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09-28-2017, 03:28 PM | #1 |
Join Date: May 2010
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[Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
So the Azrael-class world-killer in Spaceships 3 is described as a "dinosaur killer", not something that would literally make a planet go kaboom Death Star style. But what if you fired it at a moon? Gravitational binding energy calculations suggest the Azrael doesn't have enough energy to destroy a Ceres-sized moon... but could obliterate a moonlet like one of Mars' moons.
If that happens, what happens to the planet beneath? If it hits the moon from the near side, I think the gamma radiation from the impact fries anyone on one-half of the planet's surface. If it his from the other side, I think the moon's mass will protect them from the gamma rays. But will they be killed by falling rock? This is the point where my physics knowledge fails me. Does anyone know the physics required to work this out? Similar questions about say, an Azrael hitting the far side of Luna would also be interesting to have answers for. |
09-28-2017, 04:15 PM | #2 |
Hero of Democracy
Join Date: Mar 2012
Location: far from the ocean
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
One thing worth considering is that unless the vectors line up just right, the bulk of the energy will blow right past the planet the moon was orbiting.
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09-28-2017, 04:39 PM | #3 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
Depends on distance. Debris from the moon is mostly uninteresting, unless the average expansion velocity of the moon fragments exceeds the delta-V required to deorbit the moon most of the fragments just stay in orbit creating a new ring system, and if you have enough energy it's a pretty small moon and most of the energy will be wasted in fragments that miss the planet, so likely better off just hitting the planet directly. Gamma rays are also irrelevant, they'll get stopped by the atmosphere. However, the thermal flash will be dangerous at 100,000 km or so, so if the moon is relatively low altitude and the explosion is visible from the ground you might set a hemisphere on fire.
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09-28-2017, 06:04 PM | #4 | ||||
Join Date: May 2010
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
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If it helps to clarify, though, I'm interested in situations with relatively close-in moons, like Mars' phobos. Couldn't the debris from that do a lot of damage? I should also clarify that I wasn't necessarily assuming the goal is to damage the planet directly. Maybe there's an important space fortress on the moon, or maybe (as in THS' Mars) it's anchoring a space elevator. Quote:
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09-28-2017, 06:19 PM | #5 | |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
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Mass of phobos: ~1e+16kg If we figure 1e+23J is used to create fragments, that's 1e+7J/kg, or an expansion velocity of 4.5 km/s. That exceeds the orbital velocity of Phobos, so some of it is gonna hit Mars (most of it is gonna go off into deep space and never be seen again). Figure around 10^15 kg of pieces hit Mars at 6 km/s (18 MJ/kg). Total energy reaching Mars: 1.8e+22J, or about a tenth as much as the original weapon. Not great, but significantly less dangerous than a direct hit, and a lot harder to direct (you can aim the ship at stuff you want to die, such as cities; you can't aim fragments of moon). If I were gonna use an Azrael, I'd probably design it to turn into shrapnel, 10% of its mass as shrapnel across an area of 100 million square kilometers will flash the upper atmosphere and result in a ground level thermal pulse of about a megajoule per square meter, which will set the entire hemisphere on fire and kill everyone exposed to the sky. Use the remainder on harder targets. Last edited by Anthony; 09-28-2017 at 06:30 PM. |
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09-28-2017, 06:26 PM | #6 |
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Join Date: Oct 2004
Location: Forest Grove, Beaverton, Oregon
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
Sounds like turning a single slug firing shotgun to firing shot. Not great for single tough opponents, but not useless against large "squishy" foes like planet ecosystems.
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09-28-2017, 08:08 PM | #7 |
Join Date: Apr 2015
Location: Pennsylvania
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
According to my Ti-89, the tau factor need to make a 10^10 gram Azrael (nice round number) have 2x10^32 J of kinetic energy is 222,531
The tau factor at 0.99999999999c (eleven nines) is 223,607. So at that fraction of the speed of light, the KE of a Azrael exceeds the gravitational binding energy of a Earth-sized body. Does anyone want to work out how long (relative to the target planet) it would take one to reach that speed? |
09-28-2017, 10:07 PM | #8 | |
Join Date: May 2010
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
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09-29-2017, 07:01 AM | #9 | |
Join Date: Aug 2007
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
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It's already using a Cinematic ramjet to get to 0.5 c. A realistic speed limit is probably a good bit lower than that. Like magsails ramjets appear to make better brakes than accelerators.
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09-29-2017, 04:04 PM | #10 | |
Join Date: Apr 2015
Location: Pennsylvania
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Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon
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Not to mention the difficulty in actually hitting a planet sized target at 0.5c, let alone any number of nines. You can't line it up perfectly at the start, and terminal corrections at relativistic speeds are gonna be hard. |
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