03/Jun/2026 - Space Science Chats With AI

I need to properly make a new blog post as it’s been a while, but I got sidetracked with questions about space, speed, g-forces, and fixed points. I present to you, my prompts to Claude’s Opus 4.8 model, and its answers.

Science chat!

Ok, the earth is spinning, and orbiting the sun. The sun is moving around the the milky way’s galactic center. The milky way is orbiting - something. If you could somehow define a FIXED point in space - how fast am I, as a human on earth, moving per second way from that fixed point.

Right, this is a cracking question and the answer involves some beautiful layers of physics. Let me break it all down.

The “Fixed Point” Problem

First, the philosophical snag: there is no universally agreed fixed point in space. The universe has no centre, no anchor. The best we can do is use the Cosmic Microwave Background (CMB) as a reference frame - the afterglow of the Big Bang, which permeates the entire universe and is effectively the closest thing we have to an absolute frame.

Using that as your fixed point:

The Stack of Velocities

But here’s the critical bit: you cannot just add these numbers. They’re all vectors pointing in different directions, and some partially cancel depending on time of year and galactic geometry.

The Bottom Line

The Sun’s total velocity relative to the Cosmic Microwave Background, accounting for all contributions, is approximately 370 km/s.

Your rotation contribution at 52°N is tiny (~0.29 km/s), so realistically you’re sitting at somewhere in the ballpark of ~370 km/s relative to the CMB.

That’s 370 kilometres every single second. About 1.3 million km/h.

The Fun Wrinkle

Your cumulative speed through the Milky Way alone can vary from as little as 208 km/s to as much as 237 km/s depending on whether Earth is travelling in the same direction as the Solar System moves through the galaxy or the opposite direction - and this changes over the course of a year.

So your speed is literally seasonal. You’re going faster in some months than others, relative to the galaxy.

The Rabbit Hole Gets Deeper

The Milky Way itself is being gravitationally dragged toward the Great Attractor (a massive overdensity of matter about 250 million light-years away), and beyond that the Shapley Supercluster. That’s baked into the 370 km/s figure.

At the absolute largest scale? The entire observable universe is expanding in every direction. “Moving away from a fixed point” starts to become a philosophically broken concept - spacetime itself is stretching, not objects moving through space.

Einstein sends his regards. The universe is deeply weird and there’s no such thing as standing still.

When they’re going to the moon, roughly how fast per second are they moving?

Great follow-up, because the contrast is stark.

Apollo to the Moon

The rough speeds during a lunar mission:

The Counterintuitive Bit

The cruise phase slows down dramatically because Earth’s gravity is still pulling the spacecraft back the whole way. Think of throwing a ball upward - it slows as it climbs. Same principle, just over 384,000 km.

At the L1 Lagrange point - roughly 90% of the way to the Moon where lunar gravity starts winning - the spacecraft is at its slowest, around 0.8-1 km/s, before the Moon’s gravity starts pulling it back up to speed.

The Punchline vs Your Previous Question

At ~10 km/s peak velocity, the Apollo spacecraft was moving at roughly 1/37th of your speed relative to the CMB.

You, sitting completely still in your chair right now, are moving ~370 km/s relative to the CMB. The Apollo astronauts burning their engines as hard as they could managed about 2.7% of that.

Rocket technology is simultaneously one of humanity’s greatest achievements and a humbling reminder of how cosmically slow we actually are. We’re not going anywhere fast. ✨

We had a chat a few weeks ago, but I can’t seem to find it - about how, if I had a VERY good rocket and an indestructible space suit, and I wanted to go directly opposite of the earth’s orbit until my my rotational speed around the sun was zero, how long would it take to fall into the sun.

Now, what I can’t remember was, what speed do I need to get the rocket to - relative to earth - to do that? And, do we have rockets capable of that speed?

Got it - found the conversation. The answer was actually already in there. Let me pull out the exact number for you.

The Speed You Need

From our previous chat, the key figure was buried in the detail:

To come to rest relative to the Sun you need to cancel Earth’s orbital velocity of 29.78 km/s. The actual Δv requirement, accounting for also escaping Earth’s own gravity well, works out to roughly √(29.78² + 11.2²) ≈ 31.8 km/s relative to Earth, fired retrograde (directly opposite Earth’s orbital direction).

So roughly ~30 km/s is the headline number - that’s what you need to zero out your solar orbital velocity. The extra ~2 km/s is the cost of climbing out of Earth’s own gravity well first.

Can We Do That?

Short answer: not even close.

The Saturn V + Apollo stack achieved about ~15 km/s of total Δv, and most of that was spent just getting to LEO (~9 km/s). The trans-lunar injection burn was only about 3 km/s on top of that.

The fastest human-made object ever launched, the Parker Solar Probe, reaches around 163 km/s relative to the Sun - but that’s with the Sun’s gravity doing the accelerating after multiple Venus gravity assists. Its launch Δv was nowhere near that.

To actually cancel 30 km/s of velocity requires an absolutely brutal amount of fuel under the Tsiolkovsky rocket equation - propellant requirements scale exponentially with Δv. You’d need a rocket so large it essentially can’t exist with chemical propellants.

The Irony

It is genuinely harder to hit the Sun than to escape the solar system entirely. Escape needs ~42 km/s from Earth’s surface, but you’re already moving at 29.78 km/s in the right direction - so you only need to add ~12 km/s. To fall into the Sun you need to cancel that 29.78 km/s you already have. Working against your own momentum is brutal.

NASA has noted this exact problem. The Parker Solar Probe doesn’t even try to go straight in - it uses seven Venus flybys over years to bleed off velocity gradually, because a direct deceleration burn is basically impossible with current tech.

And also remind me, because I though it was cool: Let’s say I could come to that dead “stop” in relation to the sun - I’d stop orbitting. I remember you said I’d fall for about 64 days. How fast would I be moving by the time I hit the sun’s atmosphere? And what was the g-force I’d hit on impact?

From the previous chat, these numbers were in there:

Impact Velocity

After 64.5 days falling, your velocity would be ~617 km/s when you hit the solar surface.

The elegant reason: that’s the Sun’s surface escape velocity. By conservation of energy, falling into a gravity well from effectively infinity accelerates you to exactly the same speed as escaping it would require. 1 AU is close enough to “infinity” for this purpose that the maths works out cleanly.

For context: that’s about 0.2% of the speed of light.

The G-Force on Impact

This is where I need to be honest - the previous chat covered the surface gravity (28x Earth’s, so your 90kg becomes effectively ~2,500kg standing on the surface), but the impact deceleration depends entirely on how quickly you stop, which depends on the density and depth of the material stopping you.

The honest answer: the number would be so catastrophically large it’s essentially meaningless.

You’re hitting tenuous plasma at 617 km/s. The photosphere is actually less dense than Earth’s atmosphere at the very top, but it deepens rapidly. You’d be decelerating from 617 km/s to zero over some very short distance. Even if it took 100km to stop you (generous), that deceleration would be roughly:

v² / 2d = (617,000)² / (2 × 100,000) ≈ ~1.9 billion m/s²

That’s about 190 million G.

For reference, a fighter pilot blacks out at ~9G. A bullet experiences ~100,000G. You’d be experiencing forces that make a bullet impact look like a gentle hug.

Your indestructible suit would survive. You would become a very thin layer of paste distributed uniformly throughout it.

As always, you can add my RSS feed to your reader of choice and if you made it this far thanks for reading!

Chris