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 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 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 Regions on the XCom1Ruleset.rul file), which are shown on the table below.

Region	Research	Harvest	Abduction	Infiltration	Base
North America	14	17	20	20	20
Arctic	25	NA	NA	NA	35
Antarctic	25	NA	NA	NA	35
South America	10	24	12	18	22
Europe	10	10	15	20	6
North Africa	8	8	8	8	19
South Africa	7	8	8	8	9
Central Asia	5	5	6	10	8
Southeast Asia	8	13	21	13	8
Siberia	6	NA	4	NA	9
Australia	15	11	4	8	16
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 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 Waves subsection of the AlienMissions section of the XCom1Ruleset.rul file.

Alien Mission	UFO Type	UFO Count	Timer (minutes)	UFO Trajectory	Notes
Alien Research
	Small Scout	1	9000	P0	1 flyover of missionZone 1
	Medium Scout	1	7800	P2	1 landing on missionZone 1
	Large Scout	2	9000	P4	2 landings on missionZone 2
Alien Harvest
	Small Scout	1	7800	P0	1 flyover of missionZone 1
	Small Scout	1	7800	P2	1 landing on missionZone 1
	Medium Scout	1	6000	P2	1 landing on missionZone 1
	Large Scout	1	7800	P0	1 flyover of missionZone 1
	Large Scout	1	9000	P3	1 landing on missionZone 2
	Harvester	1	3000	P4	2 landings on missionZone 2
	Harvester	1	750	P4	2 landings on missionZone 2
	Battleship	1	1500	P2	1 landing on missionZone 1
Alien Abduction
	Small Scout	1	15000	P0	1 flyover of missionZone 1
	Medium Scout	1	18000	P1	1 landing on missionZone 1, 1 landing on missionZone 2
	Large Scout	1	9000	P3	1 landing on missionZone 2
	Abductor	1	9000	P3	1 landing on missionZone 2
	Abductor	2	60	P4	2 landings on missionZone 2
Alien Infiltration
	Small Scout	1	16500	P0	1 flyover of missionZone 1
	Medium Scout	1	15000	P1	1 landing on missionZone 1, 1 landing on missionZone 2
	Medium Scout	1	13800	P6	1 landing on missionZone 4
	Large Scout	1	9000	P7	1 landing on missionZone 3
	Large Scout	1	60	P7	1 landing on missionZone 3
	Terror Ship	1	60	P7	1 landing on missionZone 3
	Supply Ship	1	60	P7	1 landing on missionZone 3
	Battleship	2	60	P7	1 landing on missionZone 3
Alien Base
	Small Scout	1	13500	P1	1 landing on missionZone 1, 1 landing on missionZone 2. Landed Small Scouts only appear during Alien Base missions
	Medium Scout	1	15000	P5	2 landings on missionZone 4
	Large Scout	1	60	P5	2 landings on missionZone 4
	Supply Ship	2	60	P5	2 landings on missionZone 4
	Battleship	1	60	P6	1 landing on missionZone 4
Alien Terror
	Medium Scout	1	15000	P1	1 landing on missionZone 1, 1 landing on missionZone 2
	Large Scout	1	9600	P1	1 landing on missionZone 1, 1 landing on missionZone 2
	Terror Ship	1	12000	P0	1 flyover of missionZone 1
	Terror Ship	1	9000	P7	1 landing on missionZone 3
Alien Retaliation
	Small Scout	1	3000	P8	UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
	Medium Scout	2	3000	P8	UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
	Large Scout	3	3000	P8	UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
	Battleship	2	3000	P8	UFO goes through all different missionZones, going 1, 2, 3, 4 and 0
Alien Supply
Alien Supply	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 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)

Trajectory ID (Ground Timer)	Mission Zones	Altitude	Speed (% of UFO Max)	Notes
P0 3000	5	4	100	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
	0	3	74
	0	1	28
	1	1	47
	5	2	100
P1 4000	5	4	100	P1 consists of 2 landings, first in missionZone 1 then on missionZone 2, and travelling between missionZone 0 inbetween.
	0	2	76
	0	2	20
	1	1	44
	0	0	20
	0	2	32
	2	1	65
	5	0	100
P2 5000	5	4	100	P2 consists of arrival at missionZone0 and a single landing on missionZone. P2 is only used in Research and Abductions missions
	0	3	57
	0	2	26
	1	1	50
	5	0	100
P3 6000	5	4	100	P3 consists of arrival at missionZone0 and a single landing on missionZone 2. P2 is only used in Abductions and Harvest missions
	0	3	30
	0	2	25
	2	1	20
	5	0	100
P4 7000	5	4	100	P4 consists of arrival at missionZone0 and two landings on missionZone 2, and travelling between missionZone 0 inbetween
	0	2	67
	0	2	52
	2	1	40
	0	0	30
	0	2	20
	2	1	30
	5	0	100
P5 4000	5	4	100	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
	0	2	50
	0	2	20
	4	1	15
	0	0	20
	0	2	29
	4	1	42
	5	0	90
P6 5000	5	4	100	P6 consists of arrival at missionZone0 and a single landing on missionZone 4. P6 is only used in Infiltration and Base missions
	0	3	50
	0	2	25
	4	1	16
	5	0	100
P7 9000	5	4	100	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)
	0	3	60
	0	2	30
	3	1	20
	5	0	100
P8 6000	5	4	100	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
	1	1	60
	2	1	20
	3	1	30
	4	1	20
	0	1	42
	2	1	56
	5	4	100
P9 2000	5	4	80	P9 is a special trajectory used only by UFOs on Alien Supply missions to target the alien base
P9 2000	5	0	100
RETALIATION 1000	5	4	100
RETALIATION 1000	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.

Mission Zones

Finally, each region can have up to 6 types of Mission Zones, defined under 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.

It is possible to see the Mission Zones for all regions using Falko's WorldEditor.

Alien Missions in Enemy Unknown (OpenXcom)

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