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

Missions Frequency

The aliens will generate new missions each month according to the following pattern:

• January, 1999: 1 Alien Research, 1 Alien Terror, shooting down UFOs may trigger an Alien Retaliation mission.
• February to May, 1999: 1 random mission, 1 Alien Terror, shooting down UFOs may trigger an Alien Retaliation mission.
• June, 1999: 2 random missions, 1 Alien Terror, shooting down UFOs may trigger an Alien Retaliation mission.
• 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.
• Alien Supply missions will take place once per month for each alien base present in the globe. ^[Verify?]

The number and frequency of missions is hardcoded in OpenXCom and it can't be changed unless the .EXE file is modified.

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.

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. ^[Verify?]

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.

Missions & UFOs

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
Alien Research
	Small Scout	1	9000	P0
	Medium Scout	1	7800	P2
	Large Scout	2	9000	P4
Alien Harvest
	Small Scout	1	7800	P0
	Small Scout	1	7800	P2
	Medium Scout	1	6000	P2
	Large Scout	1	7800	P0
	Large Scout	1	9000	P3
	Harvester	1	3000	P4
	Harvester	1	750	P4
	Battleship	1	1500	P2
Alien Abduction
	Small Scout	1	15000	P0
	Medium Scout	1	18000	P1
	Large Scout	1	9000	P3
	Abductor	1	9000	P3
	Abductor	2	60	P4
Alien Infiltration
	Small Scout	1	16500	P0
	Medium Scout	1	15000	P1
	Medium Scout	1	13800	P6
	Large Scout	1	9000	P7
	Large Scout	1	60	P7
	Terror Ship	1	60	P7
	Supply Ship	1	60	P7
	Battleship	2	60	P7
Alien Base
	Small Scout	1	13500	P1
	Medium Scout	1	15000	P5
	Large Scout	1	60	P5
	Supply Ship	2	60	P5
	Battleship	1	60	P6
Alien Terror
	Medium Scout	1	15000	P1
	Large Scout	1	9600	P1
	Terror Ship	1	12000	P0
	Terror Ship	1	9000	P7
Alien Retaliation
	Small Scout	1	3000	P8
	Medium Scout	2	3000	P8
	Large Scout	3	3000	P8
	Battleship	2	3000	P8
Alien Supply
	Supply Ship	1	6000	P9

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 - 12960 min.) to the next timer. ^[Verify?] For more details check the MISSIONS.DAT page. 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 the only trajectory where UFOs don't land. 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
	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
	0	3	57
	0	2	26
	1	1	50
	5	0	100
P3 6000	5	4	100
	0	3	30
	0	2	25
	2	1	20
	5	0	100
P4 7000	5	4	100
	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
	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
	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.
	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. However, the Battleship that conducts the actual assault on the XCom base will use instead the RETALIATION trajectory.
	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 trajectories are only used for Alien Supply missions.
	5	0	100
RETALIATION 1000	5	4	100
	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. Mission Zones 5 are used exclusively as UFO entry/exit areas from the globe, while the others can be used either for travel or landings, depending on the UFO's trajectory and mission. Mission Zones 3 are the cities that appear on Geoscape and they are used by Alien Infiltration and Alien Terror missions for landings.

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.

See Also

• Ruleset Reference (OpenXcom)