When NASA's new Mars rover lands on the Red Planet this summer, it's safe to assume it'll be some time in the morning or early afternoon at the Jet Propulsion Laboratory in California, home of the rover science and engineering teams. So that means it'll be mid-afternoon on the East Coast, evening in Europe, and so on —- pretty easy to figure out the time zones. But what time will it be on Mars? What time zone will Curiosity live in -- and how can you even tell?
Timekeeping on Mars is a bit like telling time on Earth, because the planets are similar in lots of ways. But there are just enough differences to drive a person slightly crazy.
To start with, the Martian day, or sol, is 39 minutes and 35 seconds longer than a day on Earth. This isn't a lot, but it adds up quickly when you're living on Mars time -- as the Curiosity team will. And a Martian year lasts 668.59 sols, about 1.88 times an Earth year. Seasons last much longer and are much more extreme, thanks in part to Mars' deeply eccentric orbit.
"It feels like you are perpetually flying east 40 minutes every day," said Deborah Bass, a scientist at JPL who worked on the Spirit and Opportunity rovers and the Phoenix lander. "You're always jet-lagged. It's only a little bit, because an hour —- who cares, that's not so bad. But it starts to take its toll."
Just as we mark our lives according to the passage of time, so too do space missions. Most Mars missions have been solar-powered, meaning the spacecraft must do its work during daylight hours. Curiosity has a nuclear generator, but it will still be a solar craft in many ways —- its cameras and other instruments need sunlight to see, and atmospheric phenomena, like the huge temperature shift between day and night on Mars, follows the movement of the sun. So engineers need a reliable method to keep track of time on the planet. Michael Allison, an emeritus professor at the Goddard Institute for Space Studies, has made a hobby of figuring it out. "I actually know Mars time, in a way, better than Earth time," he jokes.
First, he says, you determine local noon, the point when the sun crosses the meridian overhead. That's called Local True Solar Time. But Mars (and Earth) have eccentric orbits, moving closer and farther from the sun throughout the year, so local noon can be off by a few minutes as the elevation of the sun in the sky changes. To be really accurate, then, astronomers use something called a "fictitious mean sun," which would move according to the mean position of the sun. This positioning -- given by a chart called an analemma -- gives you Local Mean Solar Time. That's what astronomers use to tell time on Mars.
The Martian prime meridian, as it happens, was chosen before Earth's was, Allison said. Astronomers picked it in 1840 so they could chart their observations, settling on a dark area that became known as Sinus Meridiani, or Meridian Bay. (The rover Opportunity landed in the western portion of this area, by the way.) Airy-0, a crater in this region, marks the true prime meridian. Astronomers chose it based on Mariner photographs in the 1960s. Airy is for the British astronomer George Airy, who built the Greenwich telescope that was eventually chosen for Earth's prime meridian, in 1884.
Just as an aside, while we're talking about organization of time, you can forget about Mars months or a Gregorian calendar. The Martian moons, Phobos and Deimos, careen around their planet so quickly that there's no point dividing up the calendar according to their phases. Instead, scientists mark the calendar using the longitude of the sun. The year begins when the sun stands directly above the Martian equator, moving north as viewed from Mars —- the start of spring. Northern winter starts when the sun is at 90 degrees, and so on.
Allison has tackled this, too, creating a Mars calendar with 10 extra months. He wanted Mars months to follow Earth months in terms of things like equinoxes, so he stuck in extra months here and there. "I have January, February, Bradbury, Clarke and March," he said with a laugh. "You couldn't construct a practical reference to the orbits of the Mars moons for a convenient seasonal division. But if people go to Mars, they may have need for some monthly reckoning. That is a convenient organization of time for us."
Allison has spent most of his career in the outer solar system, as a project scientist on Cassini, Juno and other missions. Before he got involved in Mars time, astronomers would sit down and calculate analemmas and Mars time scales, based on the position of the planet, for a few years before and after the mission's planned lifetime. Allison decided to make a general Mars time calculator, calculating the planet's position for 126 Mars years.
You can, too, by visiting the Goddard Institute's website and downloading a Mars clock that Allison's colleague, Rob Schmunk, built. Mars24 gives you a nice image of Mars and the locations of Spirit, Opportunity, Phoenix, Viking and other missions. When Curiosity lands, it'll get its own icon.
If you download Mars24, you'll notice there are no time zones. Each lander uses an estimate of local mean solar time as its frame of reference, just like cities did before standard time was created in 1884. But each lander has its own de facto time zone. Spirit and Opportunity live 12 hours apart, for instance.
At JPL, clocks on the walls keep track of these Mars times, Bass said. When a rover does something, it is cataloged as local rover time and JPL time. When the Spirit and Opportunity mission started, NASA even ordered custom Mars watches, and some team members had Mars alarm clocks.
"We have universal time, and then there is a spacecraft clock time. Then there is the local mean solar time, which is Mars time. Then we have Pacific standard time, Eastern standard time -- the conversions between time systems is pretty amazing," Bass said. "Mars time is simply another time zone, in that respect."
Very interesting - all should be this good. But it would have been helpful to explain why the Mars analemma is so different from Earth's.
Technically good read and informative!
I found extremely interesting reading about time in past civilizations and the inventions of calendars and clocks.
This article was fun!
Science sees no further than what it can sense.
Religion sees beyond the senses.
Great article. This is exactly what PopSci should be about. More please! :)
In space, no one can hear a tree fall in the forest.
Since a Martian day (sol) is almost 40 minutes longer than an Earth day, would it be like flying WEST instead of EAST? You could think of it as being similar to the switch from Daylight Savings Time to Standard Time (a 25-hour day) in the fall.
In response to Steve A.: The analogy that living on Mars time is like flying East 40 minutes every day is correct.
The reason is that, for timekeeping purposes, the longer Martian day of 24.7 Earth hours is divided into 24 Martian hours with subdivisions of 60 minutes and 60 seconds, making each "Martian second" a bit longer than an Earth second. A Mars clock ticking with Martian seconds will appear to run 40 Earth minutes slower each Earth day. NASA and JPL scientists have to shift their schedules 40 minutes forward each day to keep in sync with a schedule based on Mars time. In the same way, flying East one time zone, for example, requires you to shift your time reference one hour forward.
Guess what? There's an app for that... as they say! I have Mars Clock on my iPad and my iPod touch showing time at the Curiosity landing site; also tracks Mars distance to Earth, other data too. There's also a "lite" version.
I also found an older app for Spirit and Opportunity called Mars Surface Times, cool too, but not as detailed and sophisticated as Mars Clock.