He has flown jet fighters, travelled on the space shuttle and commanded the International Space Station. But nothing seemed to intimidate Canadian astronaut Chris Hadfield quite as much as standing next to renowned physicist Stephen Hawking.

"Let me just say how scared I am," Mr. Hadfield told a crowd of 300 people at the Canadian High Commission Thursday evening where he gave a speech and fielded a couple of questions from Mr. Hawking. "I hope the question includes part of the answer."

Mr. Hawking, who has devoted much of his research to the study of the cosmos, is a keen devotee of space travel and he has spoken about his interest in flying into space one day. On Thursday he asked Mr. Hadfield about the long-terms effects of weightlessness on trip to Mars and whether the Canadian believed the solar system would ever be colonized.

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Mr. Hadfield gave a long answer to the Mars question, saying that if the trip could be done in a month the effects could be managed. However, based on current technology it would take about six months to get there which would have a significant impact on the body and mind. The biggest challenge, he added, would be coping with the isolation. "How do you keep your crew from going crazy?" he asked. "Within a month or so you won't be able to have a real-time conversation ever again with Earth, the delay [on a call] will be so long…So that crew within weeks will become Martians psychologically, they will no longer be of Earth."

As for colonization, Mr. Hadfield, 54, said he believes the solar system will be colonized one day but only when there is a real need to do it. And he is convinced there will be some form of colonization of the Moon within his lifetime.

With Mr. Hawking looking on, Mr. Hadfield also sang David Bowie's Space Oddity, which he famously sung during his stint on the space station. And he took other questions from the audience, including what happens when someone sneezes in space? Not much, was the answer.

In an interview afterward, Mr. Hadfield, who has retired from the Canadian Space Agency, said Canada continues to play an important role in space research. "Canada is involved right across the board," he said. "China just landed on the moon. India just went to Mars so there is huge opportunity for Canada."

And he said young people are still showing a keen interest in science, math and space travel. "They are optimistic. They want to do things," he said. "We are one of the leading countries in the world in science and math. We do a wonderful job. We can do it better, sure. We sure could do it a lot worse."

It was clear that meeting Mr. Hawking, 71, had an impact on Mr. Hadfield. The two took a picture together and Mr. Hadfield said he was honoured to meet the physicist.

Later as Mr. Hadfield signed copies of his book, An Astronaut's Guide to Life on Earth, Mr. Hawking posed for pictures and answered a few questions with the help of his long-time assistant, Judith Croasdell.

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Ms. Croasdell said Mr. Hawking had followed Mr. Hadfield's journey on the space station and was eager to meet him. "We actually reject an awful lot of invitations but this was one he wanted to do," she said adding that Mr. Hawking continues to do research work at Cambridge University where he heads of the Centre for Theoretical Cosmology. "He did enjoy it. I'm driving him home tonight so we'll hear all about it."