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.
|Mars' Gusev Crater.
Credit: NASA, SpaceDaily
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.
|An illustration of the Spirit and Opportunity rovers.
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.
|Mars' Meridiani Planum region.
"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 been explored is so exciting that nobody worries much about Mars time. But by sol 89, when the mission is nearly over, he admits, "you're tired of Mars time. You want to go home and see your kids play soccer."
Down to Earth, at Light Speed
One more thing. Although MER mission participants will be living on Mars time, they won't be paying much attention to martian clocks." Martian time is somewhat standardized. There is a universal Mars time, like there is a universal time on Earth (although there is an ongoing discussion about how best to calculate it precisely). Scientists also divide Mars up into time zones similar to our own, so that at any given spot on Mars there is a local time that is offset by some number of martian hours from universal Mars time.
But when referring to the timing of MER mission events on Mars, mission controllers will modify Mars time by using a slightly Earth-centric approach to timekeeping. They will log events in Earth received time," or ERT.
ERT takes into account the time it takes for a radio signal to travel, at the speed of light, between Mars and Earth. At the beginning of the Spirit mission, this will be roughly 10 minutes. So, for example, when Spirit lands, the actual Mars local time at Gusev Crater will be 14:24 (2:24 in the afternoon). Mission Control, however, will log the landing as occurring at 14:34 LST-A (MER-A landing site time), or Gusev time.
Alas, Mars timekeeping is nothing if not complex. During the course of the 90-day Spirit mission, Earth and Mars will be moving steadily apart. As they separate, the time it will take light to travel from Mars to Earth will steadily increase. When Spirit first lands, the neighboring planets will be about 106 million miles apart, which translates to about a 10-minute lag. By the end of the Spirit mission, Earth and Mars will be separated by more than 180 million miles, resulting in a time lag of more than 16 minutes. By the end of the Opportunity mission, 20 days later, the delay will have increased to more than 17 minutes.