Like a cracked bark on a tree
One image that Petra describes of her grandmother during their conversation is that the pursing of her lips makes her skin look like a cracked bark on a tree. The bark of the tree is the protective layer that keeps out the outer elements from damaging it. A cracked bark means that the tree survived some serious damage, or it simply experienced a lot through the years. This metaphor in relation to her grandmother signifies her resilience, it is a visible proof of wisdom and experience.
Like terrified mice
Right before the spaceship takes off, a crowd of terrified people starts attacking it, trying to get on. Petra imagines them like terrified mice, scratching at the sides to get on. This comparison is significant because of the way it dehumanizes and reduces scared people to scared simple animals.
Like a younger sibling
After setting foot on Sagan, Petra immediately notices the sky and the two moons: a large one and a smaller one peeking curiously behind, like a younger sibling. This comparison is important because it reminds of the relationship between Petra and Javier, her younger brother, whom she was very protective of.
A rabbit and a rattlesnake
After reuniting with Javier, Petra agrees to read the Dreamers book to Voxy as well. In the midst of it, they are interrupted by Nyla. Voxy freezes like a rabbit, waiting for the rattlesnake to strike. Voxy was compared to a rabbit before as well, when Petra followed him into his chamber where the relics of humanity are kept, after dreaming of her grandmother urging her to follow the rabbit.
