Why do airplanes move through patches of weather, but a Space Station does not?

In orbital speed paradox we saw that height determines speed. At a given radius, staying in orbit takes a velocity determined by that radius. When you orbit the earth at at some height, you won’t pass through a space dust cloud, the way an airplane passes through rain clouds, because that dust cloud would be orbiting too. It could move right along with you, or in front of you, or behind you, but it’s not a thing you’d pass through.

Exceptions & Qualifications

This article looks at near circular orbits, not highly elliptical ones. Objects in intersecting orbits can certainly pass by each other. We see meteor showers when the earth passes through a dust trail of a comet. That dust trail consist of particles obeying the same orbital laws, in an elliptical orbit which intersects ours.
Likewise, objects might be at the same height orbiting in opposite directions. There are numerous exceptions to a rule that things at a given height travel together. A more precise phrasing: The objects we describe are all in Keplerian orbits. This article will explore the inherently different behavior between objects in Keplerian vs non-Keplerian orbits.

An Orbital Frame of Reference is boring.

Anything maintaining that same height needs the same velocity, hence immediate neighbors are near stationary relative to each other, all paying the same velocity price of admission.

Even the asteroid belt between Mars and Jupiter is dull. Firstly the asteroid belt is nothing like in sci-fi - no dodging or swerving needed. It’s close to empty, just a tiny bit less empty than the rest of the solar system.^[1]

But even if you were surrounded by space rocks in that field, they’d all hang motionless around you, once you joined that orbit.

Recap:

• Airplanes don’t orbit. They pass through weather, and can have any motion relative to each other.
• Space stations and the moon orbit earth. Things at a given height travel together
• Earth & planets & asteroids orbit the sun. Same.
• The Sun orbits the Milky Way Galaxy, and we go off the rails.

Orbits Around the Galaxy

Our neighbor stars seem to move randomly relative to each other.

	Name	Distance	Relative Velocity
Closest star	Alpha Centauri	4 ly	38 km/s
Fastest moving	Barnard’s Star	6 ly	140 km/s
Brightest	Sirius	8.6 ly	17.5 km/s

Putting these values in perspective, the Sun’s orbital velocity around the galaxy is around 230 km/s. These are not tiny variations relative to that. Shouldn’t they resemble neighbors traveling in a cohesive group, like the asteroid belt? Compared to an asteroid belt, the sun and its neighbors are zooming around in every direction, at any relative speed. Our sun does indeed pass through dust clouds, and regions of different space weather, albeit over hundreds of thousands of years.

It’s as if the rules of orbit don’t even apply.

Do the rules of orbits even apply?? Click here for the shocking answer!

No.

The rules used previously, below the scale of the galaxy, no longer apply. There are different rules here. To understand why this is different, we ask the question:

What are we actually orbiting?

You may have read that there is a supermassive black hole, named Sagittarius A** at the center of our galaxy. It’s a black hole and everyone knows that black hole gravity is special, and extra strong and spaghettifying.^[2] It’s not just a black hole, it’s massive; not just massive, it’s supermassive. It must have an iron grip on every object in the galaxy.

Does the sun orbit a SUPERMASSIVE Black Hole at the center of our galaxy?? Click here for the shocking answer!

No.

The answer is in the mass distribution. The Sun accounts for 99.86% of the mass in the solar system. Orbits in our solar system are shaped entirely by the sun’s gravity. This gravitational field is far simpler than that of our galaxy, which is diffuse, and less uniform. Sol is not orbiting a single thing, it’s following a path formed by a galaxy full of gravitational pulls. Despite the black hole and concentration of stars at the center, the Milky Way has nowhere near the mass concentration at its center. If Sagittarious A* disappeared, Sol’s orbit around the galaxy would hardly change.

Not only is the central mass of stars insignificant, even the summed gravity of all of the stars of the galaxy is a very small factor shaping orbits.

In part II we’ll explore how everything in the galaxy, in a sense, orbits Dark Matter.

* :* is pronounced “star”

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1. Suppose you want to clear out a flat patch of the asteroid belt the size of the continental US (For the new SpaceMall). On average you could drive around that area, picking up asteroids and throwing them in the back of your pickup, till the whole place was clean, and not fill the bed. ↩︎

2. No. It’s not. It’s exactly the same as other gravity, outside the event horizon. ↩︎