Luck
From Halopedia, the Halo wiki
Luck is a song included in the Halo 3: Original Soundtrack. The song plays only once, during the beginning of Halo 3 in the beginning cutscene of the level Sierra 117.
The song starts with a light melody of wind instruments with choir interspersed in between. The introduction melody comes to a dramatic climax, then turns to a more peaceful strings section. Near the end, wind once again takes the melody, which is replaced by piano at the end.
It runs for 3:23.