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Old 09-28-2017, 04:28 PM   #1
Michael Thayne
 
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Default [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.
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Old 09-28-2017, 05:15 PM   #2
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Default 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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Old 09-28-2017, 05:39 PM   #3
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Default Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon

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Originally Posted by Michael Thayne View Post
If that happens, what happens to the planet beneath?
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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Old 09-28-2017, 07:04 PM   #4
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Default Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon

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Originally Posted by ericthered View Post
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.
A majority of the energy, maybe, but any fragments that are in the right trajectory to de-orbit will be dangerous.

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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.
"Most" covers a range of values, though, from 0.5 + epsilon to 1 - epsilon. Which is a big range when you're talking about a proportion of millions of megatons of TNT equivalent.

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.

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Gamma rays are also irrelevant, they'll get stopped by the atmosphere.
Right, I forgot about that.

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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.
Ack, good point. I was confused about this until I checked just now, but thermal effects decline with the square root of distance, so when they dominate the optional "cube root" explosion damage rules become the less realistic ones. So for a low moon, that would be very bad.
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Old 09-28-2017, 07:19 PM   #5
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Default Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon

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Originally Posted by Michael Thayne View Post
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?
Total energy, assuming the math is correct: ~1.8e+23J.
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.
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Last edited by Anthony; 09-28-2017 at 07:30 PM.
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Old 09-28-2017, 07:26 PM   #6
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Default 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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Old 09-28-2017, 09:08 PM   #7
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Default 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?
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Old 09-28-2017, 11:07 PM   #8
Michael Thayne
 
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Default Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon

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Originally Posted by Anthony View Post
Total energy, assuming the math is correct: ~1.8e+23J.
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.
Ah, this math is very helpful to have. Thank you.
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Old 09-29-2017, 08:01 AM   #9
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Default Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon

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Originally Posted by cvannrederode View Post
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?
Do you want to include the time required to rip out the Ramjet and replace it with ^ Reactionless Thrusters?

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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Old 09-29-2017, 05:04 PM   #10
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Default Re: [Spaceships] [Space] Firing an Azrael-class world-killer at a planet's moon

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Originally Posted by Fred Brackin View Post
Do you want to include the time required to rip out the Ramjet and replace it with ^ Reactionless Thrusters?

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.
Ramscoops are thought to be better brakes than anything else at almost any speed, if I recall. I would consider them super science just as much as reactionless thrusters, but that's just my personal opinion. Comes down to flavor.

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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