Can terror attacks be prevented in early game?
In X-Com: UFO Defense / UFO: Enemy Unknown (the 1994 original), I've already encountered two alien terror attacks by February 2000, when I'm only on one base and at the tech level of laser weapons. They usually involve heavy casualties and result in a poor score.
Is it feasible in early game to prevent terror attacks somehow? Is it possible to detect and intercept the ships responsible for terror attacks, or otherwise disrupt them?
Yes it is!
However, early game you're unlikely to have appropriate
satelliteRadar coverage to detect the Terror ships, and with only avalanche missiles, you'll either need a lot of interceptors, or a great deal of luck (especially considering the attack range bug), since they can destroy interceptors in two shots, assuming you can even catch up to the ships in time. They are one of the two Large-class UFOs (the other being the Supply Ship).
Once you've got your detection network a little better off, you may have better odds. But until you've got Hyperwave Decoder and some better guns, your chance of detecting, interecpting, and then downing, a terrorship are remote.
You can also take "preventative" measures. Terror Missions are scouted (with Scout UFOS to start with) before the final run on a city is made. If you're consistent about shooting down these precursors, you can often delay or avoid the terror mission entirely. As before, though, hitting the right UFOs is a question of luck, before you get the Hyperwave Decoder, and the Aliens eventually use stronger ships to scout, so you still should upgrade your fighter weapons ASAP.