Half inspired by something I read on teaspoon, half the actual episode.
Spoilers for Voyage of the Damned, and beta'd by the lovely bubbles.
"On Earth, there's this tradition," and he leant forward and kissed her, aware his hand was visible through her face, a beautiful woman made of stardust; gone too soon. Like Rose.
And then he took a step back, and she began to fade, to dissolve and vanish into the space all around the Titanic, her body and being becoming part of the stars and the sky that she had longed to see and, ironically, becoming the star she was.
And the Doctor was sad, not only because someone else had died, that he'd lost another companion, but also because he'd thought of Rose, and the beach. Was that what happened when he vanished on the beach, after he ran out of time? Was that what Rose had seen?
In those few seconds, as Astrid died but started her life in the stars, the Doctor realised and remembered the impact of what had happened a year before, and in the years before that, when he'd taken the hand of another blonde, and led her from the life she was stuck in and shown her everything she could only imagine in her dreams.
Two beautiful people, one he loved, one he'd only just met, gone too soon. Rose Tyler and Astrid Peth. Two brilliant, brave, radiant women he'd let down.