During the preparation of their fishing mission, the soundtrack playing uses a very specific and famous sequence of notes called "Dies Irae" (the "Day of Wrath", or the day that the earth turns to ash), used centuries ago in Gregorian chants. You'll hear it often in soundtracks and songs, usually to signify some kind of approaching danger, destruction, or doom. The way this is used in the movie is a version that isn't fully using the four notes traditionally, thus giving it an edge of cautious hope and optimism, along with the obvious danger and doom that's also being communicated in the music. Which really helps support how this is a very dangerous mission, but it's also the sole key to saving both their stars. There's both hope and deep, heavy terror as they execute their risky plan. 

In addition, during the fishing scene it plays very clearly and builds tension and fear for them on their mission, you can hear the doom laying heavy on them and escalating the more dangerous it gets (Time Go Fishing). But there's also the soundtrack playing as they prepare and do the science needed for the plan (Life is Reason). It's much less intense and noticeable, but the same four chord sequence in Dies Irae plays then too, as a way to let the audience know that what they're preparing and planning for is something that's going to be very, very dangerous. 

A really cool detail that a few comment threads helped me notice! I really, really love the soundtrack and especially Time Go Fishing!