Atomic Clocks

"One day, we'll want to have GPS satellites around other planets, too," notes Don Strayer of NASA's Fundamental Physics Program at JPL. For example, a Martian Global Positioning System could guide explorers - both robot and human - across the Red Planet. Less likely but possible: Future farmers on Mars might want GPS to help them tend crops as their cousins on Earth do. Martian fields will definitely need special care.

Atomic clocks on board GPS satellites are stable "within 1 part in 10¹²," says Lute Maleki who supervises JPL's Quantum Sciences and Technology Group. That means an observer would have to watch a GPS clock for 10¹² seconds (32,000 years) to see it gain or lose a single second. "To guide spacecraft from planet to planet we use clocks that are even better - good to 1 part in 10¹⁴," he added.

Recently scientists have built atomic clocks that are better still - "stable to about one part in 10¹⁵," notes Maleki. They did it using a new technique called "laser cooling." In the 1990's several groups of researchers made a counter-intuitive discovery: Shining lasers on atoms can cool them to temperatures only a millionth of a degree above absolute zero. Such cold atoms make excellent "pendulums" for atomic clocks, explains Strayer, "because lower temperatures allow the natural frequency of the atom to be measured more accurately."

If cold atoms are good, then floating cold atoms are even better.

"The International Space Station is a great place for atomic clocks because the station is freely falling around the Earth," Strayer continued. Slow-moving atoms in a cooled weightless clock can be observed for a longer time, and they're less likely to hit the walls of their container in mid-oscillation.

If all goes as planned, a laser-cooled clock named PARCS will be installed on the ISS in late 2004 or 2005. Experts expect it to be the most stable clock ever, keeping time within 1 second every 300 million years (1 part in 10¹⁶).