Dear Dawnald Trump, Hillary Clindawn and all other readers,
Dawn has just completed another outstandingly successful observation campaign at Ceres. Far, far from Earth, the spacecraft has been making measurements at the alien world that were not even imagined until a few months ago. Once again, the experienced explorer has performed its complex assignments with distinction.
When Dawn arrived at Ceres in March 2015, becoming the first spacecraft to reach a dwarf planet, it was looking ahead to a very ambitious year of discovery from four different orbital altitudes. The great benefit of being able to enter orbit rather than fly by is that Dawn can scrutinize its subject over an extended period to develop a detailed, intimate portrait. Taking advantage of the ship’s ability to maneuver with its advanced ion propulsion system, mission planners had carefully selected the four orbits to enable a wide range of measurements.
By February of this year, Dawn had exceeded every one of its original mission objectives and was still going strong, accomplishing many new goals. Nevertheless, no one (at least, no one who was well informed) expected that the probe would complete its new assignments and yet still have the capability to maneuver to a fifth orbit and then undertake even more new observations. But that is exactly what occurred.
NASA / JPL-Caltech / UCLA / MPS / DLR / IDA
Dawn photographed this scene in Yalode Crater on June 15, 2016, from an altitude of 240 miles (385 kilometers). The network of fractures extends like tendrils for almost 30 miles (50 kilometers), with about 22 miles (35 kilometers) visible in this picture. Geologists think these depressions formed when stresses pulled the ground apart. The widest of the canyons is 0.9 miles (1.5 kilometers) across. This scene covers only a small part of the northwest quadrant of Yalode, which at 162 miles (260 kilometers) in diameter, is the second largest crater on Ceres. You can find it on the map presented last month centered at 43°S, 293°E. (Yalode is named for a goddess worshipped by women at the harvest rites in the African kingdom of Dahomey, in what is now southern Benin.) Full image and caption.
After more than eight months orbiting only 240 miles (385 kilometers) above the strange terrain of rock, ice and salt, Dawn ignited one of its ion engines on Sept. 2. By Oct. 6, when it had completed its graceful ascent, Dawn had made 93 spiral loops, reaching an orbit 920 miles (1,480 kilometers) high. From there, revolving once every 18.9 hours, the spacecraft has executed its new program of investigations.
With observations of Ceres from about the same altitude as a year ago in Dawn’s third mapping orbit, scientists will scour the expansive terrain, looking for changes. The most likely change is the presence of new, small craters. Everything in the solar system (including your planetary residence) is subject to strikes from rocks that orbit the sun. Ceres lives in the main asteroid belt between Mars and Jupiter, a particularly rough neighborhood, and being the largest resident there (by far) doesn’t give it any special protection or immunity. In fact, being the largest resident also makes Ceres the largest target.
NASA / JPL-Caltech / UCLA / MPS / DLR / IDA
Dawn had this view on June 6, 2016, from an altitude of 240 miles (385 kilometers). The bright material at upper left is on the northwest rim of Kerwan Crater. Geologists have cataloged well over 130 locations on Ceres that are covered with reflective material. (The most famous deposits are in Occator Crater.) The brightness is because briny ice that had been on the surface sublimated, leaving behind salts, which reflect more sunlight than other minerals on the dwarf planet. Extending 174 miles (280 kilometers) across, Kerwan is the largest crater on Ceres. It is centered at 11°S, 124°E on the map shown last month. (Kerwan is a spirit of sprouting maize among the Hopi of Arizona in the US.) Full image and caption.
In addition to remapping Ceres with all of the camera’s color filters, the flight team has given Dawn other tasks. Controlling a sophisticated interplanetary spacecraft conducting complex operations so very far from Earth is never easy (but it’s always incredibly cool). There have been many challenges throughout this ambitious mission, quite unlike any ever undertaken. One of the significant ones was observing specific targets of interest from low altitude. We have explained that orbiting so close to the ground, the spacecraft’s motion was quite difficult to predict with sufficient accuracy far enough in advance to guide the craft so that the instruments’ narrow fields of view would hit specific features. Dawn was designed to map uncharted worlds, not to conduct targeted observations.
The difficulty was compounded by the loss in 2010 and 2012 of two of the four reaction wheels, used for controlling the probe’s orientation. An important side effect of the nudges from the small hydrazine-fueled jets of the reaction control system (even in combination with the two operable reaction wheels in hybrid control mode) was tiny distortions in the spacecraft’s orbital trajectory. The cumulative effect of many jet firings over days and weeks was enough to make it quite challenging to ensure the sensors could spot the targets as Dawn sped around the rapidly rotating orb beneath it.
This is not as difficult at higher altitude both because Dawn does not need to use its jets as often and because the instruments take in a wider area. As a result, the explorer has been better able to catch sight of preselected geological features, and it has acquired valuable new data.
NASA / JPL-Caltech / UCLA / MPS / DLR / IDA
Dawn observed this area of craters, hills and canyons inside Urvara Crater on June 2, 2016, from an altitude of 240 miles (385 kilometers). The third largest crater on Ceres, it is 106 miles (170 kilometers) wide. We have seen Urvara several times before, and the crater on the right of this picture is visible in the northwestern part of Urvara shownhere and here. Urvara is on the new map at 46°S, 249°E. Full image and caption.
Dawn also has studied selected sites at several times of the Cerean day. Mission planners may determine, for example, that if Dawn points not straight down on a particular orbit at a particular time but rather partially to the side, a certain crater could be spotted soon after Ceres’ nine-hour daily rotation has brought it into sunlight. In other words, it would be early in the morning at the crater when Dawn sees it, providing a nice dawn view. On another orbital revolution, Dawn might point in a different direction to see the same location longer after it has come into sunlight (that is, longer after sunrise), so from that same crater’s point of view, it is later in the day (albeit on a different day).
The spacecraft has done more than look at some special locations at different times of the Cerean day, corresponding to different lighting conditions. In taking pictures for a new map of Ceres this month, everywhere Dawn looked, the illumination was different from the photographs for the maps it compiled in its previous orbits. The orbit now is oriented at a different angle from the sun.
When the interplanetary adventurer was at Vesta, we described the orientation of the orbits in words. Thanks to changes in the Dawn Journal site since then, now we can present a picture showing that the scenery beneath Dawn has been illuminated from a different angle at each orbital altitude. And now in the fifth orbit, by seeing the sights from the same height as in the third mapping orbit but with different lighting, we gain a new perspective on the alien terrain.
NASA / JPL-Caltech
Dawn's science orbits at Ceres
This illustrates (and simplifies) the relative size and alignment of Dawn’s five science orbits at Ceres. We are looking down on Ceres’ north pole. The spacecraft follows polar orbits, and seen edge-on here, each circular orbit looks like a line. (Orbits 1 and 2 extend off the figure to the right, on the night side. Like 3, 4 and 5, they are centered on Ceres.) The orbits are numbered chronologically. With the sun far to the left, the left side of Ceres is in daylight. Each time the spacecraft travels over the illuminated hemisphere in the different orbital planes, the landscape beneath it is lit from a different angle. Ceres rotates counterclockwise from this perspective (just as Earth does when viewed from the north). So higher numbers correspond to orbits that pass over ground closer to sunrise, earlier in the Cerean day. (Compare this diagram with this figure, which shows only the relative sizes of the orbits, with each one viewed face-on rather than edge-on.) Click on this image for a larger view.
In addition to all of its other work this month, the sophisticated robot has continued some specialized measurements it began at lower altitude. Being higher up does not cause as much of a reduction in the sharpness of some pictures as you might think. Held in a looser gravitational grip, Dawn’s orbital velocity is lower at higher altitude. As a result, observations that require a long exposure are not affected as much by the spacecraft’s movement. That’s helpful for some of the spectra and photographs. For example, Dawn has used its camera to peer into craters near the north and south poles that are in shadow continuously, every Cerean day of the Cerean year. These special locations might trap water molecules that escape from elsewhere on Ceres where it is too warm for them. With the benefits of a wider view from a higher altitude and a more predictable orbital path, Dawn’s coverage this month of these intriguing areas, faintly illuminated by sunlight reflected from crater walls, has been more complete than at lower altitude.
This fifth Ceres campaign was intricate and intensive, but it stayed right on the tight schedule. Dawn began collecting data as planned on Oct. 16 and finished transmitting its findings to Earth on Oct. 29. And it was exceedingly productive, yielding almost 3,000 photographs plus a great many infrared spectra and visible spectra containing a wealth of new information about Ceres.
This week controllers are going to check out the backup camera, as they do twice a year to confirm that it is still healthy and ready to take over should the primary camera develop a problem. Nevertheless, the primary camera remains fully functional. The team also is planning to switch to the backup set of reaction control system thrusters. Dawn has flown for so many years without a full complement of reaction wheels that these hydrazine thrusters have been used far more than anticipated when the ship was designed. They are healthy, but ever-cautious engineers do not want to overuse them.
NASA / JPL-Caltech / UCLA / MPS / DLR / IDA
Dawn took this photo of Achita Crater on June 3, 2016, from an altitude of 240 miles (385 kilometers). Departing from what may seem to be the theme above of displaying interesting landscapes in the northwestern parts of the largest craters on Ceres, this scene includes most of the 25-mile (40-kilometer) Achita. Although many craters have a mountain peak in the center, this one has an extended ridge. (We have seen other craters on Ceres with central ridges, including Haulani and Urvara here and here.) Also note the bright material at the bottom of the southwest wall and a smaller deposit on the northeast rim. Achita Crater is at 26°N, 66°E on this map. (Achita is a god of agriculture in northern Nigeria.) Full image and caption.
Dawn’s work in this fifth orbit is part of a comprehensive plan for exploring Ceres as thoroughly as possible. Surprising though it may be, we will see next month that scientists have determined that there is even more to learn about Ceres by flying to a higher altitude. So now that Dawn has accomplished all of its objectives for this phase of the mission, it is about to begin another month of maneuvering. On Nov. 4, the spaceship will once again power on ion engine #2 and start another spiral to a sixth orbital observing post.
As Earth and Ceres (accompanied by Dawn) follow their independent orbits around the sun, the distance between them is constantly changing. On Oct. 22, they were at their smallest separation in the 3.5 years from June 2014 to Dec. 2017. On that date, Dawn was a mere 1.900 AU (176.6 million miles, or 284.2 million kilometers) from its first solar system residence. Dawn never loses track of the rest of its team, still stationed on that faraway planet. But after many years of interplanetary travels and more than a year at Vesta, the denizen of deep space is now a devoted companion of Ceres, and that is where it focuses its attention. And it has more work to do as it seeks still greater insights into the nature of its mysterious and exotic home.
Dawn is 920 miles (1,480 kilometers) from Ceres. It is also 1.91 AU (178 million miles, or 286 million kilometers) from Earth, or 705 times as far as the moon and 1.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.
Dr. Marc D. Rayman 2:30 p.m. PDT October 31, 2016
P.S. Now that this Dawn Journal is complete, your correspondent can turn his attention to getting into costume for Halloween. This year, he will be disguised as someone who knew all along that Dawn would engage in a productive and innovative extended mission at Ceres. Just imagine what a great time the trick-or-treaters are going to have when they visit his home!
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