From Halopedia, the Halo wiki
Farthest Outpost is the thirteenth track (Disc 1 Track 13) included in Halo 3: Original Soundtrack, and plays during the Halo 3 campaign level The Ark. It is composed of Farthest Outpost (0:00-2:03), No Time for Sightseeing (2:03-2:49), and Make Them Pay (2:49-5:14).
Farthest Outpost (2:03) begins with the vocals singing gracefully, unaccompanied. Strings enter to echo the melody before muted percussion enter, leading into a section of bass line and percussive beats, the latter of which was played using snare drums and hi-hat. Low strings enter, playing a flowing melody as the vocals continue softly in the background. After the melody has repeated a few times, the percussion takes over while the the strings briefly plays the melody of Weight of Failure from Prologue (in Halo 2: Original Soundtrack). The piece plays in the opening cutscene, when the Fleet of Retribution arrives at Installation 00, and makes a reprise in the credits of Halo 3 in the form of the piece Duty Bound from Roll Call. The name of the piece is most likely in reference to Installation 00, which could be considered the "farthest outpost" of the Forerunners due to its remote location from the Milky Way.
No Time for Sightseeing (0:46) is a short reprise of the digital notes and bass line from the track Out of Shadow, though using muted percussion, thus resembling a reorchestration of the second part of Broken Gates in Mombasa Suite (from Halo 2: Original Soundtrack), albeit without the male vocal. This section plays when John-117 launches the attack on the the Type-27 Mantis under construction.
Make Them Pay (2:25) is a full reorchestration of Perilous Journey (from Halo: Original Soundtrack), though the sounds of the strings sections are considerably crisper compared to the original rendition. A heavy bass drum is audible in the background playing seven beats before resting and then repeating. This section plays when John-117 commandeers the vehicles at the crashed D77H-TCI Pelican.