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04-07-2017, 09:56 AM | #11 | |
Join Date: Jul 2008
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. |
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04-07-2017, 10:03 AM | #12 | |
Join Date: Mar 2013
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
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On which note, the SM for Neptune is +47 (including +2 for being a sphere), the distance is -74. Combine this with plain sight and silhouette modifiers and you get a net of +7 to detect it. Considering that the WISE mission could have detected Neptune-sized objects out to 700 AU, this is perfectly reasonable, especially when you consider that WISE could take advantage of extra time. |
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04-07-2017, 10:03 AM | #13 |
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
Because 'dim star' implies that it visibly shines, but dimly, due to reflected light.
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04-07-2017, 10:13 AM | #14 | |
Join Date: Jul 2008
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
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Of course, Spaceships spotting rules are actually for objects that are (infrared) light sources, not for spotting objects by reflected light. You really should be modifying Neptune's visibility for the illumination level!
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. |
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04-07-2017, 10:48 AM | #15 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
The +24 should really replace the +10 for in plain sight, rather than adding to it.
The standard formula for the apparent magnitude of an asteroid is roughly 15.6 - 5 x log10(diameter in km) + 5 x log10 (distance in AU) + 5 x log10 (distance from sun in AU) - 2.5 x log10(albedo) plus a phase modifier; a white 1 km asteroid at half phase at 1 AU would thus be around magnitude 16.5. If we assume a magnitude 0 star is +0, the vision modifier should be -1.2*magnitude or -20. A 1 km asteroid is SM+18 (assuming a +2 for a spherical object), a white or highly reflective object is another +2, a range of 1 AU is -66, so using basic modifiers we get a difficulty of -44, meaning we need another +24 in there to get it up to the expected value of -20. |
04-07-2017, 10:52 AM | #16 | |
Join Date: Mar 2013
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
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04-07-2017, 11:02 AM | #17 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
This is...surprisingly irrelevant, and certainly not enough to give a +10 modifier. Net IR emissions are typically similar to the white body above, though the IR background is significantly different (stars are relatively dimmer, but there's a lot of background dust).
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04-07-2017, 11:07 AM | #18 | |
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
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Some of them. There are bonuses if the target has the equivalent of at least Auxialiary power, but a target with no systems running (e.g. a long-lost derelict) gets a +0 signature. |
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04-07-2017, 11:22 AM | #19 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
Note that my above calculation did ignore active power generation. Normal 'natural' IR radiation is about 400W x 10^(SM/3), or 40 kW for a SM+6 spaceship. Based on beam output and RoF, 1 EP at SM+6 is at least 3 MW (improved major batteries fire 1/10s, 30 MJ shots); if we call it 4 MW, which is pretty generous, it's equivalent to a SM of +12, and a more likely figure is +13. Thus, the bonus for actively using 1 EP (normally doesn't happen unless using drives or weapons) is 4 + SM/2.
Now, a 100 ton thrust drive has a minimum power output of 4.5 MW x ISp; ISp is equal to (MPS/tank) * 3200. This results in some fairly ridiculous drive signatures for any interesting drive. |
04-07-2017, 11:24 AM | #20 | |
Join Date: Jul 2008
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Re: [Spaceships] The value and realism of +24 if Silhouetted Against Deep Space . . .
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You don't tend to have humans being the only light-reflecting objects in an area, but if you do (which in normal lived experience requires careful spotlighting) I think they can look pretty shiny. Satellites can look like stars, albeit suspiciously mobile ones. It's not because they're extraordinarily shiny, it's because they're well-lit against a dark background. ...Anyway, if you're looking at visual spotting of Neptune you definitely need to give it a penalty for having 900 times less intense light than we get here at Earth orbit. In principle there should be some modifier for reflectiveness (since matte black does work to make things hard to see like this) which might be more favorable for Neptune than for a human. Almost certainly more favorable than for a human in dark clothing. Yeah, but it's still not spotting by reflected light. It can't be, there would have to be a parameter for the light level.
__________________
I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. |
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Tags |
cmbr, detection, plain sight, sensors, silhouette, space, spaceships, stealth |
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