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The "rods from God" satellite. Its a satellite that contains these long darts, presumably made of a very heavy material like tungsten, that are dropped into the atmosphere. By the time they reach the surface, they're moving at hypersonic speeds, lets say Mach +10. It would strike a target with a huge amount of energy, but the question I have is "would that be a conventional explosion like from explosives, or would it basically just be a huge dart that sticks into the ground?" The rod isnt an explosive material, its just very heavy and durable. But with something as heavy as it is moving as fast as it is, the amount of force it has when it hits the ground has to be gigantic. Too big for just an anticlimactic dart piercing into the ground.

The reason I ask is that it would probably determine what kind of target this weapon would be used on. If it creates a more conventional explosion, it would probably be more effective against large groups of soldiers, vehicles, or buildings. But if it was basically a dart that punctures into the ground it would probably be a devastating bunker buster. Depending on the answer, this kind of weapon would be used on very different targets.

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    $\begingroup$ This is a cool question, but it needs details. Is the satellite geosynchronous or not? If so, then the vertical drop of the rod would reach a terminal velocity (that's likely not hyper-sonic). If not, then the rod would have considerable speed - but most of that speed would be tangential to the vertical drop, causing substantial heat on re-entry and possibly burning it up. Can you explain the satellite's orbit and how much rod mass you want to hit the ground? Or, better, can you tell us what you want to have happen and we'll tell you how it can? How big a boom do you want (specifically)? $\endgroup$
    – JBH
    May 27, 2022 at 5:29
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    $\begingroup$ "Used on very different targets": It liberates a few kilotons of TNT worth of energy. Any target in the vicinity is history. I am not sure what you think is the difference between liberating said energy from explosives, from a tactical nuke, or from a kinetic impact. Energy is energy. The effects will be the same -- very loud bang, very bright flash. The loud bang will produce a devastating overpressure shock wave in the air and a devastating mini-earthquake in the ground. The bright flash will incinerate anything around. The only difference is that the rod will also make a big crater. $\endgroup$
    – AlexP
    May 27, 2022 at 8:16
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    $\begingroup$ @sdfgeoff The satellite launch system releases its energy over the course of several minutes. The Rod from God releases it all in a fraction of a second. The difference is sort of like slowly pouring gravel on your foot, versus dropping a heavy rock of equivalent mass on your foot. A flamethrower and a hair dryer can also impart the same total energy, but over very different time scales. $\endgroup$
    – Nuclear Hoagie
    May 27, 2022 at 14:17
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    $\begingroup$ @AlexP your math must be off. At 7 km/s orbital velocity, each ton of material has a kinetic energy equivalent to 5.8 tons of TNT. Which is significant, about half the yield of a MOAB, but definitely not in the kiloton range (mv²/2 = 1000 kg*(7000 m/s)²/2 = 24.5 GJ). $\endgroup$
    – mic_e
    May 27, 2022 at 17:40
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    $\begingroup$ how does one "drop" something from orbit? $\endgroup$
    – ths
    May 27, 2022 at 19:04

4 Answers 4

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First off Mach 10+ is significantly slower than orbital velocity. Objects in low earth orbit are traveling at over 7 kilometers per second, which is roughly Mach 20.

This would be a serious explosion. We have video footage of large objects entering the atmosphere at high speeds. The Chelyabinsk meteor was estimated to be 20m and traveling at a shallow angle into the atmosphere at 19 km/s. When it burst in the air 30km above the surface, it damaged over 7 thousand buildings in 6 cities. The airburst was estimated to be equivalent to 400 kilotons of TNT, roughly 30 times the energy of the atomic bomb dropped on Hiroshima.

Keep in mind that your rod is significantly smaller, and traveling at a much slower speed. In a 2003 proposal the US military estimated that a 6 meter tungsten rod orbiting at 8km/s would hit the ground with the energetic equivalent energy of 11.5 tons of TNT. Far less than an airbursting meteor but keep in mind that when the Mythbusters blew up a cement truck they used the equivalent of roughly 600 lbs of TNT. That's 38 times less energetic than your rod. While such an energetic collision is overkill for soft targets, hard targets, or anything less than a reinforced underground bunker, you wouldn't want to be anywhere near it's point of impact.

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    $\begingroup$ Worth mentioning that part of the original concept was as an area anti-tank weapon - a few dozen crowbar-sized tungsten/depleted uranium rods plunging down from orbit with good terminal guidance could destroy a tank regiment, especially striking from above. Unfortunately (for the concept but not the tankies) no one could work out a sensor system that could see through the plasma around the nose that would be created as the crowbar plunged down from the heavens, therefore insufficient accuracy to target individual armoured vehicles. $\endgroup$
    – KerrAvon2055
    May 27, 2022 at 5:37
  • $\begingroup$ A satellite and GPS-based guidance system would get around the plasma issue you describe @KerrAvon2055. Location determined by GPS and sent to satellite, which can still see the tank(s)... No? $\endgroup$
    – user1521620
    May 27, 2022 at 21:22
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    $\begingroup$ @WHO'sNoToOldRx4CovidIsMurder - you still have the issue of communicating with the rod for guidance. $\endgroup$
    – TLW
    May 27, 2022 at 23:39
  • $\begingroup$ @TLW I believe you can talk to spacecraft in the fire with a laser in the near-UV. $\endgroup$
    – Loren Pechtel
    May 28, 2022 at 3:43
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    $\begingroup$ Interesting. Have a citation or more info? I would have thought that 10-12,000 K plasma would have enough black-body emission and absorption in the near-UV range to make things rather difficult. (12,000 K -> peak at ~240nm which is in the NUV range.) (Not necessarily impossible - but not exactly trivial either, and likely rather low bandwidth.) $\endgroup$
    – TLW
    May 28, 2022 at 18:56
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You will get a "Dark" Explosion

Non-explosive artillery can still have a significant explosive effect. That said, it will not make the sort of "fireball" explosion you get from a chemical warhead. So, you'll still get a significant explosive shockwave, a cloud of dust and debris, etc. But the heat and light from the impact will be significantly less than from a chemical explosive weapon of similar yield.

enter image description here

To understand the difference you have to realize that a kinetic weapon has to transfer energy into a target through physical interactions whereas an explosive or nuclear weapon does not. While a 5e10 Juele explosive releases all of its energy (and light) at the surface, most of the enegry released by a kinetic weapon will happen subsurface. What ever material a kinetic weapon hits only has a limited binding strength to stop the projectile before it becomes displaced. Most kinds of dirt, rock, sand, and soil have low enough of a binding energy that they become displaced before being heated up enough to visibly glow. While the metal rod will have a much higher binding energy and reach a very bright glow before loosing its integrity, nearly all of this ablation will happen underground such the the light from it will be concealed giving you only a minor flash at the moment of impact.

Also, you can not compare a rod from god to something like the Chelyabinsk meteor because a meteor can only airburst like that because it is a heterogeneous mixture of elements where internal element vaporize until they over pressure the yielding strength of the tougher elements. A pure element like tungsten will always ablate from the outside in during re-entry. Since tungsten has a very high melting point, and the rod has a very narrow cross section, it may in fact heat up enough to glow a good bit by the time it reaches the Earth, but it will still have plenty of integrity to spare when it hits which is the whole point of using tungsten to begin with.

enter image description here

The exception to this rule is when you hit somethin with more binding strength. When you strike steel for example, it will resist the kinetic impactor more before being displaced allowing it to reach temperatures needed to glow; so, you can still get a bright flash that is not concealed by non-glowing debris as shown by this tank being hit by a 30mm autocannon where strikes against the armor create bright flashes, but adjacent strikes against the ground kick up dark plumes of dirt. Striking a tank with a rod from god though will probably still kick up more dust than glowing metal because it will massively over-pernitrate the tank, but perhaps a large steel bunker or warship could still have a significant flash.

enter image description here

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  • $\begingroup$ What am I looking at in this picture of a tank? I see several different sources of light and I'm struggling to figure out the path the shell has taken and where it actually hit. Is this a real shot of a tank being hit by a 30mm shell or something simulated on e.g. a TV show or movie? $\endgroup$
    – Michael
    May 27, 2022 at 17:01
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    $\begingroup$ @Michael Not 1 30mm shell, it is an A10's 30mm autocannon. You are seeing things all over the place because you are seeing many near-simultaneous impacts. $\endgroup$
    – Nosajimiki
    May 27, 2022 at 18:33
  • $\begingroup$ Ah, gotcha... given the distance of the A10 from the tank and (to some extent) the fact that it is moving would explain why the hits aren't closer together. $\endgroup$
    – Michael
    May 27, 2022 at 18:41
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    $\begingroup$ With enough kinetic energy, the explosion will not be "dark". As a practical example: youtube.com/watch?v=58MmOpSm4LY No chemical explosives. Just a chunk of metal. The impacts are very bright. $\endgroup$
    – jdunlop
    May 28, 2022 at 0:47
  • $\begingroup$ @jdunlop You need to read my whole answer because I explained everything you are seeing in that link. Those are impacts are against metal targets which meet the criteria of the exception case I mentioned. However, that same weapon system used against concreate creates only a tiny flash from the ablating shell as it passes through as seen at 3:36 in this video: youtu.be/BS0KgdzMB5A?t=216 Like the concrete, an RfG striking the ground will cause minimal molten material to be thrown compared to the amount of non-molten earth the impact would displace to block said light.. $\endgroup$
    – Nosajimiki
    May 28, 2022 at 6:18
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Neither meteorites are made of explosive materials, but when they reach the ground they create an explosion.

Basically, an explosion is caused by the release of a large amount of energy in a very short time: it can be chemical energy in an explosive, nuclear energy in a nuke, kinetic energy in a meteorite and so on.

Whenever that happens, you have a boom.

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    $\begingroup$ Meteors often explode in the air. As they break up from aerodynamic stress, the surface area skyrockets, and so does drag, and thus so does aerodynamic heating. $\endgroup$
    – Ryan_L
    May 28, 2022 at 19:53
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As others have said, yes. It would release a ton of force which would destroy the surrounding area and create a cloud of pulverized ejecta. But it wouldn't be like an explosion from bombs, which have a lot of light. It would mostly just be dark particulates and larger debris.

However, if going fast enough, the rod would be incandescent from the heat of atmospheric entry.

How much damage, that is complex and based upon the mass, velocity, angle of impact, and the material the target is made up of. So the level of damage too hard to really calculate in a simple formula. It needs a lot of context to do accurately.

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