We have mentioned before that photography will be extremely challenging in XMO7, because of both the high speed so close to the ground and the difficulty pointing the camera accurately enough to capture a specific target. Let's take a more careful look at the nature of the orbit to understand more about the problem of trying to see any particular site.

You can think of the motion in an elliptical orbit as being somewhat like that of a swing. Imagine a girl named Dawn on a swing. Perhaps she is 10 and a half years old (like our spacecraft), usually (but not always) does what we instruct (like our spacecraft), feels energized by the light of the Sun (like our spacecraft), loves the idea of exploring uncharted worlds (like our spacecraft) and uses photomultiplier tubes coupled to a bismuth germanate crystal scintillator, lithiated glass and boron-loaded plastic to measure the spectra of nuclear radiation (okay, she is not like our spacecraft in every way).

When Dawn rides her swing, her speed is constantly changing. As she approaches the top of her arc, gravity slows her down and even brings her momentarily to a stop. She then begins to fall, accelerating as she gets lower. As soon as she passes the lowest point, her upward motion and the downward pull of gravity oppose each other, and once again she begins to slow. When her swing is pumped up (whether with her legs or by the push of her friend or her friendly ion engine), her arc will reach higher, and then she will speed through the low point even faster.

Of course, the swing does not trace out an ellipse, and the girl does not loop all the way around, but the fundamental principles of motion are the same, as methodically investigated by Galileo Galilei four centuries ago and explained by Isaac Newton in the second half of the 17th century. Dawn's elliptical orbit around Ceres will behave somewhat like the swing. At high altitude, far above the dwarf planet, the spacecraft will move at only about 120 mph (190 kph). Then, as gravity pulls it back down, the spacecraft will accelerate until it skims over the ground at 1,050 mph (1,690 kph) before starting to swing up again.

Dawn is much, much, much too far away for controllers to point its camera and other instruments as you might with a joystick or other controller in real time. Readers of the final paragraph of every Dawn Journal know that radio signals, traveling at the universal limit of the speed of light, usually take more than half an hour to complete the round trip. When Dawn is in XMO7 this summer, it will be about an hour. While the spacecraft is racing over the Cerean landscape, it can't wait for its radio signal to tell controllers what it sees and then, based on that, for a return radio signal to help it adjust the pointing of its camera. All the instructions from Earth have to be radioed in advance.

It is a very complicated process to go from measuring Dawn's orbit accurately to the probe actually aiming its camera and its spectrometers to collect new data, with many calculations and many steps in between, each of which has to be checked and double checked. The team has a special campaign planned for that purpose, and they will maneuver to XMO7 so that the best viewing will be in late June. But even when they work quickly for this dedicated attempt to get some bonus photographs of Occator, the entire process will take the better part of a week because of the spacecraft's orbital activities (e.g., while it observes Ceres, it cannot communicate with Earth), segments of its orbit where Ceres blocks its radio signal to Earth and so it is not possible to communicate, and the schedule for the large Deep Space Network antennas to shout so Dawn can then listen for what fades to become a long-distance radio whisper. Time needs to be allocated for computers and people to analyze data, to formulate and verify the new plans, to beam the instructions to Dawn and then Dawn finally to execute them. Meanwhile, even after the initial measurement of its orbit, while all this work is occurring on Earth, the ship will continue to be buffeted by the hydrazine winds and the gravitational currents, so its course will continue to change.

The consequence of all this is that by the time Dawn actually conducts its observations, its orbit will be different from what was measured days earlier. The carefully devised prediction that formed the basis of the plans could well be off one way or the other by four minutes or even more. (By the way, calculating now the credible magnitude of the error for this June campaign is a sophisticated science that, in itself, involves thousands and thousands of hours of computer calculations, performed on hundreds of computers working simultaneously. Epistemic knowledge does not come easily.)

From Dawn's perspective, descending and speeding north at 1,050 mph (1,690 kph) to the vicinity of Occator, faithfully pointing its sensors according to the plan worked out days before on a distant planet and stored in its computer, Ceres' rotation will carry the crater to the right at more than 190 mph (310 kph). Dawn's camera will take in a scene about 2.1 miles (3.4 kilometers) across, and at the spacecraft's high velocity, there won't be time to turn right and left to cover a broader swath. Even if the probe arrived at Occator's latitude a mere 20 seconds off schedule, a spot on the ground that was expected to be in the center of the camera would have moved entirely out of view and so would not even be glimpsed. If Dawn were four minutes too early or too late, the ground beneath the spacecraft (known as the ground track) would shift west or east by 13 miles (21 kilometers), and the terrain that's photographed could be entirely different from what was expected.

Occator Crater is 57 miles (92 kilometers) across, so all this work should allow GRaND, with its very wide field of view, to measure the composition in the geological unit that contains the crater. But the narrower view of the camera means we cannot be certain what features we will see. Fortunately, we already know that there is fascinating geology just about everywhere in and near Occator. Indeed, the dwarf planet is vast and varied, with a great many intriguing features. We are going to behold some amazing sights!

Before then, we will gain new perspectives from XMO6 in May. And as Dawn was getting closer to Ceres, together the pair were getting closer to the Sun until yesterday. Dawn isn't the only object in an elliptical orbit. Ceres, Earth, and all the other planets (whether dwarf or not) travel in elliptical orbits too, although they orbit the Sun. Ceres' orbit is more elliptical than Earth's but not as much as some of the other planets. The shape of Ceres' orbit is between that of Saturn's (which is more circular) and Mars' (which is more elliptical). (Of course, Ceres' orbit is larger than Mars' orbit -- it revolves farther from the Sun than the Red Planet does -- and smaller than Saturn's, but our focus here is on how much the orbit deviates from a perfect circle, regardless of the size.)