lower Body Negative Pressure tests by exercising while wearing a special outfit that simulates the effects of gravity by drawing fluids to the lower half of the body. In addition to conditioning cosmonauts for the return home, this device provides Russian researchers with data to predict how the cosmonauts will react to the full force of Earth's gravity at the end of their mission.

On Monday, Kotov, Hopkins and Ryazanskiy will bid farewell to their fellow crewmates and board their Soyuz, closing the hatches at 4:45 p.m. EDT. The trio will undock from the station at 8:02 p.m. and land southeast of the town of Dzhezkazgan in Kazakhstan at 11:24 p.m. (9:24 a.m. Tuesday, Kazakhstan time).

The departure of Kotov, Hopkins and Ryazanskiy on Monday will mark the end of Expedition 38 and the beginning of Expedition 39 under the leadership of Wakata, the first Japan Aerospace Exploration Agency astronaut to command the station. Kotov will pass the helm to Wakata during a Change of Command ceremony slated for 5 a.m. Sunday. Wakata, Mastracchio and Tyurin, who arrived at the orbiting complex Nov. 7, will remain aboard the station until mid-May.

Meanwhile the three flight engineers who will return the station's crew to its full six-person complement are in the homestretch of preparations for launch on March 25. Having completed the second of two days of final qualification exams Wednesday, NASA astronaut Steve Swanson and Russian cosmonauts Alexander Skvortsov and Oleg Artemyev conducted a pre-launch news conference at the Gagarin Cosmonaut Training Center in Star City, Russia.

Their back-ups, NASA astronaut Barry Wilmore and cosmonauts Elena Serova and Alexander Samokutyaev, joined them for the conference and traveled with them to Moscow's Red Square to lay flowers at the Kremlin Wall in honor of the Russian space icons interred there.