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.
List of Alien Missions
- Alien Research
- Alien Harvest
- Alien Abduction
- Alien Infiltration
- Alien Base
- Alien Terror
- Alien Retaliation
- Alien Supply
Alien Missions Generation
Several items of the information presented below are based on OpenXcom's work while replicating the original game's engine. This is the first attempt to compile the data from several sources at a single location.
Keep in mind that there are massive spoilers involved, so read at your own risk.
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.
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.
First the game will randomly choose a region for the mission, using weighted factors (defined on the ZONAL.DAT 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.
As said above, each region has several missions assigned. The mission will be randomly selected also based on weighted factors (defined on ACTS.DAT), 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.
Missions & UFOs
Each alien mission will consist of several UFOs being send, 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.
| 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 |
See Also
