Dawn Journal: On Course for Opposition

Dear Dawnonymous and Idawntified Readers,

Dawn has accomplished an extraordinary orbital dance. It completed the cosmic choreography with the finesse and skill that have impressed fans since its debut in space nearly a decade ago. Dawn’s latest stellar performance with Ceres took two months and four acts. (Although Ceres played an essential role in the performance, it was much easier than Dawn’s. Ceres’ part was to exert a gravitational pull, which, thanks to all the mass within the dwarf planet, is pretty much inevitable.)

In February, we presented a detailed preview of the spacecraft’s extensive orbital maneuvering with its ion engine. Now, like so many of Dawn’s cool plans, that complex flight is more than an ambitious goal. It is real. (And the Dawn project will negotiate with any theme park that would like to turn that or any of our other deep-space feats into rides. Another good candidate is here.)

But there is more to do. The reason for such dramatic changes in the orbit is not to show off the flight team’s prowess in piloting an interplanetary spaceship. Rather, it is so Dawn’s new orbital path will cross the line from the sun to the gleaming center of Occator Crater on April 29. From the explorer’s point of view at that special position, Occator will be opposite the sun, which astronomers (and readers of the last three Dawn Journals) call opposition. Last month we explained the opposition surge, in which photographing the crater’s strikingly bright region, known as Cerealia Facula, may help scientists discover details of the reflective material covering the ground there, even at the microscopic level.

Dawn is multitasking. Even as it was executing its space acrobatics, and when it measures the opposition surge later this week, its most important duty is to continue monitoring cosmic rays. Scientists use the spacecraft’s recordings of the noise from this space radiation to improve the measurements it made at low altitude of radiation emitted by Ceres.

Now that Dawn is on course for opposition, let’s take a look at the observations that are planned. Measuring the opposition surge requires more than photographing Cerealia Facula right at opposition. The real information that scientists seek is how the brightness changes over a small range of angles very near opposition. They will compare what Dawn finds for Cerealia Facula with what they measure in carefully designed and conducted laboratory experiments.

To think about Dawn’s plan, let’s consider a clock. Ceres is at the center of the face with its north pole pointing toward the 12. As in this figure, the sun is far, far to the left, well outside the 9 and off the clock. This arrangement matches the alignment in this figure.

Now let’s put the spacecraft on the tip of the second hand, so it takes only one minute to orbit around Ceres. (In reality, it will take Dawn 59 days to complete one revolution in this new orbit, but we’ll speed things up here. We can also ignore for now that Dawn’s orbit is not circular. That would correspond, for example, to the length of the second hand changing as it goes around. This clock doesn’t have that feature.) If the clock were one foot (30 centimeters) across, Ceres would be a little more than a quarter of an inch (seven millimeters) in diameter, or smaller than a pea. Dawn is at a high altitude now, which is why Ceres is so small on the clock.

With this arrangement, opposition is when the second hand is on the 9 and Occator is pointed in that direction as well, so the sun, spacecraft and crater are all on the same line. All of the opposition surge measurements need to occur within about one second of the 9, and most of them have to be within a quarter of a second of that position. This precision has created quite a challenge to the flight team for navigating to and performing the observations.

Readers have long clamored for more information on clocks in the Dawn gift shops, which we have not addressed in more than three years. (Most, of course, clamor for refunds. For that, please take your clock in person to the refund center nearest you, which usually is near the largest black hole in your galaxy.) We hope the discussion this month has filled that horological void.

The problem would be difficult enough if Ceres presented Occator to Dawn as a bright bullseye for the camera, but the dwarf planet is not that cooperative. Rather, like all planetary bodies, Ceres turns on its axis, so even if Dawn managed to hover on the line from the sun to Ceres, Occator would be visible only half the time. The rest of the time, the crater would be on the other side of Ceres, cloaked in the darkness of night (which would compromise a measurement of how much sunlight it reflects) and blocked from Dawn’s view by an opaque dwarf planet 584 miles (940 kilometers) in diameter.

Of course, Dawn can’t hover, and Occator is a moving target that’s not visible half the time. That introduces further complications. As Ceres’ rotation brings Occator from night into day (that is, it is sunrise -- dawn! -- at Occator), the crater will be on the limb from Dawn’s perspective. (Remember, Dawn is aligned with the sun.) The foreshortening would make a poor view for measuring the opposition surge. We need to have the crater closer to the center of the disc of Ceres, displaying its bright terrain for Dawn to see, not near the edge, where Cerealia Facula would appear compressed. (In November we saw a photo of Occator near the limb. When Dawn measures the opposition surge, it will be more than 13 times higher.)

Dawn’s orbit has been carefully designed so the spacecraft will cross the line from the sun to Occator when the crater is along the centerline of Ceres. That will give Dawn the best possible view. At that time, the sun will be as high as it can be that day from Occator’s perspective. Because the crater is at 20°N latitude, and Ceres’ axis is tilted only 4 degrees, the sun does not get directly overhead, but it reaches its highest point at noon.

If that is confusing, think about your own location on your planet. For most terrestrial readers, the sun never gets directly overhead (and for all, there are long stretches of the year in which it does not). But as the sun arcs across the sky from morning until evening, its highest point, closest to the zenith, is at noon. Now think about the same thing from the perspective of being far out in space, along the line from the sun to Earth, looking down on Earth as it rotates. That location will come over the limb at sunrise. (That sunrise is for someone still there on the ground. From your vantage point in space, the sun is behind you and Earth is in front of you.) Then the turning Earth will carry it to the other limb, where it will disappear over the horizon at sunset. The best view from space will be in the middle, at noon. If you have a globe, you can confirm this. Just remember that because of the tilt of Earth’s axis, the sun always stays between 23.5°N and 23.5°S. If it’s still confusing, don’t worry! You don’t need to understand this detail to follow the description of the observation plan, and you may rest assured that the Dawn team has a reasonably good grasp of the geometry.

Dawn’s orbital path is timed to make opposition occur as close as possible to 12:00:00 in the Occator Standard Time zone, and that happens to be 2:46:20 a.m. PDT on April 29. (We are glossing over many complications, but one fortunate simplification in the problem is that Cereans do not use daylight saving time. The Cerean day is only nine hours and four minutes long, but they’re so far from the sun that they don’t even bother trying to save daylight.)

Dawn will photograph Ceres extensively during the brief period around opposition. The spacecraft will be around 12,400 miles (20,000 kilometers) above Ceres, a view that would be equivalent to seeing a soccer ball 15 feet (4.7 meters) away. Occator Crater will be like a scar on the ball less than seven-eighths of an inch (2.2 centimeters) wide. The principal target, Cerealia Facula, would be a glowing pinhead, not even a tenth of an inch (about two millimeters) across, at the center of the crater.

Dawn has spent a great deal of time scrutinizing Ceres from more than 50 times closer (see this table for a summary, including comparisons with a soccer ball for other altitudes). To accomplish this new goal, however, we don’t need high resolution. There are other technical considerations that require the greater altitude. We have already seen Cerealia Facula in as much detail as Dawn will ever reveal. But thanks to the team’s creativity, we have the possibility of learning about it on a far finer scale than had ever been considered.

As we have discussed before, scientists will study the handful of pixels in each image that contain Cerealia Facula to determine how the brightness changes as the viewing angle changes. Throughout its observations, Dawn will take pictures covering a range of exposures. After all, we don’t know how large or small the surge in brightness will be. The objective is to find out. The plan also includes taking pictures through the camera’s color filters to help determine whether the strength of the opposition surge depends on the wavelength of light. (Coherent backscatter may be more sensitive to the wavelength than shadow hiding.) In addition, the probe will collect visible and infrared spectra. (Dawn’s photos and spectra will capture a great many more locations on Ceres than Cerealia Facula. Indeed, well over half of the dwarf planet will be observed near opposition. The data for all these other locations will provide opportunities for still more valuable insights.)

Although observing the opposition surge is a bonus in the extended mission, and not as high a priority as many of Dawn’s other scientific assignments, the operations team has taken extra measures to improve the likelihood of it working. Occasionally the camera experiences a glitch, perhaps from cosmic rays, that temporarily prevents the instrument from taking pictures. Therefore, for the opposition surge, the spacecraft will use both the primary camera and the backup camera. Even with well over 85,000 photos during Dawn’s exploration of Vesta and Ceres, the two cameras have been operated simultaneously only once. That was in February, and the purpose then was to verify that the cameras and all other systems (including spacecraft thermal control, data management and even extensive mission control software on distant Earth) would perform as engineers predicted. That test was successful and helped prepare for this upcoming observation.

The plan to measure the opposition surge on Ceres is complex and challenging, and the outcome is by no means assured. But that’s the nature of most efforts to uncover the universe’s secrets. After all, an expedition to orbit and explore two uncharted worlds that had appeared as little more than pinpoints of light among the stars for two centuries, the two largest bodies between Mars and Jupiter, is complex and challenging, and yet it has accomplished a great deal more than anticipated. The reward for such a bold undertaking is the thrill of new knowledge. But there are also rewards in engaging in the endeavor itself, as the spacecraft transports us far from the confines of our humble planetary residence. Such a journey fuels the fires of our passion for adventure far from home and our yearning for new sights and new perspectives on the cosmos.

Dawn is 17,800 miles (28,700 kilometers) from Ceres. It is also 3.64 AU (339 million miles, or 545 million kilometers) from Earth, or 1,505 times as far as the moon and 3.62 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT April 25, 2017

P.S. Just before this Dawn Journal was to be posted on April 24, when a scheduled telecommunications session began, the flight team discovered that the third of the spacecraft’s four reaction wheels had failed. We have written a great deal about these devices and the team’s extraordinary creativity in conducting an extremely successful mission without a full complement. The unit failed before the final, short period of ion thrusting, and the spacecraft correctly responded by entering one of its safe modes and assigning control of its orientation to the hydrazine thrusters. That meant it could not execute the brief maneuver, which would have changed the speed in orbit by 1.4 mph (2.3 kilometers per hour). The team quickly diagnosed the condition and returned the spacecraft to normal operation (still using hydrazine control) on April 25. They also determined that Dawn’s trajectory is close enough to the original plan that the opposition surge measurements can still be conducted. This experienced group of space explorers knows how to do it without the reaction wheels. (For most of the time since Dawn left Vesta in 2012, including the first year of Ceres operations, all four wheels were turned off. This will be no different.) See this mission status update for additional information. Next month’s Dawn Journal will include this new chapter in the reaction wheel story, the outcome of the attempt to observe the opposition surge and more.