Jump Distance and Stars

[Timerover51]

Does the size of a star effect the required distance from a planet for a safe Jump?

Terra's Sol is about 865,000 miles in diameter. One hundred times the diameter of the Sun would be 86,400,000 miles. Does that mean that it would not be possible to Jump from either Venus or Mercury, and that a ship would have to boost from those two planets out to a distance of 86,500,000 miles for a safe jump?

 

[salochin999]

I could be wrong but yes I think that is what is meant by jump shadow - when a Star's 100 diameter no safe jump distance extends beyond a planet's.

Either way I'm using the idea to create reasons for why some very populous world's aren't visited much. They have a massive jump shadow that makes trade uneconomic - a good excuse for a John Carter of Mars type world imo.

 

[whartung]

Jump shadows, and jump masking.

Different systems seem to treat it differently.

In Gurps, it's canon, they're really clear on that. They have tables to adjust the travel times based on whether the planet is on the wrong side of the star, etc.

The other versions don't talk about it much outside of simply the 100D rule.

And then there's the whole "shouldn't it be some tidal effect based on mass of the entity", but that's a different kettle of fish.

 

[Timerover51]

Thanks, I do not have nor have any intention of getting GURPS, but the "jump shadow" concept is what I was thinking of. It would justify the addition of a Navigator skill and Navigation Program to determine the relationship of the target planet to its sun based on a Jump from the departing system.

However, the Traveller Wiki gives the following description of Regina.

Regina is only one of seven worlds in the systems which boast a breathable atmosphere, and only one of four habitable planets in the life zone of Lusor, in orbit around the gas giant Assiniboia.

Based on this, would not the jump distance for Regina be based on the safe jump distance from the gas giant? And what is the stellar type of Lusor, given that it has such a large habitable zone, it must be a fairly large and bright star? Then you also have a dwarf companion star, Speck to worry about, and a distance third star, Darida, spectral class not given, as well.

As the gas giant, Assiniboia, is sufficiently large to retain an Earth-size planet as a satellite, that implies considerable size, probably larger than Jupiter. Jupiter's radius, per NASA, is 43,440.7 miles, so diameter is 86,881.4 miles. That would make the safe jump distance from Jupiter about 8.7 million miles. Based on this, in jumping to Regina, you have to deal with the jump shadow of the system sun, Lusor, and the jump shadow of Assinibois as well. It would appear, on the face of it, that getting to Regina is not that terribly easy, and departing from it could also be a problem, depending on where you want to jump in relationship to the location of the gas giant with respect to its sun.

 

[whartung]

Jump shadowing and Jump Masking are two different, but related things.

What you're talking about is Jump Masking, where the destination is in the Jump limit of a larger body.

So, a moon around a gas giant certainly falls in to that example.

But so can the inner worlds of a system with a large star.

So, Venus is within the 100D limit of the Sun, but Earth is not.

Jump Shadowing is where a body blocks the "Line of Jump", much like Line of Sight.

A tenet of Jump space is that when you are within the 100D limit of an object, your ship is pulled out of jump at the limit. So, if even if the Earth is not within the Jump Mask of the Sun, if you try and jump from the other side of the Sun, the sun will pull you out of jump before you even get to earth.

This, obviously, complicates travel in the game. What's the relationship between the two systems, where are the planets located, etc.

So it's fair to just "punt" on shadowing, or come up with a mechanic that Gurps has (I think they just roll dice and say "yup, shadowed, add XXX days to travel" or something).

Or, skip it, and just to Jump Masking for flavor.

 

[rancke]

Jump shadowing and Jump Masking are two different, but related things.

What you're talking about is Jump Masking, where the destination is in the Jump limit of a larger body.

No, that's jump shadowing, when a world is in the jump shadow of a larger astronomical object.

Jump Shadowing is where a body blocks the "Line of Jump", much like Line of Sight.

That's jump masking, when a destination is masked by the jump limit of an intervening object.

The problem with jump masking (and jump shadowing for that matter) is that even when there are rules for it (GT) no one actually uses it in any setting context.


Hans

 

[rancke]

Based on this, would not the jump distance for Regina be based on the safe jump distance from the gas giant?

Yes, in this case. Assiniboia and its moons are outside Lusor's jump shadow, so only Assiniboia's jump shadow affects travel to and from Regina. However, if a gas giant happened to be shadowed by its star, the star's jump limit would be the salient factor for travel to and from its moons.

And what is the stellar type of Lusor, given that it has such a large habitable zone, it must be a fairly large and bright star? Then you also have a dwarf companion star, Speck to worry about, and a distance third star, Darida, spectral class not given, as well.

The Regina System is one of a handful that have been detailed in full.

Hmm... looks like not quite full enough. The size I use for Assiniboia in my calculations appears to be something I've made up myself.

As the gas giant, Assiniboia, is sufficiently large to retain an Earth-size planet as a satellite, that implies considerable size, probably larger than Jupiter. Jupiter's radius, per NASA, is 43,440.7 miles, so diameter is 86,881.4 miles. That would make the safe jump distance from Jupiter about 8.7 million miles. Based on this, in jumping to Regina, you have to deal with the jump shadow of the system sun, Lusor, and the jump shadow of Assinibois as well. It would appear, on the face of it, that getting to Regina is not that terribly easy, and departing from it could also be a problem, depending on where you want to jump in relationship to the location of the gas giant with respect to its sun.

I once calculated that the trip from Regina orbit to Assiniboia jump limit took 14 hours at 1G.

I also once made a diagram that showed when, during a Reginan year, a jump destination was masked from Regina. It was a a lot of work, and I lost the diagram in a computer crash.

I've never gotten around to redoing it. Probably never will.


Hans

 

[aramis]

Based on this, would not the jump distance for Regina be based on the safe jump distance from the gas giant? And what is the stellar type of Lusor, given that it has such a large habitable zone, it must be a fairly large and bright star? Then you also have a dwarf companion star, Speck to worry about, and a distance third star, Darida, spectral class not given, as well.

As the gas giant, Assiniboia, is sufficiently large to retain an Earth-size planet as a satellite, that implies considerable size, probably larger than Jupiter. Jupiter's radius, per NASA, is 43,440.7 miles, so diameter is 86,881.4 miles. That would make the safe jump distance from Jupiter about 8.7 million miles. Based on this, in jumping to Regina, you have to deal with the jump shadow of the system sun, Lusor, and the jump shadow of Assinibois as well. It would appear, on the face of it, that getting to Regina is not that terribly easy, and departing from it could also be a problem, depending on where you want to jump in relationship to the location of the gas giant with respect to its sun.

There is a theorized upper limit to diameters of gas giants. Note that a 13 M_♃ GG is roughly the same diameter as a 20 M_♃ brown dwarf. In fact, the estimation is that, until about 75 M_♃, fusion as we know it is unlikely to be rapid if at all, and is limited to hydrogen only (albeit by 75 M_♃ it's getting the common isotopes of hydrogen (protium, deuterium, tritium) to fuse and starts to get enough reactions to raise the temp to visible light in significant magnitude.

And from about 1.1 M_♃ to 75 M_♃, the diameter hardly changes, based upon the infographics and articles I've found. It just compresses more.

Oh, and it looks like the hab zone of a BD and/or superjovian is pretty narrow. One can presume that a world orbiting a GG could be either (1) in the hab zone of the sub-brown-dwarf superjovian (5-13 M_♃, and under 0.01AU from it), or (2) the entire BD is within the hab zone... but then it's going to have some interesting effects, too, because it's potentially a LARGE shift in insolation density. T5 caps Jovians' diameters at 250,000 miles. (modern science puts the estimated cap somewhat closer to 110,000 to 125,000 miles, IIRC.)

Note that an F7 V (the main star, Lusor) has a pretty wide hab zone. If Asinoboia is within the HZ, and it's under 250,000 miles diameter, it could account for 2-4 habitable worlds all by itself. And, remember, 1 M_♃ is about 317 earth masses. 4 would keep the fraction low enough (around 1/79) to be a relatively stable bunch. Oh, and the jump limit of the star? Orbit 3 is blocked, but not 4. So, yes, the GG sets the limit most of the time.

T5, however, answers this for Regina. Assinoboia is in orbit 4, and regina orbits it, somewhere past 5-8 diameters (because Jupiter tidally heats those - 5 being Io, and 7 is Europa, and Assinoboia is stated to be Jupiter sized).


http://www.nasa.gov/mission_pages/hubble/news/brown_dwarfs.html
http://www.space.com/24467-brown-dwarfs-failed-stars-explained-infographic.html
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3612282/

For those not familiar (or who don't have enabling fonts available) M_♃ means Mass in multiples of the Mass of Jupiter.

 

[WistfulD]

And then there's the whole "shouldn't it be some tidal effect based on mass of the entity", but that's a different kettle of fish.

I have always padded my suspension of disbelief with the unwritten rule, "it is based on mass, not diameter, but 100D is a good rule-of-thumb to simplify the explanation for the layman."

 

[rancke]

I have always padded my suspension of disbelief with the unwritten rule, "it is based on mass, not diameter, but 100D is a good rule-of-thumb to simplify the explanation for the layman."

Unfortunately, if it's based on tidal force, 100 diameters works fine for terrestrial worlds, but ought to be around 30 diameters (IIRC) for suns.

I agree that basing jump limits on tidal force would be a really good idea. It's a thousand pities that Marc Miller disagrees.


Hans

 

[whartung]

Unfortunately, if it's based on tidal force, 100 diameters works fine for terrestrial worlds, but ought to be around 30 diameters (IIRC) for suns.

I agree that basing jump limits on tidal force would be a really good idea. It's a thousand pities that Marc Miller disagrees.

Eh, I agree with Wistful, it's a game mechanic. 100D is as good as anything, just like jump shadowing and masking complicate the system, to questionable gaming effect. From a narrative PoV, a ref can make it work either way, so is the complexity necessary.

 

[WistfulD]

Unfortunately, if it's based on tidal force, 100 diameters works fine for terrestrial worlds, but ought to be around 30 diameters (IIRC) for suns.

I agree that basing jump limits on tidal force would be a really good idea. It's a thousand pities that Marc Miller disagrees.


Hans

Yes, and for the rare instance when the star's jump-inhibition radius (have we decided, is that shadowing or masking?) is the significant factor, that will make a difference (like in the case of the subject of this thread). Otherwise, it can almost always be hand-waved away. "Gravitational tidal forces that must drop off to a point about equal to what would be 100D from a rocky world" works well for my suspension of disbelief. If you want to change the distance to 30D for stars (or stars and gas giants), more power to you. The niggling detail that the diameter of a planetary (or solar) body is not a perfect mapping of it's gravitational footprint is a detail I expect from a physics book, not a game. Your mileage may vary.

 

[rancke]

Yes, and for the rare instance when the star's jump-inhibition radius (have we decided, is that shadowing or masking?)...

Being inside a jump limit is being shadowed; having a jump limit between you and your destination is being masked from the destination.

...is the significant factor, that will make a difference (like in the case of the subject of this thread). Otherwise, it can almost always be hand-waved away. "Gravitational tidal forces that must drop off to a point about equal to what would be 100D from a rocky world" works well for my suspension of disbelief. If you want to change the distance to 30D for stars (or stars and gas giants), more power to you.

30D (or whatever is the right number, I can't remember) is the point where gravitational tidal forces of something with the density of a sun has dropped to about equal to the 100D limit of a rocky world. One follows the other.

The niggling detail that the diameter of a planetary (or solar) body is not a perfect mapping of it's gravitational footprint is a detail I expect from a physics book, not a game. Your mileage may vary.

For worlds with the same density, diameters based on tidal force do map perfectly.


Hans

 

[WistfulD]

Then we're in agreement.

 

[whartung]

30D (or whatever is the right number, I can't remember) is the point where gravitational tidal forces of something with the density of a sun has dropped to about equal to the 100D limit of a rocky world. One follows the other.

Do you have the math? I've been playing with it, and I can't get it to work at ALL, so I'm messing something up.

What is being used for calculating the tidal force?

I've been basing it on a = GM/R^2 -- a = accelerating (i.e. tidal force), G univ grav constant, M mass of Thing (rocky planet or sun), R, distance from Thing.

Some more clarification.

100D is 100 Diameters of the object, so for Earth, ~8000 miles in diameter, we're talking a sphere that's 800,000 miles in diameter, but radius, thus distance to Earth, is 400K. Right? So if I'm within 400K miles of earth, I'm within the 100D limit, vs within 800K miles, which is 200D.

"100D from something" I find confusing.

 

[Quint]

Do you have the math? I've been playing with it, and I can't get it to work at ALL, so I'm messing something up.

What is being used for calculating the tidal force?

I've been basing it on a = GM/R^2 -- a = accelerating (i.e. tidal force), G univ grav constant, M mass of Thing (rocky planet or sun), R, distance from Thing.

Some more clarification.

100D is 100 Diameters of the object, so for Earth, ~8000 miles in diameter, we're talking a sphere that's 800,000 miles in diameter, but radius, thus distance to Earth, is 400K. Right? So if I'm within 400K miles of earth, I'm within the 100D limit, vs within 800K miles, which is 200D.

"100D from something" I find confusing.

You know, I may have been reading this wrong for 30+ years, but I always took it as a sphere, the radius of which was 100x the diameter of the object. Not sure if that helps your numbers at all.

D.

 

[tjoneslo]

Do you have the math? I've been playing with it, and I can't get it to work at ALL, so I'm messing something up.

What is being used for calculating the tidal force?

I've been basing it on a = GM/R^2 -- a = accelerating (i.e. tidal force), G univ grav constant, M mass of Thing (rocky planet or sun), R, distance from Thing.

Tidal force calculation is GM/R^3, using the same constants above, but is also multiplied by the size (length) of the object in question. So a 200m scout ship and a 2,000m super-destroyer have different tidal force pressures on them.

Usually I've assume the the tidal force is over the distance of 1m so just using the formula above..

Some more clarification.

100D is 100 Diameters of the object, so for Earth, ~8000 miles in diameter, we're talking a sphere that's 800,000 miles in diameter, but radius, thus distance to Earth, is 400K. Right? So if I'm within 400K miles of earth, I'm within the 100D limit, vs within 800K miles, which is 200D.

"100D from something" I find confusing.

It is a radius of 100 times the diameter of the object. For Earth is would be within 800,000 km would be the unsafe distance.

 

[tjoneslo]

Unfortunately, if it's based on tidal force, 100 diameters works fine for terrestrial worlds, but ought to be around 30 diameters (IIRC) for suns.

If you calculate the tidal force for a standard rocky planet to match the 100D limit, the limit becomes 90D to 110D for most worlds depending upon density, about 60D to 80D for most gas giants, and 20D to 30D for most stars. In some cases, like a late stage red giant the 100D limit ends up inside the star.

The other neat thing with using tidal forces is, if you do the integration using General Relativity, in the places where a planet orbits right at the "100D limit" of the star, there are places where the tidal forces balance out, creating a safe jump zone much closer than 100D .

 

[rancke]

The other neat thing with using tidal forces is, if you do the integration using General Relativity, in the places where a planet orbits right at the "100D limit" of the star, there are places where the tidal forces balance out, creating a safe jump zone much closer than 100D .

IMO that would allow jump travel the same way having a lake big enough to float a boat would allow ocean travel. Such a spot would be surrounded by unsafe jump zones, precipitating a ship out right next to where it entered jumpspace.


Hans

 

[mike wightman]

Which is why you need a really skilled navigator and an antique 'lost jump rutter of Mc Sporran' to make the jump...

consider the adventure possibilities.

Adventurers come across the rutter in a place of your choice, the Imperium/megacorps/evil bad guys TM want it too.

They have to unlock its secrets in order to stand a chance of making the jump

Once they arrive they discover the secrets to be found on the world.