White Big ball Cross Barcelona

I see 1 White light ball crossing very slow the Sky. More Big than 3 normal planes. the object crossed the sky from left to right towards the sea. when it was moving away, the light looked more diffused as if the light was only in the lower part. I could not see the shape nor did I hear any sound. In Barcelona many planes pass by, but this object did not have position lights (red and green), it flew at a low altitude of approximately 300 meters but it was very large and silent. A minute later a plane passed by at the same height heading for the airport and I could hear the engines and the lights. the difference in size and shape is what has caused me to report this sighting.