On Mars Time
15 Dec 2003
(Source: Astrobiology Magazine)
Summary: During the upcoming Mars Exploration Rover missions, participating scientists and engineers will be waking and going to bed with the rising and setting of the sun - on Mars. There's a hitch: A day on Mars is 39.5 minutes longer than a day on Earth.
Living on Mars Time
By Henry Bortman
December 15, 2003
When NASA's Mars Exploration Rover-A, more affectionately known as "Spirit," touches down in Gusev Crater, it will be approximately 8:30 PM, January 3rd, 2004, at mission control. That's Pacific Standard Time (PST), because mission control is located on the grounds of the Jet Propulsion Laboratory (JPL), in Pasadena, California.
When time is the topic, however, Pacific Standard tells only part of Spirit's story. Scientists and engineers will also be keeping track of UTC (Coordinated Universal Time), more commonly known as Greenwich Mean Time. UTC is the basis of official timekeeping all over the world. This world, at any rate. Using UTC makes it easier for NASA and its international collaborators to synchronize their watches.
It's easy to convert between PST and UTC. UTC is eight hours later than PST. So when Spirit comes to rest on the surface of Mars, in Greenwich, England, eight time zones to the east of Pasadena, it will be 4:30 in the morning, January 4th.
So much for Earth. But what about Mars? While international flights on Earth require coordination between two time zones, interplanetary flights require time coordination between two worlds. And coordinating clocks between Earth and Mars is not a simple matter of adding or subtracting hours.
A day on Mars, which is known as a "sol," consists of 24 hours, just like a day on Earth. Each hour contains 60 minutes; each minute 60 seconds. There's nothing magical about that. Scientists simply got together and declared it to be so.
But there's a catch. A martian second is a smidge longer than what you're used to on Earth. Think of it this way: Instead of counting, "One Mississippi, two Mississippi" count "One Mississippis, two Mississippis."
Why tamper with the length of a second? Because a martian day is about 39.5 minutes longer than a terran day. By stretching each of the 86,400 seconds in a day by just a hair, those 39.5 minutes can be spread out throughout a martian sol without anyone hardly even noticing. Until, that is, it comes time to send radio signals between Mars and Earth.
Broadly speaking, the goal of the Spirit mission - and, of course, of the Opportunity mission, too; it's just easier to talk about them one at a time - is to send scientific data to Earth. But for Spirit to communicate directly with Earth, either to receive commands or to transmit data, two conditions have to be met. First, it has to be daytime on Mars, so that Spirit's transmitter is warmed up and powered by the sun's energy. Second, Earth has to be present in the martian sky above Gusev Crater. Both of these conditions are controlled primarily by Mars's rotation.
Thus, because each Mars rotation takes 24 Earth hours and 39.5 Earth minutes, on each successive sol during the Spirit mission, communication between the rover and Earth will take place 39.5 Earth minutes later than on the previous sol.
All mission acitivities are tied to this interplanetary communication schedule. Each sol, scientists will receive information, a downlink, from Spirit that will tell them what the rover did (or didn't do) in response to the previous sol's commands. They will then have only a few hours to absorb the new information, decide what they want the rover to do on the next sol, and deliver a sequence of desired activities to the engineers. The engineers will then translate the science requests into a command sequence that is ready in time for the next uplink opportunity.
Working the Night Shift
To accommodate the requirements of interplanetary communication, during the mission the Spirit science and engineering teams will have to live "on Mars time," in synch with the red planet's cycle of light and dark. This means that, here on Earth, they'll sometimes be working during daylight hours, and at other times they'll be working through the night. Over the course of a bit more than five weeks (36.5 days), the Spirit team will make a complete cycle around the clock, shifting slightly each day.
Scientists who participated in the Pathfinder mission know what it's like to live this way.
"It totally messes you up to shift every day," says Matt Golombek, who was the project scientist for Pathfinder, and is the Science Operations Working Group chair and long-term planning lead for the MER missions. "You're not going to the bank. You're not talking to your friends. You see deer more than you see people at JPL. You're on another planet."
And Pathfinder was only a 30-day mission. Spirit is a 90-day mission. So participating scientists and engineers will be going through this round-the-clock cycle not once, but nearly two-and-a-half times. Talk about jet lag!
It gets even more complex. Once Opportunity lands, 20-some sols into the Spirit mission, a second team will begin a similar routine. Similar, but opposite, because the Opportunity landing site is on the opposite side of Mars from the Spirit landing site. When it's daylight at Gusev Crater (the Spirit site), it will be night at Meridiani Planum (the Opportunity site). When Earth is in the sky over Meridiani, it will be incommunicado at Gusev. Thus, the activities of the Opportunity team will be on an exactly opposite schedule from those of the Spirit team, offset by some 12 hours and 20 minutes.
Living on this schedule will be grueling for those involved. At the beginning of the mission, on sol 1 or 2, says Golombek, "you just want to be there." Seeing the first images of a place that has never before.