Momentum refers to the speed at which Terraforming is taking place throughout a game. Depending on various factors, games can typically last from around 7 to around 15 generations, and the length of the game affects how useful different cards can be.
Thinking about momentum and the factors that influence it allows you to make a prediction of game length as early as the first hand when you're choosing your corporations. If you're approximately correct, that prediction will inform your decision throughout the game because you'll be able to more accurately assess the relative values of different cards.
Imagine your opening hand contains Adapted Lichen and you are trying to decide if it is worth the investment. At a cost of 12 total Megacredits for one plant production, if you profit by a single greenery tile then you've gained at least 1 point (if you place it after Oxygen is Terraformed and without any city adjacency) or up to 3 points if you've met both of those criteria. 12 Megacredits for 1 point is not the best, but 12 Megacredits for 3 points is excellent!
The decision can be very variable in some situations, however. This time imagine that you're trying to assess Adapted Lichen, but this time you know that the game is going to last exactly 8 rounds. You'll produce 7 plants over the course of the game from the card, and come pretty close to playing your greenery tile - in fact I would be almost certain that you'll find the one extra plant and build it in the last round. By then the Oxygen could be Terraformed, so it might be only worth 1 or 2 points. In this scenario the choice is borderline - maybe yes, maybe no. You might be better off playing an asteroid for 14 money instead of spending 12 on a possible future greenery tile, which would generate you 7 megacredits and in some situations the same amount of points.
Once again put yourself back into your opening hand, but this time you are appraising Adapted Lichen in the context of a 15 round game. You can guarantee the first greenery tile, and you also know that you'll get it around the midgame, so most likely the oxygen isn't Terraformed. Not only that, you'll generate an extra 6 plants after you've built your first guaranteed greenery tile, which is in most games sufficient to reach another greenery tile that will probably reach 2 points. So in this context you've gained pessimistically 3 points, but up to a maximum of 6 points. All this for the same cost as in the shorter game!
The takeaway point is that the value of many cards vary based on the context of the game that you are playing, and the major determinant of their value is the length of a game. Since the cost of the card is static, sometimes the length of the game will tip the card from being a good choice into a bad choice.
Short Games
In a short game, cards that act to increase production and cards that are engine-focused are worth less than they are in longer games. This is because with fewer generations they generate fewer resources and points, meaning they may not justify the initial investment. It doesn't mean that these cards have no use at all, but they are less useful than they could be in a longer game.
When there isn't time to get an efficient engine running, a lot of the focus is on maximising your share of the points available for Terraforming. This is why cards that directly Terraform or increase the efficiency of Terraforming are particularly worthwhile.
Long Games
On the other hand, in a long game there are enough generations for production cards and engine-focused cards to be more valuable. In particular cards that collect resources for points such as animals, microbes, and floaters, are sometimes almost useless in a short game, but can be very effective in a long game.
If the game is going to be long, then rapid Terraforming is less viable. One individual Terraforming by themselves in a long game isn't going to significantly speed up the game, and the advantage to engine builders will remain.
Note that there is no absolute distinction between the aggressive Terraforming strategy and the Engine-building Strategy. In any game you'll probably have a combination of the two. The important thing to keep in mind is the relative value of different cards.
Given that the length of the game affects the relative utility of cards differently, it would be incredible if you knew exactly how many generations a game would last so that you can make a game plan. Unfortunately, the variability in game length could be anywhere from around 7 generations to around 15 generations! The following discussion revolves around the factors that influence the game's momentum, so that you can make a better guess about whether the game will be long or short, and this allows you to make decisions about which cards you decide to keep.
Player Count
The most important factor in a game's momentum is the number of players. With more players, there are more resources generated in each production phase, and more cards are drawn. The increase in resources means that on average more cards, standard projects, and resource-based Terraforming steps will be played each generation, making the game finish in fewer generations. Drawing more cards means that the high-value Terraforming cards are more likely to come into play, which results in faster progression up the Terraforming tracks.
The result of this is that in 2- and 3- player games, the longer length of the game typically means that victory will require some kind of engine building Tactic. Have a look at the Tactic sections to see some of the ways that engines can be built. It can take the form of blue card engines to generate pure points, or resource production engines to generate Terraforming steps or other resources to play more cards. It also includes things like science tag accumulation, card draw, and more extensive ground game than in shorter games. Typically, if you focus on aggressive Terraforming in a 2- or 3- player game, you'll be overtaken in final scoring by players who have also invested in some aspect of engine-building.
In 4- and 5- player games on the other hand, the game take fewer generations and as a result the engine-building Tactics are relatively less valuable. In this context you should be focusing on cards that directly increase the global parameters, or indirectly help you achieve those aims such as heat production, plant production, and megacredit production. These games often turn into a rush to complete as many Terraforming steps as possible, so that you gain those points and deny them to your opponents.
Corporations
Expansions and Variants
Opponents