JPL has grown a great deal since this photo was taken in 1950. Just compare the photo to any current map web site or app, and notice the roads and buildings that have been moved, added to, or are no longer there. The JPL Archives collections of online maps, telephone books, and photo albums can help us explore JPL’s past – when there were wind tunnels on Lab, the JPL Store was a cafeteria, and near the parking structure site there was a water-filled towing channel.
With the help of the Huntington Library, we can go back even farther, to 1931. The Huntington Digital Library has a photo of the JPL site, taken from across the lake behind the Devil’s Gate Dam. Zoom in on the foothills on the right side of the photo to see the area that would become the Jet Propulsion Laboratory. Within five years, the lake was a dry river bed, the Arroyo Seco, and was chosen as the site of the famous rocket motor tests that led to the beginning of JPL. Four years after that, the first small wood frame buildings appeared along the edge of the arroyo.
Dear Fine and Dawndy Readers,
The Dawn spacecraft is performing flawlessly as it conducts the first exploration of the first dwarf planet. Each new picture of Ceres reveals exciting and surprising new details about a fascinating and enigmatic orb that has been glimpsed only as a smudge of light for more than two centuries. And yet as that fuzzy little blob comes into sharper focus, it seems to grow only more perplexing.
Dawn is showing us exotic scenery on a world that dates back to the dawn of the solar system, more than 4.5 billion years ago. Craters large and small remind us that Ceres lives in the rough and tumble environment of the main asteroid belt between Mars and Jupiter, and collectively they will help scientists develop a deeper understanding of the history and nature not only of Ceres itself but also of the solar system.
Even as we discover more about Ceres, some mysteries only deepen. It certainly does not require sophisticated scientific insight to be captivated by the bright spots. What are they? At this point, the clearest answer is that the answer is unknown. One of the great rewards of exploring the cosmos is uncovering new questions, and this one captures the imagination of everyone who gazes at the pictures sent back from deep space.
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Other intriguing features newly visible on the unfamiliar landscape further assure us that there will be much more to see and to learn -- and probably much more to puzzle over -- when Dawn flies in closer and acquires new photographs and myriad other measurements. Over the course of this year, as the spacecraft spirals to lower and lower orbits, the view will continue to improve. In the lowest orbit, the pictures will display detail well over one hundred times finer than the RC2 pictures returned a few days ago (and shown below). Right now, however, Dawn is not getting closer to Ceres. On course and on schedule for entering orbit on March 6, Earth's robotic ambassador is slowly separating from its destination.
"Slowly" is the key. Dawn is in the vicinity of Ceres and is not leaving. The adventurer has traveled more than 900 million miles (1.5 billion kilometers) since departing from Vesta in 2012, devoting most of the time to using its advanced ion propulsion system to reshape its orbit around the sun to match Ceres' orbit. Now that their paths are so similar, the spacecraft is receding from the massive behemoth at the leisurely pace of about 35 mph (55 kilometers per hour), even as they race around the sun together at 38,700 mph (62,300 kilometers per hour). The probe is expertly flying an intricate course that would be the envy of any hotshot spaceship pilot. To reach its first observational orbit -- a circular path from pole to pole and back at an altitude of 8,400 miles (13,500 kilometers) -- Dawn is now taking advantage not only of ion propulsion but also the gravity of Ceres.
On Feb. 23, the spacecraft was at its closest to Ceres yet, only 24,000 miles (less than 39,000 kilometers), or one-tenth of the separation between Earth and the moon. Momentum will carry it farther away for a while, so as it performs the complex cosmic choreography, Dawn will not come this close to its permanent partner again for six weeks. Well before then, it will be taken firmly and forever into Ceres' gentle gravitational hold.
The photographs Dawn takes during this approach phase serve several purposes. Besides fueling the fires of curiosity that burn within everyone who looks to the night sky in wonder or who longs to share in the discoveries of celestial secrets, the images are vital to engineers and scientists as they prepare for the next phase of exploration.
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The primary purpose of the pictures is for "optical navigation" (OpNav), to ensure the ship accurately sails to its planned orbital port. Dawn is the first spacecraft to fly into orbit around a massive solar system world that had not previously been visited by a spacecraft. Just as when it reached its first deep-space target, the fascinating protoplanet Vesta, mission controllers have to discover the nature of the destination as they proceed. They bootstrap their way in, measuring many characteristics with increasing accuracy as they go, including its location, its mass and the direction of its rotation axis.
Let's consider this last parameter. Think of a spinning ball. (If the ball is large enough, you could call it a planet.) It turns around an axis, and the two ends of the axis are the north and south poles. The precise direction of the axis is important for our mission because in each of the four observation orbits (previews of which were presented in February, May, June and August), the spacecraft needs to fly over the poles. Polar orbits ensure that as Dawn loops around, and Ceres rotates beneath it every nine hours, the explorer eventually will have the opportunity to see the entire surface. Therefore, the team needs to establish the location of the rotation axis to navigate to the desired orbit.
We can imagine extending the rotation axis far outside the ball, even all the way to the stars. Current residents of Earth, for example, know that their planet's north pole happens to point very close to a star appropriately named Polaris (or the North Star), part of an asterism known as the Little Dipper in the constellation Ursa Minor (the Little Bear). The south pole, of course, points in exactly the opposite direction, to the constellation Octans (the Octant), but is not aligned with any salient star.
With their measurements of how Ceres rotates, the team is zeroing in on the orientation of its poles. We now know that residents of (and, for that mater, visitors to) the northern hemisphere there would see the pole pointing toward an unremarkable region of the sky in Draco (the Dragon). Those in the southern hemisphere would note the pole pointing toward a similarly unimpressive part of Volans (the Flying Fish). (How appropriate it is that that pole is directed toward a constellation with that name will be known only after scientists advance their understanding of the possibility of a subsurface ocean at Ceres.)
The orientation of Ceres'; axis proves convenient for Dawn's exploration. Earthlings are familiar with the consequences of their planet's axis being tilted by about 23 degrees. Seasons are caused by the annual motion of the sun between 23 degrees north latitude and 23 degrees south. A large area around each pole remains in the dark during winter. Vesta's axis is tipped 27 degrees, and when Dawn arrived, the high northern latitudes were not illuminated by the sun. The probe took advantage of its extraordinary maneuverability to fly to a special mapping orbit late in its residence there, after the sun had shifted north. That will not be necessary at Ceres. That world's axis is tipped at a much smaller angle, so throughout a Cerean year (lasting 4.6 Earth years), the sun stays between 4 degrees north latitude and 4 degrees south. Seasons are much less dramatic. Among Dawn's many objectives is to photograph Ceres. Because the sun is always near the equator, the illumination near the poles will change little. It is near the beginning of southern hemisphere winter on Ceres now, but the region around the south pole hidden in hibernal darkness is tiny. Except for possible shadowing by local variations in topography (as in deep craters), well over 99 percent of the dwarf planet's terrain will be exposed to sunlight each day.
Guiding Dawn from afar, the operations team incorporates the new information about Ceres into occasional updates to the flight plan, providing the spacecraft with new instructions on the exact direction and throttle level to use for the ion engine. As they do so, subtle aspects of the trajectory change. Last month we described the details of the plan for observing Ceres throughout the four-month approach phase and predicted that some of the numbers could change slightly. So, careful readers, for your convenience, here is the table from January, now with minor updates.
|Beginning of activity in Pacific Time zone||Distance from Dawn to Ceres in miles (kilometers)||Ceres diameter in pixels||Resolution in miles (kilometers) per pixel||Resolution compared to Hubble||Illuminated portion of disk||Activity|
|Dec 1, 2014||740,000
|Jan 13, 2015||238,000
In addition to changes based on discoveries about the nature of Ceres, some changes are dictated by more mundane considerations (to the extent that there is anything mundane about flying a spacecraft in the vicinity of an alien world more than a thousand times farther from Earth than the moon). For example, to accommodate changes in the schedule for the use of the Deep Space Network, some of the imaging sessions shifted by a few hours, which can make small changes in the corresponding views of Ceres.
The only important difference between the table as presented in January and this month, however, is not to be found in the numbers. It is that OpNav 3, RC1 and RC2 are now in the past, each having been completed perfectly.
As always, if you prefer to save yourself the time and effort of the multi-billion-mile (multi-billion-kilometer) interplanetary journey to Ceres, you can simply go here to see the latest views from Dawn. (The Dawn project is eager to share pictures promptly with the public. The science team has the responsibility of analyzing and interpreting the images for scientific publication. The need for accuracy and scientific review of the data slows the interpretation and release of the pictures. But just as with all of the marvelous findings from Vesta, everything from Ceres will be available as soon as practicable.)
In November we delved into some of the details of Dawn's graceful approach to Ceres, and last month we considered how the trajectory affected the scene presented to Dawn's camera. Now that we have updated the table, we can enhance a figure from both months that showed the craft's path as it banks into orbit and maneuvers to its first observational orbit. (As a reminder, the diagram illustrates only two of the three dimensions of the ship's complicated route. Another diagram in November showed another perspective, and we will include a different view next month.)
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We can zoom out to see where the earlier OpNavs were.
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As the table and figures indicate, in OpNav 6, when Ceres and the sun are in the same general direction from Dawn's vantage point, only a small portion of the illuminated terrain will be visible. The left side of Ceres will be in daylight, and most of the hemisphere facing the spacecraft will be in the darkness of night. To get an idea of what the shape of the crescent will be, terrestrial readers can use the moon on March 16. It will be up much of the day, setting in the middle of the afternoon, and it will be comparable to the crescent Dawn will observe on April 10. (Of course, the exact shape will depend on your observing location and what time you look, but this serves as a rough preview.) Fortunately, our spacecraft does not have to contend with bad weather, but you might, so we have generously scheduled a backup opportunity for you. The moon will be new on March 20, and the crescent on March 23 will be similar to what it was on March 16. It will rise in the mid morning and be up until well after the sun sets.
Photographing Ceres as it arcs into orbit atop a blue-green beam of xenon ions, setting the stage for more than a year of detailed investigations with its suite of sophisticated sensors, Dawn is sailing into the history books. No spacecraft has reached a dwarf planet before. No spacecraft has orbited two extraterrestrial destinations before. This amazing mission is powered by the insatiable curiosity and extraordinary ingenuity of creatures on a planet far, far away. And it carries all of them along with it on an ambitious journey that grows only more exciting as it continues. Humankind is about to witness scenes never before seen and perhaps never even imagined. Dawn is taking all of us on a daring adventure to a remote and unknown part of the cosmos. Prepare to be awed.
Dawn is 24,600 miles (39,600 kilometers) from Ceres, or 10 percent of the average distance between Earth and the moon. It is also 3.42 AU (318 million miles, or 512 million kilometers) from Earth, or 1,330 times as far as the moon and 3.46 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 57 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 a.m. PST February 25, 2015
In 1964, at least two companies were working under contract to JPL on a Surveyor Lunar Roving Vehicle Study: Bendix Corporation Systems Division, and General Motors Corporation Defense Research Laboratories. This photo shows a prototype General Motors rover, one of several different approaches that were studied to determine their capabilities, limitations, and their impact on overall spacecraft design and performance. Twelve different spacecraft configurations were studied in detail, with variations in weight, power systems, communication method, and spaceframe size.
The final design of the Surveyor 1 through 7 lunar landers did not include a rover. NASA sponsored other lunar rover studies during the 1960s, with a variety of sizes and technical capabilities, and Apollo 15 astronauts became the first to drive a Lunar Roving Vehicle on the moon, during their 1971 mission. JPL continued to develop robotic spacecraft and rovers and, in 1997, landed Mars Pathfinder and its Sojourner rover on the red planet.
Dear Abundawnt Readers
The dwarf planet Ceres is a giant mystery. Drawn on by the irresistible lure of exploring this exotic, alien world, Dawn is closing in on it. The probe is much closer to Ceres than the moon is to Earth.
And now it is even closer …
And now it is closer still!
What has been glimpsed as little more than a faint smudge of light amid the stars for more than two centuries is finally coming into focus. The first dwarf planet discovered (129 years before Pluto), the largest body between the sun and Pluto that a spacecraft has not yet visited, is starting to reveal its secrets. Dawn is seeing sights never before beheld, and all of humankind is along for the extraordinary experience.
We have had a preview of Dawn’s approach phase, and in November we looked at the acrobatics the spacecraft performs as it glides gracefully into orbit. Now the adventurer is executing those intricate plans, and it is flying beautifully, just the way a seasoned space traveler should.
Dawn’s unique method of patiently, gradually reshaping its orbit around the sun with its ion propulsion system is nearly at its end. Just as two cars may drive together at high speed and thus travel at low speed relative to each other, Dawn is now close to matching Ceres’ heliocentric orbital motion. Together, they are traveling around the sun at nearly 39,000 mph (almost 64,000 kilometers per hour), or 10.8 miles per second (17.4 kilometers per second). But the spaceship is closing in on the world ahead at the quite modest relative speed of about 250 mph (400 kilometers per hour), much less than is typical for interplanetary spaceflight.
Dawn has begun its approach imaging campaign, and the pictures are wonderfully exciting. This month, we will take a more careful look at the plans for photographing Ceres. Eager readers may jump directly to the summary table, but others may want to emulate the spacecraft by taking a more leisurely approach to it, which may aid in understanding some details.
While our faithful Dawn is the star of this bold deep-space adventure (along with protoplanet Vesta and dwarf planet Ceres), the real talent is behind the scenes, as is often the case with celebrities. The success of the mission depends on the dedication and expertise of the members of the Dawn flight team, no farther from Earth than the eighth floor of JPL’s building 264 (although occasionally your correspondent goes on the roof to enjoy the sights of the evening sky). They are carefully guiding the distant spacecraft on its approach trajectory and ensuring it accomplishes all of its tasks.
To keep Dawn on course to Ceres, navigators need a good fix on where the probe and its target are. Both are far, far from Earth, so the job is not easy. In addition to the extraordinarily sophisticated but standard methods of navigating a remote interplanetary spacecraft, using the radio signal to measure its distance and speed, Dawn’s controllers use another technique now that it is in the vicinity of its destination.
From the vantage point of Earth, astronomers can determine distant Ceres’ location remarkably well, and Dawn’s navigators achieve impressive accuracy in establishing the craft’s position. But to enter orbit, still greater accuracy is required. Therefore, Dawn photographs Ceres against the background of known stars, and the pictures are analyzed to pin down the location of the ship relative to the celestial harbor it is approaching. To distinguish this method from the one by which Dawn is usually navigated, this supplementary technique is generally known as “optical navigation.” Unable to suppress their geekiness (or, at least, unmotivated to do so), Dawn team members refer to this as OpNav. There are seven dedicated OpNav imaging sessions during the four-month approach phase, along with two other imaging sessions. (There will also be two more OpNavs in the spiral descent from RC3 to survey orbit.)
The positions of the spacecraft and dwarf planet are already determined well enough with the conventional navigation methods that controllers know which particular stars are near Ceres from Dawn’s perspective. It is the analysis of precisely where Ceres appears relative to those stars that will yield the necessary navigational refinement. Later, when Dawn is so close that the colossus occupies most of the camera’s view, stars will no longer be visible in the pictures. Then the optical navigation will be based on determining the location of the spacecraft with respect to specific surface features that have been charted in previous images.
To execute an OpNav, Dawn suspends ion thrusting and turns to point its camera at Ceres. It usually spends one or two hours taking photos (and bonus measurements with its visible and infrared mapping spectrometer). Then it turns to point its main antenna to Earth and transmits its findings across the solar system to the Deep Space Network.
While it is turning once again to resume ion thrusting, navigators are already starting to extract information from the images to calculate where the probe is relative to its destination. Experts update the design of the trajectory the spacecraft must follow to reach its intended orbital position and fine-tune the corresponding ion thrust flight plan. At the next communications session, the revised instructions are radioed back across the solar system, and then the reliable robot carries them out. This process is repeated throughout the approach phase.
Dawn turned to observe Vesta during that approach phase more often than it does 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 sentient colleagues at JPL, along with excellent support from Orbital Sciences Corporation, have applied their remarkable creativity, tenacity and technical acumen to devise a strategy that allows all the original objectives of exploring Ceres to be met regardless of the condition of the wheels, even the (currently) healthy ones. Your correspondent refers to this as the “zero reaction wheel plan” 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. Guided by their successful experience at Vesta, experts determined that they could accommodate fewer OpNavs during the approach to Ceres, thus saving turns. (We will return to the topic of hydrazine conservation below.)
The images serve several purposes besides navigation. Of course, they provide a tantalizing preview of the intriguing world observed from Earth since 1801. Each picture whets our appetite! What will Ceres look like as it comes into sharper focus? Will we see evidence of a subsurface ocean? What unexpected shapes and structures will we find? What strange new features will show up? Just what is that bright spot? Quite simply: we don’t know. It would be a pretty good idea to send a spacecraft there to find out!
Scientists scrutinize all the photos for moons of Ceres, and OpNavs 3-7 will include many extra images with exposures chosen to help reveal moons. In addition, hundreds more pictures will be taken of the space around Ceres in the hours before and after OpNav 3 to allow an even more thorough search.
On two occasions during the approach, Dawn will take images and spectra throughout a complete Ceres rotation of slightly over nine hours, or one Cerean day. During that time, 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, exhibiting most of the surface. These “rotation characterizations” (known by the stirring names RC1 and RC2) will provide the first global perspectives.
As Dawn flies into orbit, it arcs around Ceres. In November, we described the route into orbit in detail, and one of the figures there is reproduced here. Dawn will slip into Ceres’ gravitational embrace on the night of March 5 (PST). But as the figure shows, its initial elliptical orbit will carry it to higher altitudes before it swoops back down. As a result, pictures of Ceres will grow for a while, then shrink and then grow again.
Because of the changing direction to Ceres, Dawn does not always see a fully illuminated disk, just as the moon goes through its familiar phases as its position relative to the sun changes. The hemisphere of the moon facing the sun is bright and the other is dark. The half facing Earth may include part of the lit side and part of the dark side. Sometimes we see a full moon, sometimes gibbous, and sometimes a thin crescent.
The table shows what fraction of Ceres is illuminated from Dawn’s perspective. Seeing a full moon would correspond to 100 percent illumination. A half moon would be 50 percent, and a new moon would be zero percent. In OpNav 6, when Ceres is 18 percent illuminated, it will be a delicate crescent, like the moon about four days after it’s new.
OpNav images of a narrow crescent won’t contain enough information to warrant the expenditure of hydrazine in all that turning. Moreover, the camera’s precision optics and sensitive detector, designed for revealing the landscapes of Vesta and Ceres, cannot tolerate looking too close to the sun, even as far from the brilliant star as it is now. Therefore, no pictures will be taken in March and early April when Dawn is far on the opposite side of Ceres from the sun. By the end of April, the probe will have descended to its first observational orbit (RC3), where it will begin its intensive observations.
The closer Dawn is to Ceres, the larger the orb appears to its camera, and the table includes the (approximate) diameter the full disk would be, measured in the number of camera pixels. To display greater detail, each pixel must occupy a smaller portion of the surface. So the “resolution” of the picture indicates how sharp Dawn’s view is.
We also describe the pictures in comparison to the best that have been obtained with Hubble Space Telescope. In Hubble’s pictures, each pixel covered about 19 miles (30 kilometers). Now, after a journey of more than seven years through the solar system, Dawn is finally close enough to Ceres that its view surpasses that of the powerful telescope. By the time Dawn is in its lowest altitude orbit at the end of this year, its pictures will be well over 800 times better than Hubble’s and more than 600 times better than the OpNav 2 pictures from Jan. 25. This is going to be a fantastic year of discovery!
|Beginning of activity in Pacific Time zone||Distance from Dawn to Ceres in miles (kilometers)||Ceres diameter in pixels||Resolution in miles (kilometers) per pixel||Resolution compared to Hubble||Illuminated portion of disk||Activity|
|Dec 1, 2014||740,000
|Jan 13, 2015||238,000
Some of the numbers may change slightly as Dawn’s trajectory is refined and even as estimates of the strength of Ceres’ gravitational tug improve. (Dawn is already feeling that pull, even though it is not yet in orbit.) Still, this should help you fill out your social calendar for the next few months.
To get views like those Dawn has, you can build your own spaceship and fly it deep into the heart of the main asteroid belt to this intriguing world of rock and ice. Or you can visit our Ceres image gallery to see pictures as soon as they are released. If you chose the first option, use your hydrazine wisely!
As we discussed above, to explore Ceres without the use of the reaction wheels that were essential to the original design, mission controllers have worked very hard to conserve hydrazine. Let’s see how productive that effort has been. (You should be able to follow the story here without careful focus on the numbers. They are here for the more technically oriented readers, accountants and our old friends the Numerivores.)
Dawn launched in Sept. 2007 with 101 pounds (45.6 kilograms) of hydrazine. The ship escaped from Vesta in Sept. 2012, four weeks after the second reaction wheel failed during the climb out of Vesta’s gravitational hole. (By the way, Dawn is now more than one thousand times farther from Vesta than it is from Ceres. It is even farther from Vesta than Earth is from the sun!) At the beginning of the long interplanetary flight to Ceres, it still had 71.2 pounds (32.3 kilograms) left. As it had expended less than one-third of the original supply through the end of the Vesta expedition, that might seem like plenty. But it was not. Without the reaction wheels, subsequent operations would consume much more hydrazine. Indeed, engineers determined that even if they still had the entire amount that had been onboard at launch, it would not be enough. The Ceres objectives were at serious risk!
The flight team undertook an aggressive campaign to conserve hydrazine. They conceived more than 50 new candidate techniques for reducing hydrazine consumption in the 30-month journey to Ceres and the 18 months of Ceres operations and systematically but quickly assessed every one of them.
The team initially calculated that the long interplanetary flight between the departure from Vesta and the beginning of the Ceres approach phase would consume 27.6 pounds (12.5 kilograms) of hydrazine even if there were no errors, no glitches, no problems and no changes in the plans. Following the intensive conservation work, they determined that the spacecraft might instead be able to complete all of its assignments for only 9.7 pounds (4.4 kilograms), an astonishing 65 percent reduction. (Keep track of that mass through the end of the next paragraph.) That would translate directly into more hydrazine being available for the exploration of Ceres. They devised many new methods of conducting the mission at Ceres as well, estimating today that it will cost less than 42.5 pounds (19.3 kilograms) with the zero reaction wheel plan. (If the two remaining wheels operate when called upon in the lowest orbit, they will provide a bonus reduction in hydrazine use.)
Dawn’s two years and four months of interplanetary cruise concluded on Dec. 26, 2014, when the approach phase began. Although the team had computed that they might squeeze the consumption down to as low as 9.7 pounds (4.4 kilograms), it’s one thing to predict it and it’s another to achieve it. Changes to plans become necessary, and not every detail can be foreseen. As recounted in October, the trip was not entirely free of problems, as a burst of cosmic radiation interrupted the smooth operations. Now that the cruise phase is complete, we can measure how well it really went. Dawn used 9.7 pounds (4.4 kilograms), exactly as predicted in 2012. Isn’t flying spacecraft through the forbidding depths of the interplanetary void amazing?
This success provides high confidence in our ability to accomplish all of the plans at Ceres (even if the remaining reaction wheels are not operable). Now that the explorer is so close, it is starting to reap the rewards of the daring 3.0-billion-mile (4.9-billion-kilometer) journey to an ancient world that has long awaited a terrestrial emissary. As Dawn continues its approach phase, our growing anticipation will be fueled by thrilling new pictures, each offering a new perspective on this relict from the dawn of the solar system. Very soon, patience, diligence and unwavering determination will be rewarded with new knowledge and new insight into the nature of the cosmos.
Dawn is 121,000 miles (195,000 kilometers) from Ceres, or half the average distance between Earth and the moon. It is also 3.63 AU (338 million miles, or 544 million kilometers) from Earth, or 1,390 times as far as the moon and 3.69 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour to make the round trip.
Dr. Marc D. Rayman
7:00 p.m. PST January 29, 2014
In 1962, JPL conducted research in low-density gas dynamics, studying the drag on a sphere in a supersonic low-density flow environment, at various temperatures and speeds (Mach 1.8 to 4.4). Experiments were conducted in JPL’s Low Density Wind Tunnel. Nozzles were wrapped in a copper coil containing liquid nitrogen to cool the apparatus. A steel or bronze ball from 1/32 to 1/8 inch in size was suspended from fine tungsten wire in the jet. Two 8 mm movie projector lamps with built-in reflectors were placed at the edge of the jet and used to raise the sphere temperature to about 1,000 kelvins.
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.
Astronomer Giuseppe Piazzi was not looking for a planet on Jan. 1, 1801, but he spotted an unfamiliar dot of light that moved 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.)
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 more than 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 a large 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.
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.
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
Riley Duren, chief systems engineer for the Earth Science and Technology Directorate at NASA's Jet Propulsion Laboratory, is reporting from the 2014 United Nations Climate Conference in Lima, Peru.
I mentioned previously that Peru is home to some of the most important forests in the world in terms of their vulnerability to future impacts from climate change and development pressure as well as their potential to mitigate climate change. This underscores the importance of certain elements of the UN Framework Convention on Climate Change. In particular, the Reduction of Emissions from Deforestation and forest Degradation (REDD+) program seeks to address the second-largest human contribution to climate change after fossil fuel use (see Friday's post).
Detailed definitions vary, but deforestation generally refers to conversion of forested lands to some other use -- particularly large-scale agriculture but also mining and expansion of infrastructure and cities. Degradation is distinct and refers to a diminished capacity of forests to store carbon, support ecosystems and other services. Forest degradation is caused by human activity such as commercial logging, fuel wood collection, charcoal production, and livestock grazing as well as natural forces like storms, insect damage and wildfires.
Forests play a critical role in Earth's carbon budget because healthy, growing trees and other forest elements remove and store carbon from the atmosphere -- converting it to "biomass" in trees, shrubs and soil. This makes forests one of the most effective countermeasures for fossil fuel CO2 emissions (see graph, below).
The Earth's evolving carbon budget from the start of the Industrial Revolution through present day. Carbon dioxide (CO2) flux is shown in units of Giga (billion) tons of carbon per year (GtC/year). Fluxes of carbon emitted to the atmosphere are indicated by "+". Fluxes of carbon removed from the atmosphere are indicated by "-". The plot shows the dramatic growth in fossil fuel CO2 emissions since the mid-20th century and slight decline in emissions from deforestation and other land use change. The graph also shows the corresponding growth in the three major carbon sinks: the atmosphere, land (forests) and oceans. The variability or "jumpiness" in the land sink from year to year is likely due to changes in precipitation associated with climate variability like El Nino. The future ability of the land and oceans to remove CO2 from the atmosphere remains an area of great uncertainty. Image source: Global Carbon Project
However, when forests are degraded or destroyed, the storage potential of the forest is reduced or eliminated. Additionally, if the downed trees are burned and/or decay and forest soils are disturbed, they release their stored carbon (sometimes centuries worth) into the atmosphere. So there's an incentive to both keep forests growing to store carbon and to avoid disturbing the carbon already stored in them.
Programs like REDD+ are intended to incentivize governments and landowners to preserve and restore their forests. For example, in carbon-trading programs, governments and business can "offset" their fossil fuel CO2 emissions by purchasing credits from forest owners who can prove they're storing an equivalent amount of emissions by implementing certain protocols, including independent measurement and verification. These efforts are particularly important in the tropics, which are home to most of the world's forest carbon, as well as the countries experiencing the most rapid growth and development pressures, very similar to the period of growth the US underwent in the 1800s.
Over the weekend, I attended the Global Landscape Forum to interact with policy makers, conservation groups and scientists on the subject of forest carbon monitoring. One of the panel sessions featured JPL's Dr. Sassan Saatchi and other experts who described the current capabilities and limitations of remote-sensing tools to assess the status and health of forests, including their carbon stocks and "fluxes" (removals from and emissions to the atmosphere).
The remote-sensing methods discussed included imaging systems like the US Landsat satellites that are being used to track forest-cover change as well as future systems that will improve understanding of forest degradation such as NASA's ICESAT-2 mission, the NASA-India Synthetic Aperture Radar (NI-SAR) and the European Space Agency's BIOMASS mission. The role of flying radar and lidar (laser radar) instruments on aircraft over high priority areas was also discussed.
Of course decisions about forest management involve dimensions other than climate change mitigation -- typically involving a balance between economic growth and the value of existing ecosystem services offered by forests. Biodiversity in particular is gaining prominence in decision-making given the societal and economic value it represents. Biodiversity, which refers to the number of species in a given area, is often highest in forest ecosystems (particularly in the tropics) given they provide a combination of food, shelter and water resources. The information required to evaluate biodiversity is related to, but distinct from, the data used to assess forest carbon. (I'll try to describe the role of remote-sensing in assessing biodiversity in a future post.)
Meanwhile, closing with some personal experience, I'm posting a couple of photos I took while working on my own forest conservation and biodiversity project in Hawaii.
A cloud forest on the flank of Hualalai volcano on the Big Island of Hawaii. The giant, ancient trees and native understory plants thrive in the high-altitude, moist environment provided by the persistent presence of clouds -- providing carbon storage as well as a habitat for threatened plant and bird species. The benefits of the unique Kona weather pattern are offset by the introduction of invasive weeds and destructive feral animals like pigs and sheep.Image credit: Riley Duren
A threatened I'iwi honeycreeper, endemic to the Hawaiian Islands, sips nectar from an Ohia tree blossom. Historically, this species ranged across the Hawaiian Islands but today only survive in a few high-elevation forests given the combined pressure of deforestation and avian malaria at lower elevations from non-native mosquitoes. The I'iwi, like many other Hawaiian bird and plant species, lacks the natural defenses to withstand the combined pressure from development and climate change. Management efforts focus on conserving, restoring and building resiliency in threatened forest habitats. Image credit: Riley Duren
Riley Duren, chief systems engineer for the Earth Science and Technology Directorate at NASA's Jet Propulsion Laboratory, is reporting from the 2014 United Nations Climate Conference in Lima, Peru.
We arrived in Lima, Peru, late last night and made our way to the United Nations climate conference venue this morning -- an impressive complex known locally as the Pentagonito or “Little Pentagon.” As host country and city, Peru and Lima are representative of several key fronts in the international effort to confront climate change. Peru is home to some of the most significant tropical forests on Earth that are the focus of programs to preserve their vital role in storing carbon and critically endangered ecosystems (more about that tomorrow). With a population approaching 10 million people, Lima itself is a rapidly growing megacity -- one of many in the developing world.
The latter topic is the focus of this post and the event I’m participating in later today at the US Center: “Understanding the Carbon Emissions of Cities.” I’ll be joining colleagues from the US National Institute of Standards and Technology, Arizona State University, Laboratoire Des Sciences du Climat et de l'Environnement (France), and Universidade de São Paulo (Brazil) in presenting the motivation for and recent scientific advances in monitoring urban carbon pollution. There won’t be a live stream but the event will be recorded - keep an eye on the US State Department’s YouTube page where it should be posted this weekend.
So what is “carbon pollution” and why should we care about it? Most of us are familiar with the general topic of air pollution; just ask anybody who has asthma or knows a friend or family member with respiratory problems. Cities are notorious sources of air pollutants or smog -- including visible particles (aerosols) and invisible but caustic ozone. One can find many examples of success stories where air quality has improved in response to clean-air standards as well as horror stories in cities lacking such standards. However, this familiar topic of air quality is mostly limited to short-lived pollutants -- compounds that only persist in the atmosphere for hours or days. Those pollutants are important because of human health impacts but they’re not primary drivers of climate change. Carbon dioxide (CO2) and methane (another carbon-based molecule) on the other hand, are long-lived gases that trap heat in the atmosphere for many years. Once CO2 and methane are in the atmosphere, they remain there for a long time -- centuries, in the case of CO2. Most people are unaware of the presence of CO2 and methane because they’re invisible and odorless and don’t have an immediate impact on health, but those gases are THE big drivers of climate change.
There are many sources of CO2 on Earth, including natural emissions that, prior to the industrial revolution, were balanced by removals from natural carbon scrubbers like forests and oceans. However human activity is rapidly changing the balance of CO2 in the atmosphere, leading to an unprecedented growth rate. Most of these human CO2 emissions come from burning fossil fuels like coal and oil. These fossil emissions are responsible for about 85 percent of humanity’s CO2 footprint today and, globally, they’re continuing to accelerate. So any successful effort to avoid dangerous climate change must have fossil CO2 mitigation at its core. Managing methane is also important given its greater heat trapping potential than CO2.
Why focus on carbon from cities? It turns out that urbanization – the increasing migration of people from rural areas to urban centers – has concentrated over half the world’s population, over 70 percent of fossil CO2 emissions and a significant amount of methane emissions into less than 3 percent of the Earth’s land area! So cities and their power plants represent the largest cause of human carbon emissions. In 2010, the 50 largest cities alone were collectively the third largest fossil CO2 emitter after China and the US – and there are thousands of cities. At the same time, in many cases, emissions from cities are undergoing rapid growth because of urbanization.
But there’s also a silver lining here.
Many cities are beginning to serve as “first responders” to climate change. While national governments continue to negotiate over country-level commitments, mayors of some of the largest cities are already taking action to reduce their cities’ carbon footprints, and they’re working together through voluntary agreements. Additionally, the concentrated nature of urban carbon emissions makes measuring those emissions easier than measuring entire countries.
Measuring the carbon emissions of cities is important (you can’t manage what you can’t measure) and challenging given the number of sources and key sectors and uncertainty about how much each contributes to the total carbon footprint. For example, in a typical city, CO2 is emitted from the transportation sector (cars, trucks, airports, seaports), energy sector (power plants), commercial and industrial sectors (businesses, factories) and residential sector (heating and cooking in homes). Likewise, urban methane sources include landfills, wastewater treatment plants, and leaks in natural gas pipelines. Mayors, regional councils, businesses and citizens have a number of options to reduce their carbon emissions. Measuring the effect of those efforts and understanding where and why they’re not having the intended impact can prove critical to successful mitigation. It also has economic implications -- toward identifying the most cost-effective actions and supporting emissions trading (carbon markets) between cities and other sub-national entities.
How can we measure the carbon emissions of cities? That’s the focus of the Megacities Carbon Project and the topic of our event in Lima today. Briefly, this involves combining data from satellites and surface-monitoring stations that track concentrations of CO2, methane and other gases in the atmosphere over and around cities with other, local data sets that contain information about key sectors. Pilot efforts in Los Angeles, Paris, Sao Paulo and other cities are beginning to demonstrate the utility of these methodologies. Satellites like NASA’s Orbiting Carbon Observatory-2 and other future missions, when combined with a global network of urban carbon monitoring stations, could ultimately play an important role in enabling more effective mitigation action by the world’s largest carbon emitters: cities.
Riley Duren, chief systems engineer for the Earth Science and Technology Directorate at NASA's Jet Propulsion Laboratory, is reporting from the 2014 United Nations Climate Conference in Lima, Peru.
Today I'm en route to Lima, Peru, to join the United Nations climate conference. This is the 20th Conference of Parties (COP-20) of the UN Framework Convention on Climate Change (UNFCCC). The meeting is intended to set the stage for an international agreement next year between 195 countries on actions to address climate change. For the next two weeks, diplomats, policy makers, scientists, engineers, economists, and representatives of business and non-profit organizations are convening in Lima to discuss a wide range of options to avoid dangerous climate change and/or attempt to manage the impacts to humanity and the other species that share planet Earth. (More background, here)
As it turned out, I managed to miss my flight yesterday as result of the heavy rains and jammed freeways that ensued from the latest "atmospheric river" event in Los Angeles. But I have to admit, I was far more relieved than annoyed by this break (albeit brief) in California's persistent drought - a sentiment shared by all my neighbors and fellow travelers. Yet another reminder of the critical connections between weather, climate, and society, and what's at stake in efforts aimed at planetary stewardship.
Several JPLers are participating in the meeting given the lab's contribution of applying satellite observations to improve scientific understanding of the Earth and support societal decision-making. Collectively, the efforts of us traveling this week span sea, land and air - each reflecting part of NASA's broader mission to study the Earth as an integrated system.
My colleague Dr. Michelle Gierach is part of the NASA delegation at the US Center and will be talking about the ocean and impacts of climate change on key features like the El Nino Southern Oscillation (ENSO). Dr. Sassan Saatchi, who studies forest carbon, will be a panelist at this weekend's Global Landscapes Forum.
My own work these days is mostly focused on heat trapping or "greenhouse" gases in the atmosphere, like carbon dioxide and methane, and understanding the connections with human activity at the scale of countries, states, cities and individual pollution sources. I spend much of my time working with policy makers and scientists to understand stakeholder needs and design monitoring systems that can support practical decision making. It's a big challenge: These monitoring "system of systems" typically require a suite of Earth observing instruments from the ground, air and space - often fused with data from many other information sources. In addition to the technical challenges, after several years in this field, I continue to marvel at the diversity of perspectives, priorities, institutional cultures and ways of thinking, with implications on what data is required. The social dimensions are every bit as important as the bio-geophysical.
I'll say more in subsequent posts about some specific efforts that are underway and how they connect with events at the Lima conference.
Dear Unidawntified Flying Objects,
Flying silently and smoothly through the main asteroid belt between Mars and Jupiter, Dawn emits a blue-green beam of high velocity xenon ions. On the opposite side of the sun from Earth, firing its uniquely efficient ion propulsion system, the distant adventurer is continuing to make good progress on its long trek from the giant protoplanet Vesta to dwarf planet Ceres.
This month, let’s look ahead to some upcoming activities. You can use the sun in December to locate Dawn in the sky, but before we describe that, let’s see how Dawn is looking ahead to Ceres, with plans to take pictures on the night of Dec. 1.
The robotic explorer’s sensors are complex devices that perform many sensitive measurements. To ensure they yield the best possible scientific data, their health must be carefully monitored and maintained, and they must be accurately calibrated. The sophisticated instruments are activated and tested occasionally, and all remain in excellent condition. One final calibration of the science camera is needed before arrival at Ceres. To accomplish it, the camera needs to take pictures of a target that appears just a few pixels across. The endless sky that surrounds our interplanetary traveler is full of stars, but those beautiful pinpoints of light, while easily detectable, are too small for this specialized measurement. But there is an object that just happens to be the right size. On Dec. 1, Ceres will be about nine pixels in diameter, nearly perfect for this calibration.
The images will provide data on very subtle optical properties of the camera that scientists will use when they analyze and interpret the details of some of the pictures returned from orbit. At 740,000 miles (1.2 million kilometers), Dawn’s distance to Ceres will be about three times the separation between Earth and the moon. Its camera, designed for mapping Vesta and Ceres from orbit, will not reveal anything new. It will, however, reveal something cool! The pictures will be the first extended view for the first probe to reach the first dwarf planet discovered. They will show the largest body between the sun and Pluto that has not yet been visited by a spacecraft, Dawn’s destination since it climbed out of Vesta’s gravitational grip more than two years ago.
This will not be the first time Dawn has spotted Ceres. In a different calibration of the camera more than four years ago, the explorer descried its faint destination, far away in both time and space. Back then, still a year before arriving at Vesta, Dawn was more than 1,300 times farther from Ceres than it will be for this new calibration. The giant of the main asteroid belt was an indistinct dot in the vast cosmic landscape.
Dawn’s first photo of Ceres, taken on July 20, 2010. Image credit: NASA/JPL-Caltech/MPS/DLR/IDA
Now Ceres is the brightest object in Dawn’s sky save the distant sun. When it snaps the photos, Ceres will be as bright as Venus sometimes appears from Earth (what astronomers would call visual magnitude -3.6).
Dawn’s first extended picture of Ceres will be only slightly larger than this image of Vesta taken on May 3, 2011, at the beginning of the Vesta approach phase. The inset shows the pixelated Vesta, extracted from the main picture in which the overexposed Vesta can be seen against the background of stars. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
To conserve hydrazine, a precious resource following the loss of two reaction wheels, Dawn will thrust with its ion propulsion system when it performs this calibration, which requires long exposures. In addition to moving the spacecraft along in its trajectory, the ion engine stabilizes the ship, enabling it to point steadily in the zero-gravity of spaceflight. (Dawn’s predecessor, Deep Space 1, used the same trick of ion thrusting in order to be as stable as possible for its initial photos of comet Borrelly.)
As Dawn closes in on its quarry, Ceres will grow brighter and larger. Last month we summarized the plan for photographing Ceres during the first part of the approach phase, yielding views in January comparable to the best we currently have (from Hubble Space Telescope) and in February significantly better. The principal purpose of the pictures is to help navigators steer the ship into this uncharted, final port following a long voyage on the interplanetary seas. The camera serves as the helmsman’s eyes. Ceres has been observed with telescopes from (or near) Earth for more than two centuries, but it has appeared as little more than a faint, fuzzy blob farther away than the sun. But not for much longer!
The only spaceship ever built to orbit two extraterrestrial destinations, Dawn’s advanced ion propulsion system enables its ambitious mission. Providing the merest whisper of thrust, the ion engine allows Dawn to maneuver in ways entirely different from conventional spacecraft. In January, we presented in detail Dawn’s unique way of slipping into orbit. In September, a burst of space radiation disrupted the thrust profile. As we saw, the flight team responded swiftly to a very complex problem, minimizing the duration of the missed thrust. One part of their contingency operations was to design a new approach trajectory, accounting for the 95 hours that Dawn coasted instead of thrust. Let’s take a look now at how the resulting trajectory differs from what we discussed at the beginning of this year.
In the original approach, Dawn would follow a simple spiral around Ceres, approaching from the general direction of the sun, looping over the south pole, going beyond to the night side, and coming back above the north pole before easing into the targeted orbit, known by the stirring name RC3, at an altitude of 8,400 miles (13,500 kilometers). Like a pilot landing a plane, flying this route required lining up on a particular course and speed well in advance. The ion thrusting this year had been setting Dawn up to get on that approach spiral early next year.
The change in its flight profile following the September encounter with a rogue cosmic ray meant the spiral path would be markedly different and would require significantly longer to complete. While the flight team certainly is patient -- after all, Earth’s robotic ambassador won’t reach Ceres until 213 years after its discovery and more than seven years after launch -- the brilliantly creative navigators devised an entirely new approach trajectory that would be shorter. Demonstrating the extraordinary flexibility of ion propulsion, the spacecraft now will take a completely different path but will wind up in exactly the same orbit.
The spacecraft will allow itself to be captured by Ceres on March 6, only about half a day later than the trajectory it was pursuing before the hiatus in thrust, but the geometry both before and after will be quite different. Instead of flying south of Ceres, Dawn is now targeted to trail a little behind it, letting the dwarf planet lead as they both orbit the sun, and then the spacecraft will begin to gently curve around it. (You can see this in the figure below.) Dawn will come to 24,000 miles (38,000 kilometers) and then will slowly arc away. But thanks to the remarkable design of the thrust profile, the ion engine and the gravitational pull from the behemoth of rock and ice will work together. At a distance of 41,000 miles (61,000 kilometers), Ceres will reach out and tenderly take hold of its new consort, and they will be together evermore. Dawn will be in orbit, and Ceres will forever be accompanied by this former resident of Earth.
In this view, looking down on the north pole of Ceres, the sun is off the figure to the left and Ceres' counterclockwise orbital motion around the sun takes it from the bottom of the figure to the top. Dawn flies in from the left, traveling behind Ceres, and then is captured on the way to the apex of its orbit. The white circles are at one-day intervals, illustrating how Dawn slows down gradually at first. (When the circles are closer together, Dawn is moving more slowly.) After capture, both Ceres' gravity and the ion thrust slow it even more before the craft accelerates to the end of the approach phase. (You can think of this perspective as being from above. Then the next figure shows the view from the side, which here would mean looking toward the action from a location off the bottom of the graphic.) Image credit: NASA/JPL
If the spacecraft stopped thrusting just when Ceres captured it, it would continue looping around the massive body in a high, elliptical orbit, but its mission is to scrutinize the mysterious world. Our goal is not to be in just any arbitrary orbit but rather in the particular orbits that have been chosen to provide the best scientific return for the probe’s camera and other sensors. So it won’t stop but instead will continue maneuvering to RC3.
Ever graceful, Dawn will gently thrust to counter its orbital momentum, keeping it from swinging up to the highest altitude it would otherwise attain. On March 18, nearly two weeks after it is captured by Ceres’ gravity, Dawn will arc to the crest of its orbit. Like a ball thrown high that slows to a momentary stop before falling back, Dawn’s orbital ascent will end at an altitude of 47,000 miles (75,000 kilometers), and Ceres’ relentless pull (aided by the constant, gentle thrust) will win out. As it begins descending toward its gravitational master, it will continue working with Ceres. Rather than resist the fall, the spacecraft will thrust to accelerate itself, quickening the trip down to RC3.
There is more to the specification of the orbit than the altitude. One of the other attributes is the orientation of the orbit in space. (Imagine an orbit as a ring around Ceres, but that ring can be tipped and tilted in many ways.) To provide a view of the entire surface as Ceres rotates underneath it, Dawn needs to be in a polar orbit, flying over the north pole as it travels from the nightside to the dayside, moving south as it passes over the equator, sailing back to the unilluminated side when it reaches the south pole, and then heading north above terrain in the dark of night. To accomplish the earlier part of its new approach trajectory, however, Dawn will stay over lower latitudes, very high above the mysterious surface but not far from the equator. Therefore, as it races toward RC3, it will orient its ion engine not only to shorten the time to reach that orbital altitude but also to tip the plane of its orbit so that it encircles the poles (and tilts the plane to be at a particular orientation relative to the sun). Then, finally, as it gets closer still, it will turn to use that famously efficient glowing beam of xenon ions against Ceres’ gravity, acting as a brake rather than an accelerator. By April 23, this first act of a beautiful new celestial ballet will conclude. Dawn will be in the originally intended orbit around Ceres, ready for its next act: the intensive observations of RC3 we described in February.
North is at the top of this figure and the sun is far to the left. Ceres orbital motion around the sun carries it straight into the figure. The original approach took Dawn over Ceres' south pole as it spiraled directly into RC3. On the new approach, it looks here as if it flies in over the north pole, but that is because of the flat depiction. As the previous figure shows, the approach takes Dawn well behind Ceres in their progression around the sun. The upper part of the green trajectory is not in the same plane as the original approach and RC3; rather, it is in the foreground, "in front of" the graphic. As Dawn flies to the right side of the diagram, it also moves back into the plane of the figure to align with the targeted RC3. As before, the circles, spaced at intervals of one day, indicate the spacecraft's speed; where they are closer together, the ship travels more slowly. (You can think of this perspective as being from the side and the previous figure as showing the view from above, off the top of this graphic.) Image credit: NASA/JPL
Dawn’s route to orbit is no more complex and elegant than what any crackerjack spaceship pilot would execute. However, one of the key differences between what our ace will perform and what often happens in science fiction movies is that Dawn’s maneuvers will comply with the laws of physics. And if that’s not gratifying enough, perhaps the fact that it’s real makes it even more impressive. A spaceship sent from Earth more than seven years ago, propelled by electrically accelerated ions, having already maneuvered extensively in orbit around the giant protoplanet Vesta to reveal its myriad secrets, soon will bank and roll, arc and turn, ascend and descend, and swoop into its planned orbit.
Illustration of the relative locations (but not sizes) of Earth, the sun, and Dawn in early December 2014. (Earth and the sun are at that location every December.) The images are superimposed on the trajectory for the entire mission, showing the positions of Earth, Mars, Vesta, and Ceres at milestones during Dawn’s voyage. Image credit: NASA/JPL
As Earth, the sun, and the spacecraft come closer into alignment, radio signals that go back and forth must pass near the sun. The solar environment is fierce indeed, and it will interfere with those radio waves. While some signals will get through, communication will not be reliable. Therefore, controllers plan to send no messages to the spacecraft from Dec. 4 through Dec. 15; all instructions needed during that time will be stored onboard beforehand. Occasionally Deep Space Network antennas, pointing near the sun, will listen through the roaring noise for the faint whisper of the spacecraft, but the team will consider any communication to be a bonus.
Dawn is big for an interplanetary spacecraft (or for an otherworldly dragonfly, for that matter), with a wingspan of nearly 65 feet (19.7 meters). However, more than 3.8 times as far as the sun, 352 million miles (567 million kilometers) away, humankind lacks any technology even remotely capable of glimpsing it. But we can bring to bear something more powerful than our technology: our mind’s eye. From Dec. 8 to 11, if you block the sun’s blazing light with your thumb, you will also be covering Dawn’s location. There, in that direction, is our faraway emissary to new worlds. It has traveled three billion miles (4.8 billion kilometers) already on its extraordinary extraterrestrial expedition, and some of the most exciting miles are still ahead as it nears Ceres. You can see right where it is. It is now on the far side of the sun.
This is the same sun that is more than 100 times the diameter of Earth and a third of a million times its mass. This is the same sun that has been the unchallenged master of our solar system for more than 4.5 billion years. This is the same sun that has shone down on Earth all that time and has been the ultimate source of so much of the heat, light and other energy upon which the planet’s residents have been so dependent. This is the same sun that has so influenced human expression in art, literature, mythology and religion for uncounted millennia. This is the same sun that has motivated scientific studies for centuries. This is the same sun that is our signpost in the Milky Way galaxy. And humans have a spacecraft on the far side of it. We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty.
Dawn is 780,000 miles (1.3 million kilometers) from Ceres, or 3.3 times the average distance between Earth and the moon. It is also 3.77 AU (350 million miles, or 564 million kilometers) from Earth, or 1,525 times as far as the moon and 3.82 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.