The physics are fine. Once you have 1G Torchships, orbital mechanics becomes very irrelevant.

You basically take straight line paths between to points in space, constantly accelerating for half the journey then flipping over at the half way point to decelerate for the rest.

Earth to Mars at closest takes 46 hours at 1G. Earth to Jupiter at closest takes less than 6 days. With 1G torch ships, the entire solar systems becomes accessible in less than two weeks journey. And the Expanse has ships that have much more acceleration.

It's fun to estimate how hot the exhaust of a torch ship must be, and how much mass should it expend on constant 1G sustained for days.

Rockets exchange impulse with the exhaust at a linear rate (mv), but put the kinetic energy into it at a quadratic rate (mv²/2), so increasing mass efficiency by increasing exhaust speed / temperature costs more and more. Realistic with fusion reactors, to the extent to which compact fusion reactors themselves are realistic. Or even with fission reactors, in the form of bombs (see project Orion).

Nothing like the current simple, low-tech chemical rockets.

There's an interesting speculation as to how an Epstein Drive could work in reality. The heat management problem is pretty central:

http://toughsf.blogspot.com/2019/10/the-expanses-epstein-dri...

The discussion at the bottom of the page has also a lot of interesting ideas.

It's a very interesting discussion, thanks!

I can't describe the feelings I feel in the last couple of weeks. It really feels like we are living in a (dystopian) future. Elon Musk is working on a brain-chip, people are wearing robot-arms and legs, space-travel gets faster and easier year by year etc.

Just crazy, I am stoked to see what new technologies and possibilities will arise in the next few decades.

The thing with the prosthetics is that I feel like there's hundreds, if not thousands of companies and individuals working on them, but there doesn't seem to be any convergence, or more importantly, mass production - meaning that prosthetics remain expensive and made-to-measure. This is a thing with a lot of things that look very high-tech or science fiction.

That said, video calling was the realm of exclusive, high-tech / demos for a long time, then without really being aware of, it's been available in everyone's pocket for a long time now. It went from near-sci-fi to mainstream in the blink of an eye.

Prosthetics haven't solved the bandwidth problem i.e. how do you connect them to the nervous system.

Well, Elon Musk's Neuralink is claiming they're close at least, but I could also imagine someone coming from the other direction entirely, and making a robot limb that was autonomous and worked with the wearer instead of being directly controlled (but the failure mode would be no fun...)

Sensory input is the other 50% of the game that remains unsolved. If Neuralink's robot works it will be a game changer for artificial eyes, limbs, everything.

Besides that a robot limb can't be autonomous in it's movement as most movements we do are asymmetrical and thus it needs input from the other limb as well.

I watch with grim satisfaction how well the cyberpunk books predicted our present and near future from 30+ years ago. (Satisfaction, because what's coming looks familiar, we've been warned.)

I only started reading and watching Cyberpunk related things about a month ago, so it's interesting to see how far we already are.

Why can there not be stealth in space? The link you gave doesn't seem to have any arguments. Not saying you're wrong, just curious.

Some simple arguments that it's not a ridiculous question:

There is stealth on earth - paint and geometry that reduces EM signature. Why couldn't there be something similar in space?

It's not like it's super easy to detect everything in space today either - we have big telescopes but regularly miss small asteroids?

Because your spaceship has to generate heat for various systems - at the very least, to keep its occupants alive. That heat, unlike on Earth, really stands out against the cold of space. You can hide the emissions of many vehicles on Earth, because Earth and its atmosphere is pretty warm compared to space in the first place. But in space all you need to do is look for heat signatures(infrared and others) and you will spot any vessel immediately.

Some Sci-Fi has tried addressing it with plausible ideas - like for instance dumping all heat into shielded internal tanks, which have to eventually be vented(so that stealth cannot be infinite because you run out of heat storage capacity on board).

Right - or maybe directionally radiating it away in a direction your adversary can't observe?

The James Webb space telescope cools it's observing side instruments to 10s of degrees above absolute zero, using active cooling. I wonder how hard it would be to detect from it's cool side, at a space combat distance?

Asteroids in the near solar system are normally 100-150 kelvin, apparently, and don't seem trivial to spot.

All that still seems to allow room for stealth in a hard sci-fi setting.

There's other problems. Even if you contain your heat or vent it from a different side, you still need to produce hot exhaust to accelerate. If you don't, your position can be inferred from your last known position/acceleration.

This and other points are discussed here: https://tvtropes.org/pmwiki/pmwiki.php/Analysis/StealthInSpa...

In the realm of hard sci-fi the detectability of an exhaust plume depends on a variety of factors including the ionization, temperature, shape, velocity, and angle of the plume wrt adversary, and the local medium.

There are a number of methods one could theoretically use to minimize these factors. As an example, imagine the detectability of a small angle coherent neutron beam or the IR exhaust of a black hole starship.

https://en.wikipedia.org/wiki/Black_hole_starship

In theory, you could fix the hot exhaust problem with a mass driver.

My personal fantasy tech for subspace is that it functions as an absolute zero heat sink. So heat engines can use absolute zero as the ultimate heat sink to build a Carnot contradiction: They don't actually sink any heat at all, and can extract all of the heat energy as work. Quantum mechanics forces them to actually sink a tiny amount of heat proportional to process entropy. Those tiny fluctuations are the "subspace signatures" that get detected anyway.

Its still more fantasy than science, but hey, so are warp drive, hyperspace, and all that jazz :)

I imagine it'd still be like stealth on Earth, but at much greater distances. I can always see a stealthy spy plane when standing next to it in a hangar. But it's purpose is to fly high above the ground.

If heat signatures are a sure way to identify space ships, then the value of stealth comes in at the limit of detection range. If an enemy can only detect me at X units away, but I can detect him at X+1 units, I have an advantage.

If I'm spouting nonsense, I'd be legitimately delighted to hear why it can't work.

this is more or less consistent with everything I've read on the topic, but X can be a surprisingly long distance (billions of km under thrust, or millions of km in "cold running" mode). you would be detectable from pretty much anywhere in the solar system while burning, and still quite a long way away in "stealth mode". even if you had really effective stealth in cold mode, adversaries that observed you while burning could accurately predict your trajectory.

X+1 detection range isn't much of an advantage if X already includes most of the places you can actually be and/or X is already an order of magnitude past effective weapons range.

There is a relatively famous and long blog about this: http://www.projectrho.com/public_html/rocket/spacewardetect....

IIRC in the series they turn off everything to stop emitting anything that can be detected - mostly IR radiation, I would guess.

The rest of stealth in space is relying on the vastness of it. A spaceship a few kilometers away is already hard to see.

IIRC, they don't turn everything off. they turn off the reactor and run the ship's systems off of battery power. against a nearly 0 K thermal backdrop, there's no way an object like this could completely evade detection.

they do actually address this in the show. you can infer from the dialogue that they don't expect to be "invisible"; rather, they are trying to be indistinguishable from space junk.

I do think people are being a bit harsh in this thread. the show isn't perfect, but it's about as hard sci-fi as you can be while still getting renewed each season.

> None of the orbits of the planets or the space ships follow realistic path.

I noticed that, but it is obviously a necessity for the TV version so that the plotlines are not interrupted. I’ll allow it

Well said. I recently got into a "the Expanse is not hard sci-fi" discussion, and the politics of space (between distant planets) was an interesting angle we hadn't considered.