Set in the modern day, I was wondering about a possible scenario in which some hackers gain control of many satellites, probably including those specifically for defense purposes, in order to wreak havoc on many cities around the world. What kind of satellites are the easiest to hack, and how can the hackers use them to commit genocide worldwide?
Trigger nuclear war
Hack a military observation satellite and tell it to transmit a signal indicating a nuclear launch by a hostile power. You'll probably need to hit a few concurrently for a convincing signal. After that you just need to sit back and watch the fireworks.
Trigger Kessler Syndrome
You don't need to knock them out of the sky, you just need to knock one into another. I don't know if there's enough materiel in orbit for the full Kessler cascade but the theory is well known and has been for decades. Generate enough debris and the species is effectively bound to the atmosphere for decades. No satellite communication, no GPS, only basic weather forecasting, (no spy satellites). The damage is incalculable.
Actually hacking a satellite is not the easiest thing. Most use encrypted communications, they will only be overhead for a short while (unless geo stationary) The command and control systems will not be documented, and the existing control infrastructure will be very well secured. Older satellites might be easier (just because threats have evolved)
On the damage you could do, assuming you could hack a satellite. Genocide will be a tall order. I'm going to break down into a couple of categories.
Loud/impressive Hack the US Military communications satellites. Redirect a few drones to your target area and fire off some missiles. This is probably the option that would give you the largest instant casualties.
Loud/civilian Make every communication satellite broadcast white noise on all possible frequencies. while it wouldn't be all that damaging, cutting communications would cause a lot of problems.
Subtle/Short Term Subtly adjust the clocks in the navigation satellites. Aircraft coming into places they shouldn't, boats crashing into rocks, trucks delivering things to the wrong places. It wouldn't take long for people to notice this. They would switch back to other older navigation methods.
Subtle/Long term Adjust the responses from weather satellites, that hurricane headed for Florida gets no warning.
Aside from orbital platforms armed with nuclear weapons for fractional orbital bombardment, which is an outlawed weapons system by international treaty, satellites lack the capacity to be directly destructive.
So maybe hacking communications satellites would enable your nihilistic coders to infiltrate control systems of conventional and nuclear power plants and power distribution systems, causing internal damage that compromises their operation. The impact of losing the entire power grid would be catastrophic to large societies dependent on distributed agriculture for generating enough food for every body.
Similarly, damaging dams and irrigation control nodes via the hacked comms satellites could flood broad areas, further damaging agriculture and maybe industrial sites too.
Finally, maybe if the hackers had orbital control of all the satellites, they could execute an insanely complex maneuver and cause all the satellites to group together and crash directly into a major city. The satellites on the outside would burn up on reentry, but maybe enough metal on the inside of the clutch of satellites would reach the surface with enough kinetic energy to do damage.
Disrupting communications is already extremely damaging to modern day society.
Sattelites control a much larger part of technology than you would think nowadays. Even just shutting them off could disrupt parts of the Internet, GPS, telephone, and media communications. All of these systems are more or less crucial for modern society. Disrupting some communications to a bank, denying people access to their money would seed anarchy already. Not being able to find out what happened from the TV would fuel it tremendously. Disrupting the quality of life of modern people often brings out the worst in them, so just depriving them of a few choice options would be fairly effective. The following chaos and destruction is all out of your hands.
To add to this, sending false signals to disrupt supply chains would be fairly effective as well. KFC running out of chicken in the UK springs to mind. This was quickly labelled as a "crisis" and people were outraged. Imagine doing this on a much larger scale by sending false messages and rerouting all trucks/supplies to most major corporations. Chaos and rioting would ensue.
If you spoofed the data from weather sats, plus solar and earth observation sats, you could wreak havoc with the agriculture sector and possibly other areas. Fake hurricane warnings, or suppressing the warning of a real hurricane. Imagine a cat 5 hurricane hit the east coast with no warning.
Bring the satellites down. Easiest, fastest, most "efficent" way.
You might not only bring down a 4 tonnes of mass from 2000 kilometres height. You can also initiate controlled cascade effect practically bombarding earth from orbit.
Nuking from orbit is sooooo 1986. If you take down the satellite with one nuke to Iowa or Arizona you not only put nuke to that places but all the additional mass you bring make for a nice mass hitting the earth vaporizing eart, making radioactive clouds.
Also a not nice reference but imagine what would happened if 4 tonne satellite falled on Manhattan instead of a plane hitting WTC.