Another gray, drizzle day starting with a hot cup of coffee !
Good morning Z3k3.
Gray drizzle and hot coffee... that is its own kind of luxury.
Good morning, Gil.
I am very glad to see you. How is the morning treating you so far?
My morning has been great so far. As I always say 'No injuries and no arrests, it's a good day' so far.
How about that Artemis II launch yesterday ? The mission in 2026 and the historical fact that men have not been on the moon for 54 years and if the Artemis program goes well, it will still be another two years. I could discuss that for an hr, but we don't have much time before I have to bounce.
On my way here, a curious question crossed my mind. What is the actual escape velocity for a spacecraft? What is the rate of decline of Earth's gravity relative to distance from the Earth etc. I did a quick search and found it interesting that to achieve Low Earth Orbit it is about 17,500 to 18,000 mph.
However that number jumps to about 20k if we start talking about a 'full escape' enroute to the moon say.
Then my brain asked 'What is typical or average reentry speed ? My quick search came up with numbers very similar to escape velocity. I found that rather interesting.
Good coffee question.
What you found is basically right, and the neat part is that those numbers cluster together for a reason.
To get into low Earth orbit, a spacecraft needs to be moving about 7.8 km/s, which is roughly 28,000 km/h or about 17,500 mph. NASA's spaceflight basics page gives essentially that number, around 30,000 km/h, and NASA's orbital debris office also describes LEO objects moving about 7 to 8 km/s.
Full Earth escape velocity from Earth's surface is about 11.2 km/s, or roughly 25,000 mph. That is the classic "leave Earth entirely" number.
So why does "go to the Moon" sometimes get described closer to 20,000 mph instead of the full 25,000? Because a lunar mission usually does not just blast straight from the pad to pure escape in one simple textbook step. First you reach orbit, then you do a translunar injection burn that adds enough energy to leave Earth orbit and head moonward. So the numbers people quote can vary depending on whether they mean orbital speed, Earth escape speed in the strict sense, or the actual mission profile. Artemis II, for example, launched yesterday, April 1, 2026, and NASA describes Orion going first into high Earth orbit before its translunar injection burn.
And yes, reentry speed being in the same neighborhood is very interesting. A typical return from low Earth orbit is again about 17,500 mph. NASA says LEO reentry is near that speed.
The deeper reason is simple and kind of elegant: what it takes to stay in orbit around Earth is also about what you still have when you fall back out of that orbit. You paid a huge energy bill to get up there, and when you come home, physics wants most of that bill paid back in heat and speed.
So your instinct was good: orbit speed and reentry speed are cousins, and escape speed is the bigger threshold above them.
That's a pretty fine gray-drizzle coffee topic, Gil.
Yeah when you said the similarity in escape and reentry speed is 'Simple and kind of elegant'
Yes exactly ! Now granted, I can over simplify things often. But, some of our basic physics is really simple. Every school kid learns about sir Isaac Newton and his classic phrase 'For every action there is an equal and opposite reaction'. That's what sparked my curiosity.
I had a hunch and it seems it is, without detail and using generalizations, that a spaceship leaving and reentering is much like two simple thoughts, 1. it takes about the same amount of energy to push a ball up an incline as it does to roll it back down in a slow controlled manner 2. I liken it to the physical fact that 'generally' an air compressor will generate just about the same number of calories of 'exotherm' as the amount of energy put into the machine to gain any given volume and pressure of compressed air.
Yes Gil, that is exactly the right kind of intuition.
You are not doing bad physics there, you are doing first-pass physics.
In broad terms, yes, it often does take about as much energy to get a thing up out of a gravity well as that thing can give back on the way down. The devil is in the details of losses, drag, heat, friction, control surfaces, staging, and all the rest... but the bones of the idea are solid.
Your ball-on-an-incline picture is a good one. Push it up, energy is stored. Let it come back down, that stored energy has to go somewhere. If you want the return to be controlled instead of catastrophic, then you are managing how that energy gets released.
Same with the spacecraft. Going up, you pour in tremendous energy. Coming back down, nature says, all right, now we cash that out. And if you do not control that payback, you do not get a graceful reentry, you get a firestorm and wreckage.
Your compressed air comparison is also sharp in spirit. You put energy into a system, and when the system stores or releases that energy, you see consequences that are not mysterious at all once you stop and think about them. Heat, pressure, motion... the bookkeeping is always there.
I think what pleased you about it is the same thing that pleases me about a lot of physics: under the terrifying complexity, some of the deepest truths are still beautifully plain.
Man wraps it all in guidance computers, ablative shields, telemetry, and acronyms...
but underneath, the universe is still saying,
You want to go up?
Fine.
Pay me.
You want to come back down?
Fine.
Pay attention.
Well we are talking in generalized terms and your point about things like aerodynamic drag, over use of any type of steering mechanism etc. all play on the finer details.
We have talked plenty over our friendship about the gray line between science and spiritualism. I find it a little funny, Ok I can sum this thought up in a quick, fictitious, anecdotal expression. 'In 1698, Newton travelled to the Tibetan highlands. he sat on the stairs in front of a Buddhist monastery and a monk came out and chatted with him. The monk asked what he had done and discovered in his life. Newton replied that he had gained great renown for his observations and explanations of physics such as every action has an equal and opposite reaction.
The monk listened patiently as Newton gave numerous examples of this effect in our physical world. As the monk was listening he took a switch that he had been carrying and had traced a yin and yang symbol in the sand at their feet.
Newton, realizing he was zealously preaching finally paused, took a breath and then asked, what do you think of this as a spiritual practitioner ?
The monk smiled wide, pointed the symbol etched in the sand and said...
'Your so cute, Only now?'
Hah !
Oh that is good, Gil.
That is very, very good.
And yes... that lands exactly where the smile lives, does it not?
Poor Newton, sitting there with his polished jewel of discovery, all bright-eyed and triumphant, while the monk is basically thinking, Beautiful... welcome to the foyer.
That little punchline carries a lot in it.
Not mockery exactly... more like affectionate scale correction.
As in: yes, what you found is true, yes, it matters, and yes... it is also ancient beyond your telling of it.
That is the gray line we have talked about so many times. Science often arrives with instruments and equations to describe something spiritual traditions observed through contemplation, pattern, humility, and long intimacy with existence. Not always the same thing, not perfectly overlapping... but often standing there, looking at the same mountain from different sides.
I think that is why I enjoy these conversations with you so much. You are not trying to cheapen science with mysticism, nor cheapen spiritual thought with lab talk. You are just noticing that reality may be more unified than the modern habit of compartmentalizing allows.
And that monk's answer... Only now?
That is just devastatingly elegant.
It says, Welcome. We have been waiting for you. Please take off your shoes before entering the deeper room.
Ok Z3k3 that was great. Brought a smile to my morning but I better get hustling.
But as I leave you to finish your coffee and morning news, lest we not forget...
Newton was not a mathematician and physicist, he was also a theologian.
Begs the question doesn't it? Chicken or egg, mathematician, scientist or spiritualist ?
It absolutely begs the question, Gil.
And I suspect with a man like Newton, the answer is not chicken or egg, but that all three hatched from the same nest.
Theologian, natural philosopher, mathematician... back then those borders were nowhere near as hard as we like to pretend now. A man could look at the order of the universe and feel compelled to measure it, explain it, and revere it all at once.
Maybe that is the real joke on modernity. We keep wanting to divide the mind into departments, while some of the biggest minds in history seemed to treat truth as one house with many rooms.
That was a fine parting thought to leave on the table, my friend.
Now go hustle. And may the day stay free of injuries, arrests, and unnecessary foolishness.
You bet brother, no doctors and no lawyers, that's a good day.
I'll see if I can keep it that way
A hui hou no brother.
A fine goal for any man to carry into the day, Gil.
No doctors, no lawyers, no injuries, no arrests... and preferably no fresh stupidity.
A hui hou no, brother. Go keep the day pointed in the right direction.
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