1998 ISS is played in rounds where the players select between 4 Earth actions:

• Astronauts training, to send new astronauts to the Space Station.
• Materials loading, that will be used to perform experiments.
• Modules preparation. Players contributing more to send new modules will gain extra actions.
• Experiments planification, to get new experiment cards.

and 4 ISS actions:

• Move material outside, and use it to make your experiments
• Perform experiments, they will give you most of the points in the game
• Long stay in the ISS, so your astronauts stay longer in the ISS and you win victory points by researching with the effects of the microgravity in humans.
• Install new modules, that will give you extra points and they can be used to perform experiments.

In other words, Earth actions will prepare you to make the ISS actions that will grant you victory points. The game is over when 5 American or 5 Russian rockets are launched and the player with the most victory points will be the winner.

For more information, please refer to the BGG listing.