Can we nuke the alien invaders using land based ICBMs? Do they have enough thrust to break the Earth’s gravity well? I know they have a range of only a couple tens of thousands of miles but if they get coasting in space that’s good enough right? Can we just shoot a couple thousand at the Big Bad Alien Mothership in Lunar orbit and trigger them to blow up when they get close or would we be helpless outside of launching a couple rockets specifically designed for this sort of thing? Stranger on a Train? 
No. ICBMs can’t reach lunar orbit. Actually currently we don’t have any rocket capable of reaching lunar orbit.
ICBM can go up to some 1200 km. Most satellites and almost all human flight are approximately the same. Furthest we are sending stuff nowadays (with little exceptions) is geosynchronous orbit - ~40.000km. Moon is almost ten times further.
Though, given enough time, we could rig one - say, make Atlas V and fit it with some modified warhead.
How are the Mars landers/rovers and other such probes launched these days?
And risk turning one large falling object into many?
Atlas V, if I remember correctly. According to wikipedia “available within 30 months from order”. Hence my “given enough time” corollary.
Not true. We don’t have any rocket capable of lifting a human-carrying spacecraft to lunar orbit in a practical timeframe.
The Atlas V/Delta IV family can reach lunar orbit no problem; they just wouldn’t get there with anything worth having in lunar orbit (well, probes and stuff, sure-- but I’m talking Apollo-caliber missions).
Ditto deep space missions, although in those cases we don’t do direct shots-- we use gravitational assists to slingshot probes to other planets. Bigger rocket = fewer slingshots, faster travel time.
That said, I’m not sure that even a Minuteman III ICBM could get to lunar orbit even with no payload of consequence, let alone a warhead. But an Atlas V/Delta IV carrying a warhead? No problem at all, from a lift perspective at least.
(As for the 30 months to delivery. . . that’s a cold start construction time. The urgency of a warhead mission would simply bump a lower-priority payload off of a rocket that’s ready, or nearly so. Payload integration is usually a long careful process, but corners can be cut in a hurry. Couple of weeks, maybe even less, in a crisis, I figure).
Yeah, but the point is still - we don’t have them now. Can get one if need arise, sure. But it’s not as simple as enter new target coordinates into ICBM. We still have to make a rocket (or appropriate and modify existing one), and make warhead (or appropriate and modify existing one), and then integrate it into working package, transport to Cape Canaveral or wherever capable launchpad exists and pray that it don’t blow during launch - which is highly probable taking into account rushed construction. I seriously doubt we could do it within weeks. Months maybe.
Not to mention that in meantime waves of these alien invaders are getting closer every time they reach border of the screen.
Can ICBMs even be given an arbitrary trajectory on short notice? I could easily see the software on a warhead bus not even accepting a non-earth surface destination as a valid parameter.
Plus, if they’re invaders from space, they obviously have much better tech than we do, and probably have spent more than a little time planning, gathering intelligence, and deciding which reptile admiral gets Natalie Portman for his harem/humanburger. They’re going to be ready for ICBMs.
This was the immediate problem that struck me. Weapons are designed to operate within a very tight envelope set by their mission requirements. You couldn’t just arbitrarily make an ICBM with a specialized guidance system designed specifically to guide it to earth based targets using certain methods into a spacecraft interceptor with a little tweaking.
Well, it’s safe to assume they have better tech. But should we assume their intelligence gathering, military leadership and political agenda make any sense? For all we know, they cherry-picked their intel, outfitted their troops with gear designed for the last conflict, and are coming after us to bolster mid-term election numbers.
You are oddly persuasive.
This is what I was afraid of. I was just wondering if it even technically possible to launch an ICBM at a space target. I remember reading about how missiles like this aren’t something you just fiddle with on the weekend. So a missile may have a couple of targets you could switch from but telling it to nuke somewhere else on short term notice is a big problem. I was hoping if we saw the fleet coming we could make preparations maybe…
I’m guessing when the alien invasion force is descending into the atmosphere we couldn’t have the nukes explode on their ascent and take them out, could we?
Well, you go to war with the spacefaring reptilian hordes you have, not the spacefaring reptilian hordes you want.
We probably couldn’t. The Russians might be able to; they have megaton range nuclear tipped interceptor missiles to defend Moscow last I heard.
Another point I didn’t mention is that while we have nuclear warheads, and we have rockets that can be adapted to carry loads into orbit, AFAIK we don’t have suitable orbital vehicle for final interception.
You see, just lifting stuff very high isn’t enough. We need to hit that mothership. Since in space there is no medium for blastwave, and spaceships by definition are shielded quite well against radiation, we need to score direct hit - miss by kilometer or two, and our nuke fizzle harmlessly. What that means, that we need vehicle that when lifted into moon orbit by that Atlas V will deliver this W87 warhead into trajectory colliding with trajectory of mothership. I don’t think we have something like that, and judging by development of ASAT weapons it’s very hard thing to do. Not impossible, but very, very difficult.