Asteroids and spaceships are kinetic bombs and how to prevent catastrophe

Spacecraft are generally not constructed to be able to penetrate deeply into an atmosphere at high velocity - reentry vehicles do, but witness what happened when the Hayabusa spacecraft and its small sample-return capsule hit the atmosphere together. The bit meant to reach the ground keeps going, everything else shatters into lots of tiny fragments immediately.

As such I can't help but think that anything human-constructed will tend to fall apart very high up, and dump its energy many kilometers from the ground and in many pieces. Consider the recent meteor in Russia - that was probably a solid lump 15+ meters wide weighing in at 10,000 ish tons, and it lost integrity and fell apart 30 kilometers up. Not to mention that that object was 50-100 times as massive as a 747 and carried ~200 times as much kinetic energy as the total chemical energy of the best rocket ever built. You need some truly epic spacecraft before they become dangerous as anything but (potentially radioactive depending on contents) litter downrange of where they smack the air.

Redirecting small already-earth-crossing rocks by a quarter meter per second such that their path coincides with the Earth ten years later could be another matter all together. But that would be patently obvious as something with enough delta V approached a known object. And if something unofficial can move it, something official can doubtlessly move it back...

Almost nobody wants to do terrorist attacks.

There are very few terrorist attacks, even though doing a terrorist attack is not very hard. A smart person who really wants to blow up a building, and doesn't mind death or life imprisonment, can generally do so. But very few buildings are blown up. Therefore, even though terrorist attacks are highly available thanks to news reports and fiction, almost nobody wants to do them.

For a crime to occur requires means, motive, and opportunity. We can consider these to be factors of production, or the reagents that go into making a crime happen. Which of them is the limiting reagent on the production of terrorist attacks? Not the means; guns, explosives, and other weapons aren't difficult to come by or create. Not the opportunity; we do not armor all possible terrorist targets nor track every potential terrorist. Rather, the limiting reagent on the production of terrorist attacks is motive — people's interest in doing them.

Aside from ground targets, if lots of people are in space, then there are space-borne targets of high value.

They'll have to look out a lot more carefully.

A reality of physics, and one that doesn't get much play in science fiction, is that as soon as humanity gains space travel, anyone in the asteroid mining or space travel business will have city-busting capabilities at their fingertips.

How do you know that's the reality? Could you go into more detail? Especially if you have enough telescope satellites to monitor all mining activities and some nuclear weapons to attack asteroids that get redirected?

Will a company like Planetary Resources, Inc have the ability to blow up a city when it gets it's mining working?

The principles involved here are not dependent on spaceflight or any new technology, but apply with current technology just as much. In fact, almost twelve and a half years ago, terrorists actually did this exact thing, but with airplanes rather than spaceships.

From this, we can try to extrapolate to the possibility of this happening on a larger scale. So I have two points: The first is that that attack was by far the largest and most destructive terrorist attack ever. If it involved spaceships rather than airplanes, it would have been even larger and more devastating. The second is that since then, not a single attack has managed to get even close to that one in devastation. Modern security measures against attacks like that one seem to be working, and that shows that effective measures against attacks like this are possible.

I'm not an expert on astrophysics, or millitary technology. Luckily, I have a friend who is an expert on both and has super-hard science fiction world building as his hobby. So I'm just going to quote him on why this isn't as big a deal as people think.

A related issue is the use of spacecraft as weapons against ground targets. This is unlikely, to say the least. As mentioned above, a sectional density of 10 tons/m2 is required to make it through the atmosphere with a reasonable amount of velocity remaining. While common ships might reach this threshold, they are almost certain to fail structurally long before impact. When they break up, the pieces are unlikely to retain sufficient sectional density to do damage. The result is a high-altitude airburst, which, given expected vessel sizes, is unlikely to do significant damage. It has even been suggested that vessels be intentionally designed to break up on atmospheric entry to reduce the risk to people on the ground.

This is from a recent paper meant as an introduction the theoretical space warfare community -of course that is a thing- consensus with an emphasis on what are called plausible mid-future scenarios.

See also Heinleins The Moon Is a Harsh Mistress and Arthur C. Clarke's The Hammer of God.

Well, something like this was the motivation for the space race: if you can send a rocket to the moon, you can send a bomb around the world. Even though this reason isn't usually raised by space advocates.

How effective is a spaceship as a kinetic weapon, anyway? They're not very massive.

I think we need to be specific about scenarios here. For example, right now we don't need to worry about extremely high speeds, so defending against inbound objects should be fairly easy. I think for the near future, (US, Russian, and Chinese) interceptor missile technology will outpace abuse-of-spaceships technology, so once there are a bunch of spaceships flying around and/or mining operations get started, it may be easiest to just keep track of relevant objects and shoot them down. I think that should be more than enough, since I just don't think there are very many people who'd want to use spaceships as makeshift weapons who don't have access to better weapons already. If we make it to the far future (where we might expect significant-fraction-of-c speed ships) without singularity things happening for some reason, on the other hand, more authoritarian external control may become necessary.