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Occator Crater

A veteran explorer is leisurely orbiting the only dwarf planet in the inner solar system. Measuring space radiation high over Ceres, Dawn revolves once every 30 days in its gravitational master's firm grip. Dawn is well-known for its patience, and the pace of its activities has been decidedly relaxed in this orbit. That is about to change. There is now only one revolution to go before the spacecraft begins the final campaign of its long and rewarding deep-space adventure.

For eight months in 2015-2016, Dawn circled Ceres once every 5.4 hours at only 240 miles (385 kilometers). (The orbit has been variously designated as LAMO, then XMO1, and often as "the lowest orbit.") It then flew higher to pursue new objectives. The probe's orbit now takes it from slightly under 2,800 miles (4,400 kilometers) up to 24,300 miles (39,100 kilometers) and then back down again. (These values are a little different from what we presented in December, principally because the Sun's gravity gradually alters the orbit.) The orbit is known to people who call it extended mission orbit 5, or XMO5, as "extended mission orbit 5" or "XMO5" (following the nomenclature described here). XMO5 is illustrated in a figure below.

In contrast to the distant, serene probe, the operations team has been working quite intensively to prepare for a bold new phase of the mission. They have been assiduously working through all the tasks necessary to prepare for piloting this unique spaceship, late in its life and low on supplies, through maneuvers it was never designed for and to conduct observations never conceived of prior to late last year. Since the previous Dawn Journal, the team has generated more than 45,000 trajectories to study how to fly Dawn to two new orbits. Often there are more than 100 computers operating simultaneously to perform the necessary calculations. Many thousands more trajectories are yet to be computed and analyzed. If all goes well, by June, the probe will have followed an intricate flight plan that will allow it to glide a mere 22 miles (35 kilometers) above the alien landscapes almost every day in an orbit dramatically and poetically designated XMO7 (but occasionally summarized as "Whoa, that's low!").

Juling  Crater in LAMO
Dawn took this picture of Juling Crater in LAMO from an altitude of 240 miles (385 kilometers) on April 30, 2016. When we presented a different view of Juling, taken four months later, we described the surprising discovery of ice there. In October 2016, in XMO2, Dawn successfully accomplished the challenging assignment of acquiring infrared spectra of Juling's north wall, where the ice had been spotted, at three different times of the Cerean day. Comparing these five observations, scientists have determined that the area of ice increased from 1.4 square miles (3.6 square kilometers) to 2.1 square miles (5.5 square kilometers). In other words, the ice grew by 470 acres (190 hectares) over those six months. This is the first detection of a change on Ceres during Dawn's exploration. Scientists attribute the change to a seasonal cycle of solar heating of the crater floor. During that period, late in southern hemisphere winter, the Sun was moving south (toward Juling, which is at 36°S). As the ground warmed, it released water vapor. The vapor then condensed on the colder north wall of the crater, which faces away from the Sun. The crater wall acts as a "cold trap," collecting ice. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/ASI/INAF

Let's take a look at some of the plans the flight team is developing. As always, we will provide more details when Dawn is executing its complex assignments. In addition, as some parts of the plan are still being refined, there may be a few changes, and we will keep you updated on those as well. But plans are firm enough now that a preview is warranted.

On April 17, the spacecraft will fire up ion engine #2 and begin a downward spiral, gradually shrinking its elliptical orbit. Along the way to its final space destination, XMO7, the ship will moor at an intermediate orbit. On May 14, when it is in an orbit that ranges from about 235 miles (375 kilometers) to almost 3,000 miles (4,800 kilometers), it will shut down the engine. (This orbit is illustrated in the next two figures below.)

It is only coincidental that the lowest altitude of this intermediate orbit, XMO6, is so close to height of the lowest orbit so far. Indeed, the lowest point is not the most important point. The motivation for stopping in XMO6 is to collect infrared spectra and take pictures in the southern hemisphere in a range of about 900 miles (1,500 kilometers) to 1,600 miles (2,500 kilometers). It just so happens that when flying from XMO5 to XMO7, an orbit that provides that viewing opportunity dips down to the height of LAMO/XMO1 elsewhere in the orbit.

Ceres
This figure illustrates two of Dawn's operational orbits around Ceres. The spacecraft has been operating in the outer one, XMO5, since June 2017. It will fly to the inner one, XMO6, to make new observations in May. (Next month we will see the probe's flight path between the two orbits.) The spacecraft will revolve counterclockwise, as seen from this vantage point, and the Sun would be far to the right. XMO6 is also shown in the figure below. Image credit: NASA/JPL-Caltech

The XMO6 altitude in the south was chosen to be comparable to the altitude from which Dawn observed Ceres so extensively in its third and fifth mapping orbits (known as HAMO and XMO2, respectively). XMO6 will afford the probe views of the terrain with the illumination of southern summer that will make for the best comparison with what it has already observed farther north on the dwarf planet. Dawn photographed all of Ceres in full color in those earlier orbits, but it was not possible then to cover the vast surface with the infrared mapping spectrometer, which has a much smaller field of view than the camera. Therefore, scientists had focused their spectral mapping in the northern hemisphere, taking advantage of the lighting then. While some of the southern hemisphere was studied in infrared as well, the opportunity now to observe more of it will allow a more complete understanding of the distribution of minerals.

In XMO6 the spacecraft will fly over the south pole and then head north over the hemisphere of Ceres facing the Sun. It will go lower and lower as it does so. The lowest point in the orbit will occur between 50° and 60°N. Dawn already mapped that territory from LAMO/XMO1, but now it will take advantage of being low again to acquire some new color photography in the northern hemisphere.

As the spacecraft continues farther north, the altitude will increase again. It will sail higher as it travels over the night side before beginning its fall back down. It will take about 37 hours to complete one elliptical revolution.

Some readers may recall that for all of the mapping orbits at Vesta and Ceres, Dawn traveled south over the sunlit side and north over the hemisphere shrouded in the dark of night. (Readers who don't recall that are invited to trust that it's true.) Experts readily recognize that it is very, very difficult to reverse the orbital direction. Dawn did so, however, with the extensive maneuvering in February-April 2017 that allowed it to make the unique observation of opposition. Those who are interested can review the skilled piloting that reversed the direction.

The explorer will observe Ceres on 10 consecutive orbits in XMO6. To conserve precious hydrazine, Dawn will turn to point its main antenna to Earth and radio its findings after every other transit over the sunlit landscapes. In the other orbits, it will wait patiently, saving both data and hydrazine onboard for later.

On May 31, the spaceship will resume maneuvering. It will take about a week of ion thrusting to push down to the final orbit of the mission.

In XMO7 (shown in the two figures below), Dawn will range from as high as 2,500 miles (4,000 kilometers) to as low as about 22 miles (35 kilometers). (The minimum altitude will vary by a few miles, or kilometers, from revolution to revolution, for reasons we will explain in a future Dawn Journal.) It will take a little more than a day to complete one loop.

Ceres
The two solid ellipses illustrate the relative sizes of Dawn's next two science orbits around Ceres. The outer one is XMO6, which is the inner orbit in the figure above. (As in that figure, the spacecraft orbits counterclockwise here, and sunlight comes from the right.) After completing its work in XMO6 in May, Dawn will set sail for its final orbit, XMO7. The dashed circle represents Dawn's lowest orbit so far, LAMO/XMO1. It demonstrates that XMO7 is low! But assuming the absence of tall trees (or giant Cerean spiders), the operations team will be prepared to pilot the spacecraft safely. Dawn complies with planetary protection protocols, which prohibit coming in contact with Ceres, even for decades after the mission concludes. XMO7 is also shown in the figure below. Image credit: NASA/JPL-Caltech

We have described before that photography will be very challenging, both because of the difficulty pointing the camera accurately enough to capture specific targets and the high speed so close to the ground. We will return to this problem in an upcoming Dawn Journal.

At the high point of XMO7, Dawn will move at only about 120 mph (190 kph). Then as gravity pulls it back down, the spacecraft will accelerate until it streaks northward at 1,050 mph (1,690 kph) above a relatively narrow strip of ground before starting to soar up again. Dawn was designed for mapping uncharted worlds, not making specialized observations under such conditions, and traveling so fast and so low means it cannot take pictures as sharp as you might expect. Nevertheless, even with a little bit of motion-induced blur at low altitude, any sights we photograph certainly will reveal finer details than we have seen before. This is going to be exciting!

The highest priority measurements will be the nuclear spectra, giving scientists the opportunity to take a sharper picture of the elemental composition of the faraway world, making a more accurate map of the concentration of atomic species that are important for Ceres' geology and chemistry. Dawn's gamma ray and neutron detector (GRaND) is not subject to the limitations of pointing accuracy and blur that can affect the photography. You can think of GRaND's gamma ray vision and its neutron vision as being broader but less acute than the camera's visible-light vision. Getting closer to the ground will help ensure the instrument sees a stronger nuclear signal than ever before and takes a clearer picture.

As the spacecraft races over the ground, GRaND will measure gamma rays and neutrons escaping into space from the atoms down to about a yard (meter) underground. It collected a large volume of such data from LAMO/XMO1, but being so much lower in XMO7 will allow scientists to identify and locate elements more accurately.

There are several GRaND (if not grand) objectives for XMO7. One is to see how the elemental composition differs at different latitudes. The instrument has already revealed that water is more plentiful near the surface at higher latitudes than near the equator, and now it may be able to refine this finding. One of the properties of XMO7 is that the low point will shift almost 2° of latitude south on each revolution. That is, each time Dawn swoops down to its lowest point, it will be south of the low point on the previous orbit. That will provide GRaND the opportunity to survey the concentration and distribution of underground ice at different latitudes. GRaND also may tell us more about other constituents, providing clues about the geological processes that shaped this exotic world.

Ceres
This illustrates how XMO7 shifts from one revolution to the next. It will take a little more than one day for one revolution. Each time Dawn loops around Ceres, the low point of its orbit will be about 2° south of the previous time. From the perspective in this figure, even as Dawn travels counterclockwise around Ceres, the point at which it comes closest to the dwarf planet will progress clockwise. To trace the orbital motion, start in the lower right. Image credit: NASA/JPL-Caltech

Of course, as Dawn orbits Ceres, Ceres turns on its axis, pirouetting beneath her admiring companion. So each time Dawn zooms down for a close look, it will not only be farther south than the time before but it will also be at a different longitude. The next Dawn Journal will focus on this and what it means for GRaND and for photography.

Controlling Dawn's orientation in the zero-gravity of spaceflight is harder at low altitude, where Ceres' gravitational pull is stronger. Dawn will use hydrazine much more quickly in XMO7 than at any other part of the mission, and the last of the propellant will be expended before the end of this year.

Dawn just celebrated the third anniversary of arriving at its permanent residence in the solar system. In the natural perspective of its current home, Dawn arrived about two-thirds of a Cerean year ago, or nearly 3,000 Cerean days ago. The explorer has now completed 1,600 orbits. Although hydrazine is dwindling, and the adventure is nearing its end, there is still plenty to look forward to. Stay onboard as Dawn prepares to delve further into the unknown. It's going to be a great ride!

Dawn is 10,800 miles (17,400 kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 740 times as far as the Moon and 1.88 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.

Dr. Marc Rayman
9:15 am PDT March 20, 2018

TAGS: DAWN, CERES

  • Marc Rayman
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NASA visualization of the gas giant, Gamma Cephei A b

A massive gas giant more weighty than Jupiter, orbiting an orange star some 45 light years away, might be the most important exoplanet you’ve never heard of.

A massive gas giant more weighty than Jupiter, orbiting an orange star some 45 light years away, might be the most important exoplanet you've never heard of.

The planet, called Gamma Cephei A b – "Tadmor" for short – achieved its 15 minutes of fame in 1988. At least, among astronomers. It was the first planet to be discovered outside our solar system.

Or it would have been. The discovery was withdrawn by the Canadian team that announced it in 1992, after the data backing it up was determined to be too wobbly for astronomers to be sure the planet was real. Tadmor was added to a growing list of mistaken exoplanet detections that began as far back as the 19th century.

In this case, "wobbly" turns out to be the right word. The astronomers who thought they'd found the first exoplanet had developed a technique that allowed them to track the subtle motions of stars. The amount of "wobble" would reveal the mass of an object orbiting the star, tugging it first this way, then that. The researchers' major advance was precision measurement – capturing stellar movements as small as 43 feet (13 meters) per second. That kind of precision was needed to pick up the tiny wobbles, back and forth, that a large orbiting planet caused the star to make.

Despite their advance, the research team, Bruce Campbell, Gordon Walker and Stephenson Yang, worried that periodic changes in the star's magnetic activity might have looked to them like the gravitational tugs of a planet – in other words, that they might have mistaken jitters within the star for a planet in orbit around it.

They bid goodbye to Tadmor.

Riffle forward through the calendar, and stop in 2002. On-again, off-again Tadmor was on again – this time, its presence solidly confirmed. A team of astronomers that included the original discoverers captured strong evidence of the planet. They used four separate data sets from high-precision "wobble" measurements, known as radial velocity, spanning the period from 1981 to 2002.

The radial velocity method today has notched hundreds of exoplanet discoveries. It's been overshadowed only by the "transit" method, responsible for thousands, that looks for a tiny dip in the light from a star as a planet passes in front of it.

And although the list of confirmed exoplanets was just beginning to grow in the early 2000s, Tadmor already had been eclipsed. A planet called 51 Pegasi b, discovered by Michel Mayor and Didier Queloz, stole most of the spotlight in 1995. It was the first confirmed exoplanet detection to capture worldwide public attention.

Tadmor, of course, continues to orbit its big orange sun, somewhere in the constellation Cepheus, presumably unaware of its near-fame on a small blue planet dozens of light-years away. Time rolls on. Happy 30th anniversary, Tadmor.

TAGS: EXOPLANET, TADMOR

  • Pat Brennan
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Dr. Henry Richter

In 2018 JPL celebrates the 60th anniversary of America’s first satellite, Explorer 1.

Henry Richter started working at JPL in 1955 as an engineer and Supervisor for the New Circuit Elements Group. Later he was a Staff Engineer for the Deep Space Network and then Chief of the Space Instruments Section (322). During the Explorer Project Dr. Richter was project manager for the satellite design, in charge of JPL experiments for the International Geophysical Year, and was liaison between the Satellite Instrumentation Group and the Operations and Data Groups. He published a book in 2015 –America’s Leap into Space: My Time at JPL and the First Explorer Satellites.

On Wednesday, January 31 at 3:30, Dr. Richter will present his JPL Story in the Hub (111-104), followed at 4:30 by a book signing. He’ll share the story of JPL’s role working for the Army/Caltech and of the remarkable people who were part of the Explorer team. During the late 1950s, JPL extended rocket engineering to spacecraft design, using components that were on the cutting edge of technology. When they were finally given the chance to combine the instruments, upper stages, and launch vehicle, they accomplished the task in just a few months.

The JPL documentary Explorer 1 and the 1958 film X Minus 80 Days will be shown in the 111 Hub on Tuesday, January 30 from 12:00-1:15.

For more information about the history of JPL, contact the JPL Archives for assistance.

TAGS: HENRY RICHTER, DEEP SPACE NETWORK, JPL

  • Julie Cooper
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Occator Crater

Dawn has now logged 4 billion miles (6.4 billion kilometers) on its unique deep-space adventure. Sailing on a gentle breeze of xenon ions, the ambitious explorer journeyed for nearly four years to what had been only a small, fuzzy orb for over two centuries of terrestrial observations. Dawn spent more than a year there transforming it into a vast, complex protoplanet. Having sent its Vestan riches safely back to distant Earth, Dawn devoted another 2.5 years to reaching another blank canvas and there created another masterpiece of otherworldly beauty. Permanently in residence at dwarf planet Ceres, Dawn is now preparing to add some finishing touches.

The Dawn flight team at JPL did not even take notice as the odometer rolled over to 4,000,000,000. They have been focused on intensive investigations of how to maneuver the spaceship to lower altitudes than ever anticipated and operate there. For more than eight months in 2015-2016, Dawn circled 240 miles (385 kilometers) above the exotic Cerean landscape. From there, the team piloted the probe to higher orbits to undertake new studies, not anticipating that they might devise new methods to safely go much lower.

Occator Crater
Occator Crater, with its famously bright regions (Cerealia Facula in the center and Vinalia Faculae on the left), is seen from the north looking south. A bright region on a planet is known as a facula. The crater is 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep. This view and the one above were constructed by combining well over 500 of Dawn's photos taken from an altitude of 240 miles (385 kilometers). (Many of the pictures were taken to provide stereo views to reveal the topography.) Click on the picture to zoom in and see more details of the topography. We have presented quite a few views of Occator Crater before, most recently here, but the landscape never fails to intrigue. You can find this site at 20°N, 239°E on the map provided in September and on a different map below, which plots the locations of many bright areas on the dwarf planet. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

There are many challenges to overcome in flying closer to the dwarf planet, and although progress has been excellent, much more work lies ahead before maneuvering can begin. Indeed, even as some team members took time off in December, work never stopped. Many computers operated continuously, running sophisticated trajectory calculations. Engineers will assess the results when they return at the dawn of the new year and then set the computers to work on the next set of problems.

Meanwhile, Dawn waits patiently, safe and healthy in an orbit that ranges from a little more than 3,000 miles (4,800 kilometers) to nearly 24,000 miles (39,000 kilometers). It takes 30 days to complete one revolution. The spacecraft will continue operating in this elliptical orbit at least until April, the earliest opportunity to start its descent.

Having lost the use of the reaction wheels that controlled its orientation, Dawn now relies on hydrazine propellant fired from the small jets of its reaction control system. But after years of interplanetary travels and extensive maneuvering to observe Ceres, the remaining supply is very low. There simply is not enough left for a circular orbit lower than the one the spacecraft has already operated in. Dawn has plenty of xenon propellant to perform all the thrusting with its ion engine to change its orbit, but the available hydrazine is insufficient to perform all the necessary turns and to maintain a stable orientation for pointing its ion engine, solar arrays, antenna and sensors.

To fly low with a paucity of hydrazine, controllers are devising plans for an elliptical orbit. In the previous Dawn Journal, we saw that they might try to steer Dawn down to less than 125 miles (200 kilometers). While more work remains (including all those calculations that are occupying a cluster of computers), the progress has been encouraging. They are now analyzing orbits in which Dawn might even dive below 30 miles (50 kilometers) and then glide up to about 2,500 miles (4,000 kilometers) almost once a day. With many analyses still to perform and plans to refine, engineers anticipate that Dawn has enough hydrazine to maneuver to and operate in such an orbit for two months, and perhaps even a little longer.

Map of Ceres' Bright Spots
Although the brightest features on Ceres are in Occator Crater, shown above, the dwarf planet has many more such areas, or faculae. This map charts more than 300. All are composed of salts that reflect more sunlight than the rest of the material on the ground. Here they are categorized according to whether they are found on the floor of a crater, as in Occator; on a crater rim or wall; in the surrounding blanket of material ejected when a crater was excavated by the impact of an asteroid; or on the slopes of the cryovolcano Ahuna Mons. (We have seen and discussed the mysterious Ahuna Mons before, most recently here.) You can identify more features on this map by comparing it with the map here. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

If Dawn does go so low, it will be an exciting ride. How cool to skim so close to an alien world! But controllers must be careful that the spaceship doesn't dip too low. We have described before that Dawn complies with a set of protocols called planetary protection (not entirely unrelated to the Prime Directive). The team must ensure that the final orbit is stable enough that Dawn will not contaminate the astrobiologically interesting Ceres even for decades after the mission concludes.

The primary reason to plunge down so close to the mysterious landscapes of rock, ice and salt -- apart from pure awesomeness -- is to sense the nuclear radiation emanating from Ceres with greater clarity than ever before. With its gamma ray and neutron detector (GRaND), Dawn's measurements of this radiation provide insight into the atomic constituents down to about a yard (meter) underground. We have discussed this before in detail, including how the measurements work and why after operating so close to Ceres, Dawn flew to a higher orbit to improve its data.

The radiation is so faint, however, that some elements can only be detected from much closer range than Dawn has been. This is akin to looking at a very dim object or taking a picture of it. From far away, where little light reaches your eyes or your camera, colors are difficult to discern, so the view may be nearly black and white. But if you could move in close enough to capture much more light, you could see more colors. If Dawn can move in much closer to capture more of Ceres' nuclear glow, it may be able to see more of the elements of the periodic table -- in effect, taking a more colorful picture.

We see most objects by reflected light that originates either on the sun or artificial light sources. The nuclear radiation Dawn sees from Ceres is principally caused by cosmic rays. Cosmic rays are a form of radiation that fills space and originates far outside our solar system, mostly from supernovas elsewhere in the Milky Way Galaxy. The brighter the cosmic rays, the brighter Ceres will seem to be. The atoms on and underground don't reflect cosmic rays that strike them. Rather, the cosmic rays cause them to emit neutrons and gamma rays that escape back into space and carry with them the identities of the atoms. So, we can think of this as cosmic rays illuminating a scene, and Dawn will make nuclear photographs, revealing more details of Ceres' composition.

In addition to the advantage of going very low, it turns out that there is a special benefit to performing these measurements in 2018. The sun's magnetic field, which reaches out far beyond the planets, weakens cosmic rays entering our solar system, partially dimming the illumination. But our star's magnetism waxes and wanes in a cycle of 11 years. The sun now is entering the part of this regular cycle in which the magnetic field is weak. And it just so happens that this is an unusually weak solar cycle, so the sun's ability to hold cosmic rays at bay is less than at any time in the history of space exploration. Cosmic rays will be copious in the solar system. This won't matter much for people on or near Earth, because our planet's magnetic field (which extends well above where astronauts, cosmonauts and taikonauts work) resists most of the cosmic rays, and the thick blanket of atmosphere stops the rest. Ceres, like most residents of the solar system, does not have such protections. Thanks to the combination of the forecast of uniquely bright cosmic rays and the latest technology, 2018 will the best year so far in the history of solar system exploration to measure gamma rays or neutrons. Flying so close to the ground, Dawn should get superb readings.

In a future Dawn Journal we will discuss more of the specific objectives for the measurements and what they may reveal about Ceres, but now let's not forget about Dawn's other sensors. What about photography, infrared spectroscopy, visible spectroscopy, and gravity measurements?

In a previous Dawn Journal, we saw one photo of exotic landscape that included Samhain Catenae. Scientists used many more pictures, including stereo pictures, to construct this perspective of that set of fractures, which average more than 125 miles (200 kilometers) in length. Stresses generated within Ceres' interior created underground fractures as well as the ones we see here. The tectonic activity that created these structures may have been caused by convective upwelling of material. Good theoretical studies show that convection could have taken place in the interior. We speculated that convection could produce visible structures, and studies of Samhain Catenae now provide evidence of internal geology. The analysis indicates the fractured outer layer in this region is about 36 miles (58 kilometers) thick. (The global average may be about 9 miles, or 14 kilometers, thinner than that.) You can find Samhain Catenae between 27°S, 210°E and 22°N, 295°E on this map. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We can look forward to some remarkable pictures. Some will be sharper than the best so far, but not by as much as you might expect. When it is in the low altitude segment of its orbit, Dawn will be moving faster than ever at Ceres. If you were in a plane traveling hundreds of miles (kilometers) per hour, it would not be hard to take a picture of the ground six miles (10 kilometers) beneath you. But if you were in a car driving at that speed or even faster, despite being closer to the ground, your pictures might not be better. (That wouldn't be the greatest of your worries, but the Dawn team is devoting a great deal of work to ensuring the ship's safety, as we'll discuss below.) The situation on Dawn isn't that severe, so the photography certainly will improve somewhat on what we already have.

Because the camera's field of view is so small and the hydrazine imposes such a stubborn limitation on Dawn's lifetime, we will see only a very small fraction of the dwarf planet's vast landscape with the improved clarity of low altitude.

In previous Dawn Journals (see, for example, this one), we have delved into details of how difficult it can be to predict the orbit with great accuracy. The dominant (but not exclusive) cause is that every time the hydrazine jets fire, whether to maintain a stable orientation or to turn (including to keep the sensors pointed at Ceres while Dawn swoops by in its elliptical orbit), they push the probe a little and so distort its orbit slightly. Predicting the subtleties of the changes in the spacecraft's orbit is a very complex problem. Although the outcome is not yet clear, the flight team is making progress in investigating methods to manage these orbital perturbations well enough to be able to have some control over where GRaND measures the atomic composition, because its gamma ray spectrometer and neutron spectrometer have broad views. They can tolerate the deviations in the orbit. But Dawn probably will not have the capability to capture any specific targets with its other spectrometers or cameras. Rather, controllers will take pictures of whatever terrain happens to be in view of the cameras. But on a world with as much fascinating diversity as Ceres, intriguing new details are likely to be discovered.

Dawn took this picture showing part of Kokopelli Crater and its surrounding from an altitude of 240 miles (385 kilometers) during its first extended mission. (Kokopelli is a deity of agriculture, fertility and other fields of responsibility for many groups who have lived in what is now the southwestern United States. Representations of him are familiar to many people even now, but they bear little resemblance to the scenery in this picture.) The crater is 21 miles (33 kilometers) in diameter. The wavy terrain outside Kokopelli is a remnant from the powerful impact that created the enormous Dantu Crater. The many smaller craters here are scars from huge rocks blasted out when Dantu and Kokopelli formed. This scene is at 20°N, 123°E on the map here. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Along with studying the potential for improvements in pictures and spectra, the team is investigating refinements in Ceres' gravity field. They have already measured the gravity much more accurately than expected before Dawn arrived. Whether flying very close to some regions will allow them to improve their determination of the structure deep underground is the subject of ongoing work.

We will see in a Dawn Journal in a few months that the team will try to use certain properties of the orbit besides low altitude to provide attractive scientific opportunities. Nevertheless, it is clear that some goals simply will not be possible to achieve. To accomplish other objectives that are not feasible in that low ellipse, the team is analyzing the merits of pausing the ion-propelled spiral descent for a few weeks before reaching the final orbit. This could allow the spacecraft to view some regions of Ceres with the illumination of southern hemisphere summer, as we described in the previous Dawn Journal.

To ensure our distant ship remains ready to undertake extensive new observations, the infrared spectrometer, visible spectrometer, primary camera and backup camera each will be activated in January and run through their standard health checks and calibrations. For many of the observations in 2018, the two cameras will be used simultaneously to take as many pictures as possible, just as they were for special observations in 2017. Prior to this year, Dawn never used them concurrently.

With the help of a team of dedicated controllers, Dawn has shown itself to be a fantastically capable and resourceful explorer. Many new questions have to be answered and many challenges overcome for it to undertake another (and final) year in its bold expedition. But we can be hopeful that the creativity, ingenuity, and passion for knowledge and adventure that have propelled Dawn so very far already will soon allow it to add rich new details to what is already a celestial masterpiece.

Dawn is 17,200 miles (27,700 kilometers) from Ceres. It is also 1.77 AU (165 million miles, or 265 million kilometers) from Earth, or 705 times as far as the moon and 1.80 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 30 minutes to make the round trip.

Dr. Marc Rayman
4:30pm PST December 27, 2017

TAGS: DAWN, CERES, VESTA, ASTEROID BELT

  • Marc Rayman
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Dawn's long and productive expedition in deep space is about to enter a new phase.

Building on the successes of its primary mission and its first extended mission, NASA has approved the veteran explorer for a second extended mission. Dawn will undertake ambitious new investigations of dwarf planet Ceres, its permanent residence far from Earth.

It was not a foregone conclusion that Dawn would conduct further operations. In part, that's because it is only one of many exciting and important missions NASA has underway, and more are being designed and built. But the universe is a big place, as you may have noticed if you've ever gazed in awestruck reflection at the night sky (or had to search for a parking space in Los Angeles). It simply isn't possible to do everything we want. Entrusted with precious taxpayers' dollars, NASA has to make well-considered choices about what to do and what not to do.

In addition, as we have discussed in detail, Earth's ambassador to two giants in the main asteroid belt has had to contend with severe life-limiting problems. Dawn's reaction wheels have failed, and now it has consumed most of its original small supply of hydrazine that it uses in compensation. It has also expended most of the xenon propellant for its uniquely capable ion propulsion system. It was not clear that a truly productive future would be possible for this aged, damaged ship with some supplies that are so limited. Fortunately, Dawn has endless supplies of creativity, ingenuity, dedication and enthusiasm.

For several months, the flight team has been studying the feasibility of flying the spaceship closer to Ceres than had ever been seriously considered. Dawn spent more than eight months in 2015-2016 circling about 240 miles (385 kilometers) above the dwarf planet. It had spectacular views of mysterious landscapes and acquired a wealth of data far beyond what the team had anticipated. Then Dawn flew to a higher altitude during its first extended mission for new observations. Now engineers are making progress on ways to operate the spacecraft in an elliptical orbit that would allow it to swoop down to below 125 miles (200 kilometers) for a few minutes on each revolution. Their results so far are very encouraging. There are still many complex technical problems to solve, and months of additional work remain. Dawn can wait relatively patiently in its current orbit, where it expends hydrazine quite parsimoniously as it measures cosmic rays.

Axomama Crater on Ceres
Dawn had this view from an altitude of about 240 miles (385 kilometers) on July 24, 2016, during its first extended mission. A segment of the western wall of Dantu Crater is visible at lower left. Pressure from underground liquid water is one of the possible explanations for the origin of the fractures visible here in Dantu's floor. (We have seen other views of Dantu, most recently in June. The scene above is in the lower left part of Dantu in that previous photo.) The crater below and right of center is Axomama. (Axomama, literally "potato mother," was an Incan goddess of potatoes.) At three miles (five kilometers) in diameter, Axomama's sharp rim indicates the crater was excavated in the recent geological past. This scene is visible at 23°N, 131°E on the map presented last month. Axomama is one of the newly named features on that map. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The promising potential for observing Ceres in elliptical orbits from closer than ever before makes a second extended mission there extremely attractive. NASA and the panel of scientists and engineers convened to provide an independent, objective assessment concluded that further exploration of Ceres would be the most valuable assignment for the spacecraft. It is noteworthy that Dawn is the only spacecraft ever to orbit two extraterrestrial destinations and even now, having significantly exceeded its original objectives, has the capability to leave Ceres and pay a brief visit to a third (although it does not have enough xenon left to orbit a third), but the prospects for new discoveries at Ceres are too great to pass up.

Ceres is not only the largest object between Mars and Jupiter but also certainly one of the most intriguing. In fact, motivated by what Dawn has found, there is now great interest in the possibility of sending a lander there someday. Anything more Dawn can do to learn about Ceres or to help pave the way for a subsequent mission will be of great importance.

Ceres is just too fascinating to abandon! Dawn has already revealed the dwarf planet to be an exotic world of ice, rock and salt, with organic materials and other chemical constituents, and now we can look forward to more discoveries. After all, the benefit of having the capability to orbit a distant destination, rather than being limited to a quick glimpse during a fleeting flyby, is that we can linger to scrutinize it and uncover even more of the secrets it holds. (Some readers may also draw inspiration from Ceres' ingredients to concoct recipes for treats to give out to Halloween visitors.)

In addition to the possibility of observing Ceres from unprecedentedly close, there are other benefits to keeping our sophisticated probe at work there. For now, let's consider two of them, both related to how long it takes Ceres to complete its stately orbit around the sun. One Cerean year is 4.6 terrestrial years.

The dwarf planet carries its robotic moon with it as it follows its elliptical path around the sun. In fact, all orbits, including Earth’s, are ellipses. Ceres’ orbit is more elliptical than Earth’s but not as much as some of the other planets. The shape of Ceres’ orbit is between that of Saturn (which is more circular) and Mars (which is more elliptical). (Of course, Ceres’ orbit is larger than Mars’ and smaller than Saturn’s, but here we are considering how much each orbit deviates from a perfect circle, regardless of the size.)

When Dawn arrived at Ceres in March 2015, they were 2.87 AU from the sun. That was well before the dwarf planet's orbit carried them to the maximum solar distance of 2.98 AU in January 2016. Now, with the second extended mission, the spacecraft will still be operating when Ceres reaches its minimum solar distance of 2.56 AU in April 2018. Dawn will keep a sharp eye out for any changes caused by being somewhat closer to the sun.

Xevioso Crater on Ceres
Dawn photographed this scene from an altitude of 915 miles (1,470 kilometers) on Oct. 15, 2015, from its third mapping orbit. Near the lower left is the cryovolcano Ahuna Mons, the highest mountain on the dwarf planet. (We have seen many view of Ahuna Mons before, most recently here.) Near the center top is Xevioso Crater, with bright material that was blasted when the crater formed. (Xevioso was a god of thunder who, among his other talents, nourished the land for the Fon people in the Dahomey Kingdom in what is now Benin.) The presence of the ejected material on Xevioso's left must be because the object that hit the ground came from the right. At 5.3 miles (8.5 kilometers) in diameter, Xevioso is relatively small, suggesting that this highly reflective material was relatively shallow. This scene is centered at 4°S, 314°E on the map presented last month. Like Axomama above, Xevioso is one of the recently named features on that map. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The extension also will give scientists the opportunity to examine Ceres with the different lighting caused by the change of seasons. Ceres' slower heliocentric orbit than Earth's means seasons last longer on that distant world. It was near the end of autumn in the southern hemisphere when Dawn took up residence at Ceres. Winter came to that hemisphere on July 24, 2015, when the sun reached its greatest northern latitude. The sun crossed the equator, bringing spring to the southern hemisphere, on Nov. 13, 2016, and summer begins on Dec. 22 of this year. Autumn, when the sun will leave the southern hemisphere, is more than one (terrestrial) year later. Most of Dawn's observations so far were made with the sun in the northern hemisphere. Now Dawn will have new opportunities to see the southern hemisphere with similar illumination.

In the coming months, as the team develops and refines its plans, we will describe how they will pilot the ship down to very low altitudes and what new measurements they will make. Before the new phase gets underway, however, you can explore Ceres (and other planets) yourself with Google maps (some functions don't work in some web browsers). Even though it does not use Dawn's sharpest photos, it should be more than adequate for most of your navigational needs. (It isn't quite adequate for Dawn's needs, but that's no cause for worry, because JPL navigators employ somewhat more sophisticated and accurate methods.)

What will Dawn find when it ventures closer to the ground than ever before? What will the new perspectives reveal about a strange world from the dawn of the solar system? What new challenges will the adventurer confront as it pushes further into uncharted territory? We don't know, but stay onboard as we find out together, for that is an essential element both of the tremendously successful process of science and the powerful thrill of exploration.

Dawn is 21,600 miles (34,700 kilometers) from Ceres. It is also 2.47 AU (229 million miles, or 369 million kilometers) from Earth, or 970 times as far as the moon and 2.49 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
2:30 p.m. PDT October 31, 2017

TAGS: DAWN, CERES, EXTENDED MISSION

  • Marc Rayman
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An artist’s illustration of a possible ninth planet in our solar system

The super Earth that came home for dinner

It might be lingering bashfully on the icy outer edges of our solar system, hiding in the dark, but subtly pulling strings behind the scenes: stretching out the orbits of distant bodies, perhaps even tilting the entire solar system to one side.

If a planet is there, it’s extremely distant and will stay that way (with no chance – in case you’re wondering – of ever colliding with Earth, or bringing “days of darkness”). It is a possible Planet Nine, a world perhaps 10 times the mass of Earth and 20 times farther from the sun than Neptune. The signs so far are indirect, mainly its gravitational footprints, but that adds up to a compelling case nonetheless.

One of its most dedicated trackers, in fact, says it is now harder to imagine our solar system without a Planet Nine than with one.

“There are now five different lines of observational evidence pointing to the existence of Planet Nine,” said Konstantin Batygin, a planetary astrophysicist at Caltech whose team may be closing in. “If you were to remove this explanation, and imagine Planet Nine does not exist, then you generate more problems than you solve. All of a sudden, you have five different puzzles, and you must come up with five different theories to explain them.”

Batygin and his co-author, Caltech astronomer Mike Brown, described the first three breadcrumbs on Planet Nine’s trail in a January 2016 paper, published in the Astronomical Journal. Six known objects in the distant Kuiper Belt, a region of icy bodies stretching from Neptune outward toward interstellar space, all have elliptical orbits pointing in the same direction. That would be unlikely – and suspicious – enough. But these orbits also are tilted the same way, about 30 degrees “downward” compared to the pancake-like plane within which the planets orbit the sun.

The Bering Strait
Caltech professor Mike Brown and assistant professor Konstanin Batygin have been working together to investigate Planet Nine. Credit: Caltech/Lance Hayashida

Breadcrumb number three: Computer simulations of the solar system with Planet Nine included show that there should be more objects tilted with respect to the solar plane. In fact, the tilt would be on the order of 90 degrees, as if the plane of the solar system and these objects formed an “X” when viewed edge-on. Sure enough, Brown realized that five such objects already known to astronomers fill the bill.

Two more clues emerged after the original paper. A second article from the team, this time led by Batygin’s graduate student, Elizabeth Bailey, showed that Planet Nine could have tilted the planets of our solar system during the last 4.5 billion years. This could explain a longstanding mystery: Why is the plane in which the planets orbit tilted about 6 degrees compared to the sun's equator?

“Over long periods of time, Planet Nine will make the entire solar-system plane precess or wobble, just like a top on a table,” Batygin said.

The last telltale sign of Planet Nine’s presence involves the solar system’s contrarians: objects from the Kuiper Belt that orbit in the opposite direction from everything else in the solar system. Planet Nine’s orbital influence would explain why these bodies from the distant Kuiper Belt end up “polluting” the inner Kuiper Belt.

“No other model can explain the weirdness of these high-inclination orbits,” Batygin said. “It turns out that Planet Nine provides a natural avenue for their generation. These things have been twisted out of the solar system plane with help from Planet Nine and then scattered inward by Neptune.”

The Bering Strait
The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Moreover, when viewed in 3-D, the orbits of all these icy little objects are tilted in the same direction, away from the plane of the solar system. Credit: JPL-Caltech/R. Hurt

The remaining step is to find Planet Nine itself. Batygin and Brown are using the Subaru Telescope in Hawaii’s Mauna Kea Observatory to try to do just that. The instrument is the “best tool” for picking out dim, extremely distant objects lost in huge swaths of sky, Batygin said.

But where did Planet Nine come from? Batygin says he spends little time ruminating on its origin – whether it is a fugitive from our own solar system or, just maybe, a wandering rogue planet captured by the sun’s gravity.

“I think Planet Nine’s detection will tell us something about its origin,” he said.

Other scientists offer a different possible explanation for the Planet Nine evidence cited by Batygin. A recent analysis based on a sky mapping project called the Outer Solar System Origins Survey, which discovered more than 800 new “trans-Neptunian objects,” or TNOs, suggests that the evidence also could be consistent with a random distribution of such objects. Still, the analysis, from a team led by Cory Shankman of the University of Victoria, could not rule out Planet Nine.

If Planet Nine is found, it will be a homecoming of sorts, or at least a family reunion. Over the past 20 years, surveys of planets around other stars in our galaxy have found the most common types to be “super Earths” and their somewhat larger cousins – bigger than Earth but smaller than Neptune.

Yet these common, garden-variety planets are conspicuously absent from our solar system. Weighing in at roughly 10 times Earth’s mass, the proposed Planet Nine would make a good fit.

Planet Nine could turn out to be our missing super Earth.

TAGS: PLANET NINE, PLANET, KUIPER BELT

  • Amanda Hendrix
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Orrery

This orrery was built for NASA/JPL by Pre-Mec Engineering, Inc. and was designed by JPL engineer Raymond A. McCreary (Design Section, 356 – part of the Engineering Mechanics Division).

The scale of Earth and its moon was approximately 1 cm = 6000 km, but the scale of orbits, the Sun, and other moons varied.

Computer animations did not exist in the early 1960s, and like a trajectory model, this orrery helped engineers plan, visualize, and demonstrate the expected flight path, flyby, or landing to be made by a spacecraft. Missions in development at this time were Ranger and Surveyor (lunar missions), Mariner 2 to Venus, and Mariner 4 to Mars.

For more information about the history of JPL, contact the JPL Archives for assistance. [Archival and other sources: Section 321 photo album and index, and JPL/Caltech phone directories

TAGS: ORRERY, TRAJECTORY MODEL, SOLAR SYSTEM

  • Julie Cooper
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Ceres

A decade after leaving its first home in the solar system, Dawn is healthy and successful at its current residence.

Even as the veteran explorer orbits high over dwarf planet Ceres and looks forward to continuing its mission, today it can reflect upon 10 exciting and productive years (or equivalently, with its present perspective, 2.17 exciting and productive Cerean years).

The ambitious adventurer embarked on an extraordinary extraterrestrial expedition on Sept. 27, 2007. With its advanced ion propulsion system, Dawn soared past Mars in 2009. The spacecraft took some of the Red Planet’s orbital energy around the sun to boost itself on its journey. (Nevertheless, this extra energy amounts to less than a quarter of what the ion engines have provided.) Ever a responsible citizen of the cosmos, Dawn fully adheres to the principle of the conservation of energy. So to compensate for speeding up, it slowed Mars down.

Dawn launch
Dawn launched at dawn (7:34 a.m. EDT) from Cape Canaveral Air Force Station, Sept. 27, 2007. Note the sun rising on the right edge of the picture. The intricate sequence of activities between the time this photo was taken and Dawn separated from the rocket to fly on its own is described here. Image credit: KSC/NASA

In 2011, the spacecraft arrived at Vesta, the second largest object in the main asteroid belt between Mars and Jupiter. Dawn gracefully entered into Vesta’s firm but gentle gravitational embrace. The probe maneuvered extensively in orbit, optimizing its views to get the best return possible from its photography and other observations. During 14 months in orbit, Dawn completed 1,298 revolutions around Vesta, taking nearly 31,000 pictures and collecting a wealth of other scientific measurements. From the perspective it had then, Dawn was in residence for nearly a third of a Vestan year (or almost 1,900 Vestan days). The explorer revealed a strange, ancient protoplanet, now recognized to be more closely related to the terrestrial planets (including the one Dawn left 10 years ago) than to the typical and smaller asteroids.

Unlike all other deep-space missions, Dawn had the capability to leave its first orbital destination and voyage to and enter orbit around another. After smoothly disengaging from Vesta, the interplanetary spaceship flew more than 900 million miles (1.5 billion kilometers) in 2.5 years to Ceres, the largest object in the asteroid belt. Indeed, prior to Dawn’s arrival, that dwarf planet was the largest body between the sun and dwarf planet Pluto that a spacecraft had not yet visited. And just as at Vesta, thanks to the maneuverability of ion propulsion, Dawn did not have to be content with a one-time flyby, gathering only as much data as possible during a brief encounter. By going into orbit around Ceres, the spacecraft could linger to scrutinize the exotic, alien world. And that is exactly what it has done.

Both Vesta and Ceres have held secrets since the dawn of the solar system, and both have beckoned since they were first spotted in telescopes at the dawn of the 19th century. For the next two centuries, they appeared as little more than faint smudges of light amidst myriad glittering stellar jewels, waiting for an inquisitive and admiring visitor from Earth. Finally, Dawn answered their cosmic invitations and eventually developed richly detailed, intimate portraits of each.

As the last stop on a unique interplanetary journey of discovery, Ceres has proven well worth the wait. Since arriving in March 2015 (more than half a Cerean year ago, or nearly 2,500 Cerean days ago), Dawn has completed 1,595 revolutions. It has beheld mysterious and fascinating landscapes and unveiled a complex world of rock, ice and salt, along with organic compounds and other intriguing constituents. The dwarf planet may have been covered by an ocean long ago, and there might even be liquid water underground now. The 57,000 pictures and numerous other measurements with the sophisticated sensors will keep scientists busy for many years (both terrestrial and Cerean).

By early 2016, during its ninth year in space, Dawn had accomplished so much that it exceeded all of the original objectives established for it by NASA before the ship set sail. Along the way, Dawn encountered and ultimately overcame many obstacles, including equipment failures that could well have sunk the mission. Against all odds and expectations, however, when its prime mission concluded in June 2016, the spacecraft was still healthy enough that NASA decided to extend the mission to learn still more about Ceres. Since then, Dawn has conducted many investigations that had never even been considered prior to last year. Now it has successfully achieved all of the extended mission objectives. And, once again defying predictions thanks to expert piloting by the flight team (and a small dose of good luck), Dawn still has some life left in it. Before the end of the year, NASA will formulate another new set of objectives that will take it to the end of its operational life.

Dawn has flown to many different orbital altitudes and orientations to examine Ceres. Now the probe is in an elliptical orbit, ranging from less than 3,200 miles (5,100 kilometers) up to 23,800 miles (38,300 kilometers). At these heights, it is measuring cosmic rays. Scientists mathematically remove the cosmic ray noise from Dawn’s 2015-2016 recordings of atomic elements from a low, tight orbit at only 240 miles (385 kilometers).

Juling Crater
Dawn took this picture of Juling Crater on Aug. 25, 2016, during its extended mission at an altitude of 240 miles (385 kilometers). (Juling is a crop spirit of the Orang Asli in the Malay Peninsula. The word also can mean strabismus or squint in the local language, and the spirit has been called the Squinting Demon. We leave it to you to make the connection with this particular crater apart from the general Ceres naming convention.) The 12-mile (20-kilometer) diameter crater is young, as seen by its sharp features and the absence of many smaller craters inside and nearby. Dawn’s infrared mapping spectrometer spotted the clear signature of ice on the ground in Juling. Ice is not stable for long at this location, so although the crater formed in the recent geological past, the ice must have been exposed even more recently. Scientists have found ice elsewhere as well, and other measurements show that there is a vast amount underground. One of the objectives of the second extended mission orbit was to follow up on the detection of ice in Juling by observing it under different lighting conditions and at different times of the Cerean day. Juling is at 36°S, 169°E on the map below. The next picture partially overlaps with this one, displaying more of the scenery in this area. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In its present orbit, Dawn can make these measurements to clarify Ceres’ nuclear signals while being very frugal with its precious hydrazine, which is so crucial because of the loss of three reaction wheels. (The small supply was not loaded onboard with the intention of compensating for failed reaction wheels.) When the hydrazine is expended, the mission will end. So this high elliptical orbit is a very good place to be while NASA and the Dawn project are determining how best to use the spacecraft in the future.

Meanwhile, this anniversary presents a convenient opportunity to look back on a remarkable spaceflight. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the tenth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to investigate Dawn’s ambitious journey in detail may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh, eighth and ninth anniversaries.

In its 10 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,109 days (5.8 years), or 58 percent of the time (and 0.000000042 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 908 pounds (412 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 69 of the 71 gallons (262 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.

Dawn launch
Dawn observed this rugged terrain on Aug. 24, 2016, during its extended mission at an altitude of 240 miles (385 kilometers). The upper crater is Juling (which we saw above), and the one at lower right is Kupalo, which is 16 miles (26 kilometers) in diameter. Although this and the picture above overlap, they were taken more than 27 hours apart, during which Dawn completed five orbital revolutions of Ceres. This scene is at 38°S, 169°E on the map below. We have seen other views of Kupalo and the area around it, most recently on the ninth anniversary of Dawn’s launch. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The thrusting since then has achieved the equivalent of accelerating the probe by 25,400 mph (40,900 kilometers per hour). 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. Dawn has 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.) It is remarkable that Dawn’s ion propulsion system has provided nearly the same change in speed as the entire Delta rocket.

Since launch, our readers who have remained on or near Earth have completed 10 revolutions around the sun, covering 62.8 AU (5.8 billion miles, or 9.4 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 42.4 AU (3.9 billion miles, or 6.3 billion kilometers). As it climbed away from the sun, up the solar system hill to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the 10 years since Dawn began its voyage, Vesta has traveled only 40.5 AU (3.8 billion miles, or 6.1 billion kilometers), and the even more sedate Ceres has gone 37.8 AU (3.5 billion miles, or 5.7 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the 10 years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)

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 even 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. (If you prefer not to skip it, click here.) 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.

Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family (including Earth, Dawn, Vesta and Ceres) follow their individual paths around the sun, they sometimes move closer and sometimes move farther from it.

Dawn launch
Dawn’s interplanetary trajectory (in blue). The dates in white show Dawn’s location every Sept. 27, starting on Earth in 2007. Note that Earth returns to the same location, taking one year to complete each revolution around the sun. When Dawn is farther from the sun, it orbits more slowly, so the distance from one Sept. 27 to the next is shorter. In addition to seeing Dawn’s progress on this figure on previous anniversaries of launch, we have seen it other times as well, most recently in May. (This graphic also will be at a Dawn flight team celebration this afternoon, but it will be in a form that is much more transitory and delectable, although perhaps not much more nutritious, than the way it is displayed here.) Image credit: NASA/JPL

In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of Dawn’s journey has been changing the inclination of its orbit, an energetically expensive task.)

Now we can see how Dawn has done by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sept. 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.

Minimum distance
from the Sun (AU)
Maximum distance
from the Sun (AU)
Inclination
Earth’s orbit 0.98 1.02 0.0°
Dawn’s orbit on Sept. 27, 2007 (before launch) 0.98 1.02 0.0°
Dawn’s orbit on Sept. 27, 2007 (after launch) 1.00 1.62 0.6°
Dawn’s orbit on Sept. 27, 2008 1.21 1.68 1.4°
Dawn’s orbit on Sept. 27, 2009 1.42 1.87 6.2°
Dawn’s orbit on Sept. 27, 2010 1.89 2.13 6.8°
Dawn’s orbit on Sept. 27, 2011 2.15 2.57 7.1°
Vesta’s orbit 2.15 2.57 7.1°
Dawn’s orbit on Sept. 27, 2012 2.17 2.57 7.3°
Dawn’s orbit on Sept. 27, 2013 2.44 2.98 8.7°
Dawn’s orbit on Sept. 27, 2014 2.46 3.02 9.8°
Dawn’s orbit on Sept. 27, 2015 2.56 2.98 10.6°
Dawn’s orbit on Sept. 27, 2016 2.56 2.98 10.6°
Dawn’s orbit on Sept. 27, 2017 2.56 2.98 10.6°
Ceres’ orbit 2.56 2.98 10.6°

For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that six years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the stalwart adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.

Dawn launch
This map of Ceres has all 138 feature names approved so far by the International Astronomical Union (IAU), including 25 approved last month. (We described the naming convention here.) As more features are named, this official list and map are kept up to date. The dwarf planet is 1.1 million square miles (2.8 million square kilometers). That’s about 36 percent of the land area of the contiguous United States, or the combined land areas of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. The scales for horizontal distance in this figure apply at the equator. Rectangular maps like this distort distances at other latitudes. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Even after a decade of daring space travel, flying in deep space atop a blue-green pillar of xenon ions, exploring two of the last uncharted worlds in the inner solar system, overcoming the loss of three reaction wheels, working hard to stretch its shrinking supply of hydrazine, Dawn is ready for more. And so is everyone who yearns for new knowledge, everyone who is curious about the cosmos, and everyone who is exhilarated by bold adventures into the unknown. More is to come. Dawn -- and all those who find the lure of space irresistible -- can look forward to whatever lies ahead for this unique mission.

Dawn is 16,600 miles (26,700 kilometers) from Ceres. It is also 2.92 AU (271 million miles, or 437 million kilometers) from Earth, or 1,080 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, ASTEROID BELT

  • Marc Rayman
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Mariner Jupiter Saturn 1977 (Voyager) Logo

In August and September 1977, two Voyager spacecraft were launched on a Grand Tour of the solar system. In 1973, the mission had been named Mariner Jupiter-Saturn 1977 (MJS ‘77) and was intended to go only as far as Jupiter and Saturn.

In March 1977 the mission name was changed to Voyager. In October 1978, a Voyager Fact Sheet mentioned the possibility of sending Voyager 2 to Uranus and Neptune. It would happen only if the primary science objectives were met at Saturn first. Even though the extended mission was not certain before launch, Voyager engineers (unofficially) designed and built the spacecraft to be capable of navigating to Uranus and Neptune, and surviving the longer trip. On-board computers were reprogrammed during the voyage, giving the spacecraft the ability to successfully return many more images and much more information than were expected. It’s unlikely the Voyager team imagined that both spacecraft would still be operating 40 years after launch.

For more information about the history of JPL, contact the JPL Archives for assistance. [Archival and other sources: Various Voyager and JPL History web pages; Voyager Fact Sheet, 10/6/1978; Section 260 photo album and index.

TAGS: VOYAGER, MARINER, URANUS, NEPTUNE, GRAND TOUR, JUPITER

  • Julie Cooper
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Lower Manhattan

Projecting regional changes

“Sea level scientists have a pretty good grasp on global mean sea level,” said Steve Nerem, a professor in the Aerospace Engineering Sciences Department at the University of Colorado and the team leader for NASA’s Sea Level Change Team (N-SLCT). “It’s the regional sea level change that’s the next big question, the next big step for sea level science,” he added.

Nerem and much of the rest of the N-SLCT were in New York City this July where more than 300 scientists from 42 countries gathered at Columbia University for a weeklong Regional Sea Level Changes and Coastal Impacts Conference. The international conference was organized by the World Climate Research Programme (WCRP), Climate and Ocean – Variability, Predictability, and Change (CLIVAR), and the UNESCO Intergovernmental Oceanographic Commission and was co-sponsored by NASA.

Regional sea level change is more variable, over both space and time, than global sea level change and can diverge by up to 7 inches (20 centimeters) or more from the global mean. Additionally, making regional projections about future sea level differs from making global mean sea level projections. This is due to the fact that different processes contribute to sea level change in coastal regions.

Global sea level rise is caused by thermal expansion of warmer water plus contributions from ice sheets and glaciers. Regional sea level change, especially along coastlines, is influenced by additional factors, including vertical land movements, waves and tides, and winds and storms. So in order to estimate sea level inundation and flood risk, scientists have to understand all the factors that contribute to extreme water levels such as local sea level rise, land subsidence, tides, waves and storm surge.

Members of the N-SLCT understand the importance of studying coastal sea level change and improving the accuracy of regional projections. Ben Hamlington, assistant professor in the Ocean, Earth and Atmospheric Sciences Department at Old Dominion University in Norfolk, Virginia, and upcoming team leader for N-SLCT is serious about understanding sea level.

“The overarching theme of my scientific research,” he said, is “to consistently improve regional sea level projections.” Manhattan, where the conference was held, for example, lies within a few feet of sea level, and furthermore, the U.S. East Coast has some of the highest amounts of projected sea level increase.

“Global means aren’t very useful for someone who’s on the coast of Virginia where I live,” Hamlington said. A main part of the challenge of predicting regional sea level is that what causes the sea level changes and the flooding varies dramatically from place to place. Hamlington described a term called “nuisance flooding,” which is a type of persistent tidal flooding that leads to public inconveniences like road closures and backed-up storm water systems.

“Basically it means your path to work has to change because a certain road is blocked or impassable. You can still get to work, but it might take longer,” he explained. Right now, these nuisance-flooding events occur multiple times a year. But as sea level continues to rise, the nuisance flooding will get more and more frequent and will become even more of a problem. “Where I live, it’s hard to separate the pure science from the applications. With all this flooding, the broader significance of your work is very clear,” he said.

In Norfolk, Virginia, glacial isostatic adjustment (GIA) is around 0.04 inches (1 millimeter) per year, another millimeter per year of subsidence is due to slow subsidence into the Chesapeake Bay Meteor Impact Crater plus ground water pumping. Finally add 0.08 inches (2 millimeters) per year from the ocean rising and “You get the long-term tide gauge rate of relative sea level rise of just lower than 0.20 inches (5 millimeters) per year over the last 100 years. That’s a pretty high rate of sea level rise over a long period of time,” Hamlington explained. “Beyond nuisance flooding, there are also extreme events,” he continued. “During a storm event, you can get several feet of water in some parts of Norfolk.”

Actionable science

Stakeholders and decision makers are the ones driving the demand for improved regional sea level projections, Hamlington continued. “They’re the ones driving the discussion toward regional projections and that’s what’s needed for planning efforts.” These stakeholders include state and local public works officers responsible for infrastructure such as stadiums, roads, seawalls, and dykes plus pumps, water utilities, other utilities, businesses, and coastal inhabitants.

Scientists are responsible for helping society. This is why decision makers and scientists have come together to co-produce actionable science, to discuss how to communicate and collaborate, and to ensure that sea level science is being understood by the adaptation community.

“This is one of the biggest sea level conferences that we’ve had, when everybody who is working in different areas of the field comes together,” said Nerem. There were presentations on a variety of techniques to measure sea level change: tide gauges, measurements in marshes, paleo-sea level, corals, but from the perspective of the N-SLCT, “ We’re really focused on how to use remote sensing, satellite altimetry from Jason-1, 2 and 3 and Gravity Recovery and Climate Experiment (GRACE) combined with GPS measurements to improve regional sea level measurements and projections.”

Nerem’s project targets regions around the globe that are susceptible to inundation but don’t have much measurement infrastructure, such as Bangladesh. Many of these regions do not have detailed digital elevation models or 50 years of tide gauge measurements like we do in the United States.  “If we use our satellite techniques and test them in a place we understand, then we can go out where we don’t have that infrastructure and assess future sea level change in those regions.”

The N-SLCT hopes to leverage the satellite observations as much as possible to try to better understand future regional sea level change. This will help decision makers, coastal managers and stakeholders better adapt and prepare for the impacts of sea level rise.

According to Nerem, “We would like to produce a new assessment of future regional sea level change that benefits from the extensive record of satellite measurements collected by NASA.”

Thank you for reading,
Laura

TAGS: SEA LEVEL, REGIONAL SEA LEVEL, GLOBAL SEA LEVEL

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