Jump Arrival Point

[phavoc]

I've written up some traffic control rules to handle jumping. It seems that it's a chaotic event, and while you aren't supposed to be able to jump in on-top of another object, it still seems the total randomness and very messy.

I need to find my write-up, but it works like this - each world sets up a series of 'boxes' about 50,000 cubic km. Each box is an arrival point. there are 100km 'lanes' between the boxes that ships are to traverse inbound to the planet, and once they reach the 100D limit traffic flow and control is up to local traffic control (if present), or the individual ships. Each planet that is in range of the arrival world has it's own set of boxes, and ships departing from a world are either assigned an arrival box, or they would pick one based on dates. Each box is reserved to the jumping ship for a 48hr window to allow for the randomness of jump space. The arrival worlds can continue to add in boxes further away from orbit for as much expected traffic as they think they'll need. It's space, and it's big. Plus they can control how many slots each planet in range gets, so that helps too.

Ships that are jumping out would follow specific routes to avoid the inbound traffic, and once the were beyond the 100D limit they would be free to jump. There would be a number of corridors for them to take, so traffic probably would never be an issue, and if it were, traffic control could control the departure speeds like air traffic controllers do today and efficiently handle the lanes.

Military traffic would have it's own reserved arrival/departure areas (probably above/below the eliptical plane) that lets them jump in and out on their own without any input or knowledge from traffic control. Again, space is big so having a few million cubic kilometers to play with is nothing.

I think I even have illustrations, if anyone is interested.

 

[alex_greene]

Exactly. ATC would divide up space surrounding the mainworld into spacelanes, and any ship Jumping in would be assigned a lane and an approach vector, just as aeroplanes approaching an airport are.

The best way to minimise the risk of intercepting another incoming or outgoing Jump point, or another ship, is to Jump in-system at a good distance, broadcasting IFF and await the acknowledgement signal and approach instructions from ATC and the Dock Master.

Hail the Dock Master!

 

[phavoc]

Well, the issue with that is the ships jumping in have no knowledge of what awaits them until the re-enter realspace. At that time they could then be given directions after contacting traffic control. That's the easy part. The harder part is to make sure two ships don't try to re-enter realspace in the same place/time. While the odds are against it, a busy enough system is bound to have it occur since no ship will know when or where any other ship is arriving (excepting for ships jumping together).

 

[Bardicheart]

I think I even have illustrations, if anyone is interested.

Sure, sounds interesting and it sounds like you put a lot of thought into it. Things like that fascinate me because they get into the details of "how things work" and if I don't use any other aspect of it, its good to use in descriptions to players.

That kind of system is also what I could reasonably expect to see in high traffic systems along the major trade routes. Less so on the feeder routes and when you get beyond those to outposts you almost feel relieved when someone from the starport finally radios you (by which time you're halfway there anyway). The difference between say Heathrow vs. a small airport staffed by three people in the middle of nowhere used mostly by infrequent local traffic.

While the odds are against it, a busy enough system is bound to have it occur since no ship will know when or where any other ship is arriving (excepting for ships jumping together).

Which brings to mind two thoughts. First the mental image of that 75,000 dT cruiser suddenly coming out of j-space and plowing into the 200 dT free trader... ouch! Second, how accurate are jumps, I can't recall finding any official rule on it but here's what I'm wondering. We know the amount of time in j-space is variable by up to 36 hours. If you don't know exactly when you will come out of j-space then how precise is it as to where you come out? Say you want to come out of j-space about 110D from the planet... or okay say Earth, you want to jump in 1,408,000km from Earth, out system side and slightly trailing orbit. You plot your course, you make the jump and roughly a week later you "pop out" of j-space in the vicinity of Earth... but where exactly. Do you pop out exactly where you plotted? Or does that only happen if say you scored 6+ successes on your Astrogation check? If you don't come out exactly where you plotted, where do you come out and how is that determined? I can't recall seeing a mechanism for that in the rules.

If we assume jumps are less than accurate and that you might be 10,000km or 50,000km or 1,000,000km off target... that certainly would give a clear reason for plotting your jump re-entry point further away. If the variable were up to 1,000,000km you'd want to make sure that didn't potentially put you jumping in just above the planet with it filling your screens as you skim atmosphere and the captain frantically screaming Pull up! Pull up! That would be bad, also it would likely ruin your vacation.

I need to find the reference (it might be from an older edition of Traveller, if anyone recognizes it feel free to point me to it) that jump travel used the gravity of gas giants as a kind of beacon / landmark regarding jump space. The implication being is that you used the massive gravity of a gas giant to help you figure out when you'd arrived. But that would also imply that the ship actually controls when it comes out of j-space and the randomness comes from variability in travel time through j-space. It does make a kind of sense with all the references in Traveller to refueling at gas giants, if you were using them as a kind of "nav beacon" in j-space (perhaps such massive gravities are one of the few things a ship in j-space can detect) then you might normally "pop out" near them, refuel and head in system from there. Maybe that's how Marc Miller meant for it to work originally, though clearly that's not how its being done now.

Again, just brain storming, throwing out ideas and enjoying the conversation.

 

[darue]

don't forget, it's best practice to match velocity with the target star before jump. Stars can have significant relative velocities.

http://en.wikipedia.org/wiki/Stellar_kinematics

 

[Wil Mireu]

Of course a very small planet might have a moon outside it. But even in these cases mapped systems would include nav charts that detail the orbital periods of all planetary bodies. I would assume part of plotting a jump means plotting a course that avoids all these. Keeping these nav maps updated and accurate is part of what the Scout service does.

Going back to my original topic question; this suggests to me that if a system is particularly "crowded", maybe it has lots of planetary belts for example, may force ships to jump in system beyond them because there's just too many objects to make plotting a course around them feasible. Not official of course but its an interesting idea to make a specific system unique and add some more flavor to your games.

None of this would happen. People seem to think that systems are "crowded" but the chance of a ship accidentally intersecting the 100D sphere of anything that isn't a known planet or satellite (and people are only going to deliberately intersect the 100D spheres of those bodies) is essentially zero. Real asteroid belts aren't full of tumbling rocks like in Star Wars - asteroids and their 100D limits are tiny compared to the millions of kilometres that separate them; Voyagers 1 and 2 passed straight through the asteroid belt without coming close to any asteroids at all. Comets are separated by even greater distances in the Kuiper Belt/Oort clouds.

So nobody's going to be accidentally dropped out of jump by an asteroid they didn't know was there - the probabilities are too remote to even register. It's also not going to happen in interstellar space because the distances involved are even bigger compared to the size of the objects involved.

 

[Wil Mireu]

Could but I'm guessing in most cases the planet's moon is within the planet's 100D limit.

It distorts the 100D sphere into a shape roughly like a butternut squash lying on its side.

No, it doesn't. 100D limits are spheres, they (canonically) aren't related to gravity - only the object's radius - and they aren't themselves distorted by anything.

If you had a moon orbiting near its primary's 100D limit then you'd end up with a smaller 100D sphere bulging out from the planet's otherwise undeformed 100D sphere, and the smaller sphere would rotate around the planet with the moon.

 

[Bardicheart]

You seem to be reading more into my statement than was actually there, there was a reason I put "crowded" in quotes. I probably wasn't being clear, my previous post were pretty much unedited stream of thought. Hopefully the following will clarify what I was trying to get at.

Its true in our solar system our asteroid belts aren't very dense at all. Not only did Voyager 1 and 2 pass through safely, more recently so did Dawn, and none of them came within 1,000,000 km of any asteroid while doing it. Point of fact our main belt has been estimated to contain a total mass barely 4% of our moon (or about 1/20 of 1% of Earth)... not very much material at all really. On the other hand it also contains some sizeable chunks of rock such as Vesta which is over 500 km across and Ceres which is over 900 km across (oh just think what those space Dwarves could make of that! 8) )

Space Trivia: What percentage is Ceres of the total mass of our Main Belt? A: a little over 40% all by itself.

However, that said, there are two problems with your statement. First is the assumption that the specific density of our asteroid belts can be assumed to be a representation of all or any other asteroid belt in other star systems. While our main belt lacks even enough material to make up our moon, a hypothetical belt made out of say a planet that had been pulverized by the gravitational forces of two gas giants could be easily have a density 2,000 times that seen in our own Main Belt (more if the planet was originally larger than earth / size 8 or if there were multiple planets involved). Consider also the example of the Bowman system in the Spinward Marches where one might hazard a guess that the stellar accretion disk never coalesced into planets, instead remaining a much denser belt of asteroids / planetoids for which Bowman is known; such a massive asteroid belt could very well be far denser than our Main Belt, possibly even approaching what we often see in science fiction. Equally, and its a fair point, in some other system an asteroid belt could be even less dense than those seen in our system. composed of small debris and dust... more like a dust belt really. All of which is worth keeping in mind when describing such planetary belts in Traveller, there's potentially a lot of variety out there.

But, that aside, its not necessary for a asteroid belt to be nearly that dense to present a navigation hazard to jump travel. The issue is three fold. First, any such asteroid belt will be impossible in practical terms to catalog accurately such that navigational charts for jump travel could be produced. There will always be events within the belt due to collisions or gravity of nearby gas giants which alter trajectories in unpredictable ways. So while there may be 2,000,000 or more km between asteroids, you can never be certain exactly where they all are when you plot your jump... and that's a problem. While most asteroids may be small, there are some that can be quite large and something 3-5 km across would pose a jump hazard out to 300-500 km (while such are rare in our solar system, we cannot say with any certainty that would be true in all star systems; perhaps a belt is composed of nothing but such large asteroids, the remnants of two planets that collided and broke up) . Its also large enough that even the largest ship could be knocked out of jump space if it were unlucky enough to cross its path. And again, since you cannot be 100% certain where it is because its movements are not as predictable as large bodies such as moons or planets... this presents a navigational hazard. Third, while the odds of any such small body forcing a ship out of jump space are, I agree, small... there is that last problem that if you exit jump space in the vicinity of an asteroid belt you run the risk that you may have done so in the path of an large rock. The chance may be small but even if you are in a 75,000 dT cruiser, a 300m across rock moving at 65,000 kmph slamming into you broadside will still pretty much ruin your day (Ceres moves at 17.882 km/s or a little over 64,000 kmph; I do NOT want to be in the way of that, Dawn however will rendezvous with Ceres in 2015). Again, since you can never be 100% certain where all those chunks of rock are, and in fact you may not even be able to predict with any certainty exactly where you will exit jump space even under normal circumstances... that too presents a hazard. The problem isn't so much the density of the asteroid belt, but rather there being too many variables to make a safe jump calculation possible; and while the risk may be very small it is also potentially catastrophic if it does occur. I'm guessing most pilots would rather not take the chance (then again, there are probably some who would... it is Traveller after all :lol: ).

Lastly, since most ships are probably still under a mortgage and assuming that, like in the real world, that includes by necessity an insurance policy of some sort... I suspect said insurers and mortgagers would take a very dim view of deliberately exiting jump space within or near an asteroid belt. Should the unthinkable happen and said large chunk of rock happen to hit a ship (and assuming by some miracle the captain also survives), I would imagine the captain of the ship would find themselves in all sorts of legal and financial hot water.

So, the above being a fairly long winded way of saying that in a hypothetical system where say you have the main world in orbit 3 and just beyond it there happens to be a planetary belt of some unspecified density, my guess is the normal procedure would be for ships to jump into the system beyond the planetary belt, fly through it (which as has been noted in our solar system is hardly risky; even in a "Star Wars dense" belt, one could simply fly "over" it rather than through it since the belt tends to flatten along the solar plane) and then land on the main world. But this would force ship to jump in to the system further out than normal, increase travel times, etc. which takes us back to my original question in the thread. Whew! :lol:

Then again, it raises another question... why doesn't everyone just jump in above or below the system's star, which is conveniently centrally located and absent of all these pesky asteroids, planetary bodies and gravitational fields; then fly to whatever planetary body happens to be your destination which as it happens is likely equidistant no matter where it is in its orbital period? Other than the fact that there would be nothing nearby and no way to refuel if anything went wrong meaning you'd have to signal GK and hope for rescue (perhaps another reason for the apparent focus on being able to refuel at gas giants).

Still leaves me thinking that there needs to be some rules added regarding where you exit jump space, what kind of variability there is and so forth. Knowing that would affect these kinds of discussions.

 

[Wil Mireu]

First is the assumption that the specific density of our asteroid belts can be assumed to be a representation of all or any other asteroid belt in other star systems.

Some asteroid belts may be denser - especially ones closer to a star - but age will thin them out (as the gas giants that are responsible for their existence slowly eject asteroids from the system over time through orbital resonances). But even in the denser belts, the probability of your path intersecting any objects in the belt are going to be tiny. I think you'd only be likely to get a significant chance of crossing the path of an object in REALLY young systems that are only a few million or tens of millions of years old.

A hypothetical belt made out of say a planet that had been pulverized by the gravitational forces of two gas giants could be easily have a density 2,000 times that seen in our own Main Belt (more if the planet was originally larger than earth / size 8 or if there were multiple planets involved).

Doesn't work like that. Asteroid belts aren't formed by planets being destroyed, they're formed by planets never forming in the first place. A neighbouring gas giant's gravity prevents the planetesimals from accreting, and further interactions and resonances will spread the debris out over as wide a belt as possible (bound by other planets) over time.

All of which is worth keeping in mind when describing such planetary belts in Traveller, there's potentially a lot of variety out there.

Yes, there will be a lot of variety out there - belts will be compositionally different based on distance from star/temperature, or denser or less dense, or dustier, or have different size distributions, or different widths - but in all but the youngest (still forming) systems individual asteroids are still going to be separated by distances far greater than their sizes, no matter what. Sorry, but it's a complete fallacy to believe that a ship has anything approaching a significant chance of randomly hitting an unknown asteroid in such situations.

As for collisions, chances are that most collisions will have pretty slow relative velocities in most systems - those won't generate a lot of debris, and most wouldn't escape from the asteroids anyway. Collisions with high relative velocity would be much more likely in extremely young, chaotic systems (again, less than a few tens of millions of years old), but then most ships won't have much business in those systems anyway.

First, any such asteroid belt will be impossible in practical terms to catalog accurately such that navigational charts for jump travel could be produced.

That isn't really an issue. As I said, the Voyagers (and Pioneers) went through the asteroid belt and went nowhere near any asteroids - known or previously unknown. Even 100D limits of 300-500km are absolutely piddly compared to the space between the asteroids. Think of it like the chance of an ship randomly hitting a Blue Whale as it crosses from one side of the ocean to the other - the probability that both will be in exactly the right place at exactly the right time are astronomically low.

If you want to say "well in my game it's a problem" then of course you can, but the fact remains that reality doesn't work like that. Unfortunately, asteroid belts just don't work the way you (and a lot of other people, so I'm not being personal here) think they do.

Should the unthinkable happen and said large chunk of rock happen to hit a ship (and assuming by some miracle the captain also survives), I would imagine the captain of the ship would find themselves in all sorts of legal and financial hot water.

I think it's more likely that it'll be filed under "Act of <insert deity here>". Insurers (and pilots) would know a lot more about the probability of randomly encountering objects in asteroid belts (i.e. that it's practically zero) and won't be too fussed about it.

Then again, it raises another question... why doesn't everyone just jump in above or below the system's star, which is conveniently centrally located and absent of all these pesky asteroids, planetary bodies and gravitational fields; then fly to whatever planetary body happens to be your destination which as it happens is likely equidistant no matter where it is in its orbital period? Other than the fact that there would be nothing nearby and no way to refuel if anything went wrong meaning you'd have to signal GK and hope for rescue (perhaps another reason for the apparent focus on being able to refuel at gas giants).

In practical terms, I think that's exactly what happens - the ship doesn't usually arrive on the ecliptic plane of the destination system. They have to, since the jump route is on a straight line from their departure system to the arrival system, and the chance that both those systems are oriented in space so that they're exactly coplanar with eachother is (again) astronomically low. In reality the ship will come out of jump at some angle to the destination's ecliptic - each system around the destination will have its own 'arrival vector' that ships will come in from, which depends on where the departure systems are in its sky.

I suppose it's possible for them to arrive at the ecliptic plane by tweaking the jump path to intersect the ecliptic outside the 100D limit (rather than aiming at the 100D sphere and precipitating out there) and then deliberately coming out of jump at the right instant, but I doubt that the targeting is going to be that accurate - the tiniest error in time or orientation would amount to missing the mark by tens or hundreds of AU.

 

[Wil Mireu]

Another illustration of this:
http://www.scienceadviser.net/2009/09/asteroid-belt-not-as-dense-as.html

Relevant quote: "According to Alan Stern of Space Daily, there is a less than 1 in 1,000,000,000 (billion) chance that a ballistic trajectory satellite would hit even one [asteroid] when passing through the asteroid belt." (Alan Stern is also PI for the New Horizons mission on its way to Pluto, so he'll know what he's talking about).

and:
"If one were to smash up all the rocky planets in the solar system and place them in the asteroid belt, there would be 10,000 times as much rock as is in the belt right now. This would only bring a ship up to a 1 in 100,000 chance of hitting something when passing through the asteroid belts, assuming that the size distribution was similar to what it is now, and that the ship doesn't dodge."

 

[Condottiere]

So how do you hide in an asteroid belt?

 

[rust]

So how do you hide in an asteroid belt?

Keep a big enough asteroid between your ship and the enemy's sensors,
just like a squirrel keeps a tree between itself and the hunter.

Otherwise, no plausible way of hiding in an asteroid belt.

 

[Condottiere]

But apparently, you're more likely to find a tree in meadow, than an asteroid in a belt.

 

[Wil Mireu]

But apparently, you're more likely to find a tree in meadow, than an asteroid in a belt.

Nope. You can detect larger asteroids pretty easily (obviously, or we wouldn't know about so many of them) - the unlikely part is just running into one by chance on a jump route. If you want to "hide" in an asteroid belt then you can either -

(a) turn all your power off and pretend to BE an asteroid (even better if you have a planetoid hull),
(b) find a known asteroid that's large enough to hide behind or inside (if it's stable enough to have caves)

 

[rust]

But apparently, you're more likely to find a tree in meadow, than an asteroid in a belt.

It is not difficult to locate one, the problem is to get there without
being detected ... :wink:

 

[alex_greene]

Curiously enough, starship stealth is a topic that has already been covered in another thread.

 

[Bardicheart]

Depends on the density of the asteroid belt. How dense you want to make it for game purposes is pretty much up to the referee, there's no game descriptor of specific density. That aside, whether trying to hide or simply a belter looking for something to mine, your best bet would be to look for families or clusters which often form around a single large body that becomes surrounded by dozens or hundreds of smaller bodies. For example in the outer part of our own Main Belt there is the Themis family which has over 500 asteroids in it (if memory serves its the most populated such family in our main belt). Such clusters can sometimes look a bit more like what is typically pictured in science fiction, but they are usually surrounded (at least in our solar system) with millions of kilometers of open space. Our solar system doesn't really have enough material in the belt to form much (it is indeed pretty sparse, you could fly right through it in most places and never realize there was an asteroid belt there at all). In other star systems it would depend on how much material there was in the belt, number and proximity of gas giants, the age of the star and the age of the asteroid belts (younger tends to be more dense, older less dense unless something unusual occurs) and other factors that go way beyond what Traveller typically deals with (T:NE had some math for such things, it was also very unpopular as I recall... too much realism in a sci-fi game can become a bad thing). Even in a very dense asteroid belt, you'd still have such clusters with large gaps between them, and by large I mean probably a few hundred thousand kilometers or more. The clusters might be more densely populated, i.e. you might have some clusters that are thousands (possibly even tens of thousands) of kilometers across and populated with thousands of bodies of varying sizes, probably with a small planetoid(s) at its center around which that family formed.

Then as Rust said, keep a rock between you and whatever you're trying to hide from. Better still grapple onto a large body and just coast with it. Keep in mind a sphere just 200m across is roughly 300,000 dT in Traveller terms, so a chunk of rock that is a mere 100-300m across is plenty big enough to hide behind (much less the big ones that may be a hundred kilometers or more across, but are also far rarer). So a 200 dT or even 800 dT ship is going to be pretty small compared to some of the rocks floating around out there. With a reduced IR signature, minimal power use, and assuming no one is actively looking for you you probably wouldn't be noticed. Unless of course your referee decides to throw a belter in who thinks you're a dirty no good claim jumper! Darn plot twists...

 

[Wil Mireu]

Depends on the density of the asteroid belt. How dense you want to make it for game purposes is pretty much up to the referee, there's no game descriptor of specific density. That aside, whether trying to hide or simply a belter looking for something to mine, your best bet would be to look for families or clusters which often form around a single large body that becomes surrounded by dozens or hundreds of smaller bodies. For example in the outer part of our own Main Belt there is the Themis family which has over 500 asteroids in it (if memory serves its the most populated such family in our main belt). Such clusters can sometimes look a bit more like what is typically pictured in science fiction, but they are usually surrounded (at least in our solar system) with millions of kilometers of open space.

Asteroid families aren't in "clusters" (as in groups that are physically close to eachother) - they're bodies that share the same orbital elements (inclination, eccentricity, semimajor axis) and a common origin (usually through a collision with the largest body). Over time the family members will spread out around the sun because they're in slightly different orbits and due to gravitation perturbations.

see: http://en.wikipedia.org/wiki/Asteroid_family

The only time you really get asteroids that are close to eachother is if one has one or more moons.

 

[Condottiere]

So a possible effective disguise is as a belter survey, mining or factory ship.

 

[Condottiere]

According to a PBS show I'm just watching, you have asteroids between the Earth and the Sun; hiding behind one of those should allow some extra stealthiness, though which ships would pass by there except passenger shuttles and possible in system freighters.