Terra stolen, replaced by new and improved Terra (WH40k/BT)

[Jedi Master Spock]

It's been a long while since I last posted a WH40k/BT thread, and a scenario occurred to me. By act of Q, the inhabited systems of the BT universe are used to replace the corresponding systems in the Warhammer 40,000 universe. This has the one-time side effect of temporarily "flattening" the Warp in the area, temporarily dislocating anything inside the Warp, or native to it, out of the affected ~1000 light year wide area. (They may return later, of course.)

This includes, most particularly, Holy Terra and everything in Sol system. Q is fond of the planet, and didn't like how much it had become polluted in the past few millenia, so he switched the solar system out for one that didn't smell as badly.

Ground rules:

• Daemons and other warp critters have absolutely no interaction with BT FTL drives.
• Q has not informed anybody of the change, and his involvement will not be explained via magical divination performed by any agent in the WH40kverse. Any attempts at divination will be met with the equivalent of elevator music and a "Please hold, your call is important to us."
• Yes, this includes Chaos Gods and Eldar.
• BT technology will continue to perform as reliably as it normally does despite having been transported to a different universe with radically different rules.
• If a system is replaced, everything in it is replaced - planets, orbital stations, and even any moving ships that happen to be inside the heliosphere.

What happens? And why?

[sonofccn]

How big territory wise are the various battletech forces? A quick google hinted at thousands of planets but nothing more exact.

Regardless Holy Terra has just been "destroyed" killing both the God-Emperor and the holy navigational beacon which is vital for "smooth" use of the warp. The remaining worlds of the Empire will quickly become fractured and form thier own private subempire as lawlessness and disorder abound. This will be enhanced and accelerated by all remaining space marines, those not "blipped" away, leaving to mount a holy crusade against the new Terra. The long and short of it is that the Imperium is effectivly destroyed ,but a new Human empire may rise from the ashes. Expect the next thousand odd years to be even bleaker than the preceeding one.

[Jedi Master Spock]

An answer and a question.

How big territory wise are the various battletech forces? A quick google hinted at thousands of planets but nothing more exact.

Sarna.net gives fairly precise figures.

The five "major" states of the Inner Sphere control hundreds of world each (200-500 or so) mostly in a 500 LY radius of Terra. Various minor powers control up to a hundred worlds or so. The population of the Inner Sphere is canonically several trillion. I think if all the minor powers and independent worlds were totalled up, you might have a total of ... maybe three thousand inhabited systems? (I get 1750 for the five biggest states according to the summaries there) - again, mostly in a 1000 LY sphere surrounding Terra. The density - and the individual populations of planets - drops off as you get further from Terra.

Regardless Holy Terra has just been "destroyed" killing both the God-Emperor and the holy navigational beacon which is vital for "smooth" use of the warp.

Would you go into a little more detail on how much of an effect that would have? I've read a few different things in other discussions, and I'm frankly a little puzzled.

On the one hand, you hear people say things like this (text lifted from Lexicanum):

Destroying the Astronomican would most likely halt all Imperial Warp travel within the galaxy, effectively bringing the Imperium to its knees.

On the other hand, clearly humanity was able to spread to the stars without the beacon, back in the Dark Age of Technology, and if I'm not mistaken, Navigators pre-date the existence of the beacon as well. So just how crippling would it be in practice?

[sonofccn]

An answer and a question

Thank you for the answer. I will attempt to digest the new data.

Would you go into a little more detail on how much of an effect that would have? I've read a few different things in other discussions, and I'm frankly a little puzzled.

I think 40k is a little puzzeled on its own. It isn't the most logical universe to ever exist. As far as I can tell the data is conflicting. From Lexicanum first

Two types of jump can be made, a calculated jump and a piloted jump.

Calculated jumps rely on the warp maintaining a constant stability during the jump, and short movements can be made without the need of a Navigator as the warp engines have a mechanical version built in. This is good enough for short jumps but on longer jumps it is too dangerous as they encounter changes in the currents of the warp.

Piloted jumps are much more efficient as they use the psychic powers of the Navigator. These special mutants can literally see the warp and make use of the Astronomican, guiding the ship from warp current to warp current. This allows for much safer and longer jumps.

Which suggests that warp travel is fine even without a psyker for local travel, how local isn't specified but logically indicates travel to nearby systems. However as you noted on the Astronmonical page we get the destroying the navigational beacon means destroying the Empire.

Now over at Warhammerwiki it states something fairly simliar to Lexicruniam

For jumps any longer than a handful of light-years, a member of the mutant psyker breed known as Navigators must be aboard. These mutant humans are able to 'see' the currents of the Immaterium, and are able to guide a ship to its intended destination with much greater accuracy than normal humans. However, for a Navigator to fulfil this role, he needs a reference point, which is provided by the Astronomican.

Which not only gives us a rough magnitude of distance travable without a pysker but makes it seem that a Navigator is only useful with the Astronomican. However Lexicranium has this to say on Navigator's origin:

The Navigator sub-species is so ancient their exact origins have been forgotten. It is known that their origins go back to the Dark Age of Technology, to a time of genetic experimentation when many kinds of mutants were engineered to fulfill roles envisaged by their creators

Which obviously predates the God-Emperor dying and being made into an astral beacon for humanity.

A quick search through some 40k forums found me the Tauonlineand this page which brings up your very question and tries to answer it. Unfortanly , as is the nature of forums, by the third post they have already gone of tangent talking about tyranids.

My best guess with the data I'm looking at is the person who wrote the lexicranium article was speaking in flowerly overtones. It won't end all warp jumping but reduce it to a crawl and would more or less sound the death knell for an already bloated and rotting empire.

In regards to the previously outlined scenario with warp travel reduced to microjumps and Holy Terra, Mar's shipyards, the Lords who run the empire and the rest of the higher tier goverment wiped out the Imperium will fracture into competing tiny empires, for a refrence think of medevial Europe, who will be hard pressed to survive much less lead a retake of the hundreds if not thousands of lost worlds the various BT interlopers just stole.

[Mr. Oragahn]

How big territory wise are the various battletech forces? A quick google hinted at thousands of planets but nothing more exact.

Regardless Holy Terra has just been "destroyed" killing both the God-Emperor and the holy navigational beacon which is vital for "smooth" use of the warp. The remaining worlds of the Empire will quickly become fractured and form thier own private subempire as lawlessness and disorder abound. This will be enhanced and accelerated by all remaining space marines, those not "blipped" away, leaving to mount a holy crusade against the new Terra. The long and short of it is that the Imperium is effectivly destroyed ,but a new Human empire may rise from the ashes. Expect the next thousand odd years to be even bleaker than the preceeding one.

Actually this could sanitize the Warp to a great extent.
If I get it right, the Warp is neither good nor bad, but a reflection of the state of the galaxy, the overall mass of emotions. A certain side of it had a tendency to grow in power and effect due to the growth of decadence of old species, and their pain.
I often wonder if in truth, if all was love in WH40K, if the Warp's demons wouldn't disappear, or morph, first into more neutrals creatures, chaos receding, and then turning into, or giving birth to the loose equivalent of angels or faeries (really).

One thing sure is that the disappearance of the crippled, dark and painful reign of the corpse Emperor is a two edged blade. It may help humanity more than people generally think and make all several factions independent, and thus force agreements when they overcome the limitations of their Immaterium drives, of they all fall into chaos and war.

The most crippled worlds, hiveworlds, and industrial worlds, will surely suffer a lot, since they heavily rely on resources from outside of their respective often depleted systems.

Many worlds will burn at will. The question is how long it takes for BT forces to make contact and offer something.
It's a very complicated situation, since at the same time, many Astartes chapters will attempt to make contact and return to Earth, with all the problem incumbent to the lack of the Astronomican and the destruction of the God-Emperor.
Many SM chapters may be completely lost about what to do, may fracture, may turn to Chaos or start something else entirely.

Then BT, with all its various opposed powers, is not exactly super stable either, but at least way calmer than the mess the Imperium was. That, and per the OP, the Warp has been quietened beyond the worlds of the periphery.

I suppose some lucky SM chapter may invade one of the BT worlds and perhaps start capturing tech, but nothing says they could really retroengineer it fast. It may happen after a while, assuming they managed to have the population's scientists work that way.
BT FTL drives may be slower, but they're probably far more reliable than the Warp drives.

What is also possible is the rise of several thousands of Space Marines uniting after years of travel to some point, leading a massive force towards Earth, probably losing several ships en route due to the hazards of the Warp.
The question being what will they do when they arrive?

As for BTL, the real problem is a question of numbers and logistics. Will they have what it takes to resist the Orks, Tyranids, other pirates and possible skirmishes with the Eldars, Dark Eldars and Necrons?

Add to that the fact that Chaos is only been temporarily been pushed back...

No, seriously, unless this sudden arrival of "niceness" (all relative) in WH40K doesn't neuterize Chaos a tad, I don't see what the gain is, safe pulling closer the date of humanity's final breath.

[Jedi Master Spock]

My best guess with the data I'm looking at is the person who wrote the lexicranium article was speaking in flowerly overtones. It won't end all warp jumping but reduce it to a crawl and would more or less sound the death knell for an already bloated and rotting empire.

I'll see if I can't dig something a little more specific up, but if anyone else can get some hard data in, that would be nice. I know we have a number of people here that know a good bit about WH40k.

It would be nice especially to have some hard numbers as to about how fast you can travel by making short-range jumps and re-orienting yourself outside the Warp, or going very shallowly into the Warp as the Tau do.

In regards to the previously outlined scenario with warp travel reduced to microjumps and Holy Terra, Mar's shipyards, the Lords who run the empire and the rest of the higher tier goverment wiped out the Imperium will fracture into competing tiny empires, for a refrence think of medevial Europe, who will be hard pressed to survive much less lead a retake of the hundreds if not thousands of lost worlds the various BT interlopers just stole.

Isn't there a significant amount of localized governance? As in, on the Segmentum level, down to sectors and sub-sectors? Segmentum Solar may be down to sector-level administrative centers, but the other four Segmentums have all their localized administrative apparatuses intact. The astropaths will still be around as well.

However, I'm going to play another card out on the table from the scenario I outlined in the OP, and it's not necessarily a very obvious one. The BT universe and the WH40k universe both are the sort in which humans mostly colonize Sun-like stars with planets around them. However, space is big. Apply some parameters from Drake's equation. Both universes use terraforming - Earth, Mars, and Titan are inhabited in WH40k according to Lexicanum, and Earth, Mars, and Venus are inhabited in BT according to Sarna.net.

They may not have the same terraforming techniques, either. Even if you're mostly leaning on stars about the size or larger than our Sun, and even if the same ones have the same planets in both universes, there's probably a pool of at least 30,000 systems near Sol that could have been colonized by BT or WH40k humans. That means that occupied BT systems are about 10% of those.

Space is big, I repeat. Big. So barring a handful that we can actually identify specifically, such as Terra, we're likely to see only around 10% of WH40k's closest systems replaced. If the Segmentum Solar was a disk 10,000 light years in diameter, the affected area would represent 0.03% of the Segmentum. In other words, if inhabited systems were roughly evenly distributed throughout the Segmentum, and there were a million systems just in Segmentum Solar, we would expect to see all of 33 IoM systems overlapped and replaced by Q. There could easily be as few as a half dozen affected systems.

[sonofccn]

I'll see if I can't dig something a little more specific up, but if anyone else can get some hard data in, that would be nice. I know we have a number of people here that know a good bit about WH40k.

I'll also keep an eye out but I'm not a big 40k fan as is obviously evident.

Isn't there a significant amount of localized governance? As in, on the Segmentum level, down to sectors and sub-sectors? Segmentum Solar may be down to sector-level administrative centers, but the other four Segmentums have all their localized administrative apparatuses intact. The astropaths will still be around as well.

Very true but in my opinion all but local will be rendered impotent.

There could never be enough warships to fortify all the millions of worlds in the galaxy that make up the Undying Emperor's domain, and some planetary populations may go decades or generations without once having the privelege of an Imperial Cruiser silently gliding into orbit overhead.

Obtained from here and originally here
The Imperium has a loose grip on its empire even with "peak" warp travel. The destruction of the beacon will limite them to a far slower speed and likely increase the voyage of a hypothetical flotilla. A far distant sector goverment, as far away now as Holy Terra once seemed, can hardly make the case against the hive world ten lightyears away who shows up with a handful of now armed transports and hired pirate forces demanding an "imperial tith". They can use an astropath to call for help and wait an unknown time for warships to be dispatched or simply swear fealty to the local boss and spare themselves from being blasted.

Space is big, I repeat. Big. So barring a handful that we can actually identify specifically, such as Terra, we're likely to see only around 10% of WH40k's closest systems replaced. If the Segmentum Solar was a disk 10,000 light years in diameter, the affected area would represent 0.03% of the Segmentum. In other words, if inhabited systems were roughly evenly distributed throughout the Segmentum, and there were a million systems just in Segmentum Solar, we would expect to see all of 33 IoM systems overlapped and replaced by Q. There could easily be as few as a half dozen affected systems.

I have a question, if I may. Are the battletech clans getting all of thier worlds and only a tiny fraction will overlap Imperium holdings or is it only worlds that overlap that get transported? That will effect the outcome greatly. Thank you in advance for your patience.

It's a very complicated situation, since at the same time, many Astartes chapters will attempt to make contact and return to Earth, with all the problem incumbent to the lack of the Astronomican and the destruction of the God-Emperor.
Many SM chapters may be completely lost about what to do, may fracture, may turn to Chaos or start something else entirely.

I see many conversion to choas and such but I don't see Marines loosing thier sense of purpuse as long as Terra is held by "non-believers". They live to fight, are over the top zealous to thier God-Emperor and he has just been vaporized. I Marines fighting to retake Terra and surronding systems or forging mini-empires to build larger armies to to retake Terra and teh surronding systems.

Actually this could sanitize the Warp to a great extent.
If I get it right, the Warp is neither good nor bad, but a reflection of the state of the galaxy, the overall mass of emotions. A certain side of it had a tendency to grow in power and effect due to the growth of decadence of old species, and their pain.

The Eldar specificly but yes. The problem is "now" the evil side is so heavily decked it creates a self-sustaining reaction. The various 40k deities have made the universe a grimdark place which creates more negative emotion which feeds the deities who... well you get the idea. The battletech group however is likely only going to be a drop in the ocean compared to the thousands of years of torment, suffering, and agony the IOM has carried out not counting the Orks,Eldar,etc have done.

The most crippled worlds, hiveworlds, and industrial worlds, will surely suffer a lot, since they heavily rely on resources from outside of their respective often depleted systems.

I would think they would have the industrial prowess to enforce thier will, taking vassal planets to support their hunger.

What is also possible is the rise of several thousands of Space Marines uniting after years of travel to some point, leading a massive force towards Earth, probably losing several ships en route due to the hazards of the Warp.
The question being what will they do when they arrive?

Well marines are not stupid, fanatical yes stupid no, and once they reach the first world that is part of the Imperium on thier charts but is full of strange nonbelievers they are going to grasp the magnitude of the problem. They will realize this is more than single chapter can handle and will attempt to form a true crusade. One thing I just thought off, JMS said the warp was evened out within one thousand light-years of Holy Terra. If that "bubble of normalcy" is still in existence when they arrive that will aid their travel. No warp storms=faster travel time.

[Mr. Oragahn]

About FTL speeds, we can look at several pieces of evidence.
First, my post from another thread, WH40K - 40K misc numbers... (SDN):

Still, coupling the inaccuracy of jumps (with an astropath btw), to the varying speeds, with storms blocking at least 10% of the star systems, forming haphazardly, half of them lasting less than a year and others for years or even centuries, planning a strategy and guaranteeing the certain arrival of a fleet either in time, or at all, is simply impossible.

I also point to the quotes provided by Connor, which generally provide, when all's going well enough, travel trips on the order of one or two digits thousands c, and sometimes faster when Warp currents are exceptionally favourable.

I'll also point out that the ancient Space Fleet source gives non-Astropath-driven trips distances of 4~5 LY. I don't know how long it takes to calculate such a jump.

From the same other thread, another post, another estimation of a typical FTL speed:

Whereas it once took centuries for human spacecraft to travel to Sol's closest neighbouring star, the same distance could suddenly be covered in just a few hours.

This refers to the change from STL drives to FTL drives. So they reduced to 2~4 hours what originally required 2~9 centuries of travel.
The closest star to Sol is Alpha Centauri, 4.37 light years away.
Clearly, back then, the ships could burn much fuel and would not come anywhere close to c.
Then they could cover 4.37 LY in a couple hours.
This would bring their fastest FTL speed at 19,153.71 c.
12,769.14 c based on three hours for the initial trip instead of two.
They'll need at the very least, on their fastest speed, 457.66 days to cover 1000 LY. Roughly 19 days.
Speeds reaching below the ten thousand c if Alpha Centauri was reached within 4 hours or more.

I can't tell if it's supposed to refer to the speeds they could achieve back then, with the use of new FTL drives, or if it's in direct reference to what they can complete with Astropaths "now".

[Mr. Oragahn]

What is also possible is the rise of several thousands of Space Marines uniting after years of travel to some point, leading a massive force towards Earth, probably losing several ships en route due to the hazards of the Warp.
The question being what will they do when they arrive?

Well marines are not stupid, fanatical yes stupid no, and once they reach the first world that is part of the Imperium on thier charts but is full of strange nonbelievers they are going to grasp the magnitude of the problem. They will realize this is more than single chapter can handle and will attempt to form a true crusade. One thing I just thought off, JMS said the warp was evened out within one thousand light-years of Holy Terra. If that "bubble of normalcy" is still in existence when they arrive that will aid their travel. No warp storms=faster travel time.

Well then it can be really, really fast. What will take them time is circumvent the Warp outside the ocean of quietness, with their usual speeds and limitations. But once inside the BT area, if the top speeds are right, they can cross the entire zone within a short day.

I also note that one of the source pointed out how the Imperium was making it sure the Astartes didn't become too powerful. Well, now that concept is really screwed.

What can BT forces hope to do in space?

[Enosh]

from what I understand it works like this

warp is like an ocean and the emperor is the biggest lighthouse in it, when you can see it you know where relative to the position of earth you are and where you have to go to reach point X

before the emperor there were a lot of smaller lighthouses, still the same idea, just with a shorter reach, in fact iirc the emperor can't reach certain parts on the edge of the galaxy in the Ultima Segmentum, so there are still beacons somewhere in the space controled by the ultramarines, those small beacons are what alowed humanity to use the same FTL they use now before the emperor

then there is the other version of FTL that is much slower and just alows ships to kinda glide along it but they never actualy go fully into the warp, this is what ships without navigators use (cargo ships etc that only need to travel small distances, there is a limited amount of navigators afterall and not every merchant needs them) and tau use the same princip

in case the emperor is gone the IoM could in theory go back to using the beacons

(this might not all be true since I am talking from memory)

the biggest problem with the emperor beign gone is the whole "he is the only thing holding the chaos gods back"

[Mr. Oragahn]

Thinking of it, wouldn't a calm warp have rather weak currents? Logically, considering how it is tricky for astropaths to get the right path through warp, getting their ship pushed forth in the right direction and time, it is logical that to any current that goes in a given direction, there is an opposite current. So the currents which generally allow the thousands or tens of thousands of c would be a manifestation of those strong favourable currents.

Therefore a calm warp would be like being stuck in the ocean with no wind and no particular strong current. Ships wouldn't actually get a fast path towards Earth. They would actually, and very much likely, take a very long time to get there.

[Jedi Master Spock]

The Imperium has a loose grip on its empire even with "peak" warp travel. The destruction of the beacon will limite them to a far slower speed and likely increase the voyage of a hypothetical flotilla. A far distant sector goverment, as far away now as Holy Terra once seemed, can hardly make the case against the hive world ten lightyears away who shows up with a handful of now armed transports and hired pirate forces demanding an "imperial tith". They can use an astropath to call for help and wait an unknown time for warships to be dispatched or simply swear fealty to the local boss and spare themselves from being blasted.

Well, the sector government has warships:

The Space Commander has direct command of a portion of the Segmentum's warfleet. A typical command comprises about 50 interstellar ships, although the number would obviously vary depending upon the needs of the sector.

I took that one from the WH40k fluff bible, so I'm forgetting which the exact source is. I think that may be cited in BFG, though, which also tells us that a "typical" [inhabited] sector is a cube 200 light years on a side, more or less. There are a fair number of inconsistencies surrounding this number, such as the fact that it would take a couple million of these sectors to fill the Milky Way.

If I pick two random points out of a 200 LY cube, the "typical" distance will be 2/3 of that, i.e., 133 LY; the capital of the sector is probably somewhere in the interior of the cube, but most worlds will be within 100 LY of the sector capital.

So if the Imperium looks, locally, like what it is supposed to, then the sector command will be not so terribly distant.

I have a question, if I may. Are the battletech clans getting all of thier worlds and only a tiny fraction will overlap Imperium holdings or is it only worlds that overlap that get transported? That will effect the outcome greatly. Thank you in advance for your patience.

The whole thing. All three thousand (ish) inhabited systems.

Well marines are not stupid, fanatical yes stupid no, and once they reach the first world that is part of the Imperium on thier charts but is full of strange nonbelievers they are going to grasp the magnitude of the problem. They will realize this is more than single chapter can handle and will attempt to form a true crusade. One thing I just thought off, JMS said the warp was evened out within one thousand light-years of Holy Terra. If that "bubble of normalcy" is still in existence when they arrive that will aid their travel. No warp storms=faster travel time.

Well, it's still the mind-bending Warp. It just happens to have been temporarily calmed and cleared out for a moment.

If the Warp is like an ocean, momentarily flattening it and clearing the weather won't last long. The wind will quickly pick up waves, clouds will start to drift back in. They should have relatively clear sailing in the first few years - or possibly even decades, given how slowly things change in WH40k - but it's still sailing out on the open ocean rather than a lake.

from what I understand it works like this

warp is like an ocean and the emperor is the biggest lighthouse in it, when you can see it you know where relative to the position of earth you are and where you have to go to reach point X

before the emperor there were a lot of smaller lighthouses, still the same idea, just with a shorter reach, in fact iirc the emperor can't reach certain parts on the edge of the galaxy in the Ultima Segmentum, so there are still beacons somewhere in the space controled by the ultramarines, those small beacons are what alowed humanity to use the same FTL they use now before the emperor

then there is the other version of FTL that is much slower and just alows ships to kinda glide along it but they never actualy go fully into the warp, this is what ships without navigators use (cargo ships etc that only need to travel small distances, there is a limited amount of navigators afterall and not every merchant needs them) and tau use the same princip

in case the emperor is gone the IoM could in theory go back to using the beacons

(this might not all be true since I am talking from memory)

They might remember how to use them, but definitely those beacons would have a much shorter range if they did. There are other things that act as beacons for Chaos, Tyrannids, et cetera:

There was no star nearby to bend space so that incoming vessels must emerge billions of kilometres short of their goal. A raider might materialize suddenly above the craftworld itself - especially if guided by such a psychic beacon as Eldrad had been obliged to light.

However, these other beacon-like devices only really seem to work as well for people drawing near them.

I think I understand. A ship without a Navigator can't travel out of sight of "land" or it'll get lost, and the crew has no idea how to rig the sails to take advantage of the wind, so they have to paddle. Hence it doesn't even go fully into the Warp. A ship with a Navigator is like a good sailing ship - the Navigator can see, understand, and respond to the conditions of the wind and waves.

However, without a beacon, the Navigator doesn't know if they're on course or not when they're on the open ocean. He has no compass or other bearing, so while he may be able to respond to local conditions, and may be travelling quickly, he'll tend to gradually drift off course, perhaps even going in circles without realizing it.

the biggest problem with the emperor beign gone is the whole "he is the only thing holding the chaos gods back"

The major theme of WH40k being that humanity is in for it, they just haven't quite managed to get wiped out quite yet because they're being tough, mean, and almost as evil as the stuff surrounding them.

Well then it can be really, really fast. What will take them time is circumvent the Warp outside the ocean of quietness, with their usual speeds and limitations. But once inside the BT area, if the top speeds are right, they can cross the entire zone within a short day.

Here I talk about Warp speeds; I looked at three different sources before concluding warp speeds were being inconsistent. The slower speeds are given as 2,000-12,000c, which would fit with it taking "hours" to travel to Alpha Centauri. The faster speed given was 90,000-365,000c, which would not be hours. I prefer the lower speed, since the citations accompanying it explain that "apparent" speed from on-board the vessels is in the range of the higher speed estimates given.

The Tau move sans Navigators, and are frequently cited as one fifth the speed of Imperial vessels. (Supposedly in the Tau Codex, but I'm having trouble finding it there). I would have assumed that's about the typical non-Navigator speed reduction. WH40k wiki says that the newest Tau vessel moves about 1/3 of the "average" warp speed.

I also note that one of the source pointed out how the Imperium was making it sure the Astartes didn't become too powerful. Well, now that concept is really screwed.

What can BT forces hope to do in space?

Well, what they can do is travel a fairly consistent ~1500c on average for long distances, making "instant" jumps of up to 30 light years once a week with commercial and military ships. Reliably and regardless of the condition of the Warp or any beacons.

What they can't do is fight naval battles with a sector fleet:

• The Clans have 300-500 between them, and the Inner Sphere, I think, doesn't have more than that. I don't think there's any "current" faction that has more than the 50 warships typical of a sector fleet.
• Most of these mass less than a million tons, although a few battleships go up to two million. They're not especially big by WH40k standards - escort sized. A Lunar-class cruiser dwarfs anything in BT.
• They have fusion engines, but no inertial compensators, and accelerate therefore at only a few gravities (at most) during combat, so they don't have a speed advantage.
• They have fairly limited effective ranges; Explorer Corps says that tops out at around 450 km. We're pretty sure WH40k ships have no trouble reaching that far, and a number of references have them shooting further.
• They don't have shields (although they have BT's ridiculous armor, which almost makes up for it) or a firepower advantage - up to kilotons, while WH40k capital ships are providing megatons.
• No inertial compensators means these ships accelerate at most several gravities during combat, so they're not going to have mobility on their side, either.

One typical IoM sector fleet could go toe-to-toe with the entire assembled warship fleet of any BT nation, I should think. Quite possibly all of them; the sector fleet might be outnumbered 10-20:1 in ships and have trouble tracking all the fighters and landing ships flying around, but I think the typical WH40k warship is going to be at least 10x the mass of the typical BT warship, with over 10x the firepower and over 10x the durability.

[Mr. Oragahn]

You killed your thread. :(

[Jedi Master Spock]

You killed your thread. :(

Really? Man, I should have held some more back, then. Because there were still quite a few things I wanted to explore about this scenario.

Especially the political end of it. I don't feel like I know very much about the sector/segmentum level of governance yet, and the question of how BT factions would interact with the de-capitalized Imperium is completely open to me.

I know they're not going to be taking them down in massed naval battles using their existing assets, but that's only a small piece of the puzzle.