The U.S. commander of space shuttle Discovery believes life probably exists somewhere in outer space, but there is a simple reason why aliens have not visited earth - the journey is too tough.

"We have seen some evidence that there is a possibility of some life on Mars in the past, so there is probably life all over the universe," astronaut Mark Kelly told a news conference in Tokyo on Tuesday, where he was joined by other members of the Discovery crew.

"From our experience, it is very difficult to travel through space, and I personally think aliens have not visited our planet."

The Discovery delivered Japan's Kibo orbital laboratory to the International Space Station in June. Kelly described the $1 billion (502 million pound), 32-ton module as a "Lexus of a space station" where everything worked perfectly.

"Certainly like a Japanese car, Kibo was very well-made," he said. "It is going to be the premier laboratory of the space station for many years to come."

Japan, the last of the 16-nation partnership to get its hardware in space, is expected to complete the three-part lab next year.

Japanese astronaut Akihiko Hoshide - whose name means "Go to the stars" - was part of the eight-member team.

"Fourteen days was a short time," Hoshide told the same news conference. "I wish I could have stayed longer."

During their mission, the crew successfully conducted three spacewalks to hook up the new lab, work on the station's cooling system and fix a problem that was hampering a pair of solar wing panels from tracking the sun for power.

Back on earth, some are more interested in the possible existence of alien life than the lab's scientific experiments.

Kelly's comments add to a lively Japanese debate over aliens and UFOs. Japanese politicians discussed the possible existence of flying saucers late last year after an opposition lawmaker brought up the topic in parliament. Chief Cabinet Secretary Nobutaka Machimura said he personally believed in UFOs, but Prime Minister Yasuo Fukuda was more cautious, saying their existence had yet to be confirmed.