The "bottom" of the egg is flattened by a permanent clash of particles, as the outbound solar wind smashes into atomic debris hurtling in from interstellar space, the scientists theorize.
Voyager 2 also crossed the "termination shock" several times within the space of a single day, showing that the boundary is in perpetual flux, like the ebb-and-flow of a tide.

We are passing into completely new territory of exploration with these probes. Oh the enlightenment that will come from a look home from so far away!