Traveling from one alien world to another, Dawn is reliably powering its way through the main asteroid belt with its ion propulsion system. Vesta, the fascinating and complex protoplanet it explored in 2011 and 2012, falls farther and farther behind as the spacecraft gently and patiently reshapes its orbit around the sun, aiming for a 2015 rendezvous with dwarf planet Ceres.
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.
Dear Dawnscerning Readers,
Nearly three times as far from Earth as the sun is, the Dawn spacecraft is making very good progress on its ambitious trek from Vesta to Ceres. After a spectacular adventure at the second most massive resident of the main asteroid belt between Mars and Jupiter, Dawn used its extraordinary ion propulsion system to leave it behind and undertake the long journey to a dwarf planet.
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.
Dr. Marc D. Rayman
7:00 p.m. PDT April 30, 2013
Dear Indawnstrious Readers,
In the depths of the main asteroid belt between Mars and Jupiter, far from Earth, far even from any human-made object, Dawn remains in silent pursuit of dwarf planet Ceres. It has been more than six months since it slipped gracefully away from the giant protoplanet Vesta. The spacecraft has spent 95 percent of the time since then gently thrusting with its ion propulsion system, using that blue-green beam of high velocity xenon ions to propel itself from one alien world to another.
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.
Dr. Marc D. Rayman
4:00 p.m. PDT March 29, 2013
Dear Impordawnt Readers,
The indefatigable Dawn spacecraft is continuing to forge through the main asteroid belt, gently thrusting with its ion propulsion system. As it gradually changes its orbit around the sun, the distance to dwarf planet Ceres slowly shrinks. The pertinacious probe will arrive there in 2015 to explore the largest body between the sun and Neptune that has not yet been glimpsed by a visitor from Earth. Meanwhile, Vesta, the fascinating alien world Dawn revealed in 2011 and 2012, grows ever more distant. The mini-planet it orbited and studied in such detail now appears only as a pinpoint of light 15 times farther from Dawn than the moon is from Earth.
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.
Dr. Marc D. Rayman
6:00 p.m. PST February 28, 2013
In 1963, spacecraft vibration tests were conducted in the Environmental Laboratory at NASA's Jet Propulsion Laboratory in Pasadena, Calif. A slab of granite, coated in oil, provided a smooth and stable base for the magnesium slip plate, test fixture and Ranger 6 spacecraft mounted on it. There were vibration exciters (shakers) on each end, capable of more than 25,000 pounds of force. The horizontal fixture at left was used for low frequency vibration testing, and the equipment was capable of testing along all three spacecraft axes.
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.
This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL's Library and Archives Group.
Dear Dawn't Look Backs,
Its long and daring interplanetary journey continuing smoothly, Dawn is making good progress in gradually reshaping its orbit around the sun. Its uniquely efficient ion propulsion system is gently bringing it closer to its next destination, dwarf planet Ceres, and ever farther from its previous one, Vesta. Although the robotic explorer's sights are set firmly ahead, let's take one last look back at the fascinating alien world it unveiled during its 14 months in orbit there.
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.
Dawn concluded 2012 almost 13,000 times farther from Vesta than it began the year. At that time, it was in its lowest orbit, circling the alien world at an average altitude of only 210 kilometers (130 miles), scrutinizing the mysterious protoplanet to tease out its secrets about the dawn of the solar system.
To conduct its richly detailed exploration, Dawn spent nearly 14 months in orbit around Vesta, bound by the behemoth's gravitational grip. In September they bid farewell, as the adventurer gently escaped from the long embrace and slipped back into orbit around the sun. The spaceship is on its own again in the main asteroid belt, its sights set on a 2015 rendezvous with dwarf planet Ceres. Its extensive ion thrusting is gradually enlarging its orbit and taking it ever farther from its erstwhile companion as their solar system paths diverge.
Meanwhile, on faraway Earth (and all the other locations throughout the cosmos where Dawnophiles reside), the trove of pictures and other precious measurements continue to be examined, analyzed, and admired by scientists and everyone else who yearns to glimpse distant celestial sights. And Earth itself, just as Vesta, Ceres, Dawn, and so many other members of the solar system family, continues to follow its own orbit around the sun.
Thanks to a coincidence of their independent trajectories, Earth and Dawn recently reached their smallest separation in well over a year, just as the tips of the hour hand and minute hand on a clock are relatively near every 65 minutes, 27 seconds. On Dec. 9, they were only 236 million kilometers (147 million miles) apart. Only? In human terms, this is not particularly close. Take a moment to let the immensity of their separation register. The International Space Station, for example, firmly in orbit around Earth, was 411 kilometers (255 miles) high that day, so our remote robotic explorer was 575 thousand times farther. If Earth were a soccer ball, the occupants of the orbiting outpost would have been a mere seven millimeters (less than a third of an inch) away. Our deep-space traveler would have been more than four kilometers (2.5 miles) from the ball. So although the planet and its extraterrestrial emissary were closer than usual, they were not in close proximity. Dawn remains extraordinarily far from all of its human friends and colleagues and the world they inhabit.
As the craft reshapes its solar orbit to match Ceres's, it will wind up farther from the sun than it was while at Vesta. (As a reminder, see the table here that illustrates Dawn's progress to each destination on its long interplanetary voyage.) We saw recently, however, that the route is complex, and the spacecraft is temporarily approaching the sun. Before the ship has had time to swing back out to a greater heliocentric range, Earth will have looped around again, and the two will briefly be even a little bit closer early in 2014. After that, however, they will never be so near each other again, as Dawn will climb higher and higher up the solar system hill, its quest for new and exciting knowledge of distant worlds taking it farther from the sun and hence from Earth.
This interactive computer-based stereo viewing system was used to analyze Mars topography images generated by the cameras on NASA's Viking 1 Mars lander. Two 17-inch video monitors faced a scanning stereoscope mounted between them on a table. Left and right lander camera image data were sent to the left and right monitors. Panning controls on the stereoscope helped align one image with the other to create a stereo image, 640 by 512 pixels in size. A mouse was used for finely controlled rotation of the monitors. An article about the system described a prototype mouse, used before this photo was taken in 1976. "The track ball is a baseball-sized sphere protruding from the top of a retaining box and capable of being rotated freely and indefinitely about its center ..."
The resulting images could be displayed on additional monitors and were used to create contour maps and other images that aided lander surface operations. The system was developed by Stanford University and NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Dawn is continuing to gently and patiently change its orbit around the sun. In September, it left Vesta, a complex and fascinating world it had accompanied for 14 months, and now the bold explorer is traveling to the largest world in the main asteroid belt, dwarf planet Ceres.
Dawn has spent most of its time since leaving Earth powering its way through the solar system atop a column of blue-green xenon ions emitted by its advanced ion propulsion system. Mission controllers have made some changes to Dawn's operating profile in order to conserve its supply of a conventional rocket propellant known as hydrazine. Firing it through the small jets of the reaction control system helps the ship rotate or maintain its orientation in the zero-gravity of spaceflight. The flight team had already taken some special steps to preserve this precious propellant, and now they have taken further measures. If you remain awake after the description of what the changes are, you can read about the motivation for such frugality.
Dawn's typical week of interplanetary travel used to include ion thrusting for almost six and two-thirds days. Then it would stop and slowly pirouette to point its main antenna to Earth for about eight hours. That would allow it to send to the giant antennas of NASA's Deep Space Network a full report on its health from the preceding week, including currents, voltages, temperatures, pressures, instructions it had executed, decisions it had made, and almost everything else save its wonderment at operating in the forbidding depths of space so fantastically far from its planet of origin. Engineers also used these communications sessions to radio updated commands to the craft before it turned once again to fire its ion thruster in the required direction.
Now operators have changed the pace of activities. Every turn consumes hydrazine, as the spacecraft expels a few puffs of propellant through some of its jets to start rotating and through opposing jets to stop. Instead of turning weekly, Dawn has been maintaining thrust for two weeks at a time, and beginning in January it will only turn to Earth once every four weeks. After more than five years of reliable performance, controllers have sufficient confidence in the ship to let it sail longer on its own. They have refined the number and frequency of measurements it records so that even with longer intervals of independence, the spacecraft can store the information engineers deem the most important to monitor.
Although contact is established through the main antenna less often, Dawn uses one of its three auxiliary antennas twice a week. Each of these smaller antennas produces a much broader signal so that even when one cannot be aimed directly at Earth, the Deep Space Network can detect its weak transmission. Only brief messages can be communicated this way, but they are sufficient to confirm that the distant ship remains healthy.
In addition to turning less often, Dawn now turns more slowly. Its standard used to be the same blinding pace at which the minute hand races around a clock (fasten your seat belt!). Engineers cut that in half two years ago but returned to the original value at the beginning of the Vesta approach phase. Now they have lowered it to one quarter of a minute hand's rate. Dawn is patient, however. There's no hurry, and the leisurely turns are much more hydrazine-efficient.
With these two changes, the robotic adventurer will arrive at Ceres in 2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply it had when it rocketed away from Cape Canaveral on a lovely September dawn in 2007. Mission planners will be able to make excellent use of it as they guide the probe through its exploration of the giant of the main asteroid belt.
Any limited resource should be consumed responsibly, whether on a planet or on a spaceship. Hydrazine is not the only resource that Dawn's controllers manage carefully, but let's recall why this one has grown in importance recently.
The spacecraft can stabilize or change its orientation using the hydrazine powered jets or reaction wheels. By electrically changing a wheel's spin rate, Dawn can start or stop rotating. When it is relying principally on these gyroscope-like devices, it still occasionally has to expend a little hydrazine to keep them from spinning too fast, as explained nearly four years ago. While thrusting (which is most of the time), the ion thruster works in concert with one of those other actuators to control the orientation.
For an ambitious and complex eight-year interplanetary expedition, Dawn's builders equipped it with backup systems. The craft was designed to use three reaction wheels at a time for normal operations, so it is outfitted with four. One of them encountered increased friction in June 2010. To preserve the life of the remaining wheels, engineers flew the spacecraft with all the wheels turned off from August 2010 until the Vesta approach phase began in May 2011, and they are doing the same during the flight from Vesta to Ceres.
As soon as the wheel had difficulty in 2010, Orbital Sciences Corporation and JPL began working on a method to operate with fewer than three, in case another one faltered. They developed software to operate in a "hybrid" mode with two wheels plus the hydrazine jets and installed it in the robot's main flight computer in April 2011 so it would be available at Vesta if needed.
The exploration of that alien orb, which exceeded all expectations not only for productivity but also for pure awesomeness, went very smoothly with the three operational wheels. As Dawn was spiraling away from the rocky behemoth in August 2012, however, another one experienced the same peculiar friction. Because the wheels had already been scheduled to be powered off shortly thereafter, the flight team continued the departure with them turned off, and it proceeded without further interruptions. With their typical swift professionalism, they immediately began working on the long-term ramifications of two wheels being unavailable in case the devices could not be recovered.
Because the hybrid control scheme uses more hydrazine than three wheels would, and using the hydrazine jets by themselves with no wheels consumes still more, operators undertook the new campaign to conserve the propellant during the journey to Ceres. Ever resourceful, engineers now anticipate that regardless of how healthy the wheels are, the probe will be able to conduct an exciting and rewarding exploration there.
Dawn will arrive at the distant and mysterious Ceres in 2015, and that allows plenty of time for the terrestrial members of the team to complete the exquisitely detailed plans for its adventures there. While that work is underway, the intrepid ship continues forging silently through the vast emptiness of space, distant and alone, patient and persistent. Despite its remoteness, the robot remains tightly bound to its human colleagues, for it is on their behalf and under the power of their ingenuity, thirst for knowledge, and hunger for adventure that it sails deeper into uncharted cosmic seas.
Dawn is 1.5 million kilometers (960 thousand miles) from Vesta and 57 million kilometers (36 million miles) from Ceres. It is also 1.59 AU (238 million kilometers or 148 million miles) from Earth, or 590 times as far as the moon and 1.61 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 26 minutes to make the round trip.
Dr. Marc D. Rayman
11:00 p.m. PST November 30, 2012
Dear Indawnspensable Readers,
Dawn is making good progress on the second segment of its cosmic travels. Following more than a year of arduous but sensationally productive and exciting work revealing the fascinating character of the giant protoplanet Vesta, it is now patiently pursuing its next target, the mysterious dwarf planet Ceres, which resides farther from the sun. For the second (and final) time in its interplanetary journey, however, Dawn is about to turn around, going closer to the sun rather than farther away.
In August 2008, we saw in detail how it could be that even as the bold explorer travels outward in the solar system from Earth, past Mars, to Vesta, and then on to Ceres, it could occasionally appear to reverse course temporarily. We present here a shorter explanation for those readers who did not memorize the log explaining this perplexing behavior (you know who you are, and we do as well, but your secret remains safe under the terms of our reader privacy agreement).
Dawn orbits the sun, as do Vesta, Ceres, the other residents of the main asteroid belt, and the planets. All orbits, whether of these objects around the star at the center of our solar system, artificial satellites or the moon in orbit around Earth, or even Dawn when it was in orbit around Vesta, are ellipses (like flattened circles). Earth, for example, orbits the sun at an average distance of 150 million kilometers (93.0 million miles), which astronomers call one astronomical unit (AU). During its year-long revolution, however, our planet comes in to 0.98 AU from the sun and goes out to 1.02 AU. Earthlings manage quite nicely with these small variations. (Note that the seasons are not caused by the changes in distance but instead are a result of the tilt of Earth's axis and thus the differing angles at which the warming rays of the sun arrive during the year. If the sun's distance were all that mattered, the northern and southern hemispheres would have the same seasons.) So, orbiting bodies move smoothly between a minimum and a maximum range from their gravitational masters rather than remaining at a constant distance.
When Dawn was in orbit around Vesta, it accompanied that world on its regular journey around the sun. The table last month showing the probe's progress over the five years of its deep space trek reminds us that Vesta's path brings it as close to the sun as 2.15 AU and takes it out to 2.57 AU.
If Dawn had remained in orbit around Vesta, it would have continued to follow the same elliptical course as its host in the asteroid belt. The pair would have reached their maximum solar distance next month and then would have fallen back to 2.15 AU in September 2014. While visiting Vesta was extremely gratifying, this explorer's ambitions are greater. It broke free of Vesta's grip, its sights set on a new and distant alien destination.
Now the spacecraft is in its own independent orbit around the sun, and the persistent but gentle pressure of its advanced ion propulsion system gradually reshapes that orbit. At any moment, the orbit is an ellipse, and an instant later, it is a slightly different ellipse, courtesy of the thrust. As Dawn departed from Vesta only last month, its orbit is not yet dramatically different, but over the course of the coming years, the effect of the thrusting will be to change the orbit tremendously. To reach Ceres in 2015, the ship will enlarge and tip its elliptical course to match the motion of the dwarf planet around the sun. (Some of the parameters characterizing each object's orbit are shown here.)
Although the ship's orbit is growing, it will reach the current high point on Nov. 1. It will then be 2.57 AU from the sun and, just as in 2008 (albeit at a smaller distance), it will begin moving closer, even as it continues to thrust.
If Dawn stopped thrusting on Nov. 1, its elliptical orbit would carry it down to 2.19 AU from the sun in September 2014. That's a higher orbit than Vesta's but still well below what it needs to be for the rendezvous with Ceres. Astute readers have already anticipated that the plan is not to stop thrusting but to continue reworking the trajectory, just as a ceramicist gradually achieves a desired shape to create the envisioned artistic result. The ongoing thrusting will raise the low point of the orbit, so if the ship follows the flight plan, it will descend only to 2.45 AU in October 2013 before sailing outward again. By May 2014 it will have risen to the same solar altitude as it is now. All the thrusting in the interim will have altered its course so much, however, that it will not turn around then; rather, it will continue ascending to keep its 2015 appointment with Ceres.
If not for its ion propulsion system, this extraordinary interplanetary expedition would be impossible. Conventional chemical propulsion does not have the requisite capability. The key to taking advantage of the unique performance of ion propulsion is patience, which Dawn demonstrates exemplarily. Whereas the vast majority of spacecraft spend almost all of their time coasting, Dawn devotes the preponderance of its time to powered flight, emitting a tenuous but very high velocity beam of blue-green xenon ions to propel itself. On Nov. 2, coincidentally around the same time it begins approaching the sun, the indefatigable robot will exceed three years of total thrust. By then, the gentle but persistent flow of ions will have imparted the equivalent of 7.28 kilometers per second (16,300 miles per hour) to the spacecraft. (As we have seen in many previous logs, such as here, this is not actually a measure of Dawn's speed. It is a convenient description of the effectiveness of a propulsion system that avoids the complications of orbital mechanics. The effect of the propulsion is not to increase velocity but rather to climb the solar system hill, just as pressing a car's accelerator while on a steep slope may not gain speed compared to driving on a level road, but it will gain elevation.)
As the ambitious adventurer forges through the asteroid belt, pushing tirelessly against the sun's incessant pull, the temporary reduction in heliocentric distance is just one element of the complex plan for a grand adventure to explore two of the last uncharted worlds in the inner solar system. Next month, we will see some of the changes the mission control team is making in how Dawn operates in order to ensure its work at Ceres is as richly productive as it was at Vesta.
Dawn is 708 thousand kilometers (440 thousand miles) from Vesta and 59 million kilometers (37 million miles) from Ceres. It is also 1.78 AU (267 million kilometers or 166 million miles) from Earth, or 660 times as far as the moon and 1.80 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 30 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 a.m. PDT October 31, 2012