Dawn Journal: Getting Elliptical

Dear Isaac Newdawn, Charles Dawnwin, Albert Einsdawn and all other science enthusiasts,

For the first time in almost a year, the Dawn mission control room at JPL is aglow with blue.

The rope lights strung around the room bathe it in a gentle light reminiscent of the beam emitted by an ion engine on the faraway spacecraft as it maneuvers in orbit around Ceres. Dawn had not thrust since June, but it is now using ion engine #2 to fly to a new orbit around the dwarf planet. Thanks to its uniquely capable ion propulsion system, Dawn has accomplished far more powered flight than any other spacecraft, and more is ahead.

Dawn has spent most of the last year revolving around Ceres once every 30 days in extended mission orbit 5 (XMO5), a designation that illustrates the team's flair for the dramatic. (Your correspondent, as passionate as anyone about the exploration of the cosmos, can imagine only a few names more inspiring than that. Fortunately, one of them happens to be "XMO7." Read on!) As the probe followed that elliptical course, it reached down to a little less than 2,800 miles (4,400 kilometers) above the alien world and up to 24,300 miles (39,100 kilometers).

Dawn flew to high altitude late in 2016. Its work there is now complete, and defying expectations, the aged adventurer still has life left in it. As we saw in last month's overview of the two upcoming orbits, Dawn's next assignment is to go much, much lower.

XMO5 and the subsequent two orbits are elliptical, as shown in the illustrations last month and the new one below. Observing Ceres from a very low altitude is possible only in an elliptical orbit, not a circular one. Dawn was not designed to operate at low altitude, and its reaction wheels, which are so important for controlling its orientation, have failed, making the problem even more difficult. We have discussed this before and will address another aspect of it this month for the lowest orbit.

Although the elliptical orbits introduce many new technical challenges for the team, Dawn still takes a spiral route from each orbit to the next, just as it did earlier at Ceres and at Vesta when the orbits were circular. In essence, the ion engine smoothly shrinks the starting ellipse until the new ellipse is the size needed. These trajectories are very complicated to plan and to execute, but with the expert piloting of the experienced team, the maneuvering is going very well. (You can follow the progress with the mission status updates.)

Dawn began its descent on April 16. On May 15, with the blue lights turned off in mission control, the veteran explorer will begin its observations in XMO6. (As suggested last month, the targeted minimum and maximum altitudes for XMO6 are being updated slightly even as Dawn is on its way. In the next Dawn Journal, we will present the actual altitude range.) If all goes well, the control room will be lit up in blue again from May 31 to June 7, as the ship sails down to XMO7.

In XMO7, Dawn will swoop down to an incredibly low 22 miles (35 kilometers) above the exotic terrain of ice, rock and salt. The last time it was that close to a solar system body was when it rode a rocket from Cape Canaveral over the Atlantic Ocean more than a decade ago. (For readers unfamiliar with solar system geography, that was Earth.) The XMO7 ellipse will then take the spacecraft up to 2,500 miles (4,000 kilometers). Each revolution will last 27 hours and 13 minutes. In considerably less time than that (assuming you read at a typical speed), we will discuss why this orbital period is important.

Last month, we described some of Dawn's planned low-altitude measurements of nuclear radiation to reveal more about Ceres' composition. As a bonus objective, scientists would like to study the elements in one of their favorite places (and perhaps one of yours as well): Occator Crater, site of the highly reflective salt deposits, famous not only on Ceres but also on Earth and everywhere else that readers follow Dawn's discoveries. Studying this one crater and the area around it (together known as a geological unit) could reveal more about the complex geology there. But doing so is quite a challenge, as Dawn would need to pass over that region 20 times to allow the gamma ray and neutron detector (GRaND) to record enough of the faint nuclear radiation. This is the equivalent of taking a long exposure with a camera when photographing a very dim scene.

Attempting to repeatedly fly low over that geological unit presents daunting obstacles, as we will discuss. It may not work, but the team will try. That's part of what makes for a daring adventure! And accomplishing such a feat requires a special trick. Fortunately, the Dawn team has several at its disposal.

Recall that Dawn will loop around Ceres, going south to north at low altitude and back to the south again at high altitude. Meanwhile, Ceres will turn on its axis toward the east, completing one rotation in just over 9 hours, 4 minutes. (Note that Ceres turns quite a bit faster than Earth. A Cerean day is much closer in duration to a day on Jupiter, which is 9 hours, 56 minutes. All three turn east.) Therefore, the flight team will synchronize the orbit so that each time Dawn swoops down to low altitude, it does so at just the right time so that Ceres' rotation will place the Occator geological unit under the probe's flight path.

We mentioned above that Dawn's orbit will take 27 hours, 13 minutes. This period is chosen to be exactly three times Ceres' rotation period. Experts (now including you) describe this as a three-to-one resonant orbit, meaning that for every three times Ceres turns, Dawn turns around it once.

If this synchronization is clear, feel free to skip this paragraph. Perhaps get a snack until it's time for the next paragraph or, better yet, use this time to gaze at the mesmerizing beauty of the night sky and contemplate the magnificence of the cosmos. If the synchronization is not clear, find a globe of Earth. Now imagine a satellite circling it, flying from the south pole to the north pole over one hemisphere and back to the south pole over the opposite hemisphere. Suppose the first passage occurs over your location. If Earth didn't rotate, the second orbit would take it over the same place. (Of course, if Earth didn't rotate, you might run out of patience waiting for tomorrow.) Now rotate the globe a little bit while your imagined satellite goes through one revolution. If it flew over your location the first time, it will not the second time. And you can see that with Earth rotating at a constant speed, it requires a carefully chosen speed for the satellite to pass over the desired target on each revolution. The Dawn flight team will work very hard to help our distant explorer have the orbit needed to achieve the three-to-one resonance.

The accuracy necessary will be difficult to achieve, even for the Dawn flight team at JPL, where the best celestial navigators in the solar system get to work. The problems that must be overcome are manifold. One of them is that, lacking functioning reaction wheels, Dawn fires its small hydrazine-fueled thrusters to control its orientation in space. Whether to turn to keep its sensors trained on the ground, even with the constantly changing altitude and velocity in the elliptical orbit, or to point its main antenna at Earth, the reaction from a little burst of hydrazine not only rotates the spacecraft but also nudges it in its orbit. (We have described this several times in great detail before.) Each small push from the thrusters distorts the orbit a little bit, desynchronizing it from the three-to-one resonance.

Another difficulty is that, just like Earth, Mars, the Moon and other solar system residents (not to mention cookie dough ice cream), Ceres is not uniform inside. Its complex geology has produced some regions of higher density and some of lower density (although not with the same delectable composition as the ice cream). The total gravitational pull on the spacecraft depends on the dwarf planet's internal structure. We have described before how scientists take advantage of it to map the interior. But we have measured the gravity from 240 miles (385 kilometers) high. When Dawn swoops down much lower, our gravity map will not be accurate enough to predict all the subtle details of the mass distribution that may cause slightly larger or slightly smaller pulls at some locations. It will take quite a while to formulate the new gravity map. That new map may reveal more about what's underground, but until then, it will be harder to keep the orbit in sync.

On two occasions in mid-June Dawn will use its ion engine to tweak its orbit (in what we have described before as a trajectory correction maneuver) to help maintain the synchronization, but there will still be residual discrepancies.

We described and depicted last month how the low point of Dawn's orbit will gradually shift southward on each successive revolution. That means we will have only a limited number of opportunities to fly over Occator before the low point is too far south. Given the complexity of the operations, the planned measurements are not at all assured.

There are other aspects of this problem as well. While we will not delve into them here, engineers have been working hard on every one of them.

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.)

In its 4.6-year-long Cerean year, Ceres, with Dawn in tow, reached the minimum solar distance of just under 2.56 AU (238 million miles, or 383 million kilometers) on April 28. Dawn also was in residence at Ceres when they were at their maximum distance from the Sun in January 2016. Although the dwarf planet's orbit is not elliptical enough that the additional solar heating is expected to have much effect, the upcoming observations in XMO6 will provide scientists with the opportunity to look for any changes just in case. (The change Dawn detected at Juling Crater is more likely related to the seasonal change of the angle of the Sun rather than the distance to the Sun.)

The solar system constantly performs a complex and beautiful choreography, with everything in motion. Dawn will complete its current elegant spiral in another two weeks, and then it will be time for the next act, XMO6 and, after that, the finale, XMO7. A great many challenges are ahead but the allure of the rich rewards of new knowledge, new insight, and a new adventure is irresistible as Dawn delves further into the unknown.

Dawn is 1,400 miles (2,300 kilometers) from Ceres. It is also 2.34 AU (218 million miles, or 350 million kilometers) from Earth, or 900 times as far as the Moon and 2.32 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.

Dr. Marc Rayman
7:30 pm PDT April 29, 2018