The watch in Steve Squyres' pocket is off by 39 minutes — and 106 million miles.
One of just a few timepieces of its kind in the world, the pocket watch chained to the Cornell University astronomer's faded jeans tells the time on distant Mars, where NASA successfully landed a six-wheeled robot over the weekend.
Squyres is the main scientist on the unmanned mission to explore Mars, where days last exactly 39 minutes and 35 seconds longer than they do on Earth. His watch, while slow on Earth, tells perfect Mars time.
To stay synchronized with Mars and a rover that works strictly by the clock, Squyres and the 280 other people working with him on the project have had to leave Earth time behind.
"We're essentially slaves to Martian time," said Firouz Naderi, manager of the Mars exploration program at the National Aeronautics and Space Administration's Jet Propulsion Laboratory.
The $820 million Mars Exploration Rover project includes a second rover, Opportunity, set to land on the opposite side of the planet from Spirit on Jan. 24.
The solar-powered rovers should do most of their work between 10 a.m. and 2 p.m., local Mars time, when the sun is at its highest in the Martian sky.
Breakfast at 10 p.m.
Throughout each 90-day mission, that four-hour window will fall each day during a slightly different period of the day on Earth, forcing mission members to adjust accordingly.
Mission member Julie Townsend copes by wearing two watches: one set to Earth time on her left wrist, a second running on Mars time on her right.
"It's very helpful, because there are some things I only know in Mars time," said Townsend, a mission avionics engineer, of her specially modified $150 Mars watch.
The length of a day is determined by how long it takes a planet to rotate on its axis. On Earth, a single spin takes 24 hours; on Mars, just a tad longer.
On any given day, the difference between a terrestrial and Martian day would be difficult to perceive.
Over the course of the three months each rover should last, the 40-minute daily lags begin to add up, quickly turning night to day and back again. Every 36 Earth days, Mars time falls another 24 hours behind.
Shifting schedules
To compensate, mission members have begun shifting the schedules of their every activity, including when they sleep and eat. The move came on the recommendation of sleep deprivation experts enlisted by the mission.
"Sometimes we'll be having breakfast at 10 p.m. and lunch at 6 a.m.," said mission science manager John Callas.
Agency officials also blacked out the windows in the mission operations center at JPL to block the sun and the bright reminder it provides of the time of day on Earth. The same has been done for the Pasadena rental apartments where 160 visiting scientists are living throughout the mission.
Also, mission members who end their shifts during daytime hours on Earth are told to wear sunglasses on their way home to bed, to further minimize exposure to daylight during what are nighttime hours on Mars, Callas said.
Coping with the outside world
The steady shift in time should wreak havoc in coordinating with an outside world that remains on Earth time, said Nagin Cox, deputy chief of the rover engineering team.
It "produces interesting challenges, schedule-wise," Cox told reporters in November. "Baby sitters don't work on Mars time."
To keep their own time on Mars, both rovers carry sophisticated digital clocks. NASA also outfitted each robot with a far simpler timepiece meant to engage children following the project: a sundial.