Alien Mission Game
Macedonio Heninger
During the long war against the alien threat, the skies are also part of the battlespace. Aliens will dispatch sorties of UFOs to further their goals during the war, and XCOM may respond in kind by sending out interceptors and the Skyranger to foil their missions in the air or on the ground. In other words, an alien mission always starts with an UFO sortie.
There are eight types of regular alien missions scheduled on a monthly basis. The type of each mission is determined by the resources the aliens have, and how threatening XCOM is perceived to be. There are also two special mission types that occur irregularly. All ten mission types are listed and discussed below, followed by a table detailing the scheduling of the regular missions.
The aliens like to focus on generating panic in countries and continents near their established bases. They prioritize any such countries with the highest panic, in order to push them over the edge and induce them to withdraw from the council.
All missions are computed and scheduled at the beginning of each calendar month. They determine what missions to do, which countries to do them in, what day and time to do them in, and which UFOs to use. Dynamic War: changes mission scheduling from vanilla to occur more or less frequently than once per month in order to produce more or less missions, depending on the INI scaling value used.
While some missions have a predetermined UFO (or set of UFOs) that can appear, many missions select one randomly. Which UFOs are deployed may depend on up to four factors: the mission type in question, the level of resources the aliens have, and the level of research the aliens have, and randomness. All of this is evaluated at the time the mission is scheduled (beginning of the month for standard campaigns).
Specifically, many mission types have its own pool of UFOs that it can field, with each UFO in the pool equally likely to be randomly selected. As the aliens gain more resources, additional UFOs can be added to the pool (again, depending on the mission type). And as the aliens gain more research, the resource thresholds required to add these additional UFOs to the pool are decreased. Specifically:
Missions themselves do not require the aliens to spend resources to run if left unchallenged, but shooting down or damaging the UFOs conducting them does cost the aliens resources to repair or replace the UFOs.
Individual UFO sorties are missions conducted by a solitary UFO. Shooting down the UFO (or assaulting it if it lands) is always sufficient to prevent the mission from succeeding. Dealing damage to the UFO sometimes, but not always, can affect the outcome.
A UFO will fly low and around the target country, attempting to confirm the existence of XCOM satellites. Successful scouting missions will be automatically followed up by a new sortie to Hunt the compromised satellite. It is possible that the hunting UFO will be stronger or weaker than the scouting one, so the priority to intercept this mission can vary depending on how tough the UFO is; if the UFO sent to scout is tough (e.g. an early game Raider), you could gamble that the UFO being sent to hunt will be weak (i.e. a Fighter). Conversely, you could be punished if a stronger UFO (i.e. a Destroyer) follows up instead. Fortunately, Hunt missions automatically fail if the UFO has less than 50% of its HP remaining, so it's slightly easier to stop then Scout missions even if you can't shoot it down.
A scout mission can fail in four ways: 1) the UFO is shot down, 2) the UFO could botch the mission due to sustained damaged, 3) the target country could fend off the mission by itself, or, if all else fails, 4) the UFO could simply get unlucky. The odds of these failures happening are as follows:
The aliens will always send at least three scouting sorties per month, and more if they have resources to spare and perceive XCOM to be a credible threat, up to a maximum of six sorties per month. So long as the aliens don't have enough resources to regularly field a battleship, Scout missions will comprise a sizeable proportion of your monthly income.
A UFO will fly at high altitude over a satellite-covered country, seeking to destroy the satellite. A Scout mission, if successful, is always immediately followed up by a Hunt mission, with a potentially different UFO. Thus, if a Scout mission presents you with a tougher UFO than you can handle, you can gamble on the followup Hunt mission being easier. Likewise, fail to shoot down an easy Scout mission and you could get a much tougher Hunt mission.
Successful hunts will destroy the satellite in question, setting XCOM back about 200 in replacement costs and removing the associated country satellite bonus (and continent bonus, if applicable) until a new satellite is brought back online. The base chance of success is 60%, or 90% on the second pass (30%/50% respectively if Stealth Satellites is researched). Damage sustained by the UFO will reduce this chance further proportional to twice the damage dealt to the UFO, and thus dealing 50% or more of its HP in damage leading to an automatic failure. Pertinent to this, a 50% HP UFO will start flaring up in the aerial combat screen, as if it was being hit by multiple missiles continuously. When this happens, XCOM can order their interceptor to disengage if need be, preventing further risks while stopping the hunt mission. That said, a downed UFO provides a hefty quantity of resources, so try to down it if at all possible, as you get nothing but lengthy repair times when you only damage it.
In addition to the replacement costs, the loss of a satellite imposes a hidden but potentially far more important cost as well: while the satellite coverage is down, all Research, Harvest, and Bomb missions that were otherwise scheduled for that target country automatically succeed, and any Scout or Hunt missions that were scheduled to target that country will be skipped, and XCOM is robbed of the opportunity to recover materials from them. Research, Scout, and Hunt missions account for a very large portion of your monthly income in a campaign, as they normally always target satellite-controlled countries (regardless of how many satellites you have or where they are located). Particularly in the early months, when you don't have much satellite coverage and the vast majority of such missions are concentrated on 1-3 countries, the loss of a single such countries satellite can set you back a ton of resources and XP for your soldiers.
However, note that the aliens intelligence on your satellites is abstracted somewhat, and Hunt missions can randomly spawn without immediately being preceded by a successful Scout mission. These "Direct" Hunt missions have a specific targeting priority:
Direct Hunt missions can only be scheduled if XCOM is seen as a threat (2+ threat) and depending on the amount of resources the aliens have this may be only a 50% chance to see one at all. The more threatening XCOM is and the more resources the aliens have, the more direct hunt missions are sent, up to a maximum of four per month.
A UFO will fly NOE, land for a while, and then fly off. This is a recon-in-force mission conducted against XCOM operations. They are always conducted within regions that XCOM maintains a Satellite presence, and mission will grant the aliens research if successful, which will slowly but permanently increase the capabilities of their ground troops and UFOs; the quantity of research earned is directly proportional to the quality and size of the UFO used to conduct them.
To stop the aliens, simply allow the UFO in question to land, and assault it; you'll prevent them from gaining any research if you manage to succeed, and you'll make out with a large amount of resources from the intact UFO. Shooting down the UFO is not necessary and even counterproductive, as it reduces the number of corpses, materials, and XP you can harvest. That said, when assaulting UFOs that have successfully landed, always be wary of trapped UFOs: such UFOs can have massive increases in the number of aliens that crew them, and the chance is roughly proportional to the amount of UFOs XCOM has shot down so far this month. Shooting down the UFO does stop it from being a trap, so it's an option, but you're robbing yourself of lots of resources, corpses, and kill XP by doing so.
A UFO will fly NOE, land for a while collecting resources, and then fly off. An unimpeded harvest grants the aliens resources, which will allow the aliens to field more UFO sorties per month as well as use greater quality UFOs.
If you manage to detect the UFO, you can assault it after it lands, preventing the aliens from obtaining the resources and bringing home a large haul of materials, more than you would for a crashed UFO. That said, always be wary of trapped UFOs: such UFOs can have massive increases in the number of aliens that crew them, proportional to the amount of UFOs XCOM has shot down so far this month. Shooting down the UFO does stop it from being a trap, so it's an option, but you're robbing yourself of lots of resources, corpses, and kill XP by doing so.
Harvest missions are one of the few UFO missions in the game (alongside Bombing missions) that can be fully completed without XCOMs knowledge or awareness, and thus XCOM may not have an opportunity to prevent them from succeeding. In addition to picking the country entirely at random, they also tend to avoid locations in which XCOM has satellite coverage. Specifically, if the targeted country has a satellite, there is a 10% chance for every 2 full points of of threat, max 40% (i.e. 0% at 0-1, 10% at 2-3, 20% at 4-5, 30% at 6-7, and 40% at 8+), that the aliens will re-roll the target country to one without satellite coverage (assuming one is available, if not they just stick with it), paying a flat 5 resources in the process. This represents the power projected by a strong XCOM forcing the aliens to settle for sub-optimal harvests. As the aliens' first choice for harvest is randomized, more satellite coverage means a higher chance that the aliens need to waste resource to even start a harvest mission, and more opportunities for you to target these craft and turn the aliens' attempts at resource-gathering into one of your own.
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