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.