Yesterday, the Curiosity mission released the video whose potential I got so excited about a couple of weeks ago: the view, from Curiosity, of Phobos transiting Deimos in the Martian sky. There is a YouTube version, but it looks quite dreadful because of compression artifacts. It's much better as an animated GIF. Here is, essentially, a flip-book of 41 individual frames taken by the Mastcam, shown at original size:
And here is a version of the animation where frames have been enlarged and interpolated to fill it out to 10 frames per second. This makes the motion appear smoother, and also shows you the motion at its actual speed. Pretty fast!
Cool. Very cool. You might wonder, as I did, how these images compare to how we see our Moon in our sky. Phobos is less than a hundredth of the diameter of the Moon (about 25 as compared to 3500 kilometers in diameter). But Phobos is also about exactly a hundred times closer to Mars' surface than our Moon is to Earth's surface (3600 as opposed to 380,000 kilometers), so the two effects largely cancel. That means it's possible for Phobos to appear almost as big as our Moon in the sky. But because Phobos orbits so close to Mars, its distance from any observer on Mars' surface changes a lot, by about a factor of two, from the time that it rises to the time that it's overhead. In fact, when Curiosity took the photos above, Phobos was 6,240 kilometers from the rover, so appears roughly half as big as it would have if it were directly overhead. Keep all that in mind when you look at this comparison image:
Whenever you see any Mars lander take images of things in the sky, you can be pretty sure that scientist Mark Lemmon is involved. Lemmon studies Mars' weather and climate, so he's interested in Mars' sky anyway. But he also has a knack for finding things for landers to study in Mars' sky that are both interesting scientifically and terrific for public outreach. I asked him several questions about this particular sequence, and he sent me (as usual) excellent replies, which I include below; I've edited some of his remarks for clarity, and added comments in [brackets and italics].
Me: Did you do this observation for coolness' sake or was there science to be gained?
Mark: Doing useful astronomy with landscape cameras on Mars is tricky. We compete with much better telescopes in orbit and at Earth. One thing we can do exceptionally well is image the moons in ways that precisely constrain their position. Images of the moons crossing in front of the Sun does this. However, like eclipses on Earth, there is a seasonality given by the 'line of nodes'. In the case of Mars, we can image transits only near the part of their orbits where the Martian equator crosses the ecliptic. So, last September and this month, we see opposite sides of the moons' orbits. Every time the moons crossed the Sun, they were in just about the same positions in their orbits around Mars. Those are the only two parts of the orbits we ever see, using the moons' transits of the Sun. With nighttime observations when the moons pass very close to each other, we can see other parts of their orbits.
We want to watch their positions around their orbits to better know the orbit. The moons have been observed for quite a long time from Earth. Right now, there are two numerical ephemerides for the moons, two different mathematical solutions predicting their future positions. These two solutions diverge by kilometers, up to an order of magnitude more than their error bars. The two solutions have systematic errors. Our data, on the moons' positions as seen from Mars, can fix that, and pin them down more precisely. (Fortunately, either of the two ephemerides is good enough to use for predicting non-grazing events.)
So, in the end, we will know more precisely where they are. More importantly, we'll get rid of systematic errors in their analysis; and by doing so we understand the forces that change their orbit. For instance, how fast is Phobos accelerating into Mars? That tells us about the interior elasticity of Mars, and maybe even the density distribution of Phobos. So, indirectly, we may be looking at what is under Stickney. Is Deimos moving away from Mars? The ephemerides differ. In our 2004 transit imaging, Deimos was 40 kilometers from where it was expected, due to incorrect assumptions about this.
When I saw the prediction that this event would happen, I was pretty excited by the coolness of the images we could get. But I was also immersed in comparing transit observations to predictions, and looking at few-kilometer differences, and thinking about how this could help with the overall orbit solution. I designed the images to be cool, as well as to allow the measurement I want to make.
Me: Has either of the Mars Exploration Rovers captured a mutual event?
Mark: Spirit has seen the moons close together in the sky, but was too far south for an actual occultation. Opportunity attempted to observe a mutual event once, in early July. We nailed the timing. But the rover's clock error defeated us. We used our usual small subframe to reduce data volume. [That means that they didn't save the whole photo, but instead cropped a small area of each photo around where they predicted the two moons to be.] We aimed at a specific position in the sky.
But the rover's knowledge of positions is based on Sun images, which is interpreted based on the rover's knowledge of where the Sun is. So it knows where the Sun is, but its clock is off. We compensated for the timing, but not the indirect affect on aim. The error cancels when we aim back at the Sun (or aim the antenna at the Earth). It didn't cancel this time. Phobos and Deimos were outside the frame. Fortunately, we took long-exposure, full-frame context images after the event; these captured the moons, and recovered most of the science without the pretty picture.
Me: Do the frames look JPEGgy because of the way that Mastcam captured the video?
Mark: The Mastcam video frames were compressed as JPEG quality 90. We could not afford to bring back all the images with lossless compression. Some versions of the released images have additional compression [like the YouTube version]. Ten images were brought down with lossless compression. Six of these are featured in the "before and after" image product:
The compressor over-performed, so similar images in the future may be quality 95 (including transits). The context images for this event are omitted from the release, but have been widely circulated. [Here is one of the context images, and here is the other one.] The longer exposure allows you to see Phobos in Mars-shine, but does introduce some motion blur. (When you blow the lossless images up, you see artifacts from the sharpness of the image.) [The context images also allow you to see background stars, but beware; most of the dots in the linked images are not stars but are instead noise and other artifacts.]
Me: Do you have hopes to do more of these, or was this a one-shot deal?
Mark: We hope to regularly observe transits, to the extent they fit in operations plans. For instance, we got Deimos and two Phobos transits from Opportunity, and have one Phobos transit from Curiosity from sol 363. There are two more opportunities that depend on whether their timing fits the rover's plans (for instance, we'll drive rather than get an extra transit video). We use the extras each year to see slightly different orbital positions (maybe 10 degrees). We use the Mars-year to Mars-year ones to increase the baseline for fitting the orbit.
There is a second mutual event for Curiosity in August; I probably will not even try it, due to a very bad geometry. Then there are none for a couple months. But they follow a different seasonality than the transits, and I expect we'll see more. This one was especially easy, which is good for a first try -- it happened to be immediately after a communication pass, with the rover awake anyway, and it could preheat the motors and camera to do the observation. And it occurred when the overall plan left some extra energy. [Nighttime work is costly in terms of energy, because Curiosity's motors need to be warmed to improve the flow of their lubricants in order to run, and the temperature drops by as much as 80 degrees Celsius over the Martian night.]
Me: If you can add anything about plans to image ISON, I'd love to hear them...
Mark: We also plan more astronomy for a variety of purposes. Comet ISON approaches. Actual planning for rover operations happens much closer in time to the event, compared to orbiters. So I cannot predict what will ultimately happen. But I do want to be ready. We have some tests planned. We actually did the first, imaging a star field around Regulus with various exposure times, right after the transit. I will also likely try to test the optimal focus and the sensitivity to diffuse sources. But if you look at predictions for ISON's brightness, it hits a detectable level after its coma should be resolvable by Mastcam-100.
One could question how good a view we might have. That said, if it is bright enough, there are some good options for scientifically valuable observations from a geometry (or composition) standpoint. And Mastcam and Pancam have similar sensitivities, but I'll try to exploit any advantages we can find.
Eventually, we may take a bunch of boring-looking star images as a way of tracking optical depth at night -- when fog forms, less starlight reaches the camera. We haven't yet hit the (relatively) water- rich time of year.
To go a little further, these are trickier than they look. The rover moves around, and needs to point a camera with a 5-degree field of view. And we like to use a subframe to manage data volume. That's easy when you point at something that's already been imaged by Navcam. It's less so when aiming blind.
Timing is tricky, too, but we seem to have the hang of that. No two clocks run at the same rate. We know when an atomic clock would say the events are. But we need observations to occur at the right time on the rover's clock. And we need to kick off a sequence, boot up the camera, and be imaging at the right time.
Finally, there is speed. Curiosity's Mastcam is designed for video, so that's not too tough. We spent a while figuring out how to get the fastest imaging from the Mars Exploration Rovers (you see this in dust devil movies and most of the later transits, but we had to compromise and shoot the last Deimos transit 'slow').
And we need to not fry Chemcam [by pointing it too close to the Sun for too long--which is obviously a serious concern for taking images of the moons transiting the Sun, since Chemcam is boresighted with the Mastcams]. We're taking no known risk for that--but it took a lot of effort to work through the scenarios and failure modes. [That is, they had to make sure not only were their planned image sequences safe for Chemcam, but also that any failure that could occur during a sequence wouldn't jeopardize Chemcam.]
Me: Thanks so much, Mark! I'm looking forward to more transit and maybe ISON photos!