WBH Tidal Lock DMs

[Dodo98]

@Geir Quick question, I was wondering about the +DM for Size in the Tidal Lock DMs section. A +DM would mean that bigger planets are more likely to be locked to their star, but shouldn't it be the other way round? Smaller planets are more likely to be tidally locked than Gas Giants.

 

[Limpin Legin]

Quick question, I was wondering about the +DM for Size in the Tidal Lock DMs section. A +DM would mean that bigger planets are more likely to be locked to their star, but shouldn't it be the other way round? Smaller planets are more likely to be tidally locked than Gas Giants.

a large moon will lock faster than a smaller moon at the same orbital distance from the planet because moon mass grows as the cube of the moon radius . A possible example of this is in the Saturn system, where Hyperion is not tidally locked, whereas the larger Iapetus, which orbits at a greater distance, is.

Tidal locking - Wikipedia

en.wikipedia.org

 

[Geir]

@Geir Quick question, I was wondering about the +DM for Size in the Tidal Lock DMs section. A +DM would mean that bigger planets are more likely to be locked to their star, but shouldn't it be the other way round? Smaller planets are more likely to be tidally locked than Gas Giants.

You'd think so, but the diameter of the smaller world matters. The bigger it is, the more the differential between the 'near' and 'far' side, so it actually has a greater tidal force applied to it. All other things being equal.

 

[Dodo98]

You'd think so, but the diameter of the smaller world matters. The bigger it is, the more the differential between the 'near' and 'far' side, so it actually has a greater tidal force applied to it. All other things being equal.

Interesting, you never stop learning

That said, I have two more questions about the DMs. Do I always subtract the DM-2 for atmospheric pressure above 2.5 bar for gas giants? I don't calculate it for gas giants, but I would have assumed that the pressure for gas giants is always above that, right?

And my other question would be, which size code should I use for gas giants? In the section about determining the size of gas giants, it says that they always have size code G + the designation of whether they are small, medium, or large. According to the eHex conversion, G would correspond to 16, is that correct?

 

[Geir]

Interesting, you never stop learning

That said, I have two more questions about the DMs. Do I always subtract the DM-2 for atmospheric pressure above 2.5 bar for gas giants? I don't calculate it for gas giants, but I would have assumed that the pressure for gas giants is always above that, right?

And my other question would be, which size code should I use for gas giants? In the section about determining the size of gas giants, it says that they always have size code G + the designation of whether they are small, medium, or large. According to the eHex conversion, G would correspond to 16, is that correct?

I want to make sure I'm answering the right question. The 'DMs for all cases' assumes the DMs for body that you're checking to see if it is tidally locked. So if you're checking to see if the a gas giant is tidally locked, then yes. But... I'd never bother to check to see if a gas giant is tidally locked, because I don't think it means anything. I mean, the solid core of the gas giant might be locked, but what we generally consider the 'surface' of a gas giant is the atmosphere, and I assume the atmosphere would be mostly affected by heat of the star if the gas giant was close enough to be locked. Maybe not if a gas giant was a moon of another gas giant, though. Still, for something that I can't reach the surface of, I wouldn't bother to check, and the rules haven't really been tested against a locked gas giant scenario. Maybe a lack of imagination on my part.

As far as size for gas giants, the Gas Giant Sizing table on p.55 gives the size of gas giants in Terra-sized units, so multiply that value by 8 to determine Traveller scale Size. (The hopefully soon coming update will use 2D6+10 for large gas giants to give an answer of 12-22⊕ , though). For instance Jupiter (at 317 Earth masses) would count as a Medium gas giant with 11 times the diameter of the Earth and so would be GMB (with B=11, but = Size 88 in normal planet Size)

 

[rinku]

We *may* have some data on hot jupiters and tidal locking, but I'd think it's still subject to a whole lot of educated guessing. In any case, the temperature differential between sunside and nightside is going to be doing a whole lot of chaos in a deep atmosphere or hydrosphere.

As Geir says, it really won't matter when you're talking about a big ball of gas, or even a planet like Venus. That comes down more to "how hard are the extreme winds blowing my ship around in the upper atmosphere?" and "has my hull melted yet?"

There is a middle ground for solid planets whose surface you can land on, though. But those are covered.

 

[Dodo98]

I want to make sure I'm answering the right question. The 'DMs for all cases' assumes the DMs for body that you're checking to see if it is tidally locked. So if you're checking to see if the a gas giant is tidally locked, then yes. But... I'd never bother to check to see if a gas giant is tidally locked, because I don't think it means anything. I mean, the solid core of the gas giant might be locked, but what we generally consider the 'surface' of a gas giant is the atmosphere, and I assume the atmosphere would be mostly affected by heat of the star if the gas giant was close enough to be locked. Maybe not if a gas giant was a moon of another gas giant, though. Still, for something that I can't reach the surface of, I wouldn't bother to check, and the rules haven't really been tested against a locked gas giant scenario. Maybe a lack of imagination on my part.

As far as size for gas giants, the Gas Giant Sizing table on p.55 gives the size of gas giants in Terra-sized units, so multiply that value by 8 to determine Traveller scale Size. (The hopefully soon coming update will use 2D6+10 for large gas giants to give an answer of 12-22⊕ , though). For instance Jupiter (at 317 Earth masses) would count as a Medium gas giant with 11 times the diameter of the Earth and so would be GMB (with B=11, but = Size 88 in normal planet Size)

Okay that makes sense. But if you go strictly by the rules, I couldn't find any indication that you don't have to check for gas giants. Perhaps a clearer instruction could be included in a second edition.

 

[rinku]

True tidal locking may also be avoided because of other bodies in the system and orbital eccentricity. Mercury is not tidal locked (as was originally thought) but in a 3:2 resonance for those reasons.

A big moon or twin planet may result in either body being locked to the other, but they may avoid being tidal locked to a primary as a result (i.e. they spin around their baycentre happily while zipping around the star in close orbit).

 

[CordwainerFish]

Mercury is not tidal locked (as was originally thought) but in a 3:2 resonance for those reasons.

Trivia: Larry Niven's first fiction sale was set on Mercury, and its plot relied on Mercury being tide locked. After it was bought but before it saw print, Mercury's 3:2 resonance was discovered. He wrote Frederik Pohl, the editor who'd bought it, if he wanted the money back; Pohl wrote back saying (roughly) "You nuts? Never offer an editor their money back!"