Planet density and diameters

[Chaos]

chaos- my calculations bring me to 99 * radius for g to equal .0001, or 50ish diameters. what formula are you using or are you using 100 radii rather than diameters? or am i doing something wrong here?

*checks math*

Woops! It is indeed radii. I must be getting old.

 

[Imperial Planetologist]

For sake of playability - just use 100 diameters and forget the rest. Remember that was the beauty of CT, just enough reality to be playable. Isn't this whole discussion about why you have the navigator make his navigation roll in the first place, to see if he(she) did his job right including obtaining all the pertinent information?

I like the cut of your jib, sir or madam.

 

[Theophilus]

I was just thinking about the effects jumping into the system would have on commerce, fuel use, and military campaigns.

No longer must you just guard the planet earth, the 100d limit gives an enemy a huge volume of space to jump into, and allows fleets to maneuver before engaging, plus the chances of a misjump when jumping from inside the 10d limit increase, since the area around a sun is much larger than just around a planet.

 

[dccarles]

For sake of playability - just use 100 diameters and forget the rest. Remember that was the beauty of CT, just enough reality to be playable. Isn't this whole discussion about why you have the navigator make his navigation roll in the first place, to see if he(she) did his job right including obtaining all the pertinent information?

I figure I'll just multiply the figures for rocky planet(oid)s by 5, and not worry about jump masking unless someone specifically uses it. It seems like a reasonable compromise, and I don't think 100 diameters is very big for an Earth-sized planet...not when Traveller: The New Era has sensors making detections at 4+ light seconds.

Edit: for the record, the sun has a 10D radius of about 47 light-seconds, i.e., 470 hexes in Brilliant Lances or Battle Rider. An averageish neutron star has a mass of about 1.5-2x that, so you'd have to be about 15 light-minutes from one to make a perfectly safe jump.

--Devin

 

[Theophilus]

So, The Sun has a 100d limit of 470 light second, of 4700 hexes in BL/BR. Which equals 7.83 light minutes, and Earth orbit at 8 light minutes?

Is this correct?

 

[far-trader]

So, The Sun has a 100d limit of 470 light second, of 4700 hexes in BL/BR. Which equals 7.83 light minutes, and Earth orbit at 8 light minutes?

Is this correct?

Sounds about right. The Earth is just outside the 100D limit of Sol. This makes it rather significantly masked (hidden/eclipsed) for jumping.

 

[rancke]

Sounds about right. The Earth is just outside the 100D limit of Sol. This makes it rather significantly hidden/eclipsed for jumping.

I've been trying to promote two terms to distinguish the two different kinds of gravity-related jump impediments.

Jump shadowing is when you're inside some object's jump limit.

Jump masking is when you're not shadowed, but there's a jump shadow between you and the place you want to go to.

'Eclipsed' is a perfectly good term too, but 'masked' has been used for some years now,


Hans

 

[far-trader]

Thanks for the note Hans, I've always been unable to keep the terms straight in my head* so rather than goof and pick the wrong one I used my own descriptive.

* just my own mental block/failure to learn them, nothing against them

 

[Icosahedron]

I ran some rough calcs on this just for curiosity. Disclaimer: I'm not immune to error.

Setting the Jump limit to a gravitational constant rather than a size constant makes more scientific sense, but whether you'd want to change Traveller that much is for you to decide.

Gravitational field strength at 100 earth Dia is 0.0001G (1/100^2)

Using this as the threshold value, I get jump limits of 35D for a size 1 world, 50D for a size 2 (the Moon) and 111D for a size 10 world. (Assuming Terrestrial density)

For gas giants (assuming density of 0.2*Earth), I get 111D for a SGG (50,000mls) and 158D for a LGG (100,000mls).

These could be workable in Traveller, but it gets dodgy when you calculate for stars: the 0.0001G jump limit for the Sun (870,000mls) is 466D, which covers the orbits of Earth and Mars and reaches almost out to Jupiter.

You'd have to fly a long way in to Earth if you precipitated at the Sun's 0.0001G limit.

The only way to precipitate in the habitable zone is to raise the gravity precipitation threshold.

In order for the Earth to be clear of the Sun's jump shadow, (100 solar diameters) you'd need a gravity threshold of 0.002G, and that would put the Earth's own jump limit at 22 planetary diameters, so a typical jump radius for a terrestrial planet would be 20D instead of 100D.


Jump limits for 0.002G:

Size 1 would be 8D, size 10 would be 25D

SGG would be 25D, LGG would be 35D

Sun would be 100D

Just some figures for you to play with.

I have no idea if 0.002G corresponds with the habitable zones for other stellar types, I'm not about to spend that much time on the calculations.

 

[rancke]

I ran some rough calcs on this just for curiosity. Disclaimer: I'm not immune to error.

Setting the Jump limit to a gravitational constant rather than a size constant makes more scientific sense, but whether you'd want to change Traveller that much is for you to decide.

The solution that I favor sets the limit to something called the tidal force (or possibly something that works the same way; this is beyond my own knowledge of physics). That is the same for all bodies of a given density. So a world with a density of 5.5 has a jump limit of 100 diameter, regardless of its size. IIRC tidal force is directly proportional to the density, but I may be wrong there. Be that as it may, the big difference comes for gas giants and stars, which will have considerably smaller jump limits. Since very few writers have ever used solar jump limits in their adventures (I could be wrong, but my own JTAS adventure "Cry "Egherz!" and let loose the Humans of War" (http://jtas.sjgames.com/login/article.cgi?1028, set on Walston (which orbits far inside the 100 diameter limit of its sun) may be the first and only published adventure that makes use of the solar jump limit as a plot element), such a change wouldn't be difficult to retcon in, and it sure makes a whole lot more sense than the current situation. Unfortunately, Marc Miller disagrees.


Hans

 

[shaunhilburn]

The average density of a rocky planet (like Earth) is about 5 tons/cubic meter[1].

Sort of off-topic, but this is a common misconception. Mean density is proportional to mass. Small solid planets cannot have a high mean density, and large ones cannot have a lower mean density.

There are mass-radius functions that define a sort "main sequence" for silicate-iron terrestrial planets. A simplified one is:

Radius (earths) r = m^x
Mass (earths) m = r^(1/x)
Density (earths) d = m/r³

Where the scaling exponent x = 0.283 for 0.1 earth masses (size 4) and 0.276 for 1.7 earth masses (size 9+).

 

[dccarles]

Sort of off-topic, but this is a common misconception. Mean density is proportional to mass. Small solid planets cannot have a high mean density, and large ones cannot have a lower mean density.

Oh, I know; it's exactly the same way with neutron stars. I was just trying to get rough values (correct within a factor of 2 or so.) Excessive accuracy in soft science fiction isn't important unless some critical plot point is deliberately based on it.

--Devin

 

[Icosahedron]

The solution that I favor sets the limit to something called the tidal force (or possibly something that works the same way; this is beyond my own knowledge of physics). That is the same for all bodies of a given density. So a world with a density of 5.5 has a jump limit of 100 diameter, regardless of its size. IIRC tidal force is directly proportional to the density, but I may be wrong there. Be that as it may, the big difference comes for gas giants and stars, which will have considerably smaller jump limits. Since very few writers have ever used solar jump limits in their adventures (I could be wrong, but my own JTAS adventure "Cry "Egherz!" and let loose the Humans of War" (http://jtas.sjgames.com/login/article.cgi?1028, set on Walston (which orbits far inside the 100 diameter limit of its sun) may be the first and only published adventure that makes use of the solar jump limit as a plot element), such a change wouldn't be difficult to retcon in, and it sure makes a whole lot more sense than the current situation. Unfortunately, Marc Miller disagrees.


Hans

Sorry, Hans, I'm not planning on working out tidal forces anytime soon - besides, I've forgotten the equations.

Fortunately, I don't have to worry what Mr Miller likes, I'm a maverick with an ATU all of my own.