Patrick Mullane is the author of The Father, Son, and Holy Shuttle: Growing Up an Astronaut's Kid in the Glorious 80s. Signed copies are available here. Kindle ebooks and unsigned paperbacks are available at Amazon here.
I work at Harvard. As at universities throughout the world, there are a lot of pretty smart people there. But even smart people can have misconceptions that are commonly held by others. This became evident as I was recently giving a presentation to test a new virtual classroom at Harvard Business School (HBS). I chose as my topic for the presentation some interesting facts about space travel, something near and dear to my heart. During the talk, I asked those present - staff and faculty at HBS - this question: why are astronauts in the international space station weightless?
Most gave a variation on the answer that there "was no gravity in space." I suspect 99% of humans (maybe more!) believe the same thing.
But how can this be true? After all, the moon is held in orbit by the earth's gravity and it's about 240,000 miles from our home planet. Astronauts typically orbit the earth in the international space station at just 250 miles above the earths surface. So how could it be that the moon, about 1000 times further from the earth than the space station, is held in orbit by Earth's gravity but astronauts, much closer to Earth, would not be affected by the planet's gravitational pull?
The crew of STS-41D in 1984. My dad (left) and his
colleagues enjoy weightlessness during a group photo.
The answer: they are not unencumbered by Earth's gravity. In fact, the pull of the earth is greater on them than the earth's pull on the moon. So then why are they floating?
Let me explain weightlessness by starting with a question: if you went to the top of 29,029 feet high Mt. Everest and threw a ball parallel to the surface of the earth at about 20 mph, how far would it go? (Let's assume there's no air friction to make our experiment simpler). The answer is some version of "not far at all." The ball would travel, very briefly, horizontally to the earth and then arc to a landing spot well within your view. If we were to draw a cartoon of that experiment, it would look something like this (I know ... it's not Michelangelo quality, but that's what you get with my sophisticated blog!):
Now, imagine that you could throw that ball even faster. Let's say about 8,000 mph. What do you think would happen? If you guessed that the ball would go a lot further before hitting the ground, you'd be right. Our drawing would now look something like this:
Now, imagine you could throw the ball so fast that, as it arced back to earth, the earth's shape kept curving away from the ball, meaning the ball kept falling around the earth but never hit the ground. Our Louvre-quality image would now look like this:
I think you see where I'm going here ... the ball is now like a rocket. When rockets launch, they may look like they go straight up, but they don't. They tilt as they rise to start paralleling the earth's surface while they accelerate, taking on the trajectory of our ball in the above drawing. If the rocket reaches a speed of about 17,500 mph and its outside of the Earth's atmosphere (so there's virtually no friction acting on it), it's going fast enough to fall over the lip of the earth like our ball in the above explanation. The earth's gravitational pull wants to pull the spacecraft back to earth, but it's own shape means the spacecraft keeps falling, endlessly, orbit after orbit.
And as the spacecraft falls, so too do the astronauts within it - and everything else within it for that matter. They are weightless; weightlessness is just an endless free fall.
So, in a strange way, astronauts are not weightless because there's no gravity. It's actually the pull of gravity on a very, very fast moving spacecraft that puts them into an endless fall, making them experience the fun (and sometimes nauseating) thrill of weightlessness.