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

by Marc Rayman

 

Dear Studawnts and Teachers,

Patient and persistent, silent and alone, Dawn is continuing its extraordinary extraterrestrial expedition. Flying through the main asteroid belt between Mars and Jupiter, the spacecraft is using its advanced ion propulsion system to travel from Vesta, the giant protoplanet it unveiled in 2011 and 2012, to Ceres, the dwarf planet it will reach in about eight months.

Most of these logs since December have presented previews of the ambitious plan for entering orbit and operating at Ceres to discover the secrets this alien world has held since the dawn of the solar system. We will continue with the previews next month. But now with Dawn three quarters of the way from Vesta to Ceres, let’s check in on the progress of the mission, both on the spacecraft and in mission control at JPL.

The mission is going extremely well. Thank you for asking.

For readers who want more details, read on…

The spacecraft, in what is sometimes misleadingly called quiet cruise, has spent more than 97 percent of the time this year following the carefully designed ion thrust flight plan needed to reshape its solar orbit, gradually making it more and more like Ceres’ orbit around the sun. This is the key to how the ship can so elegantly enter into orbit around the massive body even with the delicate thrust, never greater than the weight of a single sheet of paper.

The probe is equipped with three ion engines, although it only uses one at a time. (The locations of the engines were revealed shortly after launch when the spacecraft was too far from Earth for the information to be exploited for tawdry sensationalism.) Despite the disciplined and rigorous nature of operating a spaceship in the main asteroid belt, the team enjoys adding a lighthearted touch to their work, so they refer to the engines by the zany names #1, #2, and #3.

Dawn & TIE Fighter comparison

Artist’s comparison of Dawn spacecraft and Star Wars TIE Fighter.
Credit: NASA/JPL

Darth Vader and his Empire cohorts in “Star Wars” flew TIE (Twin Ion Engine) Fighters in their battles against Luke Skywalker and others in the Rebel Alliance. Outfitted with three ion engines, Dawn does the TIE Fighters one better. We should acknowledge, however, that the design of the TIE Fighters did appear to provide greater agility, perhaps at the expense of fuel efficiency. Your correspondent would concur that when you are trying to destroy your enemy while dodging blasts from his laser cannons, economy of propellant consumption probably shouldn’t be your highest priority.

All three engines on Dawn are healthy, and mission controllers consider many criteria in formulating the plan for which one to use. This called for switching from thruster #2 to thruster #1 on May 27. Thruster #1 had last been used to propel the ship on Jan. 4, 2010. After well over four years of inaction in space, it came to life and emitted the famous blue-green beam of high velocity xenon ions right on schedule (at 4:19:19 pm PDT, should you wish to take yourself back to that moment), gently and reliably pushing the spacecraft closer to its appointment with Ceres.

Artist concept showing Dawn thrusting with ion engine #1.

Artist’s concept illustrating the Dawn spacecraft pointing ion engine #1 in the direction needed for thrusting. Following this example, see the two images below, in which the spacecraft rotates to different orientations to ensure the other ion engines point in this same direction. The solar arrays are always aimed at the sun. Credit: NASA/JPL

Artist concept showing Dawn thrusting with ion engine #2.

Artist’s concept illustrating the Dawn spacecraft rotated to point ion engine #2 for thrusting in the same direction as in the images above and below. Credit: NASA/JPL

Artist concept showing Dawn thrusting with ion engine #3.

Artist’s concept illustrating the Dawn spacecraft rotated to point ion engine #3 for thrusting in the same direction as in the two images above. Credit: NASA/JPL

Without the tremendous capability of ion propulsion, a mission to orbit either Vesta or Ceres alone would have been unaffordable within NASA’s Discovery program. A mission to orbit both destinations would be altogether impossible. The reason ion propulsion is so much more efficient than conventional chemical propulsion is that it can turn electrical energy into thrust. Chemical propulsion systems are limited to the energy stored in the propellants.

Thanks to Dawn’s huge solar arrays, electrical energy is available in abundance, even far from the brilliant sun. To make accurate predictions of the efficiency of the solar cells as Dawn continues to recede from the sun, engineers occasionally conduct a special calibration. As we described in more detail a year ago, they command the robot to rotate its panels to receive less sunlight, simulating being at greater solar distances, as the ion propulsion system is throttled to lower power levels. Following the first such calibration on June 24, 2013, we assured readers (including you) that we would repeat the calibration as Dawn continued its solar system travels. So you will be relieved to know that it was performed again on Oct. 14, Feb. 3, and May 27, and another is scheduled for Sept. 15. Having high confidence in how much power will be available for ion thrusting for the rest of the journey allows navigators to plot the best possible course. Dawn is on a real power trip!

The reason for going to Ceres, besides it being an incredibly cool thing to do, is to use the suite of sophisticated sensors to learn about this mysterious dwarf planet. (In December, we will describe what is known about Ceres, just in time for it to change with Dawn’s observations.) Controllers activated and tested the cameras and all the spectrometers this summer, verifying that they remain in excellent condition and as ready to investigate the uncharted lands ahead as they were for the fascinating lands astern. The engineers also installed updated software in the primary camera in June and are ready to install it in the backup camera next month to enhance some of the devices’ functions. All of the scientific instruments are normally turned off when Dawn is not orbiting one of its targets. They will be powered on again in October for a final health check before the approach phase, during which they will provide our first exciting new views of Ceres.

To achieve a successful mission at Ceres, in addition to putting the finishing touches on the incredibly intricate plans, the operations team works hard to take good care of the spacecraft, ensuring it stays healthy and on course. In the remote depths of space, the robot has to be able to function on its own most of the time, but it does so with periodic guidance and oversight by its human handlers on a faraway planet. That means they need to stay diligent, keep their skills sharp, and remain watchful for any indications of undesirable conditions. On July 22, the team received information showing that Dawn was in safe mode, a special configuration invoked by onboard software to protect the spacecraft and the mission, preventing unexpected situations from getting out of control.

As engineers inspected the trickle of telemetry, they began to discover that this was a more dire situation than they had ever seen for the distant craft. Among the surprises was an open circuit in one of the pressurized cells of the nickel-hydrogen battery, a portion of the reaction control system that was so cold that its hydrazine propellant was in danger of freezing, temperatures elsewhere on the spacecraft so low that the delicate cameras were at risk of being damaged, and a sun sensor with degraded vision. To make it still more complicated, waveguide transfer switch #5, used to direct the radio signal from the transmitter inside the spacecraft to one of its antennas for beaming to Earth, was stuck and so would not move when software instructed it to. Other data showed that part of the computer memory was compromised by space radiation. As if all that were not bad enough, one of the two star trackers, devices that recognize patterns of stars just as you might recognize constellations to determine your orientation at night without a compass or other aids, was no longer functional. Further complicating the effort to get the mission back on track was an antenna at the Deep Space Network that needed to be taken out of service for emergency repairs. And the entire situation was exacerbated by Dawn already being in its lowest altitude orbit around Ceres (the subject of next month’s log), so for part of every 5.5-hour orbital revolution, it was out of contact as the world beneath it blocked the radio signal.

Confronted with an almost bewildering array of complex problems, the team of experts spent three days working through them with their usual cool professionalism, ultimately finding ways to overcome each obstacle to continue the mission. It would be extraordinarily, even unbelievably, unlikely for so many separate problems to stack up so quickly, even for a ship in the severe conditions of deep space, more than 232 million miles (374 million kilometers) from Dawn mission control on the top floor of JPL’s building 264. However, it easily can happen in an operational readiness test (ORT, pronounced letter by letter and not as a word, for those readers who want to conduct their own ORTs). The telemetry came from the spacecraft simulator, just down the hall from the mission control room, and the problems were the fiendishly clever creations of the ORT mastermind. (So now you may calm down, reassured that the scenario just described did not actually happen.)

The team conducted ORTs (and even an ORTathon) before launch in 2007, before Vesta in 2011, and as recently as May 2013. They will hold another in August.

While mission controllers exercised their skills in the ORT, the real spacecraft continued streaking through the asteroid belt, its interplanetary travels bringing it 45 thousand miles (73 thousand kilometers) closer to Ceres each day. But it is not only the Dawn team members who are part of this adventure. The stalwart explorer is transporting everyone who ever gazes in wonder at the night sky, everyone who yearns to know what lies beyond the confines of our humble home, and everyone awed by the mystery, the grandeur, and the immensity of the cosmos. Fueled by their passionate longing, the journey holds the promise of exciting new knowledge and thrilling new insights as a strange world, glimpsed only from afar for more than two centuries, is soon to be unveiled.

Dawn is 4.2 million miles (6.7 million kilometers) from Ceres. It is also 2.67 AU (248 million miles, or 399 million kilometers) from Earth, or 995 times as far as the moon and 2.63 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 44 minutes to make the round trip.

Dr. Marc D. Rayman
6:00 p.m. PDT July 31, 2014

All Dawn Journal entries

 


4 Responses to “Dawn Journal | July 31”

  1. Mark Raphael says:

    Hi Marc

    Ok, I know you’ll hate this one, but when can we get to see a picture of Ceres? It seems like we are under one fifth the distance from the closet approach of Venus to Earth, (which we can see with the naked eye) and we are about 20 times moon distance. I know from reading your blogs, and common sense, that any view of Ceres will be much less detailed than earth based telescopes. You’ve explained before that space based cameras have fixed focal points. However, after dillegently following your blogs and website, there is something emotional about see a few fuzy pixles of light from Dawn that makes us feel a wee bit closer to the mission from our living room computers.

    Thanks
    Mark

    • Marc Rayman says:

      Hi Mark,

      That’s a fine question. I like it, because it shows you are interested in the mission!

      The short answer is that we will take our first pictures of Ceres early in 2015 during the approach phase, which I described in some detail in December. Now let me give you the longer answer…

      I could not agree with you more enthusiastically about the emotional appeal of the first few pixels of this distant, alien world. It was the first dwarf planet discovered (129 years before Pluto), and the journey there across the interplanetary seas is long and challenging. Finally glimpsing the approach shore will be very gratifying indeed. So, why don’t we take pictures sooner? There are several considerations.

      Dawn does not carry a powerful telescope. Its cameras are designed for mapping uncharted worlds from orbit, not spying long-range targets. As I mentioned in my March 28, 2010 Dawn Journal, the cameras have a broad field of view for that purpose, with a magnification of only about three compared to our eyes. To put this in perspective, today Ceres would appear to be less than two pixels across. Even when we take our first scheduled images, a month and a half before Dawn enters orbit, Ceres will be about 24 pixels across. That’s still pretty fuzzy and will not reveal as much detail as the images we already have from the Hubble Space Telescope (which you can see in my February 2014 Dawn Journal and read a bit more about in some comments in June).

      Now, the most important consideration is that Dawn has lost two of its four reaction wheels. As I’ve written many times (including in the answer to the comment below), this could have had very, very severe implications for the mission. Your reliable and creative team of mission controllers, however, managed to devise and implement some extremely clever schemes to compensate, greatly stretching the supply of hydrazine. But that means we shouldn’t spend one gram of the precious propellant on turning the spacecraft when we don’t have to. Dawn was conceived, funded, designed, built, launched, and guided on its deep space journey to explore Vesta and Ceres. It was spectacularly successful at Vesta, and we are going to do our very best to accomplish all of the original objectives at Ceres, regardless of the condition of the reaction wheels. So we must remain disciplined, focusing on the greater rewards that will come when Dawn is in orbit.

      With a little more patience, we will have some fuzzy images, and then very soon after that, the exotic world of rock and ice will come into sharper focus. I think it will be well worth the wait!

      I hope this answers your question.

      Marc

  2. Pierre ARPIN says:

    How much xenon fuel will remain at the end of nominal mission ?

    How do plan to end the misson at Ceres ?

    Do you plan to land it on Ceres as NEAR-Shoemaker did on Eros or keep it forever in a stable orbit when xenon fuel will be exausted ?

    • Marc Rayman says:

      Hi Pierre,

      Thank you for your questions. I tried to answer your earlier question about this last month, but perhaps you have not seen it.

      There will be more than 50 pounds of xenon at the end of the mission, but xenon propellant is not what limits the duration of Dawn’s expedition. Having lost two of its four reaction wheels, the spacecraft’s operational lifetime will be limited by the hydrazine propellant used to turn and stabilize the ship. Note that for some missions, losing reaction wheels can have very serious consequences. With a great deal of effort and ingenuity, the Dawn team has been able to stretch the remaining supply of hydrazine far enough to give us confidence we can accomplish all of the objectives at Ceres, but doing so will consume nearly all the remaining hydrazine.

      It is physically impossible for Dawn to accomplish a controlled landing on Ceres. Comparisons with the very tiny asteroid Eros are misleading. Ceres is a massive dwarf planet with significant gravity, and Dawn does not carry the kind of hefty propulsion system that would be needed to touch down safely. Moreover, as I’ve described previously, including in an answer to a question in February, we need to protect the surface of Ceres from any terrestrial contamination on Dawn.

      So, the spacecraft will indeed be left in a stable orbit at Ceres. Without hydrazine, it will be unable to point its solar arrays at the sun or its antenna at Earth. It will no longer function, but as I posted in my previous reply to you, it will become a celestial monument representing human curiosity, creativity, ingenuity, and passion for adventure and knowledge.

      Thank you again for your continuing interest!

      Marc

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