Hmm, not sure. It’s heavy, but it’s wide and not very dense. Any physics wonks out there ? Else we could just set up the practical experiment and save ourselves some maths 
Shit. I blame the decaf.
Dropping heavy objects from orbit is not nearly as powerful as a large nuclear warhead. E.g, a 100 meter diameter sphere of iron is 524000 cubic meters and iron weighs about 8000 kg per cubic meter, for a total mass of 4.19 billion kg, or 10,000 times the mass of ISS.
According to this Impact Effect Calculator if that was dropped from orbital velocity it would be less than a 60 megaton bomb: Impact Earth
The OP question was what is practically achievable. There is no way putting something in orbit with 10,000 times the mass of ISS is practical with current technology.
Also an orbital path only covers one specific ground track on earth. If the target is not directly below it can’t be hit. Changing orbital inclinations is very propellant-intensive, especially for a heavy object. While orbital or suborbital kinetic weapons are widely discussed and might have some future tactical use, they are not remotely equivalent to a large nuclear bomb.
You can see an actual unarmed Minuteman kinetic impact here. It’s not that big: https://www.youtube.com/watch?v=o8n59S8j2C0
What about using nukes to free up a small bit of the moon, like a mountain top, and sending it earthward?
Assuming an impact speed of 8 km/s, about 7.6 million tons.
If you launch a Falcon Heavy every day, it’ll take 370 years to get that much mass into orbit.
If you drill into a mountain and explode a nuclear bomb, all you do is create a lot of rubble. I don’t see how you can make a solid piece come off of the mountain and fly upwards in one piece.
You’d have to find that much tungsten first-the U.S. only has about 140,000 tons of it stored up, and in 2010 the total produced worldwide was only 68,000 tonnes. Source.
The problem with bioweapons (and to a lesser degree, toxins) is that their effects tend to be self-limiting, especially if relying on infected people as the continuing vector for dispersion. Once people start dying on mass, the remaining population takes measures to quarantine and protect themselves, and of course infectious bioagents are rarely 100% lethal. There is no current nanotechnology that could pose a lethal hazard to the general population, and the limiting factor of so-called “grey goo” scenarios is fundamenal thermodynamics; anything that can spread fast enough to be a hazard is going to heat up and waste a lot of energy; anything slow enough to be efficient will move slow. And synthetic nanotechnology has not gotten anywhere close to self-replicating devices, so there are geometric limits to expansion.
Taking a medium sized asteroid, grinding it into a fine powder, and dropping it into low Earth orbit would both take out satellite communications and then fill the stratosphere with dust that would occlude sunlight, and would remain until it falls into the troposphere and is precipitated out of the atmosphere. Sustain this for long enough (e.g. a growing season or longer) and you would essentially doom all complex animal and much plant life to extinction.
Yes, larger bombs essentially just push the atmosphere up and out, producing more impressive fireballs but with only limited radial effects.
A few more speculative ideas:
[ul]
[li]Fill the oceans with hyperoxygenating engineered algae which push the atmospheric O[SUB]2[/SUB] to toxic levels (>30%) [/li][li]Introduce a stable black hole or some kind of energetic exotic matter which causes matter to accumulate and create a local high gravity gradient, eventually consuming the atmosphere or planet[/li][li]Fill the atmosphere with a gaseous hydrocarbon and combust the entire surface in one giant flamefront[/li][li]Turn off gravity and watch the Earth be flung out of orbit before internal pressure blows it apart[/li][/ul]Of course, none of these are practicable or (currently) technologically feasible, but it’s only a matter of time before my research, I mean, someone’s research, comes up with a solution, then global extortion, profit, and retirement to a nice geyserside pressure dome on Triton.
Stranger
The trick is you make a spine and skeleton structure out of this material called vibranium, which you have to get from a secretive African nation…
In that timeframe it would make a lot more sense to locate and process extraterrestrial sources of tungsten and other refractory metals. This is obviously not trivial, but it would at least give a sense of purpose to our space program vice “Let’s put a flag on Mars.”.
Stranger
Per E. E. “Doc” Smith: Hit the planet with two planet-sized masses heading in opposite directions, each at .75C.
Per the OP: What weapons could be built (practically)…
Well, a few hundred megatons of TNT should do the trick.
Yeah, I really didn’t get that plotline in Avengers: Age of Ultron. Then again, I only got to see it once because my daughter was so pissed at what they did to Black Widow.
What they did to Black Widow? Heck, Maria Hill is #2 in command of what remains of S.H.I.E.L.D. and she barely got more lines than Claw’s henchmen. The MCU is still kind of lagging behind in establishing its feminist credentials or realizing that both young girls and adult women also like to see superheroes in action (the latter seemingly more for Chris Hemsworth’s abs than the mean) but hopefully they’l catch up and do right with Captain Marvel, although I’d love to see a “Women of the MCU Kick Ass On Their Own” film with Hayley Atwell front and center.
Meanwhile, poor Hawkeye still ain’t ever going to get his own film. Harley the Potato Gun Kid is going to get his own film before Jeremy Renner. And yet, nobody cares about “arrow guy”.
Stranger
Though, in theory, a nanomachine could one day be built which is able to self-replicate and spread, like a virus, but engineered to not do anything damaging to any of its hosts until a set date. You could give the bots a year to spread across the Earth, giving no sign of their presence, and then kill every human on Earth in a synchronized attack that kills everyone but a few hermits living in the middle of nowhere, with limited human contact.
Just having the ability to mass manufacture a simple device that can release a toxin or biohazard after a scheduled delays of a year or more would do it. It may not be a way to kill every person on earth, but if there are cycled attacks civilization is going to fall apart. Assuming the deadly stuff doesn’t last forever and keep killing people the survivors will have a decent shot of rebuilding from the point of the industrial revolution onward if they can stay together.
The OP will have to clarify what he meant, but I took the word “powerful” to refer to an explosive weapon, not a chemical or biohazard. Impacting large objects from space certainly seems like the most feasible approach. Of course, you wouldn’t lift the objects out of the earth’s gravity well and them drop them back; you want to start with an object that is already out there, like an asteroid. Launching your payload from the moon may be easier than redirecting an asteroid (reference: The Moon is a Harsh Mistress).
–Mark
The way to do it is to use a mass-driver on the Moon to project lumps of Moon matter at the Earth. Guy named Heinlein wrote the handbook on this one.
Note that Gerard K O’Neill seriously proposed the construction of solar-powered mass-drivers on the Moon as part of his program for space colonisation, in his book The High Frontier; I wonder if he realised the potential for warfare (probably he did).
I’m curious - ignoring the practicalities of building the thing, are there inherent constraints in the physical process that you eventually reach?
Even then, it’d take a 1300-ton projectile dropped from the Moon to match the atomic bomb dropped on Hiroshima. And that’s assuming the projectile reaches the ground without breaking up or losing any speed.