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Suppose that:

1-an interplanetary crewed rocket such as SpaceX BFR is built in secret.

2-the launch station can be arbitrary located.

3-the rocket is launched without any warning.

4-the rocket is directed to another planet, say Mars. No return trip is needed.

I guess that the rocket launch will be noticed in a matter of seconds. Being not authorised and potentially dangerous it is likely that the military will try to intercept it and destroy it.

Can the rocket safely made it into space? Or alternatively: are there any methods to stop a rocket directed towards deep space which launched without warning?

(I understand that conditions 1 and 2 are extremely challenging, but consider them as given assumptions)

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    $\begingroup$ Don't try this if you're in Norway, you'll get the Americans killed. There have been a couple instances of launches that weren't "premeditated" between nations, and sometimes the thought isn't "intercept" it's "retaliate". en.wikipedia.org/wiki/Missile_defense_systems_by_country are the systems for missile interception. $\endgroup$ – Magic Octopus Urn Oct 13 '18 at 13:59
  • $\begingroup$ Most countries also fear the EMP of a nuclear missile, and interception is difficult on hyper-sonic missiles that travel out of the atmosphere/to the top of the atmosphere. But if they saw it was a passenger shuttle, I doubt they'd be as worried (if they saw it). The Norwegian scare was a problem because the missile looked and acted like a trident missile (nuke launched from a sub). $\endgroup$ – Magic Octopus Urn Oct 13 '18 at 14:07
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Given 2 I think it is safe to assume that the rocket cannot be intercepted before reaching orbit. If the operators were silly enough to locate their launch station in the area covered by an anti-ballistic missile system it would be a different story, but those are just a few areas.

There might be a chance for an intercept if it stayed in parking orbit around Earth for long enough. An ICBM would be able to reach its altitude (no need for orbital velocity) and could hope to set off a warhead in close proximity. If command and control was fast and flexible enough that might be possible in a matter of minutes. On the other hand parking orbit is not really essential, so they could avoid this.

The big problem arrives on the long coast to Mars. If we assume that the people trying to stop them have at least equally good launchers, and can alunch one within a day or two, they can launch a similar rocket with a payload comprising a few nukes and a hypergolic upper stage with lots of fuel. Because their ultimate payload (nukes) is fairly low mass, they can have a much larger delta-V reserve than their target, which means they can eventually match orbits, run the fugitives out of maneouvering fuel and destroy them.

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    $\begingroup$ I am not even sure nukes are needed. They are fairly useless in space since there is no atmosphere to generate a shockwave with. A simple and well tested gun might also do the trick, is a lot lighter and just disabling the target is as good as a kill. $\endgroup$ – Hennes Oct 13 '18 at 14:55
  • $\begingroup$ @Hennes: Guns aren't well-tested in space. But anti-satellite weapons are. The basic principle still holds: just hit it, the vacuum of space will be sufficient to finish the destruction. No ICBM needed; these things are operational. But that is also down the downside of the whole idea. Whoever is launching this thing won't be bothering with a parking orbit. Or at least not long enough to matter; even the militaries of the world will need time to seek permission. $\endgroup$ – MSalters Oct 13 '18 at 20:40
  • $\begingroup$ I think we both can agree that any decent weapon can be fatal. No nuke needed. Gun (and small hard to detect bullets), nukes (buit only if almost a direct hit), high powered lasers (harder than in SciFI) , ... Almost anything will do. And if you do not need the delta V to slow even ramming will do. $\endgroup$ – Hennes Oct 13 '18 at 20:59
  • $\begingroup$ @Hennes Nukes in space are quite deadly because of the radiation not be attenuated by the air. Dangerous radiation levels occur at 8-17 times the distance that they do in the Earth's atmosphere. $\endgroup$ – zeta-band Oct 15 '18 at 17:58
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Realistically, it's not going to be intercepted.

1) Launch phase: Depending on the location there might be some sort of interceptor that could catch it as interceptors generally boost much harder than space rockets. However, the window is very narrow and I can't imagine anyone actually taking the shot. Just because they don't know what's going on doesn't mean shooting is the right thing to do and by the time the warning is kicked up the chain to the point where someone can be sure it's not simply something secret the window will have passed. There is an incident from US law enforcement history that gives some indication as to the problem--the police and air traffic control wanted a plane shot down. It was unmanned (a drug runner, the crew was observed to have jumped when they were intercepted) and certain to crash and it was heading for a big city. Trying to sort this out before it reached the city proved impossible, fortunately it overflew the city and went down in the ocean beyond.

2) Orbit phase: Intercept certainly is possible--some countries have ASATs that could do it and a repurposed ICBM probably could, also--but with catastrophic EMP effects. (Setting off nukes near an atmosphere is a very bad thing as we learned from the Starfish Prime nuke test back in the 60s. That one caused problems 1600 miles from the bomb and semiconductors are far, far more sensitive than tubes.) However, once it's reached orbit it poses basically zero risk. Why would anyone expend an interceptor against it? As for those who question the need for an orbit phase--while a direct burn for Mars is possible and actually slightly more efficient (the burn is done lower, you get a bit more from Mr. Oberth) it also means an instantaneous launch window.

3) Deep space: Not possible. Sure, a lighter rocket would be going faster and could nail it but that's assuming you have such an interceptor sitting around. In practice nobody has anything that could do the job sitting around and by the time you build one it's too late, you're not going to catch it before it reaches Mars. Also, why go to the cost? A rocket heading for Mars is no threat to Earth.

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  • $\begingroup$ The last sentence pretty much sums it up for me… "Also, why go to the cost? A rocket heading for Mars is no threat to Earth." $\endgroup$ – DeveloBär Oct 15 '18 at 19:32

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