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, 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
- 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
- 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
- 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 |
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. For more details check the MISSIONS.DAT page. [Verify?]
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 1 |
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 |
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 |
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 |
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 for UFOs Alien Supply missions to target the alien base |
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.