In the Rogue One trailer—as well as the actual film itself—we see a Star Destroyer hovering directly over a city on the planet’s surface.
How is it possible?
I thought that all capital ships were built in space, because they were too heavy to lift from the planet’s gravitation after completion.
In the movie, the Star Destroyer hovers over the location for a long time. There are no Repulsorlift that could hold that much weight in the air, so how is that possible?
From the Battlefront: Twilight Company novel, which is part of new canon:
"We're in atmosphere," [the Star Destroyer captain] said, embarrassed at his own urgent tone. "We need full power to stay aloft. Any disruption at all - " Star Destroyers were extraordinary vessels, capable of razing mountains and carrying armies. But their mass was measured in millions upon millions of tonnes, and their energy requirements were vast.
Battlefront: Twilight Company Chapter 38
It implies that Star Destroyers are perfectly capable of operating within atmospheres, it just draws a significant amount of power (presumably repulsorlift technology is used to keep aloft, though there may also be ventral thrusters). This is supported by numerous depictions of Star Destroyers operating within atmospheres in Rebels, and Star Wars: Attack of the Clones Incredible Cross-Sections (which is Legends) noting that the massive Lucrehulk core ships use repulsorlift technology to land.
This image, from Star Wars Rebels:
Rebels is canon, so therefore Star Destroyers are quite capable of operating in a planet's atmosphere.
It may be a Victory Class. The Victory Class was an early Star Destroyer class that could operate in planetary atmospheres. They were also considerably smaller than the Imperial Class Star Destroyer being only 900 meters long. There were two versions of this class, the Victory I and Victory II.
Sources http://starwars.wikia.com/wiki/Victory_I-class_Star_Destroyer http://starwars.wikia.com/wiki/Victory_II-class_Star_Destroyer
