A moment in time
Posted by Emily Lakdawalla
06-05-2010 17:39 CDT
Note, May 12: There is a followup post here.
May 2, 15:00 UTC. Sunday morning. Los Angeles was just waking up (or, for a hard-living few, just going to bed); New York was mostly up and about, doing weekend chores. Berliners were enjoying the last few hours of their weekend; in Beijing, it was nearly midnight, past bedtime for those who had to report to work on Monday morning. And in Australia, it was already the wee hours of Monday morning. Even in this modern world, the angle of the Sun above the horizon dictates the rhythm of our existence. The New York Times' Lens blog documented this moment by soliciting photos from people around the world at exactly this moment, a project they called A Moment in Time.
On Mars, at 15:00 local true solar time on May 2, a solitary rover gazed southward across her own dusty deck and snapped three photos, actually three sets of three photos, which were combined to make this view. As the Mars Exploration Rover Opportunity shot these images of Meridiani planum, the same Sun that dictates the rhythms of our human lives was sinking toward the rover's western horizon, casting a lengthening shadow toward the east from the upright gnomon on the little sundial on the rover's rear deck. That sundial is engraved with the words "Two Worlds, One Sun" to mark the unity of Earth and Mars as part of the same solar system.
That's the caption submitted along with this photo to the New York Times Lens blog's A Moment in Time project, which solicited people around the world to take a photo near 15:00 UTC on May 2 to document the variety of human experience at a given moment. Humans' presence has extended across the solar system in the form of dozens of autonomous robotic spacecraft. The Mars Exploration Rover Opportunity is now more than six years into a mission originally designed only to last 90 days. It has roved more than 20 kilometers during those six years, and is in the middle of a long trek toward the largest crater it has explored yet. The trek takes it across the wind-whipped dune fields of a region on Mars called Meridiani Planum. The sands are made of grains worn from Mars' basalt volcanic rocks, so the sand is very dark. Here and there, in the troughs separating dunes, lighter-colored bedrock shows through.
The horizon is tilted because the rover was titled, paused in the act of crossing a dune; in reality, Opportunity's horizon is extremely flat. In the foreground, Opportunity's sundial is visible. The sundial, also known as the MarsDial, is both an educational project and an important component of the camera instrument; it serves as a calibration target for the camera. When Opportunity first landed on Mars, the MarsDial's face was different shades of gray, and four different color chips occupied its corners, while its gnomon (the vertical stick) was black. After six years on the Martian surface, the whole thing is covered with a layer of dust that mutes the colors.For most of the Earthbound participants in "A Moment in Time," getting a photo on May 2 at 15:00 was a piece of cake; they took their phones out of their pockets, aimed, shot, and were likely able to submit the photo to the Internet from the same device. More than 13,000 photos were submitted. But Opportunity's photo took a lot more work -- and a lot more than just one person's involvement. Here's how the Opportunity photo happened.
A couple of weeks ago, I learned via Twitter about "A Moment In Time." The Lens blog asked everybody around the world to take a photo documenting what was happening in their location at 15:00 UTC on Sunday, May 2.
Wherever you are, we hope you'll have a camera -- or a camera phone -- in hand. And we hope you'll be taking a picture to send to Lens that will capture this singular instant in whatever way you think would add to a marvelous global mosaic; a Web-built image of one moment in time across the world.
We extend the invitation to everyone, everywhere. Amateurs. Students. Pros. People who've been photographing for a lifetime or who just started yesterday.I immediately wondered: Why limit it to Earth? What about Mars? What will Opportunity be doing at 15:00 UTC on May 2?
I dialed up the Mars24 application to check out what time that would be for Opportunity. Hmm, I thought -- 15:00 UTC corresponded to 17:21 at Meridiani Planum. Rovers have certainly taken photos at that time of day before, but it's nearly the winter solstice and Opportunity's power levels are low. Still, 17:21 was really close to a possibly active hour. Opportunity is in the middle of a long journey across the Meridiani sand dunes; and at 15:00 UTC on May 2, 2010 it would be the end of another day's work, with the Sun setting in the west just as it sets on us on Earth. I fired an email off to Mars Exploration Rover camera team leader Jim Bell (who is, incidentally, also the President of The Planetary Society's Board) just pointing out the New York Times feature, saying, you know, wouldn't it be cool if we could take this project to a whole 'nother planet?
Jim didn't immediately say no, even though the time-of-day problem was worse than I realized. Because of Mars' elliptical orbit, over the course of the Martian year the local true solar time wanders around in time with respect to a clock whose rhythm is unchanging and independent of such things as Mars' varying orbital speed, and 15:00 UTC corresponded to 18:14 local true solar time. But Jim's a photographer and he understands the power of pictures to tell a story.
Jim took the idea to the rest of the rover team. When I say "team," it's a cast of dozens, split into two main groups: the scientists (that's Jim's side; they choose where the rover should go, and what kinds of photos and other data it should take of what targets) and the engineers (that's the side that includes the rover drivers like Scott Maxwell; they figure out how to command the rover to do what the scientists want it to do, and do it safely, pushing the spacecraft to its limits while simultaneously preserving it for the next days and months of work). Although the science and engineering sides of a mission can sometimes be at odds, on the Mars Exploration Rovers there has always been an unusually warm relationship between the two sides. Jim told me that both sides were enthusiastic in their support for Opportunity to participate in "A Moment in Time."
Once they decided to participate, though, they had to make sure they did it safely. 15:00 UTC was just too late for Opportunity in midwinter. Instead, they took it at 15:00 local solar time, a few hours earlier. Although this is a bit of a fudge, there are two ways in which the 15:00 UTC time is significant in the capturing of the photo from Mars: firstly, the bits were coming down from Mars at a time much closer to 15:00 UTC, so that's when humans were first able to see the view. Secondly, the rover is located near Mars' prime meridian; UTC is a time referenced to Earth's prime meridian. So in any future time zone defined for Mars, Opportunity would likely be sitting inside the one corresponding to the "prime" time. It's a stretch, but it works for me! Pancam Payload Uplink Lead Emily Dean wrote the sequence of commands for the rover, and it was included in the day's plan.
Mars was kind to the rover; the image sequence worked, and all nine photos were taken successfully. The rover relayed the data to Mars Odyssey when the orbiter passed overhead a little while later. Odyssey then relayed the data to Earth. The downlink worked fine, and the Deep Space Network received the data fine, with no data dropouts. The data arrived at the Pancam headquarters at Cornell University, and Jonathan Joseph combined the nine individual images into three color frames in a procedure that carefully calculates calibrated color values from the data bits returned in each individual frame. Then Joseph merged the three frames into a view that takes in everything from the horizon to the deck, including that little dusty Sundial.
I am still a little surprised that this photo happened. I think, in the context of the New York Times' project, it serves as a really powerful reminder that every day, every hour, every minute of our lives here on Earth, the rovers (and all other spacecraft) are also "living" out their adventures across the solar system. Pick any moment in time, and every spacecraft is somewhere, with some view of the solar system, whether it be starlit blackness or the surface of another world.
For the curious, here is the original data for the image above, after calibration but before being composited and reprojected into the color mosaic.