Ion propulsion is not a source of power for this interplanetary spaceship. Rather, the craft needs a great deal of power to operate its ion propulsion system and all other systems. It needs so much that...

We crave power!!

The ion propulsion system is power-hungry. The process of ionizing xenon and then accelerating it to high velocity consumes a significant amount of electrical power, all of which is provided by the spacecraft's huge solar arrays. With these two wings and its ion tail, Dawn resembles a celestial dragonfly. But this extraterrestrial odonate is a giant, with a wingspan of 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest tip-to-tip length of any probe NASA had ever dispatched on an interplanetary voyage. (Some such spacecraft have had flexible wire-like antennas that reach to greater lengths.) The large area of solar cells is needed to capture feeble sunlight in the remote asteroid belt to meet all of the electrical needs. Each solar array wing is the width of a singles tennis court, and the entire structure would extend from a pitcher's mound to home plate on a professional baseball field, although Dawn is engaged in activities considerably more inspiring and rewarding than competitive sports.

To sail the ship to its intended destination, navigators plot a complex course on the solar system sea. The thrust delivered by the ion engine depends on the power level; higher power translates into higher (but still ever so gentle) thrust. The farther Dawn is from the luminous sun, the less power is available, so the thrust is lower. Therefore, to keep it on its itinerary, mission planners need to know the thrust at all times in the future. It would not be a recipe for success to propel the spacecraft to a position in space from which it could not achieve enough thrust to accomplish the rest of the carefully designed journey to Ceres.

To formulate the flight plan then requires knowing how much power will be available even as the probe ventures farther from the sun. Engineers make mathematical predictions of the power the solar arrays will generate, but these calculations are surprisingly difficult. Well, perhaps some readers would not be surprised, but it is more complicated than simply reducing the power in proportion to the intensity of the sunlight. As one example, at greater distances from the sun, the temperature of the arrays in the cold depths of space would be even lower, and the efficiency of the solar cells depends on their temperature. In 2008, the operations team devised and implemented a method to refine their estimates of the solar array performance, and that work enabled the deep-space traveler to arrive at Vesta earlier and depart later. Now they have developed a related but superior technique, which the faithful spacecraft executed flawlessly on June 24.

The only way to measure the power generation capability of the arrays is to draw power from them. With the ion thrust off, even with all other systems turned on, the spacecraft cannot consume as much power as the arrays can provide, so no meaningful measurement would be possible.

In typical operations, Dawn keeps its solar arrays pointed directly at the sun. For this special calibration, it rotated them so the incident sunlight came at a different angle. This reduced the total amount of light falling on the cells, effectively creating the conditions the spacecraft will experience when it has receded from the sun. As the angle increased, corresponding to greater distances from the brilliant star, the arrays produced less power, so the ion engine had to be throttled down. (The engines can be operated at 112 different throttle levels, each with a different input power and different thrust level.)

Engineers estimated what the maximum throttle level would be at each of the angles as well as the total power all other systems would consume during the test and then programmed it so the ion propulsion system would throttle down appropriately as the solar array angle increased. Of course, they could not know exactly what the highest throttle level at each angle would be; if they did, then they would already know the solar array characteristics well enough that the calibration would be unnecessary. Fortunately, however, they did not need to determine the perfect levels in advance. The sophisticated robot is smart enough to reduce by a few throttle levels if it detects that all systems combined are drawing more power than the solar arrays generate.

Under normal circumstances, the spacecraft doesn't need to adjust the ion throttle level on its own. Engineers know the solar array performance well enough that they can predict the correct setting with high accuracy for a typical four-week sequence of commands stored onboard. It is only for the much greater distances from the sun in the years ahead that the uncertainty becomes important. In addition, during regular operations, if the spacecraft temporarily needs to use more heaters than usual (more than 140 heaters are distributed around the ship, each turning on and off as needed), thereby increasing the power demand, its battery can make up for the difference. That avoids unnecessary throttle changes.

Over the course of the exercise, the arrays were positioned at five angles, each for an hour, and the main computer recorded their output power and other pertinent measurements. Initially, when the wings were pointing directly at the sun, a glowing orb 2.48 AU (371 million kilometers, or 230 million miles) away, together they could generate more than two kilowatts. The ion propulsion system then was thrusting at level 53, consuming 1,368 watts. When the arrays were tipped to their maximum angle of 47 degrees, the insolation was the same as it would be at 3.00 AU (449 million kilometers, or 279 million miles), and the system yielded more than 1,300 watts. By then, the program engineers had stored onboard had throttled the ion drive down to level 24, where it drew 753 watts, and the spacecraft autonomously reduced it still further. When the activity was finished, the wings were turned back to their usual orientation, facing the distant sun so they could generate the maximum power possible, and the Brobdingnagian dragonfly could resume its normal flight pattern.

The calibration will be repeated occasionally as Dawn proceeds on its deep-space trek. Engineers will use the resulting data to continue to refine their plans for reaching Ceres and for maneuvering in orbit once there. Yet this is just one of the myriad details that must be worked out with exquisite care to ensure that the exploration of that enigmatic world is as richly productive, as tremendously rewarding, as outstandingly successful as the investigation of Vesta.

It is thanks to the extraordinary scrupulousness of their work that Dawn's human counterparts are able to accomplish this ambitious adventure. And although they are responsible for ensuring that the craft achieves its objectives, this endeavor extends far beyond the members of the team. This is a mission of humankind. Everyone who ever gazes in wonder at the night sky is part of it. Everyone who is curious about nature and about the reality of the universe is part of it. Everyone who hungers for knowledge and insight is part of it. Everyone who feels the passion for pursuing bold dreams and the exhilaration of discovery is part of it. Everyone who feels the lure of the unknown is part of it. Everyone who appreciates the great challenges and the great rewards of aiming beyond the horizon is part of it. So as Dawn continues its audacious exploits, anyone can be part of it.

Dawn is 18 million kilometers (11 million miles) from Vesta and 50 million kilometers (31 million miles) from Ceres. It is also 3.47 AU (519 million kilometers or 322 million miles) from Earth, or 1,310 times as far as the moon and 3.42 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, DWARF PLANETS, SOLAR SYSTEM, SPACECRAFT, MISSION

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

-- The Radar Altimeter (ALT) measured wave height at the subsatellite point and the altitude between the spacecraft and the ocean surface. The altitude measurement was precise to within ±10 cm (4 in.). The altitude measurement, when combined with accurate orbit determination information, produced an accurate image of the sea surface topography.

-- The Seasat (Fan-Beam) Scatterometer System (SASS) measured sea surface wind speeds and directions at close intervals from which vector wind fields could be derived on a global basis.

-- The Scanning Multichannel Microwave Radiometer (SMRR) measured wind speed, sea surface temperature to an accuracy of ±2°C, and atmospheric water vapor and liquid water content.

-- The Synthetic Aperture Radar (SAR) was an imaging radar that provided images of the ocean surface from which could be determined ocean wave patterns, water and land interaction data in coastal regions, and radar imagery of sea and fresh water ice and snow cover.

-- The Visual and Infrared Radiometer (VIRR) objective was to provide low-resolution images of visual and infrared radiation emissions from ocean, coastal and atmospheric features in support of the microwave sensors. Clear air temperatures were also measured.

This 1978 illustration was based on a painting, probably by artist Ken Hodges. He created artwork for many different Jet Propulsion Laboratory missions in the 1970s and 1980s, before computer aided animation was used for mission presentations and outreach.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL's Library and Archives Group.

TAGS: SEASAT, EARTH, SPACECRAFT, CLIMATE, WEATHER, MISSION,

• 

  ABOUT THE AUTHOR

  Julie Cooper, Certified Archivist

  Julie Cooper is a certified archivist who identifies and processes collections for the JPL Archives, and helps researchers find information about the history of JPL.

We have seen in many logs that this adventure would be quite impossible without its advanced ion propulsion system. Even a mission only to orbit Vesta, which Dawn has accomplished with such stunning success, would have been unaffordable in NASA's Discovery Program without ion propulsion. This is the only probe ever to orbit an object in the main asteroid belt between Mars and Jupiter. But now, thanks to this sophisticated technology, it is going beyond even that accomplishment to do something no other spacecraft has attempted. Dawn is the only mission ever targeted to orbit two extraterrestrial destinations, making it truly an interplanetary spaceship.

Ion propulsion is 10 times more efficient than conventional chemical propulsion, so it enables much more ambitious missions. It uses its xenon propellant so parsimoniously, however, that the thrust is also exceptionally gentle. Indeed, the ion engine exerts about as much force on the spacecraft as you would feel if you held a single sheet of paper in your hand. At today's thrust level, it would take more than five days to accelerate from zero to 60 mph. While that won't rattle your bones, in the frictionless, zero-gravity conditions of spaceflight, the effect of the thrust gradually accumulates. Instead of thrusting for five days, Dawn thrusts for years. Ion propulsion delivers acceleration with patience, and patience is among this explorer's many virtues.

To accomplish its mission, Dawn is outfitted with three ion engines. In the irreverent spirit with which this project has always been conducted, the units are fancifully known as #1, #2, and #3. (The locations of the thrusters were disclosed in a log shortly after launch, once the spacecraft was too far from Earth for the information to be exploited for tawdry sensationalism.) For comparison, the Star Wars TIE fighters were Twin Ion Engine ships, so now science fact does one better than science fiction. On the other hand, the TIE fighters employed a design that did seem 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 isn't the most important consideration.

At any rate, Dawn only uses one ion engine at a time. Since August 31, 2011, it has accomplished all of its thrusting with thruster #3. That thruster propelled Dawn along its complex spiral path down from an altitude of 2,700 kilometers (1,700 miles) to 210 kilometers (130 miles) above Vesta's dramatic landscape and then back up again. Eventually, the engine pushed Dawn out of orbit, and it has continued to work to reshape the spacecraft's heliocentric course so that it ultimately will match Ceres's orbit around the sun.

Although any of the thrusters can accomplish the needed propulsion, and all three are still healthy, engineers consider many factors in deciding which to use at different times in the mission. Now they have decided to put #2 back to work. So on June 24, after its regular monthly hiatus in thrusting to point the main antenna to Earth for a communications session, the robotic explorer turned to aim that thruster, rather than thruster #3, in the direction needed to continue the journey to Ceres. Despite not being operated in nearly two years, #2 came to life as smoothly as ever. It is now emitting a blue-green beam of xenon ions as the craft has its sights set on the mysterious alien world ahead.

Some readers (surely including our hungry friends the Numerivores) may be interested in the numbers that illustrate the amazing performance of the ion propulsion system, so we will include a few morsels here. Spacecraft using conventional propulsion coast the great majority of the time, using their main engines for minutes or a small number of hours over the entire course of their missions. (Note that most natural objects coast as well, including the moon orbiting Earth, Earth and other planets and asteroids orbiting the sun, and the sun and other stars orbiting within the Milky Way Galaxy.) Dawn has spent 63 percent, almost two-thirds, of its time in space in powered flight, or more than 3.6 years. (This is well in excess of any other spacecraft's total thrust time.) Engine #3 has accomplished slightly more than half of that, or 1.8 years. Engine #1 completed more than 10 months of thrusting, and engine #2 is now at 11 months and steadily increasing. (A partial summary of the history of thruster use is here.)

In all that time maneuvering through the solar system, Dawn has expended only 305 kilograms (672 pounds) of xenon. That's equivalent to less than 2.7 milligrams per second. So averaged over its deep space travels so far, the ship has consumed only half a pound of xenon per day of thrusting. What extraordinary efficiency!

That thrust has been enough to change Dawn's speed by about 8.3 kilometers per second (18,500 mph). That is nearly double the previous record for propulsive velocity change set by Deep Space 1, the first interplanetary mission to use ion propulsion.

Although it has already maneuvered far more than any other spacecraft, it still has much more ahead to reach and explore Ceres. Indeed, remarkable though the ion propulsion is, being so efficient, gentle, and persistent, it is a tool. Its importance is in what it allows the spacecraft to accomplish. Ion propulsion is taking Dawn to giants of the main asteroid belt. Vesta and Ceres have been espied from Earth since the beginning of the 19th century (and were considered planets until scientific knowledge advanced enough to change their designation). After more than 200 years, we finally have the capability to turn those smudges of light among the stars into complex, richly detailed worlds. Revealing not only fascinating secrets about the dawn of the solar system, the explorer also unveils vistas that excite everyone who is curious about the nature of the universe. Far more powerful than ion propulsion is the drive within us to undertake grand adventures, to push our boundaries, to overcome our limitations, and to challenge our imagination and our ingenuity in pursuit of noble rewards. Perched atop its blue-green pillar of xenon ions, Dawn is a dynamic symbol of humankind's insatiable drive to know the cosmos.

Dawn is 15 million kilometers (9.5 million miles) from Vesta and 53 million kilometers (33 million miles) from Ceres. It is also 3.41 AU (510 million kilometers or 317 million miles) from Earth, or 1,300 times as far as the moon and 3.35 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 57 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, SOLAR SYSTEM, DWARF PLANETS, MISSION, SPACECRAFT

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

I've also taken on a role in my local Sierra Club chapter's energy efficiency campaign. I think caring for our planet goes hand-in-hand with science and exploration. It is something that is very important to me.

In February, I rode a bus to Washington, D.C., with 40 other people to attend the Forward On Climate Change rally. The bus ride was a little over 24 hours, but it didn't feel that long at all. I had so much fun meeting and talking with people who had similar passions and motivations to those that I have. The rally and its immense energy opened my eyes to the things I could accomplish in my own community.

In May, I visited my grandparents in Beijing, China. Saying goodbye is always hard, because I absolutely love seeing them, talking to them, and being with them.

This week, I started my internship at JPL, one of my favorite places in the entire world. I am sure this will be the first of many letters that I will write to you. I can't wait to tell you more about my experiences in Pasadena as my summer continues.

With love, Clara

My Timeline:

Aug. 5, 2012: Curiosity lands in Gale Crater. I watch from Earth, crying and shouting on the edge of my seat.

Sept. 27, 2012: Curiosity finds evidence of an ancient streambed. I play my third tennis match of the season, and share the rover's exciting discovery with my parents when I get home from school.

March 12, 2013: A rock sample analysis shows ancient Mars could have supported microbial life. I write about it in my chemistry assignment.

June 10, 2013: This is the first day of my internship at JPL, a day I have been dreaming about since I was a little girl. I don't know how I got to be so lucky.

TAGS: MARS, CURIOSITY, SOLAR SYSTEM, INTERNS, EDUCATION, MISSION, SPACECRAFT

• 

  ABOUT THE AUTHOR

  Clara Ma, Student

  At the age of 12, Clara Ma won the Mars Science Laboratory naming contest with the entry of, "Curiosity." She has since interned at JPL twice.

The stalwart adventurer has recently completed its longest uninterrupted ion thrust period yet. As part of the campaign to conserve precious hydrazine propellant, Dawn now suspends thrusting once every four weeks to point its main antenna to Earth. (In contrast, spacecraft with conventional chemical propulsion spend the vast majority of time coasting.) Because of details of the mission operations schedule and the schedule for NASA's Deep Space Network, the thrust durations can vary by a few days. As a result, the spacecraft spent 31.2 days thrusting without a hiatus. This exceeds Deep Space 1's longest sustained powered flight of 29.2 days. While there currently are no plans to thrust for longer times, the unique craft certainly is capable of doing so. The principal limitation is how much data it can store on the performance of all subsystems (pressures, temperatures, currents, voltages, valve positions, etc.) for subsequent reporting to its terrestrial colleagues.

Thanks to the ship's dependability, the operations team has been able to devote much of its energies recently to developing and refining the complex plans for the exploration of Ceres. You might be among the privileged readers who will get a preview when we begin describing the plans later this year.

Controllers also have devised some special activities for the spacecraft to perform in the near future, accounts of which are predicted to be in the next two logs.

In addition, team members have had time to maintain their skills for when the spacecraft needs more attention. Earlier this month, they conducted an operational readiness test (ORT). One diabolical engineer carefully configured the Dawn spacecraft simulator at JPL to behave as if a pebble one-half of a centimeter (one-fifth of an inch) in diameter shooting through the asteroid belt collided with the probe at well over twice the velocity of a high-performance rifle bullet.

When the explorer entered this region of space, we discussed that it was not as risky as residents of other parts of the solar system might assume. Dawn does not require Han Solo's piloting skills to avoid most of the dangerous rocky debris.

The robot could tolerate such a wound, but it would require some help from operators to resume normal operations. This exercise presented the spacecraft team with an opportunity to spend several days working through the diagnosis and performing the steps necessary to continue the mission (using some of the ship's backup systems). While the specific problem is extremely unlikely to occur, the ORT provided valuable training for new members of the project and served to keep others sharp.

One more benefit of the smooth operations is the time that it enables your correspondent to write his third shortest log ever. (Feel free to do the implied research.) Frequent readers can only hope he strives to achieve such a gratifying feat again!

Dawn is 13 million kilometers (7.9 million miles) from Vesta and 54 million kilometers (34 million miles) from Ceres. It is also 3.25 AU (486 million kilometers or 302 million miles) from Earth, or 1,275 times as far as the moon and 3.20 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 54 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, DWARF PLANETS, SOLAR SYSTEM, MISSION, SPACECRAFT

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Ceres orbits the sun outside Vesta's orbit, yet Dawn is now closer to the sun than both of these alien worlds. How can it be that as the probe climbs from one to the other, it seems to be falling inward? Perhaps the answer lies in the text below; let's venture on and find out!

On Halloween we discussed why Dawn is heading in toward the sun, but this question is different. Vesta also is getting closer to the sun, but what's of interest now is that Dawn, despite its more remote destination, has been approaching the sun more quickly. That earlier log stands out as the best one ever written on this exciting mission in the entire history of October 2012, but if you prefer not to visit it now, we can summarize here the explanation for the spacecraft moving toward the sun. Like all members of the sun's entourage, Vesta and Ceres follow elliptical orbits, their distances from the master of the solar system growing and shrinking as they loop around it. Even Earth's orbit, although nearly round, certainly is not perfectly circular. Our planet is a little closer to the sun in the northern hemisphere winter (southern hemisphere summer) than it is in the summer (southern hemisphere winter). Dawn's orbit is elliptical as well, so it naturally moves nearer to the sun sometimes, and now is such a time. But that does not address why it is currently closer to the sun than Vesta, even though it is seeking out the more distant Ceres.

Because it will orbit Ceres, and not simply fly past it (which would be significantly easier but less valuable), Dawn must make its own orbit around the sun be identical to its target's. But that is not the entire story. After spending 14 months orbiting Vesta, Dawn's challenge is more than to change the shape of its orbit to match Ceres's. The spacecraft also must be at the same place in Ceres's heliocentric orbit that Ceres itself is.

It would not be very rewarding to follow the same looping path around the sun but always be somewhere else on that path. You can visualize this if you have one of the many defective -- er, exotic clocks from the Dawn gift shop on your planet that have two minute hands. If the clock starts with one hand pointed at 12 and another pointed at 1, they will take the same repetitive route, but neither hand will ever catch up with the other. For Dawn's goal of exploring Ceres, this would not prove satisfying. Therefore, part of the objective of the ion thrusting is to ensure the spacecraft arrives not only on the same heliocentric course as Ceres but is there when Ceres is also.

This is a problem familiar to all readers who have maneuvered in orbit, where the principles of orbital mechanics are the rules of the road. To solve it, we rely on one of the laws that we have addressed many times in these logs: objects in a lower orbit travel faster. We described this in more detail in February, and we can recall the essential idea here. The gravitational attraction of any body, whether it is the sun, Earth, a black hole, or anything else, is greater at shorter ranges. So to balance that strong inward pull, an orbiter is compelled to race around quickly. At higher orbits, where gravity is weaker, a more leisurely orbital pace suffices.

We can take advantage of this characteristic of orbits. If we drop to a slightly lower orbit, we travel along more swiftly. That is precisely what Dawn needs to do in order to ensure that when it finishes expanding and tilting its orbit in 2015 so that it is the same as Ceres's, it winds up at the same location as its target. This would be like speeding up the minute hand that had begun at the 12, allowing it to catch up with the hand that would otherwise always be leading it.

Dawn's orbital maneuvering is a little bit more complicated than that of clock hands, but thanks to the ingenuity and creativity of the operations team and the unique capability of its ion propulsion system, the interplanetary ship is sailing on a carefully plotted course to its next celestial port. As soon as it departed from Vesta's gravitational embrace in September, it slipped in closer to the sun. Today, Vesta is 2.53 AU from the sun, and Dawn is 2.51 AU, so the spacecraft is three million kilometers (1.9 million miles) nearer to the sun. (Dawn is farther from Vesta than that, because they are not aligned with the sun. The spacecraft has also moved ahead of the rocky behemoth.)

Of course, eventually Dawn will climb to higher altitudes from the sun than Vesta, because its destination lies beyond. As they progress on their own independent orbits, with Dawn constantly reshaping its, they will be at the same solar distance on July 31, 2013. After that, the robotic explorer will never again be as close to the sun as Vesta. By then, they will be 18 million kilometers (11 million miles) apart. But they will always be connected. Dawn was Earth's first probe to take up residence in the main asteroid belt, and Vesta was its first target. The exotic world had beckoned to humankind for over two centuries before the spacecraft obtained its richly detailed view. Now what was little more than an indistinct point of light is known as a complex and fascinating place with a unique character. And as it follows its repetitive orbit around the sun, its erstwhile companion seeks to reveal the secrets of another extraterrestrial enigma, Ceres. Great treasures await Dawn as it patiently continues its extraordinary deep-space expedition.

Dawn is 10 million kilometers (6.3 million miles) from Vesta and 56 million kilometers (35 million miles) from Ceres. It is also 2.99 AU (448 million kilometers or 278 million miles) from Earth, or 1,215 times as far as the moon and 2.97 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 50 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, DWARF PLANETS, SOLAR SYSTEM, MISSION, SPACECRAFT

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

The ship set sail from Earth more than two thousand days ago, and its voyage on the celestial seas has been wonderfully rewarding. Its extensive exploration of Vesta introduced humankind to a complex and fascinating place that had only been tantalizingly glimpsed from afar with telescopes beginning with its discovery 206 years ago today. Thanks to the extraordinary capability of ion propulsion, Dawn was able to spend 14 months orbiting Vesta, observing dramatic landscapes and exotic features and collecting a wealth of measurements that scientists will continue to analyze for many years.

When it was operating close to Vesta, the spacecraft was in frequent contact with Earth. It took Dawn quite a bit of time to beam the 31,000 photos and other precious data to mission control. In addition, engineers needed to send a great many instructions to the distant adventurer to ensure it remained healthy and productive in carrying out its demanding work in the unforgiving depths of space.

Dawn is now more than 20 times farther from Vesta than the moon is from Earth. Alone again and on its long trek to Ceres, it is not necessary for the ship to be in radio contact as often. As we saw in November, the spacecraft now stops ion thrusting only once every four weeks to point its main antenna to Earth. This schedule conserves the invaluable hydrazine propellant the explorer will need at Ceres. But communicating less frequently does not mean the mission operations team is any less busy. Indeed, as we have explained before, "quiet cruise" consists of a considerable amount of activity.

Each time Dawn communicates with Earth, controllers transmit a second-by-second schedule for the subsequent four weeks. They also load a detailed flight profile with the ion throttle levels and directions for that period. It takes about three weeks to calculate and formulate these plans and to analyze, check, double check, and triple check them to ensure they are flawless before they can be radioed to Dawn.

In addition to all the usual information Dawn needs to keep flying smoothly, operators occasionally include some special instructions. As one example, over the last few months, they have gradually lowered the temperatures of some components slightly in order to reduce heater power. When Dawn stretched out its solar array wings shortly after separating from the Delta rocket on September 27, 2007, its nearly 65-foot wingspan was the longest of any NASA interplanetary probe. The large area of solar cells is needed to collect enough light from the distant sun to power the ion propulsion system and all other spacecraft systems. Devoting a little less power to heaters allows more power to be applied to ionizing and accelerating xenon, yielding greater thrust. With two and a half years of powered flight required to travel from Vesta to Ceres, even a little extra power can make a worthwhile difference to a mission that craves power.

Most temperature adjustments are only two degrees Celsius (3.8 degrees Fahrenheit) at a time, but even that requires careful analysis and investigation, because lowering the temperature of one component may affect another. Xenon and hydrazine propellants need to be maintained in certain ranges, and the lines they flow through follow complicated paths around the spacecraft, so the temperatures all along the way matter. Most of the hardware onboard, from valves and switches to electronics to structural mounts for sensitively aligned units, needs to be thermally regulated to keep Dawn shipshape.

It can take hours for a component to cool down and stabilize at a new setting, and sometimes the change won't even occur until the spacecraft has turned away to resume thrusting, when the faint warmth of the sun and the deep cold of black space affect different parts of the complex robot. Then it will be another four weeks until engineers will receive a comprehensive report on all the temperatures, so they need to be cautious with each change.

In addition to the ongoing work to keep Dawn flying true, some special activities are being developed for later this year, each of which will serve two important purposes: they will yield valuable experience in preparing for operations in orbit around Ceres, and they will provide interesting material for you to read about in future logs. Your correspondent has confidence both in the flight team to design and execute these activities and in readers throughout the cosmos to continue to follow this ambitious mission on its extraterrestrial exploits.

And to ensure that there is plenty to read about for years to come, Dawn's human colleagues are working hard to prepare for exploring Ceres when the spacecraft reaches that remote destination in 2015. As at Vesta, the probe will take advantage of the unique maneuvering capability of ion propulsion to fly to different orbits, each optimized for specific investigations to reveal the complex character of the mysterious world, ensuring a rich and gratifying experience for everyone who wonders about the nature of the solar system. As the plans mature at the end of this year and in 2014, we will delve into them here, just as we presented the Vesta strategy in 2010 and 2011, leading up to the astounding achievements of 2011 and 2012.

Meanwhile, the spacecraft itself, loyally following carefully devised and intricate plans, continues to make good progress, patiently and reliably flying onward. Unknown challenges and unknown rewards lie ahead, and together they promise that this bold mission in deep space will provide humankind with still more inspiring and exciting cosmic adventures.

Dawn is 7.7 million kilometers (4.8 million miles) from Vesta and 56 million kilometers (35 million miles) from Ceres. It is also 2.64 AU (395 million kilometers or 246 million miles) from Earth, or 1075 times as far as the moon and 2.65 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.

TAGS: DAWN, CERES, VESTA, DWARF PLANETS, SOLAR SYSTEM, MISSION, SPACECRAFT

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Climbing through the solar system atop a column of blue-green xenon ions, Dawn has a great deal of powered flight ahead in order to match orbits with faraway Ceres. Nevertheless, it has shown quite admirably that it is up to the task. The craft has spent more time thrusting and has changed its orbit under its own power more than any other ship from Earth. While most of the next two years will be devoted to still more thrusting, the ambitious adventurer has already accomplished much more than it has left to do. And now it is passing an interesting milestone on its interplanetary trek.

With all of the thrusting Dawn has completed, it has now changed its speed by 7.74 kilometers per second (17,300 mph), and the value grows as the ion thrusting continues. For space enthusiasts from Earth, that is a special speed, known as "orbital velocity." Many satellites, including the International Space Station, travel at about that velocity in their orbits. So does this mean that Dawn has only now achieved the velocity necessary to orbit Earth? The short answer is no. The longer answer constitutes the remainder of this log.

We have discussed some of these principles before, but they are counterintuitive and questions continue to arise. Rather than send our readers on a trajectory through the history of these logs even more complicated than Dawn's flight through the asteroid belt, we will revisit a few of the ideas here. (After substantial introspection, your correspondent granted and was granted permission to reuse not only past text but also future text.)

While marking Dawn's progress in terms of its speed is a convenient description of the effectiveness of its maneuvering, it is not truly a measure of how fast it is moving. Rather, it is a measure of how fast it would be moving under very special (and unrealistic) circumstances. To understand this, we need to look at the nature of orbits in general and Dawn's interplanetary trajectory in particular.

The overwhelming majority of craft humans have sent into space have remained in the vicinity of Earth, accompanying that planet on its annual revolutions around the sun. All satellites of Earth (including the moon) remain bound to it by its gravity. (Similarly, Dawn spent much of 2011 and 2012 as a satellite of distant Vesta, locked in the massive body's gravitational grip.) As fast as satellites seem to travel compared to terrestrial residents, from the larger solar system perspective, their incessant circling of Earth means their paths through space are not very different from Earth's itself. Consider the path of a car racing around a long track. If a fly buzzes around inside the car, to the driver it may seem to be moving fast, but if someone watching the car from a distance plotted the fly's path, on average it would be pretty much like the car's.

Everything on the planet and orbiting it travels around the sun at an average of 30 kilometers per second (67,000 mph), completing one full solar orbit every year. To undertake its interplanetary journey and travel elsewhere in the solar system, Dawn needed to break free of Earth's grasp, and that was accomplished by the rocket that carried it to space more than five years ago. Dawn and its erstwhile home went their separate ways, and the sun became the natural reference for the spacecraft's position and speed on its voyage in deep space.

Despite the enormous push the Delta II rocket delivered (with affection!) to Dawn, the spacecraft still did not have nearly enough energy to escape from the powerful sun. So, being a responsible resident of the solar system, Dawn has remained faithfully in orbit around the sun, just as Earth and the rest of the planets, asteroids, comets, and other members of the star's entourage have.

Whether it is for a spacecraft or moon orbiting a planet, a planet or Dawn orbiting the sun, the sun orbiting the Milky Way galaxy, or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to a sizeable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight path. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its path; if you let go of the string, the weight's natural motion would carry it away in a straight line (ignoring the effect of Earth's gravity).

The force of gravity diminishes with distance, so the sun's pull on a nearby body is greater than on a more distant one. Therefore, to remain in orbit, to balance the relentless tug of gravity, the closer object must travel faster, fighting the stronger pull. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, around 400 kilometers, or 250 miles, from the surface) must streak around the planet at about 7.7 kilometers per second (17,000 mph) to keep from being pulled down. The moon, orbiting almost 1000 times farther above, needs only to travel at about 1.0 kilometers per second (less than 2300 mph) to balance Earth's weaker hold at that distance.

Notice that this means that for an astronaut to travel from the surface of Earth to the International Space Station, it would be necessary to accelerate to quite a high speed to rendezvous with the orbital facility. But then once in orbit, to journey to the much more remote moon, the astronaut's speed eventually would have to decline dramatically. Perhaps speed tells an incomplete story in describing the travels of a spacecraft, just as it does with another example of countering gravity.

A person throwing a ball is not that different from a rocket launching a satellite (although the former is usually somewhat less expensive and often involves fewer toxic chemicals). Both represent struggles against Earth's gravitational pull. To throw a ball higher, you have to give it a harder push, imparting more energy to make it climb away from Earth, but as soon as it leaves your hand, it begins slowing. For a harder (faster) throw, it will take longer for Earth's gravity to stop the ball and bring it back, so it will travel higher. But from the moment it leaves your hand until it reaches the top of its arc, its speed constantly dwindles as it gradually yields to Earth's tug. The astronaut's trip from the space station to the moon would be accomplished by starting with a high speed "throw" from the low starting orbit, and then slowing down until reaching the moon.

The rocket that launched Dawn threw it hard enough to escape from Earth, sending it well beyond the International Space Station and even the moon. Dawn's maximum speed relative to Earth on launch day was so high that Earth could not pull it back. As we saw in the explanation of the launch profile, Dawn was propelled to 11.46 kilometers per second (25,640 mph), well in excess of the space station's orbital speed given three paragraphs above. But it has remained under the sun's control.

Now we can think of the general problem of flying elsewhere in space as similar to climbing a hill. For terrestrial hikers, the rewards of ascent come only after doing the work of pushing against Earth's gravity to reach a higher elevation. Similarly, Dawn is climbing a solar system hill with the sun at the bottom. It started part way up the hill at Earth; and its first rewards were found at a higher elevation, where Vesta, traveling around the sun at only about two thirds of Earth's speed, revealed its fascinating secrets to the visiting ship. The ion thrusting now is propelling it still higher up the hill toward Ceres, which moves even more slowly to balance the still-weaker pull of the sun.

If Dawn had been in zero-gravity and not been obligated to obey the laws of orbital motion, the thrusting to date would have accelerated it by the 7.74 kilometers per second (17,300 mph) mentioned near the beginning. Instead of making the spacecraft go faster, however, that work was designed to climb the solar system hill. If Dawn had been targeted to a destination closer to the sun than Earth, the same amount of thrusting would have helped it speed up to descend the hill, dropping into a lower solar orbit, where it would have to zip around the gravitational master of the solar system faster than Earth.

To orbit a body that orbits the sun, a spacecraft has to match its target's solar orbit. Except in science fiction, no spacecraft in history other than Dawn has been designed to orbit two different destinations around the sun. Without its ion propulsion system, this mission would be quite impossible. Tighter orbits require greater velocity in order to counterbalance the stronger pull of gravity. Mercury and Venus orbit the sun faster than Earth. Mars moves around the sun more slowly than Earth, and all residents of the more distant main asteroid belt (including Dawn) revolve at an even more leisurely pace.

Because spacecraft wind up at different speeds relative to the sun, their final velocity is not as important in their design and operation as is the amount by which they change their velocity after being released from the rocket. Because of these complexities, rocket scientists generally put all spacecraft on a level playing field (or, in this case, a zero-gravity field free of the complications of the physics of orbits) by using the change of velocity as a measure of the spacecraft's maneuvering capability.

Dawn has slowed down tremendously since it departed Earth, but what is noteworthy is the amount by which it has propulsively changed its speed. If it had begun at a starting line with all other spacecraft on that simplified playing field, by now it would be racing along at 7.74 kilometers per second (17,300 mph), far faster than any other spacecraft. By the end of its mission, it would be flying at an extraordinary 11 kilometers per second (24,600 mph).

Most satellites in low Earth orbit hardly change their speed at all, relying instead on the momentum imparted to them by the rockets that took them into space. As you can see by comparing the numbers above, a rocket to Earth orbit delivers about the same speed that Dawn has achieved already, and the rocket that sent the probe on its interplanetary course provides roughly the same speed that Dawn will attain over the coming years. (Of course, Dawn and the rocket have different objectives. For example, our spacecraft did not have to plow through Earth's atmosphere under its own thunderous power. Rockets do. Nevertheless, the more petite Dawn is gracefully accomplishing its unique space mission without the burden of enormous propellant tanks and multiple stages.)

Having changed its speed by the same amount needed to go from the surface of Earth to Earth orbit is only a coincidence. Dawn's rocket gave it an even larger boost. But for maneuvering after launch, this spaceship is in a class by itself.

Each spacecraft is designed for a specific mission. As no other spacecraft has attempted a mission like Dawn's, no other spacecraft has needed such an exceptional capability to change its own speed. (Some others have used gravitational boosts from planets to change their speed by more than Dawn. That is not a reflection of the spacecraft's capability, however, but rather the particular trajectory it follows.) Together, all the probes humankind has dispatched on interplanetary journeys have helped provide us with new perspectives and new insights on the nature of the solar system, including its origin and evolution. And the people who are interested in them cannot help but be in awe of the daunting challenges, the remarkable engineering, the vast distances, the inspiring adventures, the thrilling sights, and the amazing new knowledge. With its extraordinary ion propulsion system, Dawn is making exciting contributions to this grand endeavor. It has already conducted a richly detailed exploration of one exotic world and, as it thrusts with its ion propulsion system to climb the solar system hill to another, it looks forward to more treasures on its ambitious expedition.

Dawn is 5.8 million kilometers (3.6 million miles) from Vesta and 56 million kilometers (35 million miles) from Ceres. It is also 2.28 AU (341 million kilometers or 212 million miles) from Earth, or 910 times as far as the moon and 2.30 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 38 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, DWARF PLANETS, SOLAR SYSTEM, SPACECRAFT, MISSION

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

During the 1960s, Ranger, Surveyor and Mariner spacecraft were developed, built and tested at JPL. Because of the heavy use, a similar but smaller test fixture was used for vibration tests on spacecraft components and assemblies. Building 144 still contains test facilities, but this equipment was removed and the room now contains an acoustic chamber.

TAGS: HISTORY, TECHNOLOGY, RANGER, SURVEYOR, MARINER, SPACECRAFT, MISSION

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  ABOUT THE AUTHOR

  Julie Cooper, Certified Archivist

  Julie Cooper is a certified archivist who identifies and processes collections for the JPL Archives, and helps researchers find information about the history of JPL.

Vesta, the second most massive resident of the main asteroid belt between Mars and Jupiter, was discovered in 1807. For more than two centuries thereafter, the mysterious object appeared as little more than a fuzzy patch of light among the stars. The only one of the millions of main belt asteroids to be bright enough to be visible to the naked eye, Vesta beckoned, but its invitation was not answered until Dawn arrived in July 2011, nearly four years after it left distant Earth. The cosmic ambassador is the only spacecraft ever to have orbited an object in the main asteroid belt, and its ambitious mission would have been impossible without ion propulsion.

Dawn found a complex and exotic place, and it returned a fabulously rich collection of pictures and other measurements that will continue to be analyzed for many, many years. For now, we will simply touch on a very few of the many insights that already have been illuminated by the light of Dawn.

Scientists recognize Vesta as being more like a mini-planet than like the chips of rock most people think of as asteroids. The behemoth is 565 kilometers (351 miles) wide at the equator and has a surface area more than twice that of California (although it is populated by far fewer eccentrics, billionaires, and other colorful characters found in that state). Dawn's measurements of the gravity field provide good evidence that Vesta separated into layers, much like Earth did as the planet was forming. Vesta's dense core, composed principally of iron and nickel, may be 200 to 250 kilometers (125 to 150 miles) across. Surrounding that is the mantle, which in turn is covered by the veneer of the crust, about 20 kilometers (12 miles) thick. The once-molten core is now solid (in contrast to Earth's, which remains hot enough to be liquid), but the differentiation into layers gives Vesta a key distinction from most asteroids. Because it was likely still in the process of accumulating material to become a full-sized planet when Jupiter's immense gravity terminated its growth, scientists often refer to Vesta as a protoplanet.

Among the most prominent features of the alien landscape is a huge gouge out of the southern hemisphere so large that its presence was inferred from observations with the Hubble Space Telescope. Dawn found this gigantic crater to be even deeper and wider than expected, penetrating about 19 kilometers (12 miles) and spanning more than 500 kilometers (310 miles), or nearly 90 percent of the protoplanet's equatorial diameter.

The yawning hole is now known as Rheasilvia, after the Vestal Virgin who not only was the mythical mother of Romulus and Remus, but also surely would have been astounded by the spectacular sights on Vesta as well as the spacecraft's capability to point any user-defined body vector in a time-varying inertial direction defined by Chebyshev polynomials. As Dawn has brought Vesta into focus, cartographers have needed labels for the myriad features it has discovered. The International Astronomical Union names Vestan craters for Vestal Virgins and other famous Roman women; mountains, canyons, and other structures are named for towns and festivals associated with the Vestal Virgins.

Vesta dates to the dawn of the solar system, more than 4.5 billion years ago, and its age shows. Myriad craters tell the story of a timeworn surface that has been subjected to the rough and tumble conditions of life in the asteroid belt ever since. A virtual rain of space rocks has fallen upon it. While Rheasilvia records the most powerful punch, from an object as much as 50 kilometers (30 miles) across, there are at least seven craters, some quite ancient indeed, more than 150 kilometers (nearly 100 miles) in diameter. As the eons pass, craters degrade and become more difficult to discern, their crisp shapes eroded by subsequent impacts large and small.

The long history of cratering is particularly evident in the startling difference between the northern and southern hemispheres. The north is very densely cratered, but the south is not. Why? The titanic blow that carved out Rheasilvia is estimated to have occurred over one billion years ago. It excavated a tremendous volume of material. Much of it fell back to the surface, wiping it clean, so the cratering record had to start all over again. Recall that the crater itself is 500 kilometers (310 miles) in diameter, and scientists estimate that 50 kilometers (30 miles) outside the rim, the debris may have piled about 5 kilometers (3 miles) high. Even at greater distances, preexisting features would have been partially or completely erased by the thick accumulation. The effect did not reach to the northern hemisphere, however, so it retained the craters than had formed before this enormous impact.

Some of the rocks were ejected with so much energy that they broke free of Vesta's gravitational grip, going into orbit around the sun. They then went their own way as they were yanked around by the gravitational forces of Jupiter and other bodies, and many of them eventually made it to the part of the solar system where your correspondent and some of his readers spend most of their time: Earth. When our planet's gravity takes hold of one of these Vesta escapees, it pulls the rock into its atmosphere. Some lucky witness might even observe it as a meteor. Its blazing flight to the ground is not the end of its glory, however, for these rocks are prized by planetary geologists and other enthusiasts who want a souvenir from that impact.

Scientists now know that about 6 percent of the meteorites seen to fall to Earth originated on Vesta. Six percent! One of every 16 meteorites! This is an astonishingly large fraction. Apart from Mars and the moon, Vesta is the only known source of specific meteorites. Although rocks from Vesta had to travel much farther, they far outnumber meteorites from these other two more familiar celestial bodies.

Combining laboratory studies of the numerous samples of Vesta with Dawn's measurements at the source provides an extraordinary opportunity to gain insights into the nature of that remote world. Meteorites from Vesta are so common that they are often displayed in museums (occasionally even without the curators' awareness of their special history) and can be obtained from many vendors. Anyone who has seen or held one surely must be moved by contemplating its origin, so distant in space and time, from well beyond Mars and long before animal or plant life arose on Earth.

The impact that formed Rheasilvia partially obliterated the second largest crater on Vesta, Veneneia. That 400-kilometer (250-mile) crater was about 12 kilometers (7.5 miles) deep. It was formed more than two billion years ago, around the time life on Earth grew to macroscopic proportions and photosynthesis by cyanobacteria was still introducing oxygen to the atmosphere.

Scientists are deciphering a possible consequence of the colossal collisions that formed Rheasilvia and Veneneia. The events were so forceful that they sent shock waves reverberating through the entire world. (It is fortunate that Vesta was not destroyed, because if it had been, we would not have such a fascinating place to explore, although it may well be that other protoplanets in the asteroid belt did meet that fate.) As the energy surged through Vesta's interior, the material deformed in complex ways that just a chunk of rock could not. But Vesta is not just a chunk of rock. As a mini-planet, its interior composition and properties have been altered by the geological forces that formed and shaped the world. The seismic stretching apparently caused faults 400 kilometers (250 miles) from the impact sites, and those scars are now evident in another of the most conspicuous features: a vast network of chasms near the equator.

Eighty-six gorges mapped at the equator appear to have been caused by the Rheasilvia impact, and another seven may be from Veneneia. Individual troughs extend to as much as 465 kilometers (289 miles) in length. One is more than 39 kilometers (24 miles) wide and 4.0 kilometers (2.5 miles) deep. These dimensions rival those of the Grand Canyon. This would be as if huge impacts on Earth in Barrow, Alaska and in London (at similar latitudes but the opposite hemisphere as Rheasilvia and Veneneia, respectively) had triggered the formation of giant canyons near the equator.

Vesta has another feature that exceeds the dimensions of anything found on Earth. At the center of Rheasilvia is a mountain of staggering proportions. The summit soars to a fantastic 20 to 25 kilometers (12.4 to 15.5 miles) above the variable elevation of the terrain around it (even ignoring the smaller craters that go deep into the floor of Rheasilvia) and 180 kilometers (110 miles) across at its base. This colossus is well over twice the height of puny Mt. Everest, which rises to less than 9 kilometers (5.5 miles) above the distant seas. Vesta's peak is the second tallest known in the solar system. Olympus Mons on Mars is (slightly) higher.

Dawn's extensive stereo measurements showed that Vesta's extreme topography is not limited to Rheasilvia. This craggy world has many steep slopes. When a space rock smashes into such a slope, the crater it forms may be unstable. The uphill portion succumbs to the pull of Vesta's gravity and the resultant landslide leaves a very deformed crater, as Dawn observed in many locations.

Vesta's surface displays more variety than dramatic craters, towering mountains, expansive chasms, and other impressive topographic features. Among the surprises are strong variations in the brightness of the material itself. Dark splotches here and there are likely deposits from dark rocks that formed in a different location in the solar system and eventually crashed into Vesta. Bright areas are interpreted to be material that originated on Vesta and which has hardly changed at all since the giant protoplanet formed. Gradually it was covered by debris settling to the ground from impacts elsewhere, but occasionally an impact exposes it.

The sophisticated probe from Earth has allowed scientists to make many more wonderful discoveries, far too many to be presented here (or, at least, far too many for your correspondent to describe without exceeding his self-imposed limit of 2,173 words). Some of them have been reported in JPL/NASA news releases as well as in other websites, printed publications, news broadcasts, at dinner tables, and perhaps even on playgrounds (the kind this writer would have enjoyed anyway). And if you haven't been to Vesta to behold the marvelous sights yourself, then either go there or go here to see a few of the best views.

As half a second in a person's lifetime, for one of its nearly 4.6 billions years, Vesta had a companion from Earth, but now it is alone again. Dawn has given us stunning views of this survivor from the dawn of the solar system. Even as dazzling knowledge is gained about Vesta, that distant orb grows only fainter for the probe itself, their separation already being more than 10 times the distance between Earth and the moon. The world Dawn orbited not so long ago now appears only as a pinpoint. It glows among the stars about as bright as Canopus, the second brightest star (apart from the sun) visible from our solar system. Only Sirius is brighter, about twice as luminous. (For Dawn, as well as for terrestrial observers, the planet Jupiter also shines brighter now.)

The discoveries so far are only the beginning. Dawn's sensors returned so much data that it will take decades to mine their riches. Surprising new understandings will be gained by further studies, combined with more investigations of the meteorites from Vesta, comparisons with other worlds, and additional new information about the cosmos. One of the beauties of science is that it allows us not only to comprehend more and more about the universe but also to ask -- and ultimately answer -- more and more insightful questions. Dawn has already made great contributions to this splendid enterprise. The explorer promises still more rewards as it travels ever farther on its exhilarating quest for knowledge, taking us all on a bold and exciting interplanetary adventure to uncharted worlds.

Dawn is 4.2 million kilometers (2.6 million miles) from Vesta and 57 million kilometers (35 million miles) from Ceres. It is also 1.92 AU (288 million kilometers or 179 million miles) from Earth, or 750 times as far as the moon and 1.95 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, DWARF PLANETS, SOLAR SYSTEM, MISSION, SPACECRAFT

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.