This artist's conception of the Magellan spacecraft was created in about 1983, when it was known as Venus Radar Mapper (VRM). This kind of artwork was usually based on reports and drawings provided to the artist by the project staff. By the time Magellan was launched in May 1989 aboard the space shuttle Atlantis, the configuration had changed. It was not an uncommon occurrence for the design of a spacecraft to evolve over a period of months or years, based on input from the various instrument teams and engineers working on the project. It also happened when projects encountered funding problems and were scaled down in order to meet a budget.
One 1984 VRM project document explained, "The details of the configuration of the VRM spacecraft are changing continually as the spacecraft design matures. This illustration [a line drawing that matches the configuration shown in this artwork] shows the general configuration of the VRM spacecraft .... However several details of this illustration are out of date (such as the FEM length, altimeter antenna design and placement, and the amount of STAR-48 support structure retained after VOl)." Other, less detailed drawings were quickly added to the report to show the recent updates.
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 February. Last 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.
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.
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!
The last of the Surveyor lunar landers, Surveyor 7, was launched on January 7, 1968, and operated on the surface of the moon for about six weeks. Later that year, additional geoscience studies were carried out in the Mojave Desert using a spare surface sampler arm. A four-wheel-drive camper truck simulated an automated rover and was used to study the procedures and equipment necessary for remote geoscience. The truck was equipped with various sampler instruments, four TV cameras mounted on the top of the vehicle and one portable TV camera. Inside the camper was a simulated Space Flight Operations Center, with TV monitors, controllers for the cameras and instruments, and recording equipment. The field test observer (sitting in the camper) would survey the geology of the test area and carry out sampling operations remotely. Ritchie Coryell (System Design and Integration Section), Roy Brereton (Advanced Studies Office) and Earle Howard (Lunar and Planetary Instruments Section) all worked on this field test program.
Dear Compedawnt Readers,
Less than a year from its rendezvous with dwarf planet Ceres, Dawn is continuing to make excellent progress on its ambitious interplanetary adventure. The only vessel from Earth ever to take up residence in the main asteroid belt between Mars and Jupiter, the spacecraft grows more distant from Earth and from the sun as it gradually closes in on Ceres. Dawn devotes the majority of its time to thrusting with its remarkable ion propulsion system, reshaping its heliocentric path so that by the time it nears Ceres, the explorer and the alien world will be in essentially the same orbit around the sun.
In December, we saw what Dawn will do during the "approach phase"; to Ceres early in 2015, and in January, we reviewed the unique and graceful method of spiraling into orbit. We described in February the first orbit (with the incredibly cool name RC3) from which intensive scientific observations will be conducted, at an altitude of 8,400 miles (13,500 kilometers). But Dawn will take advantage of the extraordinary capability of ion propulsion to fly to three other orbital locations from which it will further scrutinize the mysterious world.
Let’s recall how the spacecraft will travel from one orbit to another. While some of these plans may sound like just neat ideas, they are much more than that; they have been proven with outstanding success. Dawn maneuvered extensively during its 14 months in orbit around Vesta. (One of the many discussions of that was in November 2011.) The seasoned space traveler and its veteran crew on distant Earth are looking forward to applying their expertise at Ceres.
As long-time readers of these logs know so well, the ion thrust is uniquely efficient but also extremely low. Ion propulsion provides acceleration with patience. Ultimately the patience pays off, enabling Dawn to accomplish feats far beyond what any other spacecraft has ever had the capability to do, including orbiting two extraterrestrial destinations. The gentle thrust, comparable to the weight of a single sheet of paper, means it takes many weeks to maneuver from one observational orbit to another. Of course, it is worthwhile to spend that much time, because each of the orbital phases is designed to provide an exciting trove of scientific data.
Those of you who have navigated around the solar system, as well as others who have contemplated the nature of orbits without having practical experience, recognize that the lower the orbital altitude, the faster the orbital motion. This important principle is a consequence of gravity’s strength increasing as the distance between the massive body and the orbiting object decreases. The speed has to be higher to balance the stronger gravitational pull. (For a reminder of some of the details, be sure to go here before you go out for your next orbital expedition.)
While Dawn slowly reduces its altitude under the faint pressure of its ion engine, it continues circling Ceres, orbiting in the behemoth’s gravitational grip. The effect of combining these motions is that the path from one altitude to another is a spiral. And as Dawn descends and zips around Ceres faster and faster, the spirals get tighter and tighter.
The first coils around Ceres will be long and slow. After completing its investigations in RC3, the probe will spiral down to”survey orbit,”; about 2,700 miles (4,400 kilometers) above the surface. During that month-long descent, it will make only about five revolutions. After three weeks surveying Ceres from that new vantage point, Dawn will follow a tighter spiral down to the (misleadingly named) high altitude mapping orbit (HAMO) at 910 miles (1,470 kilometers). In the six-week trip to HAMO, the craft will wind around almost 30 times. It will devote two months to performing extensive observations in HAMO. And finally as 2015 draws to a close, it will fly an even more tightly wound course to reach its low altitude mapping orbit (LAMO) at 230 miles (375 kilometers), where it will collect data until the end of the mission. The ship will loop around 160 times during the two months to go from HAMO to LAMO. (We will preview the plans for survey orbit, HAMO and LAMO in May, July and August of this year, and if all goes well, we will describe the results in 2015 and 2016.)
Designing the spiral trajectories is a complex and sophisticated process. It is not sufficient simply to activate the thrust and expect to arrive at the desired destination, any more than it is sufficient to press the accelerator in your car and expect to reach your goal. You have to steer carefully (and if you don’t, please don’t drive near me), and so does Dawn. As the ship revolves around Ceres, it must constantly change the pointing of the blue-green beam of high velocity xenon ions to stay on precisely the desired winding route to the targeted orbit. The mission control team at JPL will program the ship to orient its thruster in just the right direction at the right time to propel itself on the intended spiraling course.
Aiming a thruster in the direction needed to spiral around Ceres requires turning the entire spacecraft. Each thruster is mounted on its own gimbal with a limited range of motion. In normal operation, the gimbal is positioned so that the line of thrust goes through the center of the ship. When the gimbal is swiveled to another direction, the gentle force from the ion engine causes the ship to rotate slowly. This is similar to the use of an outboard motor on a boat. When it is aligned with the centerline of the boat, the craft travels straight ahead. When the motor is turned, it continues to propel the boat but also turns it. In essence, Dawn’s steering of its thrust is accomplished by pivoting the ion engine.
A crucial difference between the boat and our interplanetary ship is that with the former, the farther the motor is turned, the tighter the curving course. (Another difference is that the spacecraft wouldn’t float.) Dawn doesn’t have that liberty. For our craft, the gimballing of the thruster needs to be carefully coordinated with the orbital motion, as if the motorboat operator needed to compensate for a curving current. This has important implications at Ceres. Sophisticated as it is, Dawn knows its own location in orbit only by virtue of information mission controllers install onboard to predict where it will be at any time. That is based on their best computations of Ceres’ gravity, the planned operation of the ion propulsion system, and many other considerations, but it will never be perfectly accurate. Let’s take a look at two of the reasons.
Ceres, like Vesta, Earth, the moon, Mars, and other planets or planetary-type bodies, has a complex gravity field. The distribution of materials of different densities within the interior creates variations in the strength of the gravitational force, so Dawn will feel a slightly changing tug from Ceres as it travels in orbit. But there is a noteworthy difference between Ceres’ gravity field and the fields of those other worlds: Ceres’ field is unknown. We will have to measure it as we go. The subtle irregularities in gravity as Dawn descends will cause small deflections from the planned trajectory. Our ship will be traversing unknown, choppy waters.
Other phenomena will lead to slight discrepancies as well. The ion propulsion system will be responsible for changing the orbit, so even tiny deviations from the intended thrust eventually may build up to have a significant effect. This is no different from any realistic electrical or mechanical system, which is sure to have imperfections. If you planned a trip in which you would drive 60.0 miles (96.6 kilometers) at 60.0 mph (96.6 kilometers per hour), you could expect to arrive in exactly 60.0 minutes. (No surprises there, as it isn’t exactly rocket science.) But even if you maintained the speedometer as close to 60 as you could, it would not be accurate enough to indicate the true speed. If your actual speed averaged 60.4 mph (97.2 kilometers per hour), you would arrive 24 seconds early. Perhaps that difference wouldn’t matter to you (and if it did, you might consider replacing your car with a spaceship), but such minuscule errors, when compounded by Dawn’s repeated spirals around Ceres, would make a difference in achieving its carefully chosen orbit.
As a result of these and other effects, mission controllers will need to adjust the complex flight plan as Dawn travels from one observational orbit to another. So it will thrust for a few days and then stop to allow navigators to get a new fix on its position. When it points its main antenna to Earth, the Doppler shift of its radio signal will reveal its speed, and the time for radio signals (traveling, as all readers know so well, at the universal limit of the speed of light) to make the round trip will yield its distance. Combining those measurements with other data, mission controllers will update the plan for where to point the thruster at each instant during the next phase of the spiral, check it, double check it, and transmit it to the faraway robot, which will then put it into action. This intensive process will be repeated every few days as Dawn maneuvers to lower orbits.
The flight team succeeded brilliantly in performing this kind of work at Vesta, but they will encounter some differences at Ceres. Sunlight is even weaker in that remote part of the asteroid belt. The giant solar arrays will generate less electrical power for the ion propulsion system, so the whisper-like thrust will be even fainter. In addition, Ceres is more massive than Vesta, so its gravitational hold is stronger. Of course, the team has developed plans to account for these and other differences as they guide Dawn from one orbit to another.
The reward for these particularly challenging parts of the mission will be new perspectives on Ceres. The distant landscapes, barely even hinted at by observations for more than two centuries, will come into sharper and sharper focus as Dawn spirals closer. At each new orbital perch, the explorer will reveal exciting new details, allowing new discoveries and new insights. Everyone who is curious about the cosmos is welcome to join the journey as human ingenuity and curiosity take us far, far from home to an uncharted world.
Dawn is 9.2 million miles (15 million kilometers) from Ceres. It is also 1.61 AU (149 million miles, or 241 million kilometers) from Earth, or 620 times as far as the moon and 1.60 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 27 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT April 30, 2014
P.S. This is the 100th Dawn Journal, so this seems like a good time to end. This will be the last one.
P.P.S. Until next month.
Dear Ardawnt Readers,
Continuing its daring mission to explore some of the last uncharted worlds in the inner solar system, Dawn remains on course and on schedule for its rendezvous with dwarf planet Ceres next year. Silently and patiently streaking through the main asteroid belt between Mars and Jupiter, the ardent adventurer is gradually reshaping its orbit around the Sun with its uniquely efficient ion propulsion system. Vesta, the giant protoplanet it unveiled during its spectacular expedition there in 2011-2012, grows ever more distant.
In December, and January, we saw Dawn's plans for the "approach phase" to Ceres and how it will slip gracefully into orbit under the gentle control of its ion engine. Entering orbit, gratifying and historic though it will be, is only a means to an end. The reason for orbiting its destinations is to have all the time needed to use its suite of sophisticated sensors to scrutinize these alien worlds.
As at Vesta, 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. As the spacecraft will not leave Ceres, there is no value in ascending from its fourth and lowest orbit.) All of the plans for exploring Ceres have been developed to discover as much as possible about this mysterious dwarf planet while husbanding the precious hydrazine propellant, ensuring that Dawn will complete its ambitious mission there regardless of the health of its reaction wheels.
All of its orbits at Ceres will be circular and polar, meaning the spacecraft will pass over the north pole and the south pole, so all latitudes will come within view. Thanks to Ceres's own rotation, all longitudes will be presented to the orbiting observer. To visualize this, think of (or even look at) a common globe of Earth. A ring encircling it represents Dawn's orbital path. If the ring is only over the equator, the spacecraft cannot attain good views of the high northern and southern latitudes. If, instead, the ring goes over both poles, then the combined motion of the globe spinning on its axis and the craft moving along the ring provides an opportunity for complete coverage.
Dawn will orbit in the same direction it did at Vesta, traveling from north to south over the side illuminated by the distant Sun. After flying over the south pole, it will head north, the surface directly beneath it in the dark of night. When it travels over the north pole, the terrain below will come into sunlight and the ship will sail south again.
Dawn's first orbital phase is distinguished not only by providing the first opportunity to conduct intensive observations of Ceres but also by having the least appealing name of any of the Ceres phases. It is known as RC3, or the third "rotation characterization" of the Ceres mission. (RC1 and RC2 will occur during the approach phase, as described in December.)
During RC3 in April 2015, Dawn will have its first opportunity for a global characterization of its new residence in the asteroid belt. It will take pictures and record visible and infrared spectra of the surface, which will help scientists determine its composition. In addition to learning about the appearance and makeup of Ceres, these observations will allow scientists to establish exactly where Ceres's pole points. The axis Earth rotates around, for example, happens to point very near a star that has been correspondingly named Polaris, or the North Star. [Note to editors of local editions: You may change the preceding sentence to describe wherever the axis of your planet points.] We know only roughly where Ceres's pole is from our telescopic studies, but Dawn's measurements in RC3 will yield a much more accurate result. Also, as the spacecraft circles in Ceres's gravitational hold, navigators will measure the strength of the gravitational pull and hence its overall mass.
RC3 will be at an orbital altitude of about 8,400 miles (13,500 kilometers). From there, the dwarf planet will appear eight times larger than the moon as viewed from Earth, or about the size of a soccer ball seen from 10 feet (3.1 meters). At that distance, Dawn will be able to capture the entire disk of Ceres in its pictures. The explorer's camera, designed for mapping unfamiliar extraterrestrial landscapes from orbit, will see details more than 20 times finer than we have now from the Hubble Space Telescope.
Although all instruments will be operated in RC3, the gamma-ray and neutron detector (GRaND) will not be able to detect the faint nuclear emissions from Ceres when it is this far away. Rather, it will measure cosmic radiation. In August we will learn more about how GRaND will measure Ceres's atomic composition when it is closer.
It will take about 15 days to complete a single orbital revolution at this altitude. Meanwhile, Ceres turns on its axis in just over nine hours (more than two and a half times faster than Earth). Dawn's leisurely pace compared to the spinning world beneath it presents a very convenient way to map it. It is almost as if the probe hovers in place, progressing only through a short arc of its orbit as Ceres pirouettes helpfully before it.
When Dawn is on the lit side of Ceres over a latitude of about 43 degrees north, it will point its scientific instruments at the unfamiliar, exotic surface. As Ceres completes one full rotation, the robot will fill its data buffers with as much as they can hold, storing images and spectra. By then, most of the northern hemisphere will have presented itself, and Dawn will have traveled to about 34 degrees north latitude. The spacecraft will then aim its main antenna to Earth and beam its prized findings back for all those who long to know more about the mysteries of the solar system. When Dawn is between 3 degrees north and 6 degrees south latitude, it will perform the same routine, acquiring more photos and spectra as Ceres turns to reveal its equatorial regions. To gain a thorough view of the southern latitudes, it will follow the same strategy as it orbits from 34 degrees south to 43 degrees south.
When Dawn goes over to the dark side, it will still have important measurements to make (as long as Darth Vader does not interfere). While the surface immediately beneath it will be in darkness, part of the limb will be illuminated, displaying a lovely crescent against the blackness of space. Both in the southern hemisphere and in the northern, the spacecraft will collect more pictures and spectra from this unique perspective. Dawn's orbital dance has been carefully choreographed to ensure the sensitive instruments are not pointed too close to the Sun.
Although it is not the primary objective of the measurements, team members are working to determine whether observations from the vantage point of the night side of RC3 might shed more light on the recent fascinating detection of water vapor around Ceres by the Herschel Space Observatory. Whether the water is lofted into space by ice sublimating on the surface or by geysers or cryovolcanoes (“cold volcanoes,” which may be active on this small, frigid world of rock and ice far from the sun) is not yet known. Scientists do not even know whether any water vapor will still be there when Dawn is. Even if it is not, it may be that signs of water will be evident on the surface from other measurements. We will discuss this intriguing possibility more in the December 2014 log.
Dawn’s controllers will take advantage of the flexibility afforded by ion propulsion to guide the spacecraft into whatever part of the RC3 orbit turns out to be most efficient, based on details of the trajectory as it closes in on Ceres. So, for example, if it spirals down to RC3 over the unlit side, its observations of the day hemisphere will first be in the north, then the equator, then the south. But if it arrives in RC3 over the low northern latitudes on the side lit by the sun, it will begin its observations over the equator and then continue in the south. After it flies north over the other side and then returns to the half of Ceres that is in daylight, it will be ready to conclude RC3 by collecting its northern hemisphere data. The flight team has formulated the plan so that the activities can be executed in whatever order is most natural. The schedule will be finalized during the approach phase, and readers may rest assured that the answer will be presented in these logs.
If all goes according to plan, which is never assured when undertaking challenging tasks in a forbidding, distant, alien environment that has never even been visited by a flyby spacecraft for an initial reconnaissance, Dawn will collect in excess of 1,000 pictures and several million spectra in RC3. After that rich bounty is securely on Earth, it will resume ion thrusting to lower its altitude to the next orbit. We will discuss the spiral descent in April and that second observation phase in May.
Dawn’s first inspection of Ceres in RC3 promises both to provide tremendous advancements in our knowledge and whet our appetites for its subsequent examinations. The most massive resident of the main asteroid belt was also the first one to be discovered. Yet for the more than two centuries since then, our glimpses from afar have shown little more than a fuzzy round dot. That distant orb, shining among the stars, has intrigued us for so long. When finally its invitation for an ambassador from Earth is answered next year, the secrets it has held since the dawn of the solar system will begin to be revealed. The rewards for the long and challenging journey will be new insights, new understanding, and new fuel for the fires that burn within everyone who feels the passion to explore.
Dawn is 14 million miles (22 million kilometers) from Ceres. It is also 1.76 AU (163 million miles, or 263 million kilometers) from Earth, or 725 times as far as the moon and 1.77 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 29 minutes to make the round trip.
Dr. Marc D. Rayman
3:00 p.m. PST February 28, 2014
Dawn is continuing its trek through the main asteroid belt between Mars and Jupiter. Leaving behind a blue-green wake of xenon from its ion propulsion system, its sights are set on dwarf planet Ceres ahead. The journey has been long, but the veteran space traveler (and its support team on distant Earth) is making good progress for its rendezvous early next year.
Last month, we had a preview of many of the activities the probe will execute during the three months that culminate in settling into the first observational orbit at Ceres in April 2015. At that orbit, about 8,400 miles (13,500 kilometers) above the alien landscapes of rock and ice, Dawn will begin its intensive investigations. Nevertheless, even during the "approach phase," it will often observe Ceres with its camera and one of its spectrometers to gain a better fix on its trajectory and to perform some preliminary characterizations of the mysterious world prior to initiating its in-depth studies. The discussion in December did not cover the principal activity, however, which is one very familiar not only to the spacecraft but also to readers of these logs. The majority of the time in the approach phase will be devoted to continuing the ion-powered flight. We described this before Vesta, but for those few readers who don't have perfect recall (we know who you are), let's take another look at how this remarkable technology is used to deliver the adventurer to the desired orbit around Ceres.
Thrusting is not necessary for a spacecraft to remain in orbit, just as the moon remains in orbit around Earth and Earth and other planets remain in orbit around the sun without the benefit of propulsion. All but a very few spacecraft spend most of their time in space coasting, following the same orbit over and over unless redirected by a gravitational encounter with another body. In contrast, with its extraordinarily efficient ion propulsion system, Dawn's near-continuous thrusting gradually changes its orbit. Thrusting since December 2007 has propelled Dawn from the orbit in which the Delta rocket deposited it after launch to orbits of still greater distance from the sun. The flight profile was carefully designed to send the craft by Mars in February 2009, so our celestial explorer could appropriate some of the planet's orbital energy for the journey to the more distant asteroid belt, of which it is now a permanent resident. In exchange for Mars raising Dawn's heliocentric orbit, Dawn lowered Mars's orbit, ensuring the solar system's energy account remained balanced.
While spacecraft have flown past a few asteroids in the main belt (although none as large as the gargantuan Vesta or Ceres, the two most massive objects in the belt), no prior mission has ever attempted to orbit one, much less two. For that matter, this is the first mission ever undertaken to orbit any two extraterrestrial destinations. Dawn's exclusive assignment would be quite impossible without its uniquely capable ion propulsion system. But with its light touch on the accelerator, taking nearly four years to travel from Earth past Mars to Vesta, and more than two and a half years from Vesta to Ceres, how will it enter orbit around Ceres? As we review this topic in preparation for Ceres, bear in mind that this is more than just a cool concept or neat notion. This is real. The remarkable adventurer actually accomplished the extraordinary feats at Vesta of getting into and out of orbit using the delicate thrust of its ion engines.
Whether conventional spacecraft propulsion or ion propulsion is employed, entering orbit requires accompanying the destination on its own orbit around the sun. This intriguing challenge was addressed in part in February 2007. In February 2013, we considered another aspect of what is involved in climbing the solar system hill, with the sun at the bottom, Earth partway up, and the asteroid belt even higher. We saw that Dawn needs to ascend that hill, but it is not sufficient simply to reach the elevation of each target nor even to travel at the same speed as each target; the explorer also needs to travel in the same direction. Probes that leave Earth to orbit other solar system bodies traverse outward from (or inward toward) the sun, but then need to turn in order to move along with the body they will orbit, and that is difficult.
Those of you who have traveled around the solar system before are familiar with the routine of dropping into orbit. The spacecraft approaches its destination at very high velocity and fires its powerful engine for some minutes or perhaps even about an hour, by the end of which it is traveling slowly enough that the planet's gravity can hold it in orbit and carry it around the sun. These exciting events may range from around 1,300 to 3,400 mph (0.6 to 1.5 kilometers per second). With ten thousand times less thrust than a typical propulsion system on an interplanetary spacecraft, Dawn could never accomplish such a rapid maneuver. As it turns out, however, it doesn't have to.
Dawn's method of getting into orbit is quite different, and the key is expressed in an attribute of ion propulsion that has been referred to 63 times (trust or verify; it's your choice) before in these logs: it is gentle. (This example shows just how gentle the acceleration is.) With the gradual trajectory modifications inherent in ion propulsion, sharp changes in direction and speed are replaced by smooth, gentle curves. The thrust profiles for Dawn's long interplanetary flights are devoted to the gradual reshaping of its orbit around the sun so that by the time it is in the vicinity of its target, its orbit is nearly the same as that of the target. Rather than hurtling toward Vesta or Ceres, Dawn approaches with grace and elegance. Only a small trajectory adjustment is needed to let its new partner's gravity capture it, so even that gentle ion thrust will be quite sufficient to let the craft slip into orbit. With only a nudge, it transitions from its large, slow spiral away from the sun to an inward spiral centered around its new gravitational master.
To get into orbit, a spacecraft has to match speed, direction and location with its target. A mission with conventional propulsion first gets to the location and then, using the planet's gravity and its own fuel-guzzling propulsion system, very rapidly achieves the required speed and direction. By spiraling outward from the sun, first to the orbit of Vesta and now to Ceres, Dawn works on its speed, direction and location all at the same time, so they all gradually reach the needed values at just the right time.
To illustrate this facet of the difference between how the different systems are applied to arrive in orbit, let's imagine you want to drive your car next to another traveling west at 60 mph (100 kilometers per hour). The analogy with the conventional technology would be similar to speeding north toward an intersection where you know the other car will be. You arrive there at the same time and then execute a screeching, whiplash-inducing left turn at the last moment using the brakes, steering wheel, accelerator and adrenaline. When you drive an ion propelled car (with 10 times higher fuel efficiency), you take an entirely different path from the start, one more like a long, curving entrance ramp to a highway. As you enter the ramp, you slowly (perhaps even gently) build speed. You approach the highway gradually, and by the time you have reached the far end of the ramp, your car is traveling at the same speed and in the same direction as the other car. Of course, to ensure you are there when the other car is, the timing is very different from the first method, but the sophisticated techniques of orbital navigation are up to the task.
In March or April 2015, as the probe follows its approach trajectory to Ceres, their paths will be so similar they will be racing around the sun at nearly the same speed (38,500 mph, or 17.2 kilometers per second) and in the same direction. But what matters is their relative velocity. When at a range of 30,000 miles (48,000 kilometers), the spacecraft will be closing in on its destination at less than 85 mph (37 meters per second). The combination of distance and velocity will allow Ceres to take Dawn in its grasp. The spacecraft will not even notice the difference, but it will be in orbit around its second and final celestial target, even as it continues ion thrusting to spiral to its first planned orbital altitude two and a half weeks later.
Unlike missions that use conventional chemical propulsion, there is no sudden change on the spacecraft and no nail-biting on Earth. If you were in space watching the action, you probably would be hungry, cold and hypoxic, but you would not notice anything unusual about the scene as Ceres smoothly and tenderly takes Dawn into an invisible gravitational embrace.
If instead of being in deep space, you had been in Dawn mission control watching the action when the spacecraft entered orbit around Vesta in July 2011 you would have been in the dark and all alone (until JPL Security arrived to escort you away). Your correspondent was out dancing, and other members of the team were engaged in activities similarly unrelated to controlling a probe hundreds of times farther away than the moon. There was no need to have radio contact with the reliable spaceship. It had already been thrusting for 70 percent of its time in space, so it was performing a very familiar function. It should be no different at Ceres (although the dance program may not be exactly the same). When Dawn enters orbit, no one is tense or anxious; rather, all the drama is in the promise of the spectacular discoveries in exploring uncharted worlds, the rewards of new knowledge, and the thrill of knowing that humankind is reaching far, far from home in a grand effort to know the cosmos.
Dawn is 16 million miles (26 million kilometers) from Ceres. It is also 2.05 AU (191 million miles, or 307 million kilometers) from Earth, or 855 times as far as the moon and 2.08 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 34 minutes to make the round trip.
Dr. Marc D. Rayman
2:00 p.m. PST January 31, 2014
Now more than halfway through its journey from protoplanet Vesta to dwarf planet Ceres, Dawn is continuing to use its advanced ion propulsion system to reshape its orbit around the sun. Now that the ship is closer to the uncharted shores ahead than the lands it unveiled astern, we will begin looking at the plans for exploring another alien world. In seven logs from now through August, we will discuss how the veteran adventurer will accomplish its exciting mission at Ceres. By the time it arrives early in 2015 at the largest object between Mars and Jupiter, readers will be ready to share not only in the drama of discovery but also in the thrill of an ambitious undertaking far, far from Earth.
Mission planners separate this deep-space expedition into phases. Following the "launch phase" was the 80-day "checkout phase". The "interplanetary cruise phase" is the longest. It began on December 17, 2007, and continued to the "Vesta phase," which extended from May 3, 2011, to Sept. 4, 2012. We are back in the interplanetary cruise phase again and will be until the "Ceres phase" begins in 2015. (Other phases may occur simultaneously with those phases, such as the "oh man, this is so cool phase," the "we should devise a clever name for this phase phase," and the "lunch phase.") Because the tasks at Vesta and Ceres are so complex and diverse, they are further divided into sub-phases. The phases at Ceres will be very similar to those at Vesta, even though the two bodies are entirely different.
In this log, we will describe the Ceres "approach phase." The objectives of approach are to get the explorer into orbit and to attain a preliminary look at the mysterious orb, both to satisfy our eagerness for a glimpse of a new and exotic world and to obtain data that will be helpful in refining details of the subsequent in-depth investigations. The phase will start in January 2015 when Dawn is about 400,000 miles (640,000 kilometers) from Ceres. It will conclude in April when the spacecraft has completed the ion thrusting necessary to maneuver into the first orbit from which it will conduct intensive observations, at an altitude of about 8,400 miles (13,500 kilometers). For a reason to be revealed below, that orbit is known by the catchy cognomen RC3.
(Previews for the Vesta approach phase were presented in March 2010 and May 2011, and the accounts of its actual execution are in logs from June, July, and August 2011. Future space historians should note that the differing phase boundaries at Vesta are no more than a matter of semantics. At Vesta, RC3 was described as being part of the approach phase. For Ceres, RC3 is its own distinct phase. The reasons for the difference in terminology are not only unimportant, they aren't even interesting.)
The tremendous maneuverability provided by Dawn's uniquely capable ion propulsion system means that the exact dates for events in the approach phase likely will change between now and then. So for those of you in 2015 following a link back to this log to see what the approach plan has been, we offer both the reminder that the estimated dates here might shift by a week or so and a welcome as you visit us here in the past. We look forward to meeting you (or even being you) when we arrive in the future.
Most of the approach phase will be devoted to ion thrusting, making the final adjustments to Dawn's orbit around the sun so that Ceres's gravity will gently take hold of the emissary from distant Earth. Next month we will explain more about the unusual nature of the gradual entry into orbit, which will occur on about March 25, 2015.
Starting in early February 2015, Dawn will suspend thrusting occasionally to point its camera at Ceres. The first time will be on Feb. 2, when they are 260,000 miles (420,000 kilometers) apart. To the camera's eye, designed principally for mapping from a close orbit and not for long-range observations, Ceres will appear quite small, only about 24 pixels across. But these pictures of a fuzzy little patch will be invaluable for our celestial navigators. Such "optical navigation" images will show the location of Ceres with respect to background stars, thereby helping to pin down where it and the approaching robot are relative to each other. This provides a powerful enhancement to the navigation, which generally relies on radio signals exchanged between Dawn and Earth. Each of the 10 times Dawn observes Ceres during the approach phase will help navigators refine the probe's course, so they can update the ion thrust profile to pilot the ship smoothly to its intended orbit.
Whenever the spacecraft stops to acquire images with the camera, it also will train the visible and infrared mapping spectrometer on Ceres. These early measurements will be helpful for finalizing the instrument parameters to be used for the extensive observations at closer range in subsequent mission phases.
Dawn obtained images more often during the Vesta approach phase than it will on approach to Ceres, and the reason is simple. It has lost two of its four reaction wheels, devices used to help turn or stabilize the craft in the zero-gravity, frictionless conditions of spaceflight. (In full disclosure, the units aren't actually lost. We know precisely where they are. But given that they stopped functioning, they might as well be elsewhere in the universe; they don't do Dawn any good.) Dawn's hominin colleagues at JPL, along with excellent support from Orbital Sciences Corporation, have applied their remarkable creativity, tenacity, and technical acumen to devise a plan that should allow all the original objectives of exploring Ceres to be met regardless of the health of the wheels. One of the many methods that contributed to this surprising resilience was a substantial reduction in the number of turns during all remaining phases of the mission, thus conserving the precious hydrazine propellant used by the small jets of the reaction control system.
When Dawn next peers at Ceres, nine days after the first time, it will be around 180,000 miles (290,000 kilometers) away, and the pictures will be marginally better than the sharpest views ever captured by the Hubble Space Telescope. By the third optical navigation session, on Feb. 21, Ceres will show noticeably more detail.
At the end of February, Dawn will take images and spectra throughout a complete Ceres rotation of just over nine hours, or one Cerean day. During that period, while about 100,000 miles (160,000 kilometers) distant, Dawn's position will not change significantly, so it will be almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath its watchful eye. Dawn will see most of the surface with a resolution twice as good as what has been achieved with Hubble. (At that point in the curving approach trajectory, the probe will be south of Ceres's equator, so it will not be able to see the high northern latitudes.) This first "rotation characterization," or RC1, not only provides the first (near-complete) look at the surface, but it may also suggest to insightful readers what will occur during the RC3 orbit phase.
There will be six more imaging sessions before the end of the approach phase, with Ceres growing larger in the camera's view each time. When the second complete rotation characterization, RC2, is conducted on March 16, the resolution will be four times better than Hubble's pictures. The last photos, to be collected on March 24, will reveal features seven times smaller than could be discerned with the powerful space observatory.
The approach imaging sessions will be used to accomplish even more than navigating, providing initial characterizations of the mysterious world, and whetting our appetites for more. Six of the opportunities also will include searches for moons of Ceres. Astronomers have not found moons of this dwarf planet in previous attempts, but Dawn's unique vantage point would allow it to discover smaller ones than would have been detectable in previous attempts.
When the approach phase ends, Dawn will be circling its new home, held in orbit by the massive body's gravitational grip and ready to begin more detailed studies. By then, however, the pictures and other data it will have returned will already have taught Earthlings a great deal about that enigmatic place. Ceres has been observed from Earth for more than two centuries, having first been spotted on January 1, 1801, but it has never appeared as much more than an indistinct blob amidst the stars. Soon a probe dispatched by the insatiably curious creatures on that faraway planet will take up residence there to uncover some of the secrets it has held since the dawn of the solar system. We don't have long to wait!
Dawn is 20 million miles (32 million kilometers) from Vesta and 19 million miles (31 million kilometers) from Ceres. It is also 2.42 AU (225 million miles, or 362 million kilometers) from Earth, or 1,015 times as far as the moon and 2.46 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 40 minutes to make the round trip.
Dr. Marc Rayman
3:00 p.m. PST December 31, 2013
This blog entry from John Grotzinger, the project scientist for NASA's Curiosity Mars rover, was originally prepared for use by the Planetary Society and explains the importance of some of the rover's findings.
It was fun for me to catch up with Emily Lakdawalla of the Planetary Society at the American Geophysical Union meeting, and to discuss our new Curiosity mission results. They focus on the discovery of an ancient habitable environment; we are now transitioning to the focused search for organic carbon. What's great about Emily's blog is that with her strong science background she is able to take complex mission results and translate these into something that can reach a broader and more diverse audience. I'll try to do the same here.
Since we first reported our results on March 12, 2013, from drilling in Yellowknife Bay it has been my experience that lots of people ask questions about how the Curiosity mission, and future missions, will forge ahead to begin with looking for evidence of past life on Mars. There is nothing simple or straightforward about looking for life, so I was pleased to have the chance to address some of the questions and challenges that we find ourselves most frequently discussing with friends and colleagues. The Planetary Society's blog is an ideal place to take the time to delve into this.
I also need to state at the outset that what you'll read below is my opinion, as Curiosity science team member and Earth geobiologist, and not necessarily as its Project Scientist. And I have only worked on Mars science for a decade. However, I can say that many other members of the Curiosity team share this opinion, generated from their own experiences similar to mine, and it was easy for us to adopt these ideas to apply to our future mission. To a large extent, this opinion is shaped by our experience of having spent decades trying to explore the early record of life on Earth. As veterans of the Mars Exploration Rover and Curiosity missions, we have learned that while Mars has significant differences from Earth, it also has some surprising similarities that could be important in the search for evidence of ancient Martian life - a "paleobiosphere," if you will. The bottom line is that even for Earth, a planet that teems with life, the search for ancient life is always difficult and often frustrating. It takes a while to succeed. I'll try to explain why later on.
So here goes....
The Dec. 9, 2013, publication of the Curiosity team's six papers in Science provides the basis for understanding a potentially habitable environment on ancient Mars. The search for habitable environments motivated building the rover, and to that end the Curiosity mission has accomplished its principal objective. This naturally leads to the questions of what's next, and how we go about exploring for organic carbon?
To better understand where we're coming from, it helps to break down these questions and analyze them separately. With future advocacy of missions to Mars so uncertain, and with difficult-to-grasp mission objectives located between "the search for water" (everyone got that) and "the search for life" (everyone wants NASA to get on with it), the "search for habitability" and the "search for carbon" are important intermediate steps. By focusing on them scientists can identify specific materials to study with more sophisticated future missions and instruments, or to select for sample return, or to be the target of life detection experiments.
Note: You can get access to all six of these Science papers here or here. The latter site also has the papers we published back in September. Science has a policy that allows us to post a "referrer link" to our home websites. This redirects the query to AAAS, where the paper can be downloaded without cost.
Let's start with "habitability." We reported the discovery of an ancient lake, and one that formed clay minerals. The presence of clays represents more benign environmental conditions than the acid sulfates found by Spirit and Opportunity. However, clays are not the only thing needed to demonstrate habitability. The bar is high: In brief, a mission needs to demonstrate the presence of water, key elements regarded as the building blocks of life (including carbon), and a source of energy. And you need to find them all together, and at the same instant in geologic time. In turn, each one of these must be characterized further to qualify an environment as having been habitable. Finally, it's never black and white; understanding habitability is part of a broad continuum of environmental assessment, which is why orbiters and earlier rovers and landers are important assets in this process as well.
It is also important to define what group of organisms is being imagined to have inhabited the environments - their requirements will vary. Single-celled microorganisms are a great place to start based on our understanding of the early evolution of life on Earth, which was dominated by microbes for at least the first two billion years of the planet's history. More specifically, the Curiosity team has been focusing on the conditions of habitability relevant to "chemolithotrophs," a group of microbes that feeds on chemical energy available in rocks.
The water of a habitable environment should be relatively fresh, or at least not contain so much salt that the relative abundance of water is so low (what chemists call "water activity") that the osmotic pressure on cells would cause them to collapse. My favorite analog here is honey. Yes, it's an aqueous environment but no, it's not habitable: The sugar content is so high that microbes can't live in it. This is why honey doesn't spoil when not refrigerated. Salt serves the same role as sugar; too much salt inhibits life. Acidity is also important, although microbes have been shown to tolerate an extraordinary range of pH, including the very lowest values encountered in natural environments on Earth. However, more moderate pH favors a greater diversity of microorganisms, and thus more options to explore for emerging life forms. Finally, the water needs to last a long time on the surface; the longer, the better. A flow of water emerging on the surface of Mars from an underground source and boiling off in the presence of Mars' modern low atmospheric pressure is not a good scenario for life. A stable source, such as a very ancient lake, with associated streams, and water flowing through the ground beneath it, is much better. We envision for the lake/stream/groundwater system that Curiosity discovered at Yellowknife Bay that the water could have existed for millions of years potentially. But even shorter periods are viable - the qualitative point here is that the rocks at Yellowknife Bay record more than a one-time event.
Key building blocks of life.
A conventional list of key elements for life will include "CHNOPS" - carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Previous orbiter and landed missions have provided ample evidence for H, O, and S via observations of sulfate and clay minerals, and P was measured by earlier rovers and landers. Curiosity has done the same. The tricky stuff is N and C and, along with P, they must all be "bioavailable," which means to say they cannot be bound tightly within mineral structures that water and microbial chemical processes could not unlock. Ideally, we are looking for concentrated nitrogen- and phosphorous-bearing sedimentary rocks that would prove these elements were actually dissolved in the past water at some point, and therefore could have been available to enable microorganism metabolism. But in the interim Curiosity has been able to measure N as a volatile compound via pyrolysis (heating up rock powder in the SAM instrument), and P is observed in APXS data. We feel confident that N was available in the ancient environment, however we must infer that P was as well. Two of the Science papers, Grotzinger et al. and Ming et al., discuss this further.
Carbon is the elephant in the room. We'll discuss organic carbon further below, but here it's important to make one very important point: Organic carbon in rocks is not a hard-line requirement for habitability, since chemoautotrophs can make the organics they need to build cellular structures from metabolizing carbon dioxide (CO2). These organisms take up inorganic carbon as CO2 dissolved in water to build cellular structures. Organic carbon could serve as fuel if it was first oxidized to CO2, or could be used directly for biomass, or could be part of waste products. As applied to Mars it is therefore attractive to appeal directly to CO2, presumed to have been abundant in its early atmosphere. Curiosity does indeed see substantial carbon generated from the ancient lake deposits we drilled. The CO2 that was measured is consistent with some small amount of mineral carbon present in those lake mudstones. These minerals would represent CO2 in the ancient aqueous environment. Furthermore, it is possible that Martian organic sources have been mixed with inorganic sources of carbon in the mudstone; however, any organic contributions from the mudstone would be mixed with Earth-derived sources during analysis (see Ming et al. paper).
All organisms also require fuel to live and reproduce. Here it is essential to know which kind of microorganism we're talking about, since there are myriad ways for them to harvest energy from the environment. Chemolithotrophs derive energy from chemical reactions, for example by oxidizing reduced chemical species like hydrogen sulfide or ferrous iron. That's why Curiosity's discovery of pyrite, pyrrhotite, and magnetite are so important (see Vaniman et al. and Ming et al. papers). They are all more chemically reduced than their counterparts discovered during earlier missions to Mars (for example, sulfate and hematite). Chemolithotrophic microbes, if they had been present on Mars at the time of this ancient environment, would have been able to tap the energy in these reduced chemicals (such as hydrogen sulfide, or reduced iron) to fuel their metabolism. If you are interested in more detail regarding these kinds of microbial processes I can strongly recommend Nealson and Conrad (2000) for a very readable summary of the subject.
The next section describes where I think we're headed in the future. We'll continue to explore for aqueous, habitable environments at Mt. Sharp, and along the way to Mt. Sharp. And if we discover any, they will serve as the starting point for seeing if any organic carbon is preserved and, if so, how it became preserved.
Now there's a ten-dollar word. Taphonomy is the term paleontologists use to describe how organisms become fossilized. It deals with the processes of preservation. Investigations of organic compounds fit neatly in that category. We do not have to presume that organic compounds are of biologic origin. In fact, in studies of the Earth's early record of life, we must also presume that any organic materials we find may be of inorganic origin - they may have nothing to do with biology. Scientific research will aim to demonstrate as conclusively as possible that the materials of interest were biogenic in origin. For Earth rocks that are billions of years old, it's rare to find a truly compelling claim of ancient biogenic carbon. Here's why.
On a planet that teems with life, one would presume these discoveries would be ordinary. But they aren't, and that's why fossils of almost any type, including organic compounds (so-called "chemofossils"), are so cool - it's because they are rare. That's also why taphonomy emerged as an important field of study. We need to understand how biologic materials become recorded in Earth's rock record. It's important in understanding modes of organism decomposition, to interpret ancient environmental conditions, and in reconstructing ancient ecosystems. But there also is one other reason that is particularly relevant for early Earth, and even more so for Mars: If you want to find something significant, you have to know where to look.
To explore for organics on Mars, three things have to go right. First, you need to have an enrichment of organics in the primary environment where organic molecules accumulate, which is large enough so that your instrument could detect them. Second, the organics have to survive the degrading effects associated with the conversion of sediment to rock. Third, they must survive further degradation caused by exposure of rock to cosmic radiation at Mars' surface. Even if organics were once present in Martian sediment, conversion to rock and exposure to cosmic radiation may degrade the organics to the point where they can't be detected.
Organics degrade in two main ways. The first is that during the conversion of sediment to rock, organics may be chemically altered. This generally happens when layers of sediment are deposited one on top of the other, burying earlier-deposited layers. As this happens, the buried sediment is exposed to fluids that drive lithification - the process that converts sediment to rock. Sediments get turned into rocks when water circulates through their pores, precipitating minerals along the linings of the pores. After a while the sediment will no longer feel squishy and it becomes rigid - lithified.
During the process of lithification, a large amount of water may circulate through the rock. It can amount to hundreds, if not thousands, of times the volume of the pore space within the rock. With so much water passing through, often carrying other chemicals with it, any organics that come into contact with the water may be broken down. Chemically, this occurs because organics are reduced substances and many chemicals dissolved in water are oxidizing. Those two chemical states don't sit well together, and this tends to drive chemical reactions. Simply put, organics could be broken down to the point where the originally organic carbon is converted into inorganic carbon dioxide, a gas that can easily escape the lithifying sediment. Water on Mars may be a good thing for habitability but it can, paradoxically, negatively affect the preservation of organics.
Now, if any organics manage to escape this first step in degradation, then they are still subject to further degradation when the rock is exhumed and exposed to the surface of Mars. There it will be bombarded by cosmic radiation. I won't go into the details here, but that is also bad news for organics because the radiation tends to break apart organic molecules through a process called ionization. The upper few meters of a rock unit is the most susceptible; below that the radiation effect rapidly dies away. Given enough time the organics could be significantly degraded.
The Hassler et al. paper just published in Science reports that the surface radiation dose measured by Curiosity could, in 650 million years, reduce the concentration of small organic molecules, such as amino acids, by a factor of 1000, all other factors being equal. That's a big effect - and that's why we were so excited as a team when we figured out how to measure the cosmogenic exposure age of rocks we drilled (see Emily's blog and the Farley et al. paper). This gives us a dependable way to preferentially explore for those rocks that have been exposed for the shortest period of time. Furthermore, it is unlikely that organics would be completely eliminated due to radiation effects and the proof of this is that a certain class of meteorites - the carbonaceous chondrites - have been exposed to radiation in space for billions of years and yet still retain complex organics. This provides hope that at least some types of organics should be preserved on Mars.
Being able to account for the radiation history of rocks that Curiosity might drill is a very big step forward for us in the search for organic molecules. It is a big step forward in learning how to explore for past life on Mars (if it ever existed there). Now we have the right tools to guide the search for rocks that might make the best targets for drilling. Coupled with our other instruments that measure the chemistry and mineralogy of the rocks, to help select those that might have seen the least alteration of organics during burial, we have a pretty good sense of what we need to do next. That's because we have been through this before on Earth.
Over the years Emily has written many blogs dedicated to the discovery of interesting minerals on Mars. There are many reasons for this, but I'll suggest one more that may grow in importance in years to come.
Believe it or not, the story starts with none other than Charles Darwin. In pondering the seemingly instantaneous appearance of fossils representing complex and highly differentiated organisms in Cambrian-age rocks (about 500 million years ago), Darwin recognized this as a major challenge to his view of evolution. He explained the sudden appearance of fossils in the record by postulating that Cambrian organisms with no known antecedents could be explained by "record failure" - for some unknown reason, older rocks simply didn't record the emergence and evolution of life's beginning. Conditions weren't suitable to preserve organisms as fossils.
Most of that story goes on in the direction of evolutionary biology, and we'll skip that, rather focusing instead on learning more about taphonomy. What is important for Mars was the discovery of minerals that could preserve evidence of early microorganisms on Earth. (For a good read on Precambrian paleobiology, try Andy Knoll's "Life on a Young Planet: The First Three Billion Years of Evolution on Earth.")
We now know that pre-Cambrian time represents about 4 billion years of Earth's history, compared to the 540 million years represented by Cambrian and younger rocks that Darwin had studied. (See Emily's blog on the Geologic time scale.) We also know now that the oldest fossil microbes on Earth are about 3.5 billion years old, and that in between there is a compelling, but very sparse record of the fossil organisms that Darwin had anticipated. However, what's even more remarkable is that it took 100 years to prove this. And this was with hundreds, maybe thousands, of geologists scouring the far corners of the Earth looking for evidence.
The big breakthrough came in 1954 with the discovery of the "Gunflint microbiota" along the shores of Lake Superior in southern Canada. A University of Wisconsin economic geologist, Stanley Tyler, discovered microscopic threads of what we now understand to be fossil bacteria in a kind of rock called "chert". Chert is a microcrystalline material formed of the mineral quartz, or silicon dioxide, which precipitates very early in waters that contain microbial colonies. It forms so early that it turns the sediment almost instantly into rock, and any microbes become entombed in a mineral so stable it resists all subsequent exposure to water, and the oxidizing chemicals dissolved in water, for billions of years.
As it turned out, this was the Rosetta stone that helped decipher the code to the field of pre-Cambrian paleontology. It took almost 10 years for the discovery to be fully appreciated (the initial report in Science was viewed with much skepticism), but once it was confirmed, in the mid-1960s, the field exploded. Once geologists and paleontologists knew what to search for, they were off to the races. Since that initial discovery, other magic minerals have been found that preserve ancient microbes, sometimes with spectacular fidelity. But chert is still the mineral of choice, and I never pass by it in the field without collecting some.
We don't know yet what magic minerals exist on Mars that could have trapped and preserved organics. Clays and sulfates hold promise, and that's why we're so interested in them. Silica, perhaps similar to terrestrial chert, has been observed from orbit at a few places on Mars, and in Spirit rover data from Gusev crater. The great thing about Gale crater as a landing site is that we have so many choices in this trial-and-error game of locating a mineral that can preserve organic carbon.
The figure below provides some sense of the impact of this discovery. It is modified from a similar figure published in a very nice summary by Bill Schopf, a Professor of Paleontology at UCLA. Bill also was a very early participant in this race for discovery and has made a number of very significant contributions to the field.
In studying Mars, the importance of this lesson in the search for life preserved in the ancient rock record of Earth cannot be overstated. Curiosity's discovery of a very Earth-like ancient habitable environment underscores this point. With only one or two rovers every decade, we need to have a search paradigm: something to guide our exploration, something to explain our inevitable failures. If life ever evolved on Mars, we need to have a strategy to find it. That strategy begins with the search for organics, and regardless of their origin - abiotic or biotic, indigenous to Mars or not - they are important tracers for something more significant. Curiosity cannot see microfossils, but it can detect organic compounds. And just as with microfossils on Earth, we first have to learn where organics on Mars might be preserved. So that's what we're going to try and do.
In August 1964, this test fixture was used by the Spacecraft Design Section at NASA's Jet Propulsion Laboratory in Pasadena, Calif., to study spin stabilization of spacecraft - in this case, Rangers 8 and 9 (part of the Ranger Block 3 design). Many spacecraft had used spin stability for attitude control during acceleration or thrust, and it was found that a slower spin provided better stability for the coasting phase.
One method of decreasing the spin of a spacecraft, or de-spinning, was the deployment of yo-yo devices. Weights were attached to rigid or stretch cords, then released while the fixture was spinning. The cords would unwind, like the arms of a figure skater extending to slow a spin, and then the cords were released. In this photo, the cables and weights can be seen, attached to the outside of the white circle. The test fixture is surrounded by what appear to be bales of paper and trash to absorb the impact of the weights when they were released from the spinning test fixture.
Gliding smoothly through the main asteroid belt between Mars and Jupiter, Dawn continues to make good progress on its ambitious mission of exploration. It is patiently but persistently pursuing Ceres, the second destination on its interplanetary itinerary.
Protoplanets Ceres and Vesta, the two most massive residents of the asteroid belt, were discovered at the beginning of the 19th century, and they have tantalized astronomers and others curious about the nature of the universe ever since. (Indeed, Ceres was the first dwarf planet discovered, having been found 129 years before Pluto.) They have waited patiently for a visitor from Earth since the dawn of the solar system. Dawn's objective is to turn these uncharted orbs from tiny smudges of light amidst the stars into richly detailed places. It succeeded spectacularly at Vesta in 2011 - 2012, and it remains on course and on schedule for doing so at Ceres in 2015.
Next month, the adventurer will pass an invisible milestone on its celestial journey. On Dec. 27, it will be equidistant from these behemoths of the asteroid belt as all three follow their own independent heliocentric paths. The spacecraft will be 0.21 AU (19.4 million miles, or 31.3 million kilometers) away from each world, the one already visited and the one yet to be reached. And as the indefatigable ship sails on the cosmic seas with its sights set on Ceres, our anticipation for glimpsing the alien landscape ahead grows and grows, while the now-familiar scenery of Vesta shrinks into the distance, fading over the horizon.
The next day, Dawn will be equidistant from two other solar system bodies, both of which have been known (to our human readers, at least) for somewhat longer than Ceres and Vesta have. On Dec. 28, our celestial ambassador will be 2.46 AU (229 million miles, or 368 million kilometers) from Earth and the sun. (We cannot specify in which century either of them was discovered.)
Its complex route through the solar system has already taken the spacecraft farther from each of these bodies before. In the current phase of the mission, it is receding from the sun again, climbing the solar system hill from Vesta to Ceres. (It approached the sun in late 2012 and 2013 as part of the strategy for arriving at Ceres's orbit when Ceres itself was there.) Having attained its greatest distance from Earth for the year in August, the spacecraft is temporarily getting closer to its planet of origin. (More precisely, as we discussed then, Earth is currently moving toward Dawn, because Earth travels faster in its solar orbit than Dawn does in its much more remote orbit.)
Dawn will reach two more impressive milestones in December, although neither pertains to its location. Soon the craft will surpass four years of ion thrust. While most spacecraft rely on conventional propulsion and hence coast most of the time (just as planets, moons, and asteroids do), Dawn's mission would be impossible if it did that. In order to orbit and explore two distant destinations, the only terrestrial probe ever to attempt such a feat, it must accomplish a great deal of maneuvering. It spends the majority of its time using its uniquely efficient and capable ion propulsion system, constantly putting a gentle pressure on its trajectory to gradually reshape it. Although the spacecraft has already accumulated far more time in powered flight than any other mission, it still has a great deal more ahead.
And in December, that thrusting will push the craft's speedometer past an extraordinary 20,000 mph (8.94 kilometers per second). (As we have seen in many previous logs, such as this one, this measure of the speed does not represent the actual spacecraft velocity. Nevertheless, it is a useful metric that avoids the complicating effects of orbital mechanics.) That is more than twice the previous record for propulsive velocity change set by Deep Space 1, the first interplanetary mission to use ion propulsion.
Dawn spends most of its time emitting a lovely blue-green beam of high-velocity xenon ions to propel itself. As foretold in the prophecy commonly known as the October log, however, we are now in one of just two periods of the long mission in which coasting is better for the trajectory than thrusting. Mission controllers took advantage of this time to instruct the robot to perform some special activities that would have been less convenient during routine ion thrusting. The reliable ship completed all of them flawlessly.
For more than 27 hours on Nov. 12 and 13, Dawn operated in a mode that had not even been conceived of when it was designed and built. It controlled its orientation in the frictionless, zero-gravity conditions of spaceflight using a scheme that was developed long after it left Earth. This "hybrid" control method operated perfectly, validating the extensive work engineers have invested in it and verifying its readiness for use at Ceres.
When it embarked on its bold journey more than six years ago, the ship was outfitted with four reaction wheels. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can turn or stabilize itself. It generally used three at a time, with a fourth kept in reserve. For such a long and complex expedition, extending to well over one million times farther from Earth than the International Space Station, backup systems are essential.
One of the wheels experienced increased friction in June 2010, but the mission continued with the other three. A second met the same fate in August 2012, as Dawn was climbing away from Vesta. Other spacecraft have encountered similar issues with their reaction wheels as well, and the consequences can be dire.
We have described the operations team's swift and productive responses to the regrettable behavior of the reaction wheels in a number of logs (see, as one example, here). As soon as the first wheel faltered, JPL and Orbital Sciences Corporation began working on a method to operate with fewer than three in case another one had difficulty. They developed software to operate in a hybrid mode of two wheels plus the small hydrazine-powered jets of the reaction control system and installed it in the craft's main computer in April 2011 so it would be available at Vesta if needed.
Given the problems with reaction wheels on Dawn and other spacecraft, engineers do not have high confidence that the two remaining units will operate for long (although it certainly is possible they will). Thanks to their remarkable ingenuity and resourcefulness, the team has devised a detailed plan that should allow Dawn to complete its extraordinary mission using only the hydrazine thrusters, achieving all of its objectives in exploring Ceres regardless of the condition of its wheels. (Note that it is not even obvious that doing so is possible, but then again, it isn't obvious that sending probes so far from our home planet is possible either. Part of the thrill of a solar system adventure is overcoming the extremely daunting challenges.) So now, hybrid control would provide an enhancement, extending the supply of precious hydrazine propellant and giving the spacecraft the opportunity to operate even longer at Ceres than it would without the two functioning wheels. When the hydrazine is exhausted, the mission will conclude.
Dawn will use hybrid control only in its lowest altitude orbits at Ceres, the final phase of the mission. (Beginning in December and continuing in 2014, we will describe all phases of the Ceres plan in detail.) Hybrid control will be called upon to perform three kinds of tasks for the spacecraft: train the suite of sophisticated sensors at the mysterious world beneath it, point the main antenna to distant Earth to transmit its findings and receive updated instructions, and rotate from one orientation to another. The innovative system has now unerringly demonstrated its capability to accomplish all three by executing exactly those functions earlier this month.
The confirmation that hybrid control works as intended is not the only task Dawn is carrying out during this coasting period. All of its scientific instruments (including even the backup camera) are being powered on and given thorough health checks, verifying that they remain fully functional and ready to reveal Ceres's secrets. Engineers also conducted some tests with the ion engine that has operated the longest of the three to confirm expectations of how it will perform at Ceres.
On Dec. 9, Dawn's four-week coast period will end. Once again it will turn to point an ion engine in the direction needed to push forward to its rendezvous with the distant and exotic world ahead. As the probe nears and then passes the halfway point on its remarkable journey from Vesta to Ceres, it is pulled by forces even more powerful than ion propulsion: the attraction of discovery, the lure of the unknown, and the draw of tremendous new insights and profound new understandings to be gained in a daring adventure far, far from home.
Dawn is 18 million miles (29 million kilometers) from Vesta and 22 million miles (35 million kilometers) from Ceres. It is also 2.78 AU (258 million miles, or 415 million kilometers) from Earth, or 1,125 times as far as the moon and 2.82 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 p.m. PST November 30, 2013