NASA has developed a portable atomic clock that would allow deep space probes to navigate on their own. As Geoff Manaugh notes here, when you’re traveling in space with no access to a frame of reference, travel time from a point of origin is how one orients:

One might say that the ship is navigating time as much as it is traveling through space—steering through the time between things rather than simply following the lines that connect one celestial object to another.

The general problem of ship orientation and navigation in deep space is a fascinating one, and it has led to ideas like using “dead stars” as fixed directional beacons, a kind of thanato-stellar GPS. This is “the long-sought technology known as pulsar navigation,” Nature reported last year. “For decades, aerospace engineers have dreamed of using these consistently repeating signals for navigation, just as they use the regular ticking of atomic clocks on satellites for GPS.” You head toward something that’s only consistent because it’s dead.