View source for Alien Missions in Enemy Unknown (OpenXcom)

UFOs are sent out periodically to conduct a number of types of '''Alien Missions''', some of which the purpose is fairly inscrutable, others are fairly obvious in their reasoning. Especially once the [[Hyper-wave Decoder]] is deployed, knowing the mission of the aliens can help make strategic plans for further actions that the aliens may attempt in the near future.

The information presented below describes how Alien Missions work in the original Enemy Unknown game, based upon [[OpenXcom]]'s XCom1 ruleset file and other sources, namely posts at OpenXcom's forums and pages about the original game files. 

Keep in mind that there are massive spoilers involved, so read at your own risk. 

==List of Alien Missions==

* [[Alien Research]]
* [[Alien Harvest]]
* [[Alien Abduction]]
* [[Alien Infiltration]]
* [[Alien Base]]
* [[Alien Terror]]
* [[Alien Retaliation]]
* [[Alien Supply]]

==Alien Missions Generation==
===Mission Frequency===
The aliens will generate new missions each month according to the following pattern: 
* January, 1999: 1 Alien Research on the region of the initial XCom base, 1 Alien Terror
* February to June, 1999: 1 random mission, 1 Alien Terror
* July, 1999: 2 random missions, 1 Alien Terror
* Once an Infiltration mission starts on a region it will repeat itself indefinitely on that region, and it will keep spawning bases after all countries have signed pacts with the aliens
* Starting on the first month, each shootdown of a UFO can trigger a retaliation mission by the aliens, with the chance being determined by difficulty level, at Easy being 4%, then increasing each difficulty level by 4%, until it reaches 20% on Superhuman 
** The chance of the Retaliation taking place in the region of the shootdown is determined by the formula (50 - 6 * getDifficultyCoefficient). If this fails the aliens will assign the Retaliation to a likely point of origin of the interceptor, based on its approach vector
** There can only be 1 active Retaliation mission per region
* After [[The Martian Solution]] is researched OR depending the difficulty level (March 2000 on Easy or November 1999 on Superhuman): 1 Alien Retaliation mission is added, in addition to the above
** The detection chance for Retaliation missions is given by the formula: (completedFacilities / 6 + 15) / (mindShields + 1)
* There is a 6% chance for a supply mission to be generated each day, for each alien base

===Missions Selection===
The game will create an array of possible missions for each region of the world to be used when randomly selecting missions for each month. The random missions assigned to each region can be of 5 kinds: Alien Research, Base, Abductions, Harvest and Infiltration. Alien Terror, Retaliation and Supply missions are special cases and are not present in this array since those missions are automatically generated. 

Once a particular mission is selected it will be removed from the array and won't appear again for that region. If all of the missions in the array are used then the game will restart this process and refill the mission array, usually somewhere around 2000. (this is unique to openxcom, and didn't occur in the original)

First the game will randomly choose a region for the mission, using weighted factors (defined on the [[ZONAL.DAT]] file in the original game and in [[Ruleset Reference (OpenXcom)#Regions|Regions]] on the XCom1Ruleset.rul file) to influence the probability of each region being selected: 
* North America: 18
* Arctic: 4
* Antarctic: 4
* South America: 10
* Europe: 16
* North Africa: 12
* South Africa: 8
* Central Asia: 12
* Southeast Asia: 15
* Siberia: 10
* Australia: 10
* Pacific: 5
* North Atlantic: 5 
* South Atlantic: 5 
* Indian Ocean: 5 

The 3 last regions don't have any random missions assigned and they won't see any Alien Terror missions since they have no cities present. However, it will be possible to see Alien Retaliation missions if any UFOs are shot down over those areas. This is because the target region for the retaliation is randomly picked between the UFO's mission region and the intercepting craft's point of origin. In the event that the interceptor's point of origin was one of these regions, a nearby region may be substituted in. Original handling would default the region to North America.

As said above, each region has several missions assigned. The mission will be randomly selected also based on weighted factors (defined on [[ACTS.DAT]] and in [[Ruleset Reference (OpenXcom)#Regions|Regions]] on the XCom1Ruleset.rul file), which are shown on the table below. 

{|class="wikitable" width="70%" 
| align="center" style="background:#f0f0f0;"|'''Region'''
| align="center" style="background:#f0f0f0;"|'''Research'''
| align="center" style="background:#f0f0f0;"|'''Harvest'''
| align="center" style="background:#f0f0f0;"|'''Abduction'''
| align="center" style="background:#f0f0f0;"|'''Infiltration'''
| align="center" style="background:#f0f0f0;"|'''Base'''
|- align="center"
| North America||14||17||20||20||20
|- align="center"
| Arctic||25||NA||NA||NA||35
|- align="center"
| Antarctic||25||NA||NA||NA||35
|- align="center"
| South America||10||24||12||18||22
|- align="center"
| Europe||10||10||15||20||6
|- align="center"
| North Africa||8||8||8||8||19
|- align="center"
| South Africa||7||8||8||8||9
|- align="center"
| Central Asia||5||5||6||10||8
|- align="center"
| Southeast Asia||8||13||21||13||8
|- align="center"
| Siberia||6||NA||4||NA||9
|- align="center"
| Australia||15||11||4||8||16
|- align="center"
| Pacific||15||NA||NA||NA||27
|-
|}

The information on the table can be used together with any Alien Activity shown on the game's Graphs to make an educated guess on what the aliens are up to in certain areas. For instance, alien activity in Northern Africa, Arctic, Antarctic or the Pacific have a higher chance of an Alien Base being established on those regions.

Finally, the race of the aliens present for each mission will be randomly selected according to their [[Alien Appearance Ratios]]. In the XCom1Ruleset.rul this can be changed under [[Ruleset Reference (OpenXcom)#Alien Missions|Alien Missions]], on the raceWeights subsection. 

===UFO waves===
Each alien mission will consist of a wave of several UFOs, each of a specific type and with a particular flight pattern. The type and number of UFOs and the time interval between them are defined on the [[MISSIONS.DAT]] file (original game) or under the [[Ruleset Reference (OpenXcom)#Waves|Waves]] subsection of the AlienMissions section of the XCom1Ruleset.rul file. 

{| class="wikitable" width="70%" 
| align="center" style="background:#f0f0f0;"|'''Alien Mission'''
| align="center" style="background:#f0f0f0;"|'''UFO Type'''
| align="center" style="background:#f0f0f0;"|'''UFO Count'''
| align="center" style="background:#f0f0f0;"|'''Timer (minutes)'''
| align="center" style="background:#f0f0f0;"|'''UFO Trajectory'''
| align="center" style="background:#f0f0f0;"|'''Notes'''
|- align="center"
| rowspan="4"| Alien Research
|- align="center"
| Small Scout||1||9000||P0||1 flyover of missionZone 1
|- align="center"
| Medium Scout||1||7800||P2||1 landing on missionZone 1
|- align="center"
| Large Scout||2||9000||P4||2 landings on missionZone 2
|- align="center"
| rowspan="9"| Alien Harvest
|- align="center"
| Small Scout||1||7800||P0||1 flyover of missionZone 1
|- align="center"
| Small Scout||1||7800||P2||1 landing on missionZone 1
|- align="center"
| Medium Scout||1||6000||P2||1 landing on missionZone 1
|- align="center"
| Large Scout||1||7800||P0||1 flyover of missionZone 1
|- align="center"
| Large Scout||1||9000||P3||1 landing on missionZone 2
|- align="center"
| Harvester||1||3000||P4||2 landings on missionZone 2
|- align="center"
| Harvester||1||750||P4||2 landings on missionZone 2
|- align="center"
| Battleship||1||1500||P2||1 landing on missionZone 1
|- align="center"
| rowspan="6"| Alien Abduction
|- align="center"
| Small Scout||1||15000||P0||1 flyover of missionZone 1
|- align="center"
| Medium Scout||1||18000||P1||1 landing on missionZone 1, 1 landing on missionZone 2
|- align="center"
| Large Scout||1||9000||P3||1 landing on missionZone 2
|- align="center"
| Abductor||1||9000||P3||1 landing on missionZone 2
|- align="center"
| Abductor||2||60||P4||2 landings on missionZone 2
|- align="center"
| rowspan="9"| Alien Infiltration
|- align="center"
| Small Scout||1||16500||P0||1 flyover of missionZone 1
|- align="center"
| Medium Scout||1||15000||P1||1 landing on missionZone 1, 1 landing on missionZone 2
|- align="center"
| Medium Scout||1||13800||P6||1 landing on missionZone 4
|- align="center"
| Large Scout||1||9000||P7||1 landing on missionZone 3
|- align="center"
| Large Scout||1||60||P7||1 landing on missionZone 3
|- align="center"
| Terror Ship||1||60||P7||1 landing on missionZone 3
|- align="center"
| Supply Ship||1||60||P7||1 landing on missionZone 3
|- align="center"
| Battleship||2||60||P7||1 landing on missionZone 3
|- align="center"
| rowspan="6"| Alien Base
|- align="center"
| Small Scout||1||13500||P1||1 landing on missionZone 1, 1 landing on missionZone 2. Landed Small Scouts only appear during Alien Base missions
|- align="center"
| Medium Scout||1||15000||P5||2 landings on missionZone 4
|- align="center"
| Large Scout||1||60||P5||2 landings on missionZone 4
|- align="center"
| Supply Ship||2||60||P5||2 landings on missionZone 4
|- align="center"
| Battleship||1||60||P6||1 landing on missionZone 4
|- align="center"
| rowspan="5"| Alien Terror
|- align="center"
| Medium Scout||1||15000||P1||1 landing on missionZone 1, 1 landing on missionZone 2
|- align="center"
| Large Scout||1||9600||P1||1 landing on missionZone 1, 1 landing on missionZone 2
|- align="center"
| Terror Ship||1||12000||P0||1 flyover of missionZone 1
|- align="center"
| Terror Ship||1||9000||P7||1 landing on missionZone 3
|- align="center"
| rowspan="5"| Alien Retaliation
|- align="center"
| Small Scout||1||3000||P8||UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
|- align="center"
| Medium Scout||2||3000||P8||UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
|- align="center"
| Large Scout||3||3000||P8||UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
|- align="center"
| Battleship||2||3000||P8||UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
|- align="center"
| rowspan="2"| Alien Supply
|- align="center"
| Supply Ship||1||6000||P9||UFO lands at alien base location
|-
|}

'''Additional Info'''
* The actual value of the timer between missions will be set between 50-150% of the value above displayed above. 
* Shooting down a UFO will add 1-9 days (1440 - 13340 min) to the next timer
* Those delays from shooting down UFOs can postpone Alien missions to the next month, which can result in situations such as 2 Terror Sites in the same month

===UFO Trajectories===
The [[Ruleset Reference (OpenXcom)#UFO Trajectories|UFO trajectories]] are used by the game to determine the actual flight path of the UFO, as well as their speed/altitude and any possible landings and time spent on the ground. Each UFO trajectory has the following attributes:
* Trajectory ID
* Ground Timer (minutes spend on ground before taking-off)
* List of Waypoints. Each trajectory will have several waypoints, including entry/exit of Earth's atmosphere and patrol/landing areas. Those areas are called Misson Zones and each region has several defined zones. Each waypoint is used to determine the next destination of the UFO, along with the altitude and speed while flying.  
** MissionZone
** Altitude (5 = Very High, 0 = Ground). If the altitude of the next waypoint is 0, the UFO will land up reaching the waypoint it is currently heading towards. 
** Speed (% of the UFO's maximum speed)
{|class="wikitable" width="100%" 
| align="center" style="background:#f0f0f0;"|'''Trajectory ID'''<br>''(Ground Timer)''
| align="center" style="background:#f0f0f0;"|'''Mission Zones'''
| align="center" style="background:#f0f0f0;"|'''Altitude'''
| align="center" style="background:#f0f0f0;"|'''Speed (% of UFO Max)'''
| align="center" style="background:#f0f0f0;"|'''Notes'''
|- align="center"
|  rowspan="5"| P0<br>''3000''
|5||4||100||  rowspan="5"| P0 is a trajectory where UFOs don't land and survey missionZone 1 areas for later landings. It is used mainly by Small Scouts assigned as the 1st UFO on the Abductions, Harvest, Research and Infiltration missions 
|- align="center"
|0||3||74
|- align="center"
|0||1||28
|- align="center"
|1||1||47
|- align="center"
|5||2||100
|- align="center"
| rowspan="8"|P1<br>''4000''
|5||4||100|| rowspan="8"| P1 consists of 2 landings, first in missionZone 1 then on missionZone 2, and travelling between missionZone 0 inbetween. 
|- align="center"
|0||2||76
|- align="center"
|0||2||20
|- align="center"
|1||1||44
|- align="center"
|0||0||20
|- align="center"
|0||2||32
|- align="center"
|2||1||65
|- align="center"
|5||0||100
|- align="center"
| rowspan="5"|P2<br>''5000''
|5||4||100||  rowspan="5"| P2 consists of arrival at missionZone0 and a single landing on missionZone. P2 is only used in Research and Abductions missions
|- align="center"
|0||3||57
|- align="center"
|0||2||26
|- align="center"
|1||1||50
|- align="center"
|5||0||100
|- align="center"
|rowspan="5"|P3<br>''6000''
|5||4||100|| rowspan="5"| P3 consists of arrival at missionZone0 and a single landing on missionZone 2. P2 is only used in Abductions and Harvest missions
|- align="center"
|0||3||30
|- align="center"
|0||2||25
|- align="center"
|2||1||20
|- align="center"
|5||0||100
|- align="center"
| rowspan="8"|P4<br>''7000''
|5||4||100|| rowspan="8"| P4 consists of arrival at missionZone0 and two landings on missionZone 2, and travelling between missionZone 0 inbetween
|- align="center"
|0||2||67
|- align="center"
|0||2||52
|- align="center"
|2||1||40
|- align="center"
|0||0||30
|- align="center"
|0||2||20
|- align="center"
|2||1||30
|- align="center"
|5||0||100
|- align="center"
|rowspan="8"| P5<br>''4000''
|5||4||100||  rowspan="8"| P5 consists of arrival at missionZone0 and two landings on missionZone 4, and travelling between missionZone 0 inbetween. P5 is only used by UFOs on Base missions
|- align="center"
|0||2||50
|- align="center"
|0||2||20
|- align="center"
|4||1||15
|- align="center"
|0||0||20
|- align="center"
|0||2||29
|- align="center"
|4||1||42
|- align="center"
|5||0||90
|- align="center"
|rowspan="5"|P6<br>''5000''
|5||4||100||  rowspan="5"| P6 consists of arrival at missionZone0 and a single landing on missionZone 4. P6 is only used in Infiltration and Base missions
|- align="center"
|0||3||50
|- align="center"
|0||2||25
|- align="center"
|4||1||16
|- align="center"
|5||0||100
|- align="center"
|rowspan="5"| P7<br>''9000''
|5||4||100|| rowspan="5"| P7 trajectories are only used by UFOs that land on cities, namely the Terror Ship that triggers the appearance of a Terror Site, and the last phase of the Alien Infiltration mission, where several UFOs descend on cities (missionZone 3)
|- align="center"
|0||3||60
|- align="center"
|0||2||30
|- align="center"
|3||1||20
|- align="center"
|5||0||100
|- align="center"
|rowspan="8"| P8<br>''6000''
|5||4||100||   rowspan="8"| P8 trajectories are exclusive to UFOs on Retaliation missions, who will never land while searching for XCom bases. The UFO will head towards 1 entry on each of the different zones, starting with missionZone 1. However, the Battleship that conducts the actual assault will use instead the RETALIATION trajectory to target the XCom base
|- align="center"
|1||1||60
|- align="center"
|2||1||20
|- align="center"
|3||1||30
|- align="center"
|4||1||20
|- align="center"
|0||1||42
|- align="center"
|2||1||56
|- align="center"
|5||4||100
|- align="center"
|rowspan="2"| P9<br>''2000''
|5||4||80||   rowspan="2"| P9 is a special trajectory used only by UFOs on Alien Supply missions to target the alien base
|- align="center"
|5||0||100
|- align="center"
|rowspan="2"| RETALIATION<br>''1000''
|5||4||100||    rowspan="2"| 
|- align="center"
|5||0||100
|-
|}

The following sequence of images shows a Large Scout on a Terror mission with a P1 trajectory as it moves from one waypoint to the other.

[[File:Large Scout P0 Trajectory (EU).png]]

===Mission Zones===
Finally, each region can have up to 6 types of Mission Zones, defined under [[Ruleset Reference (OpenXcom)#Regions|Regions]], corresponding to different areas on the map. Each type serves as a specific role:
* MissionZone 0 - areas used for UFOs when arriving at a specific region. Also regularly used by UFOs to transit between other missionZones. UFOs never land on this missionZone
* MissionZone 1 & 2 - used for landings on several missions
* MissionZone 3 - cities that appear on Geoscape and they are used by Alien Infiltration and Alien Terror missions for landings
* MissionZone 4 - Used for landings of UFOs on Infiltration and Base missions. Also used as the locations to spawn bases
* MissionZone 5 - used exclusively as UFO spawn/despawn areas before heading towards the regions where the mission will take place. These zones are usually located in a different world region

As an example, this image displays the Mission Zones for Europe. 

[[Image: Mission Zones Europe (EU).png|1000px]]

It is possible to see the Mission Zones for all regions using Falko's [http://falkooxc2.pythonanywhere.com/worldeditor WorldEditor].

==See Also==
*[[Ruleset Reference (OpenXcom)]]

[[Category:OpenXcom]]