Simplify the following text for a Grade 11 native English-speaking person.
Original: When pictures were going to be taken of the Great Dark Spot again in November 1994 by the Hubble Space Telescope , it had gone away completely , leaving astronomers to believe that it had either been covered up or that it had just disappeared .
Simplified: When the spot was to be photographed again in November 1994 by the Hubble Space Telescope , it had disappeared completely , leaving astronomers to believe that it had either been covered up or vanished .