I'm still confused. Is it still like a passenger craft inside? What is the function of this craft?
Basically, it will replace Hubble telescope.
It's a NASA research aircraft. Originally it was a Pan Am and later United Airlines passenger 747SP used for long range routes back before the 747-400 or MD-11 were available. The empennage section is unpressurized and holds the telescope. There is a pressure bulkhead built between the passenger cabin and the telescope bay that allows the front of the plane to have passengers, work stations, and equipment that support the operation of the telescope. When the telescope is in operation there is actually a large hole open in the empennage of the plane.
I'm sure NASA got this plane cheap. Once more efficient planes like the 747-400, that had the same range but lower seat mile costs, became available, most of the 747SP's were retired by the airlines. Only 45 747SP's were ever built and less than half of those are still flying.