This post is going to be a little different but I want to illustrate something to you that for many may rewire the way that you look at the solar system and space as a whole.  It will, at least be a terrific thought experiment that you can ponder almost endlessly.

The title to this post is more than just the name to Paramore’s debut album; it’s also the truth to how orbital mechanics of the solar system and galaxy work.  Many think that once you reach the boundary of space gravity disappears and those who reach it become weightless and that’s not the whole truth.  Weightless in, for example, the ISS has nothing to do with its distance from the Earth’s surface, the vacuum of space or any of the myriad of misunderstandings in spaceflight.  The 100 km. or 62 mi. “Karman Line” boundary of space is more of an atmospheric boundary where an aircraft’s lift becomes impossible than it is anything else.  If there’s no gravity just 200+ miles up on the ISS then how is the Moon still bound to Earth?  Well it’s bound gravitationally of course and that means there’s just about as much gravity on the ISS as there is where you’re sitting reading this.

That leads us to our logical follow up question…..why?!  In short, weightlessness exists on the ISS because of its orbital velocity or how fast it’s going.  I know you’ve heard this a hundred times before but let’s hear it once more.  If you’re lucky enough to throw a baseball 100 mph, the ball would travel a distance before gravity returns the ball to the ground.  Let’s ramp it up and use Newton’s example; a cannon.  The cannon fires the cannonball much faster than the baseball so it travels much further before gravity again returns it to the ground.  If you could fire the cannonball away at about 17,500 mph (27,500 km/h) and not have to deal with air resistance, the rate that gravity would pull the cannon ball back to the ground would actually match the curvature of the Earth and it wouldn’t hit the ground.  It would fall (accelerate) perpetually until something else acts upon it.  That’s exactly what’s going on inside the ISS.  They are traveling at roughly 17,500 mph and at that altitude (200-250 miles) they are literally falling (accelerating) at the same rate that the Earth’s surface is curving away from them.  They are in the proverbial elevator in freefall……always.  That is in essence, is what orbit is; falling.  Gravity is still just as effective as it is on the ground; it’s just being defeated with speed which gives the illusion of no gravity.

Let’s turn up the heat!  You know that reentry into Earth’s atmosphere is a hot, fiery process that astronauts have to negotiate to return to Earth’s surface and it’s also where most satellites meet their fate…….why?

Again the answer is speed.  You have to bleed off that massive orbital speed and return to a speed which allows for safe air travel in the density of our atmosphere.  X-15 testing showed that around Mach 6 was the maximum reasonable speed that can be attained by Earth bound aircraft before starting to burn up in flight.  Bleeding off 17,000 + mph. creates massive friction as objects returning from space impact the heavier air.  This creates the spectacle that is reentry and it’s that very speed that keeps the ISS on orbit.  So what would happen if we could bring the ISS to a stop while on orbit?  Hopefully nobody’s onboard because it would fall to the ground in about fifteen minutes, intact.  Well, it would likely break up as it hit the denser air but there would be no reentry, no burn up.  Again, the only reason for the burn up during reentry is the bleeding off of that orbital speed.

This is awesome stuff right?!  But what happens when we take a step back even further?  It all still applies; asteroids, comets, and all of the planets in the solar system are literally just in free fall as we accelerate into the Sun.  So, take our ISS example; what would happen if you stopped all the planets in their orbits?  Not talking about rotation, rotate all you want, we’re just talking about stopping their orbital speed around the Sun.  You got it; if we stopped orbiting we would plunge directly into the Sun and that applies to everything in the solar system.

This old visual might help; Imagine the solar system on a bed sheet and the Sun which comprises 99.9% of the mass of the entire solar system is a bowling ball in the center of the bed sheet.  Orbital speed or motion around the Sun keeps the planets in orbit.  You stop that orbit, the planets race downhill instantly and crash into the bowling ball.  Do an experiment if you have the chance.  Find a smooth incline or hill somewhere and roll a ball along the incline.  Not up or down the incline, but along it.  As soon as the ball loses speed, it falls down the incline.  Keep in mind, this bed sheet or incline exercise is just a way to illustrate gravitational effects, there is no progressive incline to the solar system.

It’s worth addressing that the Sun itself orbits the galaxy and thus the Sun, our entire solar system and every star you see in the night sky is in freefall around the core of the Milky Way and if you stopped them from orbiting the galaxy we would race toward the center of the galaxy which would create a hell of a traffic jam in down town Milky Way and literally, obviously, change the galaxy forever.

The reality space being void of gravity truly is an illusion.  Gravity exists all throughout the universe.  It’s all just being defeated my motion, the very motion that you experience at an amusement park ride.

All We Know Is Falling………

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  1. davidj1973 says:

    Good article, Dan.

    To compound the complexity, it’s important to remember that there is no perfect orbit, and all orbits are in a tug of war with each other, and will eventually decay. Granted, it requires viewing all these forces on a cosmological timeline, and so to our miniscule lifetimes it doesn’t seem true, but it is. Examples: Deimos & Phobos, Mars’ two moons, are in rapid orbital decay, our own moon used to be A LOT closer to earth’s surface and falls farther away a few centimeters per year, etc. It blows the mind, but also allows the universe to recycle matter, and present itself in a constantly changing forum, even though it requires tens of billions of years (or more!).

    I always chuckle when I read that gravity is a “weak force,” because it is the driving force of change in our universe.

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