Sunday, January 25, 2015

Across The Universe Newsl | December 29

Across The Universe News | December 29:

Artist's concept of formation of asteroids



Pardawn Me, Dear Readers,

Far away from Earthlings who look forward to a new year, Dawn looks forward to a new world. On the far side of the sun, the interplanetary explorer is closing in on Ceres, using its advanced ion propulsion system to match solar orbits with the dwarf planet.

Since breaking out of orbit around the giant protoplanet Vesta in September 2012, the spaceship has patiently flown in interplanetary cruise. That long mission phase is over, and now Dawn is starting the Ceres chapter of its extraordinary extraterrestrial expedition. Configured for its approach phase, the craft is following a new and carefully designed course described in detail last month. In March it will slip ever so gracefully into orbit for an ambitious and exciting exploration of the alien world ahead.

Over the past year, we have provided previews of the major activities during all the phases of Dawn’s mission at Ceres. This month, let’s take a look at Ceres itself, an intriguing and mysterious orb that has beckoned for more than two centuries. Now, finally, after so long, Earth is answering the cosmic invitation, and an ambassador from our planet is about to take up permanent residence there. Over the course of Dawn’s grand adventure, our knowledge will rocket far, far beyond all that has been learned before.



There can be two accounts of Ceres: its own history, which dates back to near the dawn of the solar system almost 4.6 billion years ago, and its history in the scope of human knowledge, which is somewhat shorter. Both are rich topics, with much more than we can cover here (or in the first log for this entire mission), but let’s touch on a few tidbits. We begin with the latter history.

In 1800, the known solar system contained seven planets: Mercury, Venus, Earth (home to some of our readers), Mars, Jupiter, Saturn and Uranus. This reflected a new and sophisticated scientific understanding, because Uranus had first been noticed in telescopes not long before, in 1781. (The other planets had been known to ancient sky watchers.) Even before William Herschel’s fortuitous sighting of a planet beyond Saturn, astronomers had wondered about the gap between Mars and Jupiter and speculated about the possibility of a planet there. Although some astronomers had searched, their efforts had not yielded a new planet.

Giuseppe Piazzi

Giuseppe Piazzi points the way to his discovery, the planet Ceres. (Dawn’s route there is more complex than Piazzi might have guessed.) Credit: Osservatorio Astronomico di Palermo
Astronomer Giuseppe Piazzi was not looking for a planet on Jan. 1, 1801, but he spotted an unfamiliar dot of light that wandered slowly among the stars. He named it for Ceres, the Roman goddess of agriculture, and if you had cereal this morning, you have already had an etymological connection with the goddess.

The Dawn project worked with the International Astronomical Union (IAU) to formalize a plan for names on Ceres that builds upon and broadens Piazzi’s theme. Craters will be named for gods and goddesses of agriculture and vegetation from world mythology. Other features will be named for agricultural festivals.

Because Ceres was fainter than the other known planets, it was evident that it was smaller. Nevertheless, many astronomers considered it to be a planet too.

It is worth noting the significance of this. Modern astronomy had chanced upon only one other planet, so Piazzi’s discovery was A Big Deal. When a new chemical element was found a couple of years later, it was named cerium in tribute to the new planet Ceres. (Uranus had been similarly honored with the 1789 naming of uranium. That element’s peculiar property of emitting radiation would not be known for another century.)

In the six years following the discovery of Ceres, three more bodies were detected orbiting between Mars and Jupiter. (One of them is Vesta, now known in spectacular detail thanks to Dawn’s extensive exploration in 2011-2012.) There then ensued a gap of more than 38 years before another was found, so for well over a generation, the sun’s family of planets was unchanged.

So if you had been reading about all this 200 years ago, there would have been at least two important differences from now. One is that your Internet connection would have been considerably slower. The other is that you might have learned in school or elsewhere that Ceres was a planet.

In 1846, a planet was discovered beyond Uranus, and we call it Neptune. Nothing else of comparable size has subsequently been seen in our solar system.

With scientific knowledge and technology progressing in the middle of the nineteenth century, new objects were glimpsed between Mars and Jupiter. As more and more were seen over the years, what we now know as the main asteroid belt was gradually recognized. Terminology changed too. One of the great strengths of science is that it advances, and sometimes we have to modify our vocabulary to reflect the improved, refined view of the universe.

By the time Pluto was sighted in 1930, Ceres had long been known as a “minor planet” and an “asteroid.” For a while thereafter, Pluto enjoyed planetary status similar to what Ceres had had. In fact, in 1940, scientists named two more additions to the periodic table of the elements neptunium and plutonium. While the histories are not identical, there is a certain parallel, with more and more objects in Pluto’s part of the solar system later being found. Terminology changed again: Pluto was subsumed into the new category of “dwarf planets” defined by the IAU in 2006. Ceres was the first body to be discovered that met the criteria established by the IAU, and Pluto was the second. (Spacecraft are now on their way to both dwarf planets: Dawn to orbit Ceres 214 years after its discovery and the wonderful New Horizons mission to fly past Pluto 85 years after it was found.)

Four views of Ceres taken by the Hubble telescope

Four views of Ceres as it rotates, as seen with Hubble Space Telescope, are the best we have so far. (The brightest feature has been exaggerated here.) As summarized in October, Dawn’s pictures will surpass this quality within a month, revealing details never seen before. Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park)
We discussed this new nomenclature in some detail shortly after it was adopted. We understand that the designation then, as now, is controversial among some scientists and the public, and there are strong emotions on this topic. We will not delve into it here (nor in the blog comments below), preferring instead to focus on the extraordinary successes of science, the great power of the scientific method and the thrill of bold adventures far from home. The Dawn team remains both unperturbed and confident in what to call this intriguing and alluring world: we call it “Ceres.” And our goal is to develop that faint smudge of light amidst the stars into a richly detailed portrait.

One of the advances of science was the recognition that Ceres really is entirely different from typical residents of the main asteroid belt. It is a colossus! There are millions upon millions of asteroids, and yet Ceres itself contains roughly 30 percent of the mass in that entire vast region of space. By the way, Vesta, the second most massive body there, constitutes about eight percent of the asteroid belt’s mass. It is remarkable that Dawn will single-handedly explore around 40 percent of the asteroid belt’s mass.

With an equatorial diameter of about 605 miles (975 kilometers), a value that Dawn will refine very soon, Ceres is the largest body between the sun and Pluto that a spacecraft has not yet visited. It is occasionally described as being comparable in size to Texas, which is like comparing a basketball to a flat sheet of paper. Ceres has a surface area 38 percent of that of the continental United States, or four times the area of Texas. (Nevertheless, until Dawn shows evidence to the contrary, we will assume Texas has more rodeos.) It is nearly a third of the area of Europe and larger than the combined lands of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. Such an expansive place offers the promise of tremendous diversity and many marvelous and exciting sights to behold. Earth is about to be introduced to a fascinating new world.

How did Ceres come to be? And why is that being phrased as a question instead of a more declarative introduction to the history and nature of this dwarf planet? For that matter, why is this paragraph composed exclusively of questions? At least this sentence isn’t a question, right? OK, really, shouldn’t we stay more on topic?

At the dawn of the solar system almost 4.6 billion years ago, the young sun was surrounded by a swirling cloud of dust and gas. Sometimes some particles would happen to hit and stick together. Then more and more and more particles would stick to them, and eventually these agglomerations would grow so large that their gravity would pull in even more material. It was through mechanisms like this that the planets formed.

But when massive Jupiter developed, its powerful gravity terminated the growth of objects nearby, leaving bits and pieces as asteroids. Ceres and Vesta, already sizable by then, might have grown to become even larger, each incorporating still more of the nearby material, had Jupiter not deprived them of such an opportunity. Not having made it to full planetary proportions, Ceres and Vesta are known as protoplanets, and studying them provides scientists with insight into the largest building blocks of planets and into worlds that are intriguing in their own rights.



Artist's concept of formation of asteroids


Dust and gas surrounding the sun coalesced into planets and asteroids, as depicted in this artist’s view. Credit: William Hartmann, Courtesy of UCLA
Ceres apparently formed far enough from the sun under conditions cool enough for it to hang on to water molecules. Indeed, scientists have good reason to believe that water (mostly in the form of ice) may make up an astonishing 30 percent of its mass. Ceres may contain more water than Mars or any other body in the inner solar system except Earth. (Comets, of course, have high proportions of water too, but they are so minuscule compared to this behemoth that each one harbors a quite negligible amount of water when measured against Ceres’ huge inventory.)

Although some of the moons of the outer planets also are ice and rock, and they display very interesting characteristics to the impressive and capable spacecraft that have flown past (in some cases repeatedly, as the craft orbited the host planet), no probe has had the capability to linger in orbit around any of them. Dawn’s in-depth exploration of Ceres will yield more detailed and complete views than we have obtained of any icy moon.

Radioactive elements incorporated into Ceres when it was forming would supply it with some heat, and its great bulk would provide thermal insulation, so it would take a very long time for the heat to escape into space. The sun, faraway though it is, adds still more heat. As a result, there may be some water warm enough to be liquid. (The concentration of any chemical impurities in the water that affect its freezing point, as salt does, may make an important difference in how much is liquid.) This distant, alien world may have lakes or even oceans of liquid water deep underground. What a fantastic possibility!

There will be no liquid on the frigid surface. Even ice on the surface, exposed to the cold vacuum of space, would sublimate before long. But ice could be just beneath the surface, perhaps well less than a yard (a meter) deep.

Ceres then may have a thin, dusty crust over a mantle rich in ice that might be more than 60 miles (100 kilometers) thick. Its warmer core is likely composed mostly of rock.

As heat dissipated from Ceres’ interior over the eons, it may have undergone convection, with the warmer material rising and cooler material sinking very slowly. This is reminiscent of what occurs in pot of heated water and in Earth’s interior. Even if it did occur at some time in Ceres’ history, it probably is not happening any longer, as too much heat would have been lost by now, so there would not be enough left to power the upward movement of warm material. But the convective process might have written its signature in structures or minerals left behind when ice sublimated after being pushed to the surface. Dawn’s photos of geological features and measurements of the composition may provide a window to forces in the interior of the protoplanet sometime in its past.

Even if convection is no longer occurring, Ceres is not entirely static. We have very tantalizing information from a marvelously productive far-infrared space telescope named for the only known astronomer who found a planet before Piazzi made his discovery. The Herschel Space Observatory recently detected a tiny amount of water vapor emanating from the distant dwarf planet. Scientists do not know how the water vapor makes it into space. It might be from ice sublimating (possibly following a powerful impact that exposed subsurface ice) or perhaps from geysers or even erupting cryovolcanoes (“cold volcanoes”) powered by heat that Ceres has retained since its formation. In any case, Herschel saw water, albeit in very, very small quantity.

It is not certain whether water vapor is there all the time. It is unknown whether, for example, it depends on solar heating and hence where Ceres is in its somewhat elliptical orbit around the sun (not as circular as Earth’s orbit but more circular than Mars’), which requires 4.6 years to complete.

Even if the water vapor is present during Dawn’s 1.3-year primary mission in orbit, it would be extremely difficult to detect. Herschel made its findings when our ship was already far, far from Earth, well along its interplanetary itinerary. The probe’s sensors were designed for studying the solid surfaces of airless bodies, not an exceedingly tenuous veil of water molecules. For context, the water vapor Herschel measured is significantly less dense than Earth’s atmosphere is even far above the International Space Station, which orbits in what most people consider to be the vacuum of space. Dawn will not need windshield wipers! Nevertheless, as we saw in February, the Dawn team, ever creative and dedicated to squeezing as much out of the mission as possible, investigated techniques this year that might be effective in searching for an exceptionally thin vapor. They have augmented the plan with many hours of observations of the space above Ceres when the spacecraft is over the night side during its first science orbit in April and May at an altitude of 8,400 miles (13,500 kilometers). It is possible that if there is some water vapor, the instruments may pick up a faint signature in the sunlight that passes through it.

Regardless of the possibility of detecting traces of water from Ceres, Dawn will focus its measurements on the uncharted surface and the interior, as it did at Vesta. Vesta displayed landscapes battered by craters from impacts during more than 4.5 billion years in the rough and tumble asteroid belt. Ceres has spent most or all of its history also in the asteroid belt, but it is possible it will not show its age so clearly. Ice, although very hard at such low temperatures, is not as hard as rock. So it may be that the surface gradually “relaxes” after an impact, just as your skin restores its shape after pressure has been removed. Craters older than a few tens of millions of years may have slowly disappeared. (That may sound old, but it is a small fraction of Ceres’ lifetime.) Near the poles, where it is colder so ice is harder, the scars of impact craters may be preserved for longer.

Ceres has more than water-ice and rock. It probably contains organic materials, some produced by chemical processes with the minerals already there and some delivered by asteroids that fell to its surface. This is noteworthy, because water and organic chemicals are ingredients for life. The combination of Ceres’ internal heat and the weak but persistent heating from the sun provides energy, which also is essential for life. Even if the possibility of life itself there is extremely remote (and it is beyond Dawn’s capability to detect), the conditions for “prebiotic” chemistry would be tremendously interesting. That is why, as we explained in August, we want to protect the special environment on the ground from contamination by the terrestrial chemicals in our orbiting spacecraft.



Image of Ceres taken by the Dawn spacecraft.


Dawn acquired this picture of Ceres on Dec. 1, as described last month. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
While there is more known about Ceres, there is much, much more that is unknown. Dawn seeks to discover many of the secrets of this unfamiliar, fascinating member of the solar system family. One of the measures of its success would be if, upon answering many of our questions about Ceres, we are left with even more questions. Now on the threshold of an old world which will be new to us, we do not have long to wait for the great rewards of new knowledge, new insight, new thrills and new mysteries to solve.

Dawn is 382,000 miles (614,000 kilometers) from Ceres, or 1.6 times the average distance between Earth and the moon. It is also 3.77 AU (351 million miles, or 564 million kilometers) from Earth, or 1,500 times as far as the moon and 3.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and three minutes to make the round trip.

Dr. Marc D. Rayman

8:00 a.m. PST December 29, 2014
Posted by Nelio Inacio at 10:19 AM 0 comments

Saturday, January 24, 2015

PLANET EARTH IN THE UNIVERSE

PLANET EARTH IN THE UNIVERSE :

PLANET EARTH IN THE UNIVERSE
PLANET EARTH IN THE UNIVERSE
Posted by Nelio Inacio at 12:53 PM 0 comments
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The Giant Asteroid, Near and Far

The Giant Asteroid, Near and Far:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft departing asteroid Vesta


Artist’s concept of NASA’s Dawn spacecraft departing the giant asteroid Vesta. Image credit: NASA/JPL-Caltech
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.

› Continue reading Marc Rayman’s Dawn Journal to learn how to approximate Dawn’s position in the sky on Jan. 21 and 22

Posted by Nelio Inacio at 11:41 AM 0 comments

For Dawn, a Little Push Goes a Long Way

For Dawn, a Little Push Goes a Long Way:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta


Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech
Dear Dawnamic Readers,

The indefatigable Dawn spacecraft is continuing its extraordinary interplanetary flight on behalf of inquisitive creatures on distant Earth. Progressing ever farther from Vesta, the rocky and rugged world it so recently explored, the ship is making good progress toward its second port of call, dwarf planet Ceres.

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 System Hot Fire Test for Deep Space 1


Ion Propulsion System Hot Fire Test for the Deep Space 1 spacecraft. Image credit: NASA/JPL-Caltech
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.

› Continue reading Marc Rayman’s Dawn Journal

Posted by Nelio Inacio at 11:40 AM 0 comments

NASA’s Dawn Spacecraft Celebrates Six Years in Space

NASA’s Dawn Spacecraft Celebrates Six Years in Space:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft


Artist’s concept of NASA’s Dawn spacecraft between the giant asteroid Vesta and the dwarf planet Ceres. Image credit: NASA/JPL-Caltech - See more at: http://blogs.jpl.nasa.gov/category/columns/dawn-journal/#sthash.kRxiDi66.dpuf Image credit: NASA/JPL-Caltech
Dear Dawnniversaries,

On the sixth anniversary of leaving Earth to embark on a daring deep-space expedition, Dawn is very, very far from its erstwhile planetary residence. Now humankind’s only permanent resident of the main asteroid belt between Mars and Jupiter, the seasoned explorer is making good progress toward the largest object in that part of the solar system, the mysterious dwarf planet Ceres. The voyage is long, and the intrepid but patient traveler will not reach its next destination until half a year after its seventh anniversary of departing Earth.

On its fifth anniversary, Dawn was still relatively close to Vesta, the giant protoplanet that had so recently held the craft in its gravitational grip. The only probe ever to orbit a main belt asteroid, Dawn spent 14 months (including its fourth anniversary) accompanying Vesta on its way around the sun. After more than two centuries of appearing to astronomers as little more than a fuzzy blob of light among the stars, the second most massive body in the asteroid belt has been revealed as a fascinating, complex, alien world more closely related to terrestrial planets (including Earth) than to typical asteroids.

Most of the ship’s first four years of spaceflight were devoted to using its ion propulsion system to spiral away from the sun, ascending the solar system hill from Earth to Vesta. Now it is working to climb still higher up that hill to Ceres.

For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the sixth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the logs from its first, second, third, fourth, and fifth anniversaries.

In its six years of interplanetary travels, the spacecraft has thrust for a total of 1,410 days, or 64 percent of the time (and about 0.000000028 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 318 kilograms (701 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 8.7 kilometers per second (19,500 mph). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished about three-quarters of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have completed six revolutions around the sun, covering about 37.7 AU (5.6 billion kilometers or 3.5 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 27.4 AU (4.1 billion kilometers or 2.5 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It will have to slow down still more to rendezvous with Ceres. Since Dawn’s launch, Vesta has traveled only 24.2 AU (3.6 billion kilometers or 2.2 billion miles), and the even more sedate Ceres has gone 22.8 AU (3.4 billion kilometers or 2.1 billion miles).

Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.

› Continue reading Marc Rayman’s Dawn Journal

Posted by Nelio Inacio at 11:39 AM 0 comments

For Dawn, a Time to Thrust and and a Time to Coast

For Dawn, a Time to Thrust and and a Time to Coast:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits


In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech
Dear All Hallows’ Dawns,

Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.

The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn’s heliocentric orbit was the same as the rocky behemoth’s. Now they are very far apart, and by early 2015, the robotic explorer’s path will be close enough to Ceres’s that they will become locked in a gravitational embrace.

Without ion propulsion, Dawn’s unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA’s Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.

During its years of interplanetary travel, Dawn has reliably followed a carefully formulated flight plan from Earth past Mars to Vesta and now from Vesta to Ceres. We discussed some of the principles underlying the development of the complex itinerary in a log written when Dawn was still gravitationally anchored to Earth. To carry out its ambitious adventure, Dawn should thrust most of the time, but not all of the time. Indeed, at some times, thrusting would be unproductive.

We will not delve into the details here, but remember that Dawn is doing more than ascending the solar system hill, climbing away from the sun. More challenging than that is making its orbit match the orbit of its targets so that it does not fly past them for a brief encounter as some other missions do. Performing its intricate interplanetary choreography requires exquisite timing with the grace and delicacy of the subtly powerful ion propulsion.

Of course Dawn does not thrust much of the time it is in orbit at Vesta and Ceres; rather, its focus there is on acquiring the precious pictures and other measurements that reveal the detailed nature of these mysterious protoplanets. But even during the interplanetary flight, there are two periods in the mission in which it is preferable to coast. Sophisticated analysis is required to compute the thrusting direction and schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the mass of the sun and the masses and orbits of Earth, Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when mission planners will not allow thrusting (e.g., during spacecraft maintenance periods).

The first interval that interplanetary trajectory designers designated as “optimal coast” was well over four years and 1.8 billion miles (2.8 billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8, 2009. During that time, the ship took some of Mars’s orbital energy to help propel itself toward Vesta. (In exchange for boosting Dawn, Mars slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters, in 180 million years.)

The second and final interval when coasting is better than thrusting begins next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit around the sun without modifying it. The timing of this coast period is nearly as important to keeping the appointment with Ceres as is the timing of the thrusting. In next month’s log, we will describe some of the special assignments the sophisticated robot will perform instead of its usual quiet cruise routine of accelerating and emitting xenon ions. We also will look ahead to some interesting celestial milestones and alignments in December.

While the spacecraft courses through the asteroid belt, the flight team continues refining the plans for Ceres. In logs in December and several months in 2014, we will present extensive details of those plans so that by the time Dawn begins its mission there, you will be ready to ride along and share in the experience.

In the meantime, as the stalwart ship sails on, it is propelled not only by ions but also by the promise of exciting new knowledge and the prospects of a thrilling new adventure in exploring an uncharted alien world.

Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million miles, or 460 million kilometers) from Earth, or 1,200 times as far as the moon and 3.10 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

– Dr. Marc D. Rayman

P.S. This log is posted early enough to allow time for your correspondent to don his Halloween costume. In contrast to last year’s simple (yet outlandish) costume, this year’s will be more complex. He is going in double costume, disguised as someone who is only pretending to be passionate about the exploration of the cosmos and the rewards of scientific insight.

› Read more entries from Marc Rayman’s Dawn Journal

Posted by Nelio Inacio at 11:39 AM 0 comments

For Dawn, a Time to Thrust and and a Time to Coast

For Dawn, a Time to Thrust and and a Time to Coast:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits


In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech
Dear All Hallows’ Dawns,

Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.

The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn’s heliocentric orbit was the same as the rocky behemoth’s. Now they are very far apart, and by early 2015, the robotic explorer’s path will be close enough to Ceres’s that they will become locked in a gravitational embrace.

Without ion propulsion, Dawn’s unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA’s Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.

During its years of interplanetary travel, Dawn has reliably followed a carefully formulated flight plan from Earth past Mars to Vesta and now from Vesta to Ceres. We discussed some of the principles underlying the development of the complex itinerary in a log written when Dawn was still gravitationally anchored to Earth. To carry out its ambitious adventure, Dawn should thrust most of the time, but not all of the time. Indeed, at some times, thrusting would be unproductive.

We will not delve into the details here, but remember that Dawn is doing more than ascending the solar system hill, climbing away from the sun. More challenging than that is making its orbit match the orbit of its targets so that it does not fly past them for a brief encounter as some other missions do. Performing its intricate interplanetary choreography requires exquisite timing with the grace and delicacy of the subtly powerful ion propulsion.

Of course Dawn does not thrust much of the time it is in orbit at Vesta and Ceres; rather, its focus there is on acquiring the precious pictures and other measurements that reveal the detailed nature of these mysterious protoplanets. But even during the interplanetary flight, there are two periods in the mission in which it is preferable to coast. Sophisticated analysis is required to compute the thrusting direction and schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the mass of the sun and the masses and orbits of Earth, Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when mission planners will not allow thrusting (e.g., during spacecraft maintenance periods).

The first interval that interplanetary trajectory designers designated as “optimal coast” was well over four years and 1.8 billion miles (2.8 billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8, 2009. During that time, the ship took some of Mars’s orbital energy to help propel itself toward Vesta. (In exchange for boosting Dawn, Mars slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters, in 180 million years.)

The second and final interval when coasting is better than thrusting begins next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit around the sun without modifying it. The timing of this coast period is nearly as important to keeping the appointment with Ceres as is the timing of the thrusting. In next month’s log, we will describe some of the special assignments the sophisticated robot will perform instead of its usual quiet cruise routine of accelerating and emitting xenon ions. We also will look ahead to some interesting celestial milestones and alignments in December.

While the spacecraft courses through the asteroid belt, the flight team continues refining the plans for Ceres. In logs in December and several months in 2014, we will present extensive details of those plans so that by the time Dawn begins its mission there, you will be ready to ride along and share in the experience.

In the meantime, as the stalwart ship sails on, it is propelled not only by ions but also by the promise of exciting new knowledge and the prospects of a thrilling new adventure in exploring an uncharted alien world.

Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million miles, or 460 million kilometers) from Earth, or 1,200 times as far as the moon and 3.10 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

– Dr. Marc D. Rayman

P.S. This log is posted early enough to allow time for your correspondent to don his Halloween costume. In contrast to last year’s simple (yet outlandish) costume, this year’s will be more complex. He is going in double costume, disguised as someone who is only pretending to be passionate about the exploration of the cosmos and the rewards of scientific insight.

› Read more entries from Marc Rayman’s Dawn Journal

Posted by Nelio Inacio at 11:33 AM 0 comments

NASA’s Dawn Fills out its Ceres Dance Card

NASA’s Dawn Fills out its Ceres Dance Card:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits


This graphic shows the planned trek of NASA’s Dawn spacecraft from its launch in 2007 through its arrival at the dwarf planet Ceres in early 2015. Image credit: NASA/JPL-Caltech
› Full image and caption
Dear Hand-Me-Dawns,

Gliding smoothly through the main asteroid belt between Mars and Jupiter, Dawn continues to make good progress on its ambitious mission of exploration. It is patiently but persistently pursuing Ceres, the second destination on its interplanetary itinerary.

Protoplanets Ceres and Vesta, the two most massive residents of the asteroid belt, were discovered at the beginning of the 19th century, and they have tantalized astronomers and others curious about the nature of the universe ever since. (Indeed, Ceres was the first dwarf planet discovered, having been found 129 years before Pluto.) They have waited patiently for a visitor from Earth since the dawn of the solar system. Dawn’s objective is to turn these uncharted orbs from tiny smudges of light amidst the stars into richly detailed places. It succeeded spectacularly at Vesta in 2011 - 2012, and it remains on course and on schedule for doing so at Ceres in 2015.

Next month, the adventurer will pass an invisible milestone on its celestial journey. On Dec. 27, it will be equidistant from these behemoths of the asteroid belt as all three follow their own independent heliocentric paths. The spacecraft will be 0.21 AU (19.4 million miles, or 31.3 million kilometers) away from each world, the one already visited and the one yet to be reached. And as the indefatigable ship sails on the cosmic seas with its sights set on Ceres, our anticipation for glimpsing the alien landscape ahead grows and grows, while the now-familiar scenery of Vesta shrinks into the distance, fading over the horizon.

The next day, Dawn will be equidistant from two other solar system bodies, both of which have been known (to our human readers, at least) for somewhat longer than Ceres and Vesta have. On Dec. 28, our celestial ambassador will be 2.46 AU (229 million miles, or 368 million kilometers) from Earth and the sun. (We cannot specify in which century either of them was discovered.)

Its complex route through the solar system has already taken the spacecraft farther from each of these bodies before. In the current phase of the mission, it is receding from the sun again, climbing the solar system hill from Vesta to Ceres. (It approached the sun in late 2012 and 2013 as part of the strategy for arriving at Ceres’s orbit when Ceres itself was there.) Having attained its greatest distance from Earth for the year in August, the spacecraft is temporarily getting closer to its planet of origin. (More precisely, as we discussed then, Earth is currently moving toward Dawn, because Earth travels faster in its solar orbit than Dawn does in its much more remote orbit.)

Dawn will reach two more impressive milestones in December, although neither pertains to its location. Soon the craft will surpass four years of ion thrust. While most spacecraft rely on conventional propulsion and hence coast most of the time (just as planets, moons, and asteroids do), Dawn’s mission would be impossible if it did that. In order to orbit and explore two distant destinations, the only terrestrial probe ever to attempt such a feat, it must accomplish a great deal of maneuvering. It spends the majority of its time using its uniquely efficient and capable ion propulsion system, constantly putting a gentle pressure on its trajectory to gradually reshape it. Although the spacecraft has already accumulated far more time in powered flight than any other mission, it still has a great deal more ahead.

And in December, that thrusting will push the craft’s speedometer past an extraordinary 20,000 mph (8.94 kilometers per second). (As we have seen in many previous logs, such as this one, this measure of the speed does not represent the actual spacecraft velocity. Nevertheless, it is a useful metric that avoids the complicating effects of orbital mechanics.) That is more than twice the previous record for propulsive velocity change set by Deep Space 1, the first interplanetary mission to use ion propulsion.

Dawn spends most of its time emitting a lovely blue-green beam of high-velocity xenon ions to propel itself. As foretold in the prophecy commonly known as the October log, however, we are now in one of just two periods of the long mission in which coasting is better for the trajectory than thrusting. Mission controllers took advantage of this time to instruct the robot to perform some special activities that would have been less convenient during routine ion thrusting. The reliable ship completed all of them flawlessly.

For more than 27 hours on Nov. 12 and 13, Dawn operated in a mode that had not even been conceived of when it was designed and built. It controlled its orientation in the frictionless, zero-gravity conditions of spaceflight using a scheme that was developed long after it left Earth. This “hybrid” control method operated perfectly, validating the extensive work engineers have invested in it and verifying its readiness for use at Ceres.

When it embarked on its bold journey more than six years ago, the ship was outfitted with four reaction wheels. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can turn or stabilize itself. It generally used three at a time, with a fourth kept in reserve. For such a long and complex expedition, extending to well over one million times farther from Earth than the International Space Station, backup systems are essential.

One of the wheels experienced increased friction in June 2010, but the mission continued with the other three. A second met the same fate in August 2012, as Dawn was climbing away from Vesta. Other spacecraft have encountered similar issues with their reaction wheels as well, and the consequences can be dire.

We have described the operations team’s swift and productive responses to the regrettable behavior of the reaction wheels in a number of logs (see, as one example, here). As soon as the first wheel faltered, JPL and Orbital Sciences Corporation began working on a method to operate with fewer than three in case another one had difficulty. They developed software to operate in a hybrid mode of two wheels plus the small hydrazine-powered jets of the reaction control system and installed it in the craft’s main computer in April 2011 so it would be available at Vesta if needed.

Given the problems with reaction wheels on Dawn and other spacecraft, engineers do not have high confidence that the two remaining units will operate for long (although it certainly is possible they will). Thanks to their remarkable ingenuity and resourcefulness, the team has devised a detailed plan that should allow Dawn to complete its extraordinary mission using only the hydrazine thrusters, achieving all of its objectives in exploring Ceres regardless of the condition of its wheels. (Note that it is not even obvious that doing so is possible, but then again, it isn’t obvious that sending probes so far from our home planet is possible either. Part of the thrill of a solar system adventure is overcoming the extremely daunting challenges.) So now, hybrid control would provide an enhancement, extending the supply of precious hydrazine propellant and giving the spacecraft the opportunity to operate even longer at Ceres than it would without the two functioning wheels. When the hydrazine is exhausted, the mission will conclude.

Dawn will use hybrid control only in its lowest altitude orbits at Ceres, the final phase of the mission. (Beginning in December and continuing in 2014, we will describe all phases of the Ceres plan in detail.) Hybrid control will be called upon to perform three kinds of tasks for the spacecraft: train the suite of sophisticated sensors at the mysterious world beneath it, point the main antenna to distant Earth to transmit its findings and receive updated instructions, and rotate from one orientation to another. The innovative system has now unerringly demonstrated its capability to accomplish all three by executing exactly those functions earlier this month.

The confirmation that hybrid control works as intended is not the only task Dawn is carrying out during this coasting period. All of its scientific instruments (including even the backup camera) are being powered on and given thorough health checks, verifying that they remain fully functional and ready to reveal Ceres’s secrets. Engineers also conducted some tests with the ion engine that has operated the longest of the three to confirm expectations of how it will perform at Ceres.

On Dec. 9, Dawn’s four-week coast period will end. Once again it will turn to point an ion engine in the direction needed to push forward to its rendezvous with the distant and exotic world ahead. As the probe nears and then passes the halfway point on its remarkable journey from Vesta to Ceres, it is pulled by forces even more powerful than ion propulsion: the attraction of discovery, the lure of the unknown, and the draw of tremendous new insights and profound new understandings to be gained in a daring adventure far, far from home.

Dawn is 18 million miles (29 million kilometers) from Vesta and 22 million miles (35 million kilometers) from Ceres. It is also 2.78 AU (258 million miles, or 415 million kilometers) from Earth, or 1,125 times as far as the moon and 2.82 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.

– Dr. Marc D. Rayman

› Read more entries from Marc Rayman’s Dawn Journal

Posted by Nelio Inacio at 11:32 AM 0 comments

NASA’s Dawn Plans for Planetary Shores Ahead

NASA’s Dawn Plans for Planetary Shores Ahead:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

NASA Dawn spacecraft between its targets, Vesta and Ceres


Artist’s concept of NASA’s Dawn spacecraft between the giant asteroid Vesta and the dwarf planet Ceres. Image credit: NASA/JPL-Caltech
Dear Clairvoydawnts,

Now more than halfway through its journey from protoplanet Vesta to dwarf planet Ceres, Dawn is continuing to use its advanced ion propulsion system to reshape its orbit around the sun. Now that the ship is closer to the uncharted shores ahead than the lands it unveiled astern, we will begin looking at the plans for exploring another alien world. In seven logs from now through August, we will discuss how the veteran adventurer will accomplish its exciting mission at Ceres. By the time it arrives early in 2015 at the largest object between Mars and Jupiter, readers will be ready to share not only in the drama of discovery but also in the thrill of an ambitious undertaking far, far from Earth.

Mission planners separate this deep-space expedition into phases. Following the “launch phase” was the 80-day “checkout phase”. The “interplanetary cruise phase” is the longest. It began on December 17, 2007, and continued to the “Vesta phase,” which extended from May 3, 2011, to Sept. 4, 2012. We are back in the interplanetary cruise phase again and will be until the “Ceres phase” begins in 2015. (Other phases may occur simultaneously with those phases, such as the “oh man, this is so cool phase,” the “we should devise a clever name for this phase phase,” and the “lunch phase.”) Because the tasks at Vesta and Ceres are so complex and diverse, they are further divided into sub-phases. The phases at Ceres will be very similar to those at Vesta, even though the two bodies are entirely different.

In this log, we will describe the Ceres “approach phase.” The objectives of approach are to get the explorer into orbit and to attain a preliminary look at the mysterious orb, both to satisfy our eagerness for a glimpse of a new and exotic world and to obtain data that will be helpful in refining details of the subsequent in-depth investigations. The phase will start in January 2015 when Dawn is about 400,000 miles (640,000 kilometers) from Ceres. It will conclude in April when the spacecraft has completed the ion thrusting necessary to maneuver into the first orbit from which it will conduct intensive observations, at an altitude of about 8,400 miles (13,500 kilometers). For a reason to be revealed below, that orbit is known by the catchy cognomen RC3.

(Previews for the Vesta approach phase were presented in March 2010 and May 2011, and the accounts of its actual execution are in logs from June, July, and August 2011. Future space historians should note that the differing phase boundaries at Vesta are no more than a matter of semantics. At Vesta, RC3 was described as being part of the approach phase. For Ceres, RC3 is its own distinct phase. The reasons for the difference in terminology are not only unimportant, they aren’t even interesting.)

The tremendous maneuverability provided by Dawn’s uniquely capable ion propulsion system means that the exact dates for events in the approach phase likely will change between now and then. So for those of you in 2015 following a link back to this log to see what the approach plan has been, we offer both the reminder that the estimated dates here might shift by a week or so and a welcome as you visit us here in the past. We look forward to meeting you (or even being you) when we arrive in the future.

Most of the approach phase will be devoted to ion thrusting, making the final adjustments to Dawn’s orbit around the sun so that Ceres’s gravity will gently take hold of the emissary from distant Earth. Next month we will explain more about the unusual nature of the gradual entry into orbit, which will occur on about March 25, 2015.

Starting in early February 2015, Dawn will suspend thrusting occasionally to point its camera at Ceres. The first time will be on Feb. 2, when they are 260,000 miles (420,000 kilometers) apart. To the camera’s eye, designed principally for mapping from a close orbit and not for long-range observations, Ceres will appear quite small, only about 24 pixels across. But these pictures of a fuzzy little patch will be invaluable for our celestial navigators. Such “optical navigation” images will show the location of Ceres with respect to background stars, thereby helping to pin down where it and the approaching robot are relative to each other. This provides a powerful enhancement to the navigation, which generally relies on radio signals exchanged between Dawn and Earth. Each of the 10 times Dawn observes Ceres during the approach phase will help navigators refine the probe’s course, so they can update the ion thrust profile to pilot the ship smoothly to its intended orbit.

Whenever the spacecraft stops to acquire images with the camera, it also will train the visible and infrared mapping spectrometer on Ceres. These early measurements will be helpful for finalizing the instrument parameters to be used for the extensive observations at closer range in subsequent mission phases.

Dawn obtained images more often during the Vesta approach phase than it will on approach to Ceres, and the reason is simple. It has lost two of its four reaction wheels, devices used to help turn or stabilize the craft in the zero-gravity, frictionless conditions of spaceflight. (In full disclosure, the units aren’t actually lost. We know precisely where they are. But given that they stopped functioning, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Dawn’s hominin colleagues at JPL, along with excellent support from Orbital Sciences Corporation, have applied their remarkable creativity, tenacity, and technical acumen to devise a plan that should allow all the original objectives of exploring Ceres to be met regardless of the health of the wheels. One of the many methods that contributed to this surprising resilience was a substantial reduction in the number of turns during all remaining phases of the mission, thus conserving the precious hydrazine propellant used by the small jets of the reaction control system.

When Dawn next peers at Ceres, nine days after the first time, it will be around 180,000 miles (290,000 kilometers) away, and the pictures will be marginally better than the sharpest views ever captured by the Hubble Space Telescope. By the third optical navigation session, on Feb. 21, Ceres will show noticeably more detail.

At the end of February, Dawn will take images and spectra throughout a complete Ceres rotation of just over nine hours, or one Cerean day. During that period, while about 100,000 miles (160,000 kilometers) distant, Dawn’s position will not change significantly, so it will be almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath its watchful eye. Dawn will see most of the surface with a resolution twice as good as what has been achieved with Hubble. (At that point in the curving approach trajectory, the probe will be south of Ceres’s equator, so it will not be able to see the high northern latitudes.) This first “rotation characterization,” or RC1, not only provides the first (near-complete) look at the surface, but it may also suggest to insightful readers what will occur during the RC3 orbit phase.

There will be six more imaging sessions before the end of the approach phase, with Ceres growing larger in the camera’s view each time. When the second complete rotation characterization, RC2, is conducted on March 16, the resolution will be four times better than Hubble’s pictures. The last photos, to be collected on March 24, will reveal features seven times smaller than could be discerned with the powerful space observatory.

The approach imaging sessions will be used to accomplish even more than navigating, providing initial characterizations of the mysterious world, and whetting our appetites for more. Six of the opportunities also will include searches for moons of Ceres. Astronomers have not found moons of this dwarf planet in previous attempts, but Dawn’s unique vantage point would allow it to discover smaller ones than would have been detectable in previous attempts.

When the approach phase ends, Dawn will be circling its new home, held in orbit by the massive body’s gravitational grip and ready to begin more detailed studies. By then, however, the pictures and other data it will have returned will already have taught Earthlings a great deal about that enigmatic place. Ceres has been observed from Earth for more than two centuries, having first been spotted on January 1, 1801, but it has never appeared as much more than an indistinct blob amidst the stars. Soon a probe dispatched by the insatiably curious creatures on that faraway planet will take up residence there to uncover some of the secrets it has held since the dawn of the solar system. We don’t have long to wait!

Dawn is 20 million miles (32 million kilometers) from Vesta and 19 million miles (31 million kilometers) from Ceres. It is also 2.42 AU (225 million miles, or 362 million kilometers) from Earth, or 1,015 times as far as the moon and 2.46 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 40 minutes to make the round trip.

› Read more entries from Marc Rayman’s Dawn Journal

Posted by Nelio Inacio at 11:31 AM 0 comments

WOW It’s All About Grace Under Pressure for Dawn’s Drop Into Orbit

It’s All About Grace Under Pressure for Dawn’s Drop Into Orbit:

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft at the dwarf planet Ceres


Artist’s concept of NASA’s Dawn spacecraft thrusting with its ion propulsion system as it approaches the dwarf planet Ceres. Image credit: NASA/JPL-Caltech
Dear Rendawnvous,

Dawn is continuing its trek through the main asteroid belt between Mars and Jupiter. Leaving behind a blue-green wake of xenon from its ion propulsion system, its sights are set on dwarf planet Ceres ahead. The journey has been long, but the veteran space traveler (and its support team on distant Earth) is making good progress for its rendezvous early next year.

Last month, we had a preview of many of the activities the probe will execute during the three months that culminate in settling into the first observational orbit at Ceres in April 2015. At that orbit, about 8,400 miles (13,500 kilometers) above the alien landscapes of rock and ice, Dawn will begin its intensive investigations. Nevertheless, even during the “approach phase,” it will often observe Ceres with its camera and one of its spectrometers to gain a better fix on its trajectory and to perform some preliminary characterizations of the mysterious world prior to initiating its in-depth studies. The discussion in December did not cover the principal activity, however, which is one very familiar not only to the spacecraft but also to readers of these logs. The majority of the time in the approach phase will be devoted to continuing the ion-powered flight. We described this before Vesta, but for those few readers who don’t have perfect recall (we know who you are), let’s take another look at how this remarkable technology is used to deliver the adventurer to the desired orbit around Ceres.

Thrusting is not necessary for a spacecraft to remain in orbit, just as the moon remains in orbit around Earth and Earth and other planets remain in orbit around the sun without the benefit of propulsion. All but a very few spacecraft spend most of their time in space coasting, following the same orbit over and over unless redirected by a gravitational encounter with another body. In contrast, with its extraordinarily efficient ion propulsion system, Dawn’s near-continuous thrusting gradually changes its orbit. Thrusting since December 2007 has propelled Dawn from the orbit in which the Delta rocket deposited it after launch to orbits of still greater distance from the sun. The flight profile was carefully designed to send the craft by Mars in February 2009, so our celestial explorer could appropriate some of the planet’s orbital energy for the journey to the more distant asteroid belt, of which it is now a permanent resident. In exchange for Mars raising Dawn’s heliocentric orbit, Dawn lowered Mars’s orbit, ensuring the solar system’s energy account remained balanced.

While spacecraft have flown past a few asteroids in the main belt (although none as large as the gargantuan Vesta or Ceres, the two most massive objects in the belt), no prior mission has ever attempted to orbit one, much less two. For that matter, this is the first mission ever undertaken to orbit any two extraterrestrial destinations. Dawn’s exclusive assignment would be quite impossible without its uniquely capable ion propulsion system. But with its light touch on the accelerator, taking nearly four years to travel from Earth past Mars to Vesta, and more than two and a half years from Vesta to Ceres, how will it enter orbit around Ceres? As we review this topic in preparation for Ceres, bear in mind that this is more than just a cool concept or neat notion. This is real. The remarkable adventurer actually accomplished the extraordinary feats at Vesta of getting into and out of orbit using the delicate thrust of its ion engines.

Whether conventional spacecraft propulsion or ion propulsion is employed, entering orbit requires accompanying the destination on its own orbit around the sun. This intriguing challenge was addressed in part in February 2007. In February 2013, we considered another aspect of what is involved in climbing the solar system hill, with the sun at the bottom, Earth partway up, and the asteroid belt even higher. We saw that Dawn needs to ascend that hill, but it is not sufficient simply to reach the elevation of each target nor even to travel at the same speed as each target; the explorer also needs to travel in the same direction. Probes that leave Earth to orbit other solar system bodies traverse outward from (or inward toward) the sun, but then need to turn in order to move along with the body they will orbit, and that is difficult.

Those of you who have traveled around the solar system before are familiar with the routine of dropping into orbit. The spacecraft approaches its destination at very high velocity and fires its powerful engine for some minutes or perhaps even about an hour, by the end of which it is traveling slowly enough that the planet’s gravity can hold it in orbit and carry it around the sun. These exciting events may range from around 1,300 to 3,400 mph (0.6 to 1.5 kilometers per second). With ten thousand times less thrust than a typical propulsion system on an interplanetary spacecraft, Dawn could never accomplish such a rapid maneuver. As it turns out, however, it doesn’t have to.

Dawn’s method of getting into orbit is quite different, and the key is expressed in an attribute of ion propulsion that has been referred to 63 times (trust or verify; it’s your choice) before in these logs: it is gentle. (This example shows just how gentle the acceleration is.) With the gradual trajectory modifications inherent in ion propulsion, sharp changes in direction and speed are replaced by smooth, gentle curves. The thrust profiles for Dawn’s long interplanetary flights are devoted to the gradual reshaping of its orbit around the sun so that by the time it is in the vicinity of its target, its orbit is nearly the same as that of the target. Rather than hurtling toward Vesta or Ceres, Dawn approaches with grace and elegance. Only a small trajectory adjustment is needed to let its new partner’s gravity capture it, so even that gentle ion thrust will be quite sufficient to let the craft slip into orbit. With only a nudge, it transitions from its large, slow spiral away from the sun to an inward spiral centered around its new gravitational master.

illustration of Dawn's orbit


This graphic shows the planned trek of NASA’s Dawn spacecraft from its launch in 2007 through its arrival at the dwarf planet Ceres in early 2015. Note how Dawn spirals outward to Vesta and then still more to Ceres. Image credit: NASA/JPL-Caltech
To get into orbit, a spacecraft has to match speed, direction and location with its target. A mission with conventional propulsion first gets to the location and then, using the planet’s gravity and its own fuel-guzzling propulsion system, very rapidly achieves the required speed and direction. By spiraling outward from the sun, first to the orbit of Vesta and now to Ceres, Dawn works on its speed, direction and location all at the same time, so they all gradually reach the needed values at just the right time.

To illustrate this facet of the difference between how the different systems are applied to arrive in orbit, let’s imagine you want to drive your car next to another traveling west at 60 mph (100 kilometers per hour). The analogy with the conventional technology would be similar to speeding north toward an intersection where you know the other car will be. You arrive there at the same time and then execute a screeching, whiplash-inducing left turn at the last moment using the brakes, steering wheel, accelerator and adrenaline. When you drive an ion propelled car (with 10 times higher fuel efficiency), you take an entirely different path from the start, one more like a long, curving entrance ramp to a highway. As you enter the ramp, you slowly (perhaps even gently) build speed. You approach the highway gradually, and by the time you have reached the far end of the ramp, your car is traveling at the same speed and in the same direction as the other car. Of course, to ensure you are there when the other car is, the timing is very different from the first method, but the sophisticated techniques of orbital navigation are up to the task.

› Continue reading Marc Rayman’s January 2014 Dawn Journal

Posted by Nelio Inacio at 11:30 AM 0 comments
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