Canadian astronaut Bob Thirsk holds willow saplings that are part of a Canadian experiment aboard the International Space Station. (Canadian Space Agency/Canadian Press)Canadian astronaut Bob Thirsk returned to Earth on Tuesday after spending 189 days — more than six months — in space, a record stay for a Canadian.

Thirsk returned to Earth on the same Russian Soyuz spacecraft that brought him to the International Space Station on May 27.

During his stay, Thirsk welcomed two other Canadians to the station, fellow astronaut Julie Payette and space tourist Guy Laliberté. The meeting of Thirsk and Payette on the ISS in July was the first time two Canadians met in space.

Thirsk has also used the station's robotic arm, Canadarm2, to assist in the station's construction. In September, he assisted in the arm's capture of a Japanese cargo vessel, the first-ever capture of a free-flying spacecraft. He called "the first Canadian cosmic catch" the most exciting moment of his mission.

Thirsk has participated in dozens of experiments, many of them studying the effects of long-term living in space on his own body, including his nutrition, changes in his immune system and the bone loss and lower red blood cell count that many astronauts experience during weightlessness.

Some of the experiments are Canadian or have Canadian contributions.

• The Marangoni Experiment in Space is a Japanese experiment on fluid physics in microgravity with a contribution from Masahiro Kawaji of the University of Toronto. The experiment involves the formation of semiconductor crystals in a liquid bridge between two solid discs. The researchers hope to better understand the basic physics behind the manufacture of semiconductors, used in computer microchips.
• The Microgravity Vibration Isolation Subsystem is a sophisticated lab environment designed to isolate delicate experiments from the vibrations that occur on the space station. MVIS uses magnets to keep its contents floating in space and untouched by the surrounding container.
• An experiment called Bodies in the Space Environment, or BISE, aims to understand how astronauts distinguish up from down in space. Thirsk is one of six astronauts participating in the York University experiment, which will continue after Thirsk leaves the station and into 2010.
• The Radi-N Neutron Field Study is a Canadian experiment to measure the increased space radiation levels seen on the station. Neutron radiation can penetrate into living tissue, causing DNA damage, which can lead to cataracts or cancer.
• IRIS is an experiment on human perception created by students at the International Space University. IRIS (short for Image Reversal in Space) will investigate whether the microgravity Thirsk experiences on the station affects his perception of optical illusions that play with how the brain sees two- and three-dimensional objects.
• SODI-IVIDIL is a European experiment with a Canadian contribution from Ziad Saghir of Ryerson University. The experiment will study the influence of vibration on the diffusion of liquids in a weightless environment.
• A Canadian experiment involving 24 willow saplings went up with the space shuttle Atlantis in November. The experiment, known as APEX-Cambium (Advanced Plant EXperiments on Orbit), will help determine the role gravity plays in the formation of different kinds of wood. The study is led by professor Rodney Savidge of the University of New Brunswick and funded by the Canadian Space Agency.

Canadian astronaut Robert Thirsk waves from his stretcher after exiting the Soyuz TMA-15 space capsule. Thirsk landed safely in northern Kazakhstan Dec. 1, ending a successful mission aboard the International Space Station. Thirsk's space-related scientific work will continue now that he has returned to Earth. He's conducting an experiment called CCISS (Cardiovascular and Cerebrovascular Control on Return from the ISS), which will study how astronauts' return to full gravity affects their bodies.

About 20 per cent of space shuttle astronauts and 80 per cent of the astronauts who spend extended periods on the International Space Station are affected by light-headedness and fainting upon returning to Earth's gravity.