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Here is the scenario:
The Martian leader is going to have a speech at certain location according to a reliable source.
Objective is to eliminate just him
But here's the catch:
To show Earth dominance over puny Martian rebels, you must kill him from Earth surface in less then a few hours.
You can use mass and/or massless projectiles.
What would be the best weapon for this mission?

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    $\begingroup$ Uhm ... you're outta luck. $\endgroup$ – AndreiROM Dec 18 '15 at 16:44
  • $\begingroup$ Just to be clear, the martian leader is on Mars, and you want to kill him from Earth? $\endgroup$ – AndyD273 Dec 18 '15 at 17:11
  • $\begingroup$ @AndyD273 yes he is on Mars surface $\endgroup$ – Ivan Dec 18 '15 at 17:22
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    $\begingroup$ How about a time machine? What level of technology do we have available? And can we assume that the angles involved are optimal, so that this guy isn't behind the planet? $\endgroup$ – DaaaahWhoosh Dec 18 '15 at 17:53
  • $\begingroup$ A slightly similar question I asked has answers that might help your storyline: worldbuilding.stackexchange.com/questions/27254/… $\endgroup$ – Mikey Dec 18 '15 at 21:02
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I think massless is the way to go here. To make the journey to Mars in three hours, we'd need to accelerate the projectile up to about 0.08c. That may not sound like a lot, but that's 25,070 km/s. For a comparison, the fastest ever man-made object to date (not counting the nuclear powered manhole cover) is the Juno probe, which pulled a blistering 40 km/s during its gravity assist on the way to Jupiter. The object would need fairly substantial mass to penetrate both the terrestrial and martian atmospheres, so the amount of energy you'd dissipate into the surrounding crowd would violate the "just him" rule.

So we turn to lasers. Let's pretend we've got some new technology that allows us to produce very high-powered laser pulses (enough to reliably kill a human) in a way that won't be immediately distorted in the atmosphere. Visible is actually the best choice here, as it's relatively high-energy and the atmosphere is mostly transparent to it (as opposed to, say, gamma, which the atmosphere strongly filters). A terawatt laser is probably the most likely, considering how far we have to project this energy. Petawatt lasers exist, but can currently only fire for such short bursts (0.5 picoseconds) that the total energy delivered is probably going to be lower than with a terawatt laser.

Now, the next big problem: focus. We can achieve very good angular resolution with large arrays of telescopes, and in principle this process will work in reverse. Aiming them all together is going to need a pretty advanced computer targeting system, but we'll need that anyway to crunch the targeting data (which is going to have to account for the orbits and rotation of the planets, atmospheric distortion, etc), so let's say we've got it. Since you're going to have to fire the weapon several minutes in advance, you don't actually want to make the resolution too fine: we're relying on the power of our laser here, and we're going to try to envelop the whole stage (because odds are over the course of the trip he'll walk back and forth some). We'll have to assume the targeting computer can account for the scattering caused by the atmosphere to reliably achieve this radius without cooking the crowd. Depending on the exact dimensions of the stage and crowd, this may not be possible.

And if you overcome all of those problems (and probably a bunch I haven't considered)... Bam! Dead rebel leader. It's that easy.

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  • $\begingroup$ Laser was my first pick. Isn't it as fast as light ? $\endgroup$ – Kii Dec 18 '15 at 20:28
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    $\begingroup$ Indeed. Laser is an acronym that stands for Light Amplification by Stimulated Emission of Radiation; ie a laser is light, and light always travels at the speed of light, although it doesn't always travel at c (the speed of light in a vacuum). Light slows down very slightly when it passes through matter, such as air or water. $\endgroup$ – realityChemist Dec 18 '15 at 21:12
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    $\begingroup$ Of course that laser will pierce the roof of the compartment the speech is happening in, so you better have a few agents on Mars that can covertly prepare for a repair group standing by, so the hole can be fixed before too much air leaves; otherwise you'll have a few more dead people on Mars due to suffocation. $\endgroup$ – celtschk Dec 19 '15 at 9:43
  • $\begingroup$ From this answer even in vacuum the laser would attenuate way to much to be lethal from 139 million miles, especially with going through two atmospheres $\endgroup$ – AndyD273 Jan 3 '16 at 20:03
  • $\begingroup$ @realityChemist, what does "massless" mean? $\endgroup$ – Len Feb 7 '18 at 0:47
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Better than eliminating him would be to project a wormhole from Earth (or Earth orbit) and either funnel him into the mouth of the wormhole or have a "snatch squad" jump out and grab him, pulling him back into the wormhole (and to custody on Earth) in front of a live audience and all the people who have logged on to see the podcast live.

Thos would most certainly demonstrate the power of Earth over Mars or anyplace else in the Solar System (or the rest of the Universe, for that matter), and of course once the rebel leader is in custody then he can be questioned and information extracted to deal with the rest of the Martian rebellion (Any culture which can use a wormhole to kidnap a political opponent is probably capable of vacuuming information out of a person's head without anything so crude as "enhanced interrogation").

For a slower but more subtle means of dealing with the Martian leader, the EarthForce will target the audience with memes that invalidate the message of the leader and discredit him in the eyes of the audience. Something much like this happens with social media today in terms of developing followers, getting likes and so on, with a few hundred years of research, memes could be created with high degrees of precision.

Since the Intelligence service would have this guy under close surveillance, and have developed a clear picture of the audience, the development and targeting of the meme isn't guesswork. Knowing the time and place of the speech gives the EarthForce time to craft and plant the meme (or more likely a chain of related memes which build off each other) days or even weeks ahead of time. The leader gets "taken out" by being booed off the stage and another person (previously selected for the role by Intelligence) is swept into the position in a wave of popular support.

And hey, those Earth guys are an all right bunch once you get to know them. That last guy was just blowing things right out of proportion and creating trouble where there was none.....

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When Greg Bear wrote about a separatist Mars movement in his novel Moving Mars, Earth's main weapon in the end was "locusts": nanotech fabrication systems planted surreptitiously on the Martian surface well in advance of any rebellion.

"How do you control an entire planet from across the Solar System? Seed it with nano factories that can build a variety of automatic weapons, self-directing warbeiters. Mars's soil is ideal. High silicate and aluminum, high ferrous content. Choose old mines or seemingly depleted sites, still rich with the basic materials, open to deep exploration and concealment without triggering alarm. Sprinkle nano factory seeds from orbit. A single small ship could do it. We have no defense against such an atrocity."

(In the novel, an "arbeiter" is (basically) an autonomous robot, so a "warbeiter" is a weaponized arbeiter.)

Obviously this requires some forethought on Earth's part. But if this infrastructure was already in place before the rebellion, it could be activated within the span of a few hours as requested. Presumably there would be a satellite in orbit to relay instructions to the ground-based fabrication units; Earth sends the signal to the satellite, which activates the nearest nanotech factory, which creates a small drone that can assassinate the separatist leader.

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  • $\begingroup$ You still have to move the drone to the separatist leader location, which may take a lot of tunneling $\endgroup$ – SJuan76 Dec 18 '15 at 21:23
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If you have massless projectiles you might be able to pull it off, since a massless projectile would be able to travel at the speed of light.

The average distance between Mars and Earth is around half an hour at the speed of light. So you would have about that long between when the shot is fired to when it hits.
Because it's massless and moving at the speed of light you aren't going to be able to do any orbiting, so the location with the gun and the location where the target is have to be facing toward each other.

This massless projectile is going to need some kind of guidance system, since on average Mars is 139 million miles away, and you're trying to hit a moving target a foot across.

Avoiding other casualties is going to be hard, because even though it is massless it's still moving really really fast, and will be displacing air as it travels toward the target, which will make a heck of a shock wave. Also, at those speeds, the planet isn't going to slow it down much so it'll enter the atmosphere, generate a shockwave that will cause damage to the crowd gathered to hear the speech, pass through the target, pass through the planet causing more shockwaves and damage, exit the other side causing a huge crater, and proceed to exit the solar system. The entry and exit points are going to know what happened.

If you made the projectile have some mass and shot it at any significant fraction of C, the damage would be even worse:
https://what-if.xkcd.com/1/
Another even more relevant example:
https://what-if.xkcd.com/20/

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  • $\begingroup$ What does "massless" mean? $\endgroup$ – Len Feb 7 '18 at 0:48
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The question of focusing the laser needs some expansion. Aiming, too.

First, distances. Since the average distance of Mars from the Sun is 228 million km, and for Earth it's 149 million, at conjunction the distance between the two is 79 million km, and we'll use this as the easiest example - call it X. In order to get a small focus spot (diameter d) for the laser, it will need to be expanded to some larger size D when it is launched, and this will be done with a mirror system. Let's figure a d of 1 meter, although this has problems of its own. What is D? This is determined by the equation $$D = \frac{4 \lambda X}{\pi d}$$ If we assume an NIR laser with a wavelength $\lambda$ of 1 micron ($10^{-6}$ meters) $$D = \frac{4\times 10^{-6} \times 79 \times 10^9}{\pi \times 1} = 100 \text{ km}$$ This may be a bit of a struggle to construct in a few hours, so I'll let you get on with it. If you think it will help, you could try a UV laser with a 100 nm wavelength, and the mirror will only need to be 10 km across. Granted, this will not penetrate the earth's atmosphere well, so you'll need a LOT of power, plus you'll get massive beam distortion, but I'm trying to be helpful.

As I mentioned, the small desired spot size has a problem or two. The most obvious is aiming. To hit a spot with an accuracy of 1/2 meter at a range of 79 million km requires a pointing accuracy of $$\theta = \frac{0.5}{79 \times{10^9}}= 6\times 10^{-12}\text{ radians} = 0.0000000034 \text{ degrees}$$ Roughly.

The orbital velocity of Earth is about 30 km/sec, while Mars is about 24 km/sec, so at conjunction the difference in velocity is about 6 km/sec. This means that the time of transit of the laser pulse must be (for 1/2 meter accuracy) accurate to $$t = \frac{0.5}{6\times{10^6}} = 83 \text{ nsec}$$ In other words, you will have to point an enormous mirror with extraordinary accuracy AND fire the laser with considerable precision of timing. Good luck.

Oh yes, and one more obvious problem. If the two planets are in conjunction, and the speech is being given at night (local time) you're just going to be out of luck with the laser thing. Your target is on the other side of the planet, and unless you think you can drill a very large hole you're out of luck.

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