A toaster size atomic clock has been launched few days ago by NASA to undergo a series of tests in orbit. It was tested for over a year on the ground and it proved able to remain within the specification of maximum drift of 1 nanosecond in 10 days (a billionth of a second). That means a maximum drift of 1 second in 27 million years of operation (actually it is much more accurate since the drift is not necessarily going in the same direction over time but I guess we won’t be there to see it has drifted more than one second).
We already have atomic clocks with this kind of precision on Earth, based on the vibration of rubidium atoms. The problem with these clocks is their size, too bulky and heavy to become part of an orbiting object. The Deep Space Atomic Clock is based on mercury atoms and has the size of a toaster.
You are most likely to have an atomic clock on your wrist, a quartz clock, that is using the vibration of a quartz crystal (made up by many many atoms) when a voltage is applied (that’s why you need a battery and you need to change it after a few years…). These watches are quite accurate if you compare them to the mechanical watches of your grandparents, but still they are not accurate enough when you have to deal with precise positioning in space. With the precision of a quartz watch, even with the best one you can get, after six weeks in space the drift would be around one ms, unnoticeable to you but resulting in a positioning error of about 300km.
That is why current space navigation is using Earth based atomic clocks and the time it takes for a signal to go back and forth from the space vehicle to Earth. Since the speed of the electromagnetic field is constant (speed of light) by measuring the time the signal takes to reach the vehicle and come back we can know the distance with very high precision and by using several receivers on Earth we can triangulate and pinpoint its position in space. The problem with this system is that the signals take quite some time, and the further the spaceship is from Earth the longer it takes, meaning that it becomes impossible to steer the spacecraft in real time (it takes between 4 and 24 minutes for a signal to reach Mars from Earth, depending on their relative position, twice that time to have the signal coming back).
We are already using atomic clocks on GPS satellites orbiting the Earth but they have to be synchronised twice a day to keep them in synch. Future space travel will need a GPS like support and for that we need ot have much more stable atomic clocks that can be sent on spaceships. This is the goal of the NASA Deep Space Atomic Clock. If all goes well it will become part of future spaceships providing GPS service in their voyage to other planets, Mars being the first one we will aiming at.