This room is located at the very top of the Phazon Core and is sealed by a Red Door. It contains a Missile Station that appears to be fueled by a giant mass of Red Phazon above it, which itself is supported by several orange tendons. In the NTSC version, it is possible that either the Space Pirates placed the Missile Station there after penetrating the Artifact Temple's Cipher, or that it was there because Metroid Prime had assimilated some of the Pirates' technology.
The former idea would not be possible in the PAL and all subsequent versions of Metroid Prime, due to changes made to certain Logbook entries that retconned Pirate logs stating they were able to breach the Cradle. Therefore, it is possible that the Chozo placed it here, for the Entrusted One, Samus Aran, to use in preparation for her battle with the Worm, Metroid Prime.