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 the later track Roll Call. The opening strings were later featured in the track From the Vault from Halo: Reach Original Soundtrack.
No Time for Sightseeing
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 rearrangement of the second section of Broken Gates (from Mombasa Suite in 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
Make Them Pay (2:25) is a full rearrangement 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.