Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL
Dawn Journal | May 31

by Marc Rayman


Dear Dawnosaurs,

Silently streaking through the main asteroid belt, emitting a blue-green beam of xenon ions, Dawn continues its ambitious interplanetary expedition. On behalf of creatures on distant Earth who seek not only knowledge and insight but also bold adventure, the spacecraft is heading toward its appointment with Ceres. In about 10 months, it will enter orbit around the ancient survivor from the dawn of the solar system, providing humankind with its first detailed view of a dwarf planet.

This month we continue with the preview of how Dawn will explore Ceres. In December we focused on the “approach phase,” and in January we described how the craft spirals gracefully into orbit with its extraordinary ion propulsion system. The plans for the first observational orbit (with a marvelously evocative name for a first examination of an uncharted world: RC3 — is that cool, or what?), at an altitude of 8,400 miles (13,500 kilometers), were presented in FebruaryLast month, we followed Dawn on its spiral descent from each orbital altitude to the next, with progressively lower orbits providing better views than the ones before. Now we can look ahead to the second orbital phase, survey orbit.


This figure shows Dawn’s second observational orbit, “survey orbit,” at the same scale as the size of Ceres. At an altitude of 2,730 miles (4,400 kilometers), the spacecraft will make seven revolutions in about three weeks. Credit: NASA/JPL

In survey orbit, Dawn will make seven revolutions at an altitude of about 2,730 miles (4,400 kilometers). At that distance, each orbit will take three days and three hours. Mission planners chose an orbit period close to what they used for survey orbit at Vesta, allowing them to take advantage of many of the patterns in the complex choreography they had already developed. Dawn performed it so beautifully that it provides an excellent basis for the Ceres encore. Of course, there are some adjustments, mostly in the interest of husbanding precious hydrazine propellant in the wake of the loss of two of the spacecraft’s four reaction wheels. (Although such a loss could have dire consequences for some missions, the resourceful Dawn team has devised a plan that can achieve all of the original objectives regardless of the condition of the reaction wheels.)

We had a preview of survey orbit at Vesta four years ago, and we reviewed the wonderfully successful outcome in September 2011. When we develop the capability to travel backwards in time, we will insert a summary of what occurred in survey orbit at Ceres here: _______…… Well, nothing yet. So, let’s continue with the preview.

As in all phases at Ceres (and Vesta), Dawn follows what space trajectory experts (and geeks) call a polar orbit. The ship’s course will take it above the north pole, and then it will sail south over the side bathed in the light of the sun. After flying over the south pole, Dawn will head north. Although the surface beneath it will be dark, the spacecraft will be high enough that it will not enter the dwarf planet’s shadow. The distant sun will constantly illuminate the large solar arrays.

The leisurely pace in survey orbit allows the explorer to gather a wealth of data during the more than 37 hours on the day side. It will train its science camera and visible and infrared mapping spectrometer (VIR) on the surface lit by the sun. The camera will collect hundreds of images using all seven of its color filters. It will reveal details three times finer than it observed in RC3 orbit and 70 times sharper than the best we have from the Hubble Space Telescope. VIR will acquire millions of spectra to help scientists determine the minerals present as well as the temperature and other properties of the surface. While the sensors are pointed at the surface, the main antenna cannot simultaneously be aimed at Earth, so the robot will store its pictures and spectra.

One Cerean day, the time it takes Ceres to rotate once on its axis, is a little over nine hours. (For comparison, Earth, as some of its residents and visitors know, takes 24 hours. Jupiter turns in just under 10 hours, Vesta in five hours and 21 minutes, and your correspondent’s cat Regulus in about 0.5 seconds when chasing a laser spot.) So as Dawn travels from the north pole to the south pole, Ceres will spin underneath it four times. Dawn will be close enough that even the wide field of view of its camera won’t capture the entire disc below, from horizon to horizon, but over the course of the seven orbits, the probe will see most of the surface. As in developing the plan for Vesta, engineers (like certain murine rodents and male humans) are keenly aware that as careful, as thorough, and as diligent as they are, their schemes don’t always execute perfectly. In the unknown, forbidding depths of space with a complex campaign to carry out, glitches can occur and events can go awry. The plan is designed with the recognition that some observations will not be achieved, but those that are promise great rewards.

Artist's concept of Dawn orbiting Ceres

Artist’s concept of Dawn in its survey orbit at dwarf planet Ceres. Credit: NASA/JPL

Most of the time, the spacecraft will gaze straight down at the alien terrain immediately beneath it. But on the first, second, and fourth passages over the day side of Ceres, it will spend some of the time looking at the limb against the blackness of space. Pictures with this perspective will not only be helpful for establishing the exact shape of the dwarf planet but they also will provide some very appealing views for eager sightseers on Earth.

In addition to using the camera and VIR, Dawn will measure space radiation with its gamma ray and neutron detector (GRaND). GRaND will still be too far from Ceres to sense the nuclear particles emanating from it, but recording the radiation environment will provide a valuable context for the sensitive measurements it will make at lower altitudes.

When Dawn’s orbit takes it over the dark side, it will turn away from the dwarf planet it is studying and toward the planet it left in 2007 where its human colleagues still reside. With its 5-foot (1.52-meter) main antenna, it will spend most of the day and a half radioing its precious findings across uncounted millions of miles (kilometers) of interplanetary space. (Well, you won’t have to count them, but we will.)

In addition to the instrument data it encodes, Dawn’s radio signal will allow scientists and engineers to measure how massive Ceres is. By observing the Doppler shift (the change in frequency caused by the spacecraft’s motion), they can determine how fast the ship moves in orbit. Timing how long the signals (traveling at the universal limit of the speed of light) take to make the round trip, navigators can calculate how far the probe is and hence where it is in its orbit. Combining these (and including other information as well) allows them to compute how strongly Ceres pulls on its orbital companion. The strength of its gravitational force reveals its heft.

By the end of survey orbit, Dawn will have given humankind a truly extraordinary view of a dwarf planet that has been cloaked in mystery despite more than 200 years of telescopic studies. As the exotic world of rock and ice begins to yield its secrets to the robotic ambassador from Earth, we will be transported there. We will behold new landscapes that will simultaneously quench our thirst for exploration and ignite our desire for even more. It is as humankind reaches ever farther into the universe that we demonstrate a part of what it means to be human, combining our burning need for greater understanding with our passion for adventure and our exceptional creativity, resourcefulness and tenacity. As we venture deeper into space, we discover much of what lies deep within ourselves.

Dawn is 7.2 million miles (12 million kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 695 times as far as the moon and 1.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.

Dr. Marc D. Rayman
11:00 p.m. PDT May 31, 2014

P.S. This is the 101st Dawn Journal. Only 99 more to go before cake and balloons again!

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8 Responses to “Dawn Journal | May 31”

  1. Jon Wilcox says:

    Your reference to “how strongly Ceres pulls” on Dawn made me wonder about the opposite effect.
    To what extent will the orbit of Ceres be perturbed by the Dawn mission, say after 100 years?

    • Marc Rayman says:


      Thank you for posting an interesting question!

      Ceres is so massive that Dawn’s effect on it is quite unmeasurable. Unlike small asteroids that come near Earth and which might be moved slightly by the gravity of a spacecraft (one of the techniques sometimes considered for preventing an impact with Earth), Ceres is gargantuan. It is about 600 miles in diameter.

      Dawn’s gravitational effect on Ceres depends on how far apart they are, so let’s take survey orbit, the subject of this Dawn Journal. As Ceres and Dawn pull on each other, Dawn will complete an orbit every three days and three hours at an altitude of 2,730 miles. Ceres will loop around too, at the same rate. To find how large its motion will be, I did a simplified calculation that shows that, thanks to Dawn, Ceres will be shifted by less than one tenth of the diameter of a hydrogen atom.

      I think that’s an interesting and fun result, and I hope it answers your question. It also may set at ease anyone concerned about the solar system environmental impact of putting a probe in orbit around that alien world.


  2. Scott says:

    Ok, Marc, you’re having WAY too much fun here. But seriously, can you expand a bit on how we will control Dawn with only 2 reaction wheels? 3, I can get, but 2 seems to lead to a lot of hydrazine. Or is there some other “secret” you’re not telling us?


    • Marc Rayman says:


      Of course I’m having fun! With a spacecraft farther away than the Sun, exploring some of the last uncharted worlds in the inner solar system, revealing dramatic, alien landscapes on places that have been observed only with telescopes from afar for more than two centuries, how could it be anything other than fun? I hope you and other readers feel the same good fortune in being part of an experience this.

      I’ve referred in several prior logs to the Dawn team’s development of a “hybrid” control scheme. (See, for example, my November 2013 Dawn Journal.) The details are too dry to warrant including here — most readers wouldn’t find them to be very fun — although the design has been published in a technical paper. The essential idea is that the two healthy reaction wheels control two axes of the spacecraft (you can think of that as the left/right axis, or pitch, and the front/back axis, or roll, although it isn’t quite that), and the hydrazine jets control the third axis (in this example, that would be the up/down axis, or yaw). Note that this does not mean moving in those directions, but rather rotating in place around those axes. Ion propulsion moves the spacecraft from one place to another. Hybrid control may sound simple, but to design, verify, and install a new control system for a spacecraft already in deep space, especially on a tight schedule (I wanted it in place before Vesta in case we lost a second wheel there, especially in our low altitude orbit), is challenging.

      I have many “secrets” but none on the topic you raise. Hybrid control does indeed consume more hydrazine than using three reaction wheels but less than pure hydrazine control does. That is not the only way the skilled Dawn team has managed to continue this remarkable mission despite the loss of two reaction wheels. When the second wheel misbehaved, we already had the hybrid control software on the spacecraft, but we undertook an intensive campaign to reduce hydrazine expenditures. We quickly came up with and then investigated about 50 creative methods of conserving hydrazine. The flight team really did an outstanding job. We have implemented many of the ideas, and it is the combined effect of all of them that has allowed us to devise a plan with such good prospects for accomplishing all of our objectives at Ceres. I have written about some, such as in November 2012, and I will write about more in the future (including in June). There is plenty more fun ahead!


  3. Erut Gudahl says:

    The so called “orbit diagram” showing a simple circle around Ceres is an insultingly oversimplification of the spacecraft trajectory to the dwarf planet, one wonders why you bothered to create the illustration at all. Although The illustrations that were and continue to be provided for the Cassini mission are a wonderful illustration of how the public image of NASA space missions can be enhanced by communicating the complexity of work that went into the mission planning. The New York Times illustration for Cassini was wonderful

    But even the more simplistic initial approach diagrams from the mission plans

    added value and understanding to a science interested public. Why can’t the Dawn Mission team put some simple illustrations of the orbital insertion and spirals down to different planned orbits?
    You appear to have no trouble talking about the mission plan:

    >Dawn will take advantage of the extraordinary capability of its ion propulsion system to maneuver extensively in orbit at Ceres. During the course of its long mission there, it will fly to four successively lower orbital altitudes, each chosen to optimize certain investigations. (The probe occupied six different orbits at Vesta, where two of them followed the lowest altitude)

    The Japanese Hayabusa mission amply demonstrated the complexity and difficulty in making rendezvous and orbit with low gravity bodies. The ability of the Dawn mission team to plan and execute such complex maneuvers at not just one, but two different asteroids is nothing short of amazing. It is painful to see the lack of illustration of these accomplishments. Surely there must be some underworked intern at one of the NASA orbital dynamics centers of competence who could put together a decent diagram of the Ceres mission orbital plan? Note too from the NYT diagram of Cassini, the time scale provided for the orbit. How long does it take for Dawn to orbit once around Ceres? What are your apogee and perigee point distances from the body? Details like these would do much to increase the sense of remotely “being there” that are key to peaking interest in the mission.

    • Marc Rayman says:

      Thank you for your comments. I always appreciate constructive feedback.

      It appears you were seeking something different from the broad overview of survey orbit that I tried to provide here as a preview. Perhaps the intent of the orbit diagram was not clear. It was designed to illustrate the size of survey orbit compared to the size of Ceres. I had hoped that would give readers a sense of the scale. It also turns out to be a pretty good depiction of what the actual orbit will look like. Survey orbit is circular (see more about that below) and repetitive, so the seven revolutions will overlap each other.

      I have written in other blogs about the trajectory to the dwarf planet as well as the orbital insertion and the spirals to different orbits; those were not subjects I tried to cover in this blog. As you expressed a specific interest in those other topics, however, I hope you might find the text and diagrams in my January and April blogs to be helpful. (I had thought that perhaps my second paragraph would guide interested readers to them.) In many other blogs, including March, I have discussed various aspects of the interplanetary trajectory to Ceres. In all of those, as well as my other Dawn Journals, I have made an effort to help the public understand what you so correctly describe as the complexity of the work that goes into mission planning and other aspects of flying this ambitious and exciting mission.

      Dawn’s mission profile is dramatically different from that of Cassini and Hayabusa. Those are wonderful missions but of an entirely different character. (Hayabusa, for example, went to a destination that is thousands of times smaller than Ceres, with a mass more than 25 billion times less. Hence, the nature of the rendezvous and orbits is somewhat different.)

      You posed some specific questions:

      – How long does it take for Dawn to orbit once around Ceres? As I tried to explain in my third paragraph, one orbit takes three days and three hours.

      – What are the apogee (the maximum distance) and perigee (the minimum distance) of the orbit? Survey orbit is designed to be circular, so the altitude of 2,730 miles is a good description of the orbit. (In brief but more technical terms, we target an eccentricity of zero.) Deviations from a circle will depend on subtle details that we will not measure until we are at Ceres, but they likely will be less than the thickness of the line showing the orbit. This blog was intended simply to be a preview. I will provide details on the orbit characteristics when we are in the orbit. (I did the same thing at Vesta. See, for example, my discussion of the change in orbital altitude here.)

      I appreciate your interest in details that allow readers to gain the sense of “being there.” I hope you find that these answers, as well as some of the information and perspectives proffered in my other Dawn Journals, contribute to your achieving that.