The Rainbow Bridge is the crystalline, prismatic bridge that runs from the palace at the center of Asgard to Heimdall's Observatory at the edge of Asgard. It supplies the Observatory with the rainbow-like energy that powers the Bifrost.
Thor and his companions traveled across it on horses on their way to Heimdall and to Jotunheim. On Thor's return to Asgard, he and Loki battled at the entrance to Heimdall's Observatory which resulted in Thor breaking the Bridge and destroying the Bifrost. This resulted in the outbreak of raids throughout the Nine Realms.
After the Chitauri Invasion, Thor and Loki transported back to Asgard using the Tesseract. Heimdall used the Tesseract's power to rebuild the Bifrost and transported the armies of Asgard across space to quell the uprisings.
During the Second Dark Elf Conflict, Heimdall noticed the Dark Elf ships approaching Asgard so he ran up the bridge's suspension to attack the ship and deactivate the cloaking mechanism. After he successfully destroyed the ship, several more ships uncloak and invade Asgard. Heimdall activates a force field around the palace, transferring a signal through the Rainbow Bridge.
