Other anatomical changes associated with long-duration space flight are definitely negative: the immune system weakens, the heart shrinks because it doesn't have to strain against gravity, eyesight tends to degrade, sometimes markedly (no one's exactly sure why yet). The spine lengthens as the little sacs of fluid between the vertebrae expand, and bone mass decreases as the body sheds calcium. Without gravity, we don't need muscle and bone mass to support our own weight, which is what makes life in space so much fun but also so inherently bad for the human body, long-term.

If you haven't learned to ride a bike by the time your peer group has, then suddenly it's an embarrassment and you'll avoid opportunities where you're expected to ride a bike. And then it starts shaping your behaviour. Reading is much subtler, but much more destructive if you have not - for whatever reason - learned to read by the time you should.

Still, I also know that most people, including me, tend to applaud the wrong things: the showy, dramatic record-setting sprint rather than the years of dogged preparation or the unwavering grace displayed during a string of losses. Applause, then, never bore much relation to the reality of my life as an astronaut, which was not all about, or even mostly about, flying around in space.

Almost everything worthwhile carries with it some sort of risk.

So without that Canadian invention we were grounded. And so that was a really important and key part of the mission and Canadians should take real pride in it.

Life off Earth is in two important respects not at all unworldly: you can choose to focus on the surprises and pleasures, or the frustrations. And you can choose to appreciate the smallest scraps of experience, the everyday moments, or to value only the grandest, most stirring ones.

For the last several years and culminating in six months in orbit next year, I've been training for my third space flight. This one is almost in a category completely different than the previous two, specifically to live in on the space station for six months, to command a space ship and to fly a new rocket ship.