Creating the perfect timepiece isn't really NASA's goal, though. The space agency wants to learn more about the basic nature of time and how it is affected by gravity. For this, NASA will install a second ultra-precise clock to make comparisons between the two.

The second timepiece will be a hydrogen maser, which works on the same principal as the cesium fountain but is based instead on the hydrogen atom.

The hydrogen maser actually makes a more accurate measurement of the second because the hydrogen atom moves back and forth more quickly than the cesium atom. In effect, the hydrogen maser provides more swings of the pendulum for observation. But this gain in resolution is outweighed by the hydrogen maser's tendency to become unstable within a matter of days, Sullivan explained.

By comparing the two clocks onboard the ISS, scientists will be able to test Albert Einstein's theory of general relativity, a cornerstone of modern physics, said PARCS scientist Lute Maleki of NASA's Jet Propulsion Laboratory.

The research could also improve understanding of the somewhat bizarre realm where time and space meet. According to Einstein, space and time become a single dimension, space-time, which is the true fabric of the universe.

Einstein's calculations showed large fields of gravity from massive objects like the Earth and the Sun distort space-time.

If this is true, explained Maleki, "A clock on Earth should be slower than it is in space." By comparing the clocks on the ISS with the cesium fountain clock at the National Institute of Standards and Technology in Boulder, CO, PARCS scientists hope to test this aspect of general relativity as well, he said.

The clocks' tick-tocks will be monitored on Earth via a Global Positioning System satellite link.

GPS satellites are already used to transfer time data from the 200 or so atomic clocks around the world to the French International Bureau of Weights and Measures (whose French acronym is BIPM). The data from these clocks are beamed to the GPS satellites and then back down to the BIPM, where the times are averaged together to keep what's called International Atomic Time.

International Atomic Time is then adjusted by the tempo kept by the world's two cesium fountain clocks, and by astronomers who track the eccentricity of the Earth's spin. But the two trips from clock to satellite to the BIPM can distort the original data.

Because the new clocks' signals will only be sent through the atmosphere once, PARCS scientists believe their project will be a model for a future space-based timekeeper that would cut this distortion in half.

"Time is already kept internationally," Sullivan said. "Why not do something like the Primary Atomic Reference Clock in Space, where it's accessible to everyone? This would truly reflect an international spirit for the effort of timekeeping."