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Marc Rayman

Dawn Journal: Descent to LAMO

Posted by Marc Rayman

02-12-2015 12:02 CST

Topics: asteroid 4 Vesta, mission status, asteroids, asteroid 1 Ceres

Dear Superintendawnts and Assisdawnts,

An intrepid interplanetary explorer is now powering its way down through the gravity field of a distant alien world. Soaring on a blue-green beam of high-velocity xenon ions, Dawn is making excellent progress as it spirals closer and closer to Ceres, the first dwarf planet discovered. Meanwhile, scientists are progressing in analyzing the tremendous volume of pictures and other data the probe has already sent to Earth.


NASA / JPL-Caltech

Dawn’s spiral descent from its third mapping orbit (HAMO), at 915 miles (1,470 kilometers), to its fourth (LAMO), at 240 miles (385 kilometers). The two mapping orbits are shown in green. The color of Dawn’s trajectory progresses through the spectrum from blue, when it began ion-thrusting in HAMO, to red, when it arrives in LAMO. The red dashed sections show where Dawn is coasting for telecommunications. It requires 118 spiral revolutions around Ceres to reach the low altitude (and additional revolutions to prepare for and conduct the trajectory correction maneuver described below). Compare this to the previous spiral. (Readers with total recall will note that this is fewer loops than illustrated last year. The flight team has made several improvements in the complex design since then, shortening the time required and thus allowing more time for observing Ceres.)

Dawn is flying down to an average altitude of about 240 miles (385 kilometers), where it will conduct wide-ranging investigations with its suite of scientific instruments. The spacecraft will be even closer to the rocky, icy ground than the International Space Station is to Earth’s surface. The pictures will be four times sharper than the best it has yet taken. The view is going to be fabulous!

Dawn will be so near the dwarf planet that its sensors will detect only a small fraction of the vast territory at a time. Mission planners have designed the complex itinerary so that every three weeks, Dawn will fly over most of the terrain while on the sunlit side. (The neutron spectrometergamma ray spectrometer and gravity measurements do not depend on illumination from the sun, but the camera, infrared mapping spectrometer and visible mapping spectrometer do.)

Obtaining the planned coverage of the exotic landscapes requires a delicate synchrony between Ceres’ and Dawn’s movements. Ceres rotates on its axis every nine hours and four minutes (one Cerean day). Dawn will revolve around it in a little less than five and a half hours, traveling from the north pole to the south pole over the hemisphere facing the sun and sailing northward over the hemisphere hidden in the darkness of night. Orbital velocity at this altitude is around 610 mph (980 kilometers per hour).

Last year we had a preview of the plans for this fourth and final mapping orbit (sometimes also known as the low altitude mapping orbit, or LAMO), and we will present an updated summary next month.

The planned altitude differs from the earlier, tentative value of 230 miles (375 kilometers) for several reasons. One is that the previous notion for the altitude was based on theoretical models of Ceres’ gravity field. Navigators measured the field quite accurately in the previous mapping orbit (using the method outlined here), and that has allowed them to refine the orbital parameters to choreograph Dawn’s celestial pas de deux with Ceres. In addition, prior to Dawn’s investigations, Ceres’ topography was a complete mystery. Hubble Space Telescope had shown the overall shape well enough to allow scientists to determine that Ceres qualifies as a dwarf planet, but the landforms were indiscernible and the range of relative elevations was simply unknown. Now that Dawn has mapped the topography, we can specify the spacecraft’s average height above the ground as it orbits. With continuing analyses of the thousands of stereo pictures taken in August – October and more measurements of the gravity field in the final orbit, we will further refine the average altitude. Finally, we round the altitude numbers to the nearest multiple of five (both for miles and kilometers), because, as we will discuss in a subsequent Dawn Journal, the actual orbit will vary in altitude by much more than that. (We described some of the ups and dawns of the corresponding orbit at Vesta here. The variations at Ceres will not be as large, but the principles are the same.)

Urvara Crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Urvara Crater
Dawn had this view of Urvara crater in mapping cycle #4 from an altitude of 915 miles (1,470 kilometers) during the third mapping orbit. (Urvara is a Vedic goddess associated with fertile lands and plants.) The crater is 101 miles (163 kilometers) in diameter. It displays a variety of features, including a particularly bright region on the peak at the center, ridges nearby, a network of fissures, some smooth regions and much rougher terrain. You can locate all the areas shown in this month’s photos on the Ceres map presented last month. Full image and caption.

To attain its new orbit, Dawn relies on its trusty and uniquely efficient ion engine, which has already allowed the spacecraft to accomplish what no other has even attempted in the 58-year history of space exploration. This is the only mission ever to orbit two extraterrestrial destinations. The spaceship orbited the protoplanet Vesta for 14 months in 2011-2012, revealing myriad fascinating details of the second most massive object in the main asteroid belt between Mars and Jupiter, before its March 2015 arrival in orbit around the most massive. Ion propulsion enables Dawn to undertake a mission that would be impossible without it.

While the ion engine provides 10 times the efficiency of conventional spacecraft propulsion, the engine expends the merest whisper of xenon propellant, delivering a remarkably gentle thrust. As a result, Dawn achieves acceleration with patience, and that patience is rewarded with the capability to explore two of the last uncharted worlds in the inner solar system. This raises an obvious question: How cool is that? Fortunately, the answer is equally obvious: Incredibly cool!

The efficiency of the ion engine enables Dawn not only to orbit two destinations but also to maneuver extensively around each one, optimizing its orbits to reap the richest possible scientific return at Vesta and Ceres. The gentleness of the ion engine makes the maneuvers gradual and graceful. The spiral descents are an excellent illustration of that.

Dawn began its elegant downward coils on Oct. 23 upon concluding more than two months of intensive observations of Ceres from an altitude of 915 miles (1,470 kilometers). At that height, Ceres’ gravitational hold was not as firm as it will be in Dawn’s lower orbit, so orbital velocity was slower. Circling at 400 mph (645 kilometers per hour), it took 19 hours to complete one revolution around Ceres. It will take Dawn more than six weeks to travel from that orbit to its new one. (You can track its progress and continue to follow its activities once it reaches its final orbit with the frequent mission status updates.)

Dantu Crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Dantu Crater
Dawn took this picture of Dantu crater from an altitude of 915 miles (1,470 kilometers) during the third mapping orbit, in mapping cycle #4. (Dantu is a timekeeper god who initiates the cycle of planting rites among the Ga people of the Accra Plains of southeastern Ghana. You can find Dantu, but not Ghana, on this map.) The crater is about 77 miles (125 kilometers) across. Note the isolated bright regions, the long fissures, and the zigzag structure at the center. Scientists are working to understand what these indicate about the geological processes on Ceres. Full image and caption.

On Nov. 16, at an altitude of about 450 miles (720 kilometers), Dawn circled at the same rate that Ceres turned. Now the spacecraft is looping around its home even faster than the world beneath it turns.

When ion-thrusting ends on Dec. 7, navigators will measure and analyze the orbital parameters to establish how close they are to the targeted values and whether a final adjustment is needed to fit with the intricate observing strategy. Several phenomena contribute to small differences between the planned orbit and the actual orbit. (See here and here for two of our attempts to elucidate this topic.) Engineers have already thoroughly assessed the full range of credible possibilities using sophisticated mathematical methods. This is a complex and challenging process, but the experienced team is well prepared. In case Dawn needs to execute an additional maneuver to bring its orbital motion into closer alignment with the plan, the schedule includes a window for more ion-thrusting on Dec. 11-13 (concluding on Dawn’s 2999th day in space). In the parlance of spaceflight, this maneuver to adjust the orbit is a trajectory correction maneuver (TCM), and Dawn has experience with them.

The operations team takes advantage of every precious moment at Ceres they can, so while they are determining whether to perform the TCM and then developing the final flight plan to implement it, they will ensure the spacecraft continues to work productively. Dawn carries two identical cameras, a primary and a backup. Engineers occasionally operate the backup camera to verify that it remains healthy and ready to be put into service should the primary camera falter. On Dec. 10, the backup will execute a set of tests, and Dawn will transmit the results to Earth on Dec. 11. By then, the work on the TCM will be complete.

Although it is likely a TCM will be needed, if it turns out to be unnecessary, mission control has other plans for the spacecraft. In this final orbit, Dawn will resume using its reaction wheels to control its orientation. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can control its orientation, stabilizing itself or turning. We have discussed their lamentable history on Dawn extensively, with two of the four having failed. Although such losses could have been ruinous, the flight team formulated and implemented very clever strategies to complete the mission without the wheels. Exceeding their own expectations in such a serious situation, Dawn is accomplishing even more observations at Ceres than had been planned when it was being built or when it embarked on its ambitious interplanetary journey in 2007.

Craters on Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Craters on Ceres
Dawn took this picture in its third mapping orbit at an altitude of 915 miles (1,470 kilometers) in mapping cycle #5 of its third mapping orbit. The prominent triplet of overlapping craters nicely displays relative ages, which are apparent by which ones affect others and hence which ones formed later. The largest crater, Geshtin, is 48 miles (77 kilometers) across and is the oldest. (Geshtin is a Sumerian and Assyro-Babylonian goddess of the vine.) A subsequent impact that excavated Datan crater, which is 37 miles (60 kilometers) in diameter, obliterated a large section of Geshtin’s rim and made its own crater wall in Geshtin’s interior. (Datan is one of the Polish gods who protect the fields but apparently not this crater.) Still later, Datan itself was the victim of a sizable impact on its rim (although not large enough to have merited an approved name this early in the geological studies of Ceres). Full image and caption.

Now the mission lifetime is limited by the small supply of conventional rocket propellant, expelled from reaction control system thrusters strategically located around the spacecraft. When that precious hydrazine is exhausted, the robot will no longer be able to point its solar arrays at the sun, its antenna at Earth, its sensors at Ceres or its ion engines in the direction needed to travel elsewhere, so the mission will conclude. The lower Dawn’s orbital altitude, the faster it uses hydrazine, because it must rotate more quickly to keep its sensors pointed at the ground. In addition, it has to fight harder to resist Ceres’ relentless gravitational tug on the very large solar arrays, creating an unwanted torque on the ship.

Among the innovative solutions to the reaction wheel problems was the development of a new method of orienting the spacecraft with a combination of only two wheels plus hydrazine. In the final orbit, this “hybrid control” will use hydrazine at only half the rate that would be needed without the wheels. Therefore, mission controllers have been preserving the units for this final phase of the expedition, devoting the limited remaining usable life to the time that they can provide the greatest benefit in saving hydrazine. (The accuracy with which Dawn can aim its sensors is essentially unaffected by which control mode is used, so hydrazine conservation is the dominant consideration in when to use the wheels.) Apart from a successful test of hybrid control two years ago and three subsequent periods of a few hours each for biannual operation to redistribute internal lubricants, the two operable wheels have been off since August 2012, when Dawn was climbing away from Vesta on its way out of orbit.

Controllers plan to reactivate the wheels on Dec. 15. However, in the unlikely case that the TCM is deemed unnecessary, they will power the wheels on on Dec. 11. The reaction wheels will remain in use for as long as both function correctly. If either one fails, which could happen immediately or might not happen before the hydrazine is depleted next year, it and the other will be powered off, and the mission will continue, relying exclusively on hydrazine control.

Between Fluusa and Toharu

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Between Fluusa and Toharu
Dawn recorded this view in its third mapping orbit at an altitude of 915 miles (1,470 kilometers) in mapping cycle #5. The region shown is located between Fluusa and Toharu craters. The largest crater here is 16 miles (26 kilometers) across. The well defined features indicate the crater is relatively young, so subsequent small impacts have not degraded it significantly. As elsewhere on Ceres, some strikingly bright material is evident, particularly in the walls. Full image and caption.

Dawn will measure the energies and numbers of neutrons and gamma rays emanating from Ceres as soon as it arrives in its new orbit. With a month or so of these measurements, scientists will be able to determine the abundances of some of the elements that compose the material near the surface. Engineers and scientists also will collect new data on the gravity field at this low altitude right away, so they eventually can build up a profile of the dwarf planet’s interior structure. The other instruments (including the camera) have narrower fields of view and are more sensitive to small discrepancies in where they are aimed. It will take a few more days to incorporate the actual measured orbital parameters into the corresponding plans that controllers will radio to the spacecraft. Those observations are scheduled to begin on Dec. 18. But always squeezing as much as possible out of the mission, the flight team might actually begin some photography and infrared spectroscopy as early as Dec. 16.

Now closing in on its final orbit, the veteran space traveler soon will commence the last phase of its long and fruitful adventure, when it will provide the best views yet of Ceres. Known for more than two centuries as little more than a speck of light in the vast and beautiful expanse of the stars, the spacecraft has already transformed it into a richly detailed and fascinating world. Now Dawn is on the verge of revealing even more of Ceres’ secrets, answering more questions and, as is the marvelous nature of science and exploration, raising new ones.

Dawn is 295 miles (470 kilometers) from Ceres. It is also 3.33 AU (309 million miles, or 498 million kilometers) from Earth, or 1,270 times as far as the moon and 3.37 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 55 minutes to make the round trip.

Dr. Marc D. Rayman
5:00 p.m. PST November 30, 2015

See other posts from December 2015


Or read more blog entries about: asteroid 4 Vesta, mission status, asteroids, asteroid 1 Ceres


Atom: 12/02/2015 02:53 CST

Thanx Dr. Marc, The decent to LAMO has gone very smoothly so congregations are in order for that. With the planned altitude for LAMO about twice what it was at Vesta, will the lower resolution of the observations be adequate for the neutron-gamma ray and gravity experiments? Since Ceres is much larger I assume the ratio of the LAMO orbits are very similar so the results should be similar. Please elaborate.

Marc Rayman: 12/02/2015 04:26 CST

Thank you, Atom. (I wish my parents had been that creative in naming me.) You're quite right about the ratios of the orbits. To Dawn, Ceres and Vesta will be almost exactly the same apparent size in the respective LAMOs. (Each one would appear to be about the same size as a soccer ball seen from 3.5 inches, or 9 centimeters.) Indeed, being close to that condition was one of the criteria in our selection of the altitude at Ceres. Loosely speaking, the intensity of the signals both for the neutron measurements and for the gamma ray measurements should therefore be similar at Ceres to what they were at Vesta. It's as if, from a nuclear spectroscopy standpoint, Ceres and Vesta will be similarly "bright" in LAMO. Dawn's gravity measurements at Ceres will not show as much detail as at Vesta, but they will still be of great value for understanding the interior, and this difference was part of the original plans for the project.

Chris Landau: 12/02/2015 09:48 CST

Hi Marc What are your thoughts on the last image on this blog with the area between Fluusa and Toharu that these craters are calderas from a "local hotspot" like Hawaii. The lower crater appears to have some fluid descending into the crater from all sides(striations like on Mars of theorized water flow) with no apparent impact craters in the floor.This might be a mud/ice slurry/ lava flow. It would mean that this region is "volcanically active" right now and that the heat is moving from the north in this photo to the south as that crater has fewer streaks on its caldera walls and has some small craters on its infilled caldera floor. It would certainly beat the old boring ejecta theories I have been reading about for the last few years. I would love your opinion. Thanks Chris Landau(geologist)

Marc Rayman: 12/03/2015 04:52 CST

That's an interesting idea, Chris. Cryovolcanism certainly is credible on Ceres. I'm not an expert, so my opinion isn't very valuable, but I think if these craters were calderas from the surface moving over a stationary hot spot, there likely would be other evidence of such motion. But to the key distinction of volcanism vs. impact, in principle you still could be seeing one or more calderas without invoking horizontal movement. They may not have to have a single source. Of course, many criteria would need to be considered to make a compelling case for a caldera, including showing that it is a depression on a mountain (based on the topographic data) and that the material is not ejecta (using the color images or the IR or visible spectra). It will be interesting to see whether the detailed analyses of the data are consistent with these and other attributes of calderas. As an aside, while crater formation from impacts, and resulting phenomena such as ejecta, temporary melting, etc., may be boring (unless you happen to be standing there when it occurs), it is common on planetary surfaces unprotected by dense atmospheres. In any case, I look forward to learning whether your ideas are supported by the ongoing geological investigations!

Chris Landau: 12/05/2015 10:22 CST

Hi Marc Thank you for the detailed reply. I think with all your years of studying Vesta and Ceres from getting the spacecraft into the correct orbit for repeated photos you are now more of an expert than the experts. One point on the caldera hypothesis is that as with the biggest known active caldera on Earth, at Yellowstone, if the viscosity of the "rock" is low, the calderas will not have to be on a mountain top. So slow cooling, low viscosity and slow lateral movement of a hot spot, as with Yellowstone, might be a possibility here. I agree with your multiple hot spot sources postulate for Ceres. I think it is globally heated dwarf planet and Ceres is a "warm body". I also think that Occator crater has active cryovolcanism occuring right now, but that the bright spots are like the "Old Faithful" geysers, occuring in a visible east-west joint pattern with smaller polygonal structures, outlining these micro calderas similar to Giants Causeway in Northern Island. I believe that Occator crater, unlike this "very warm" caldera is solidifying more rapidly now and only extremely low viscosity material is making its way through the visible polygonal joint pattern at high pressures to give us the "Old Faithful" bright spots. Hopefully with the lowest planned orbit, we will soon see if this is true. Ceres is also a study of localized high viscosity material which makes up the 3 mile high " pyramid"volcano like Mt Shasta. Definitely a very active world this Ceres. Thank you Marc for bringing us the beautiful worlds of Vesta and Ceres. They are amazing dynamic and active bodies that have helped me understand the driving forces and global continental shapes on Earth. I look forward to you managing the next "Ceres Rover Team" to explore Ceres for a future base for humans as they populate the solar system. The last point is that, I know there is no magnetometer on Dawn, but do you have any other instruments to detect a magnetic field on Ceres? Chris Landau(geologist)

Marc Rayman : 12/05/2015 07:39 CST

Thank you for your comment, Chris. Dawn cannot detect a magnetic field. Whether the energetic electrons I wrote about in my October Dawn Journal (search for "Exuldawnt") will turn out to be indicative of a magnetic field remains to be seen.

Chris Landau: 12/06/2015 05:46 CST

I did read your October blog, Marc now and I am sure your team did discover a magnetic field above Ceres induced by Ceres' rapid spin. Did you find a similar field on Dawn's descent to Vesta? I would have thought that Vesta would have a stronger magnetic field because of its higher density and more rapid rotation. I am sure the coupling and uncoupling of these two dwarves with Jupiter and the Sun's magnetic field as they orbit the sun has induction heated these two bodies for eternity and melted and remelted them globally? What are your thoughts? The other evidence I see for planetary wide induction volcanism is that Ceres has global flow banding at about 45 degrees to its present axis of rotation and Vesta has 2 directions of global flow banding at about 30 degrees to each other, indicating the whole body has has melted at least twice. Ceres may have an unstable axis of rotation and be changing declinations repeatedly. Sometime in the past these two bodies must have been spinning at an even greater rate than they are today to have left us such a definitive legacy. Your thoughts, Marc? Chris Landau (geologist)

Marc Rayman: 12/06/2015 02:43 CST

There are too many topics here for me to explain in the constrained format of blog comments, Chris. I can address your questions elsewhere, but I should apologize in advance that my free time for offering thoughts on wide ranging ideas is quite limited. But to close off a few simple points here: Dawn yielded no evidence of a magnetic field at Vesta, as expected given the instrument suite. The "banding" you observe on Vesta is unrelated to melting. It is a result of the two large impacts deep in the southern hemisphere that excavated the Rheasilvia and Veneneia basins. This has been written about extensively, but see, for example, my January 2013 Dawn Journal (search for "dawn't"). Thanks for your continuing interest!

Chris Landau: 12/06/2015 07:12 CST

And the banding that runs North-west south-east on Ceres at 45 degrees to the axis of spin, is that also due to an impact? Thanks for all the replies. I never thought that 2 giant hexagonal rock vortices on the north pole of Vesta as well as the giant rock vortices on the north pole had anything to do with impacts as most meteors, comets or any other bodies would be travelling in the plane of the ecliptic and not impacting the poles but the equitorial regions of planets and moons.. So that so called double impact at the poles, to me is pure nonsense. The shapes are hexagonal like those cloud vortices on the poles of Jupiter, Saturn, Uranus and Neptune. Only these are set in stone. Is that a big leap? Chris Landau(geologist)

dougforspaceexplr: 12/08/2015 08:17 CST

Hi Dr. Marc; That is good to hear that Dawn will be entering its final and closest orbit probably around the second week of this month of December. As it will probably be quite a long time before another space craft is sent to Ceres and it is the largest asteroid I would still like to know if there will be more results to determine the chemical composition of the bright spots in Ocator crater or elsewhere on Ceres or if you have seen anymore haze there and what the chemical composition of that might be. I would also be interested to know if there have been any more results what the general composition of the surface of Ceres is and what organic chemicals might be there and if this can be helpful in determining if any meteorites on earth are from Ceres. I think you mentioned before in answer to my previous similar questions that you could get more data when Dawn got to its final and closest orbit which it seems now about to be in and that one of the best ways to determine the chemical composition of these surface features is with the instrument measuring in the infrared light. How is this instrument working and will it still be helpful in determining the composition of these or other surface features on Ceres and what other instruments would be helpful and will be used for this purpose. Thank you for the groundbreaking work in helping us understand this largest asteroid and nearest dwarf planet in our solar system and for being accessible for questions and good reporting.

Marc Rayman: 12/08/2015 01:14 CST

Hi Doug, The analysis of the composition data for Occator and elsewhere on Ceres is continuing, and progress is being made. But establishing the chemical composition is notoriously difficult, because the spectra are complex and there are so many materials to use for comparisons. At any rate, the results will be announced as they become available. NASA will issue a press release (and, of course, findings will be published in scientific journals and presented at conferences), so please keep an eye out for that. The infrared mapping spectrometer is working extremely well, and (as I mentioned in several Dawn Journals, including October) we have obtained a great many spectra (significantly more even than we originally planned). The main other contributor to understanding chemical composition is the set of color images from the camera. (I wrote about this more in October as well. Search for "Exuldawnt.") The neutron spectra and gamma ray spectra both will help reveal the elemental composition.

ScienceNotFiction: 12/20/2015 02:02 CST

Hi Marc, As you may recalled that I had previously speculated that many of the bright dots on Dantu (and on other craters) are in fact tiny openings caused by fallen stalagmite columns below or unseal puncture holes from small impact rocks. And the holes on the crater wall near the top are the result of surface crust tears around the rim from the impact force. I hope DAWN's LAMO photos can show us that these are openings to the interior. As for OXO and other fibrous craters (PIA20125), my hope is that DAWN can confirm the existence if fibrous strains (or lianas) in the upcoming close up photos. This can strengthen the case for the existence of organic substance underneath the Cerean crust. That will be the climax of your entire mission, discovery of organic plant life.

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