I think its a Sheild World and this is why.
I can not say for sure what the object is, however I do have a very good theory.
If you take note of the fact that each "Halo" ring (hence forth referenced by "HR") came with, or was issued a emergency survival sphere AKA-Shield World (hence forth referenced by "SW"), then taking note of "The Ark" (hence forth referenced by "Ark") and the fact that the only way to get to the Ark was by means of the "Portal" on Earth, (which are both outside the range of any HRs) one would come to a logical conclusion. According to the "Terminals" and previous story-Lines and books, the Forerunners built the HR's to wipe the proverbial slate clean of all "bio-mass" in order to starve the Flood. They planned to save as many organisms as they could by using the SWs and the Ark. One could assume that the Ark was the only real object the Forerunners actually built themselves. The Ark would design, build, and deploy the HRs where needed to maximize their effectiveness. Taking note now of the Ark's design, you can clearly see in the game that the core of the Ark is wide open, in fact it is open enough to slide a planet through. My theory is that the Ark not only designed, built and deployed the HRs, but the SWs as well. If this is true then each HR has a SW with it. As you know we (being Master Chief) destroyed Halo Delta (Installation 04) (hence forth referenced by "HR-D"). As such the Ark is programmed to be a last step firing platform and HR replacement facility. If any HR's firing sequence is aborted as it was at the end of Halo 2, the Ark can activate all HRs simultaneously and wipe out the Flood. Once HR-D was destroyed the Ark started making a new Installation 04, which is also why the activation index from HR-D that Cortana had, worked in the new ring. All this in mind, the Ark is not responsible for the maintaining or monitoring of the SW's because of Medicant Bias's betrayal. As such when HR-D was destroyed it automatically came with a new SW. The SW is immune to the HR's energy wave and may have been constructed prier to the new ring. My theory on that in a minute. All that in mind firing the new HR-D replacement before its completion caused it to explode taking the Ark with it, however the SW was already at a safe distance awaiting the new rings completion. The Portal was shutdown when the Portal Controller on Earth was unable to detect a signal from the Ark, cutting the ship in half. So the object on approach at the end of Halo 3 is an SW. Now taking into account that Medicant Bias betrayed the Forerunners by telling the Flood the locations, purpose, and functions of all SW's, the Flood may have sent a small contingency group to the SW and may be on board. As the SW would have an engine they could use it to return to Earth. (See below about engine)
My other theory that I mentioned earlier was to the idea of ring moving and positioning. The reason the SW would be built first is because the SW has an engine and HR's do not. If you build the HR first it would simply drift away. The SW can move itself away from the Ark and move into its HR to move it. Noting that the Ark builds the SW in its center and the HR around that, then it’s plain to see that the HR would fit nicely around the SW, magnetically docking to allow for travel and placement of the ring as the Ark itself never moves. How else would the HRs be deployed?
