In the mid-1970s, JPL evaluated several techniques for determining atmospheric water vapor effects on radiometric range. These experiments allowed the signals between spacecraft and the Deep Space Network antennas to be properly calibrated. One of the experiments was the Scanning Microwave Inversion Layer Experiment (SMILE). In May 1974, this test was conducted in El Monte, California, with a radiosonde suspended beneath a weather balloon. When the balloon reached 10,000 feet (about 3 km) it began measuring absolute pressure, ambient temperature, and relative humidity, then radioed the results to ground receivers.
A veteran interplanetary traveler is writing the closing chapter in its long and storied expedition. In its final orbit, where it will remain even beyond the end of its mission, at its lowest altitude, Dawn is circling dwarf planet Ceres, gathering an album of spellbinding pictures and other data to reveal the nature of this mysterious world of rock and ice.
Ceres turns on its axis in a little more than nine hours (one Cerean day). Meanwhile, its new permanent companion, a robotic emissary from Earth, revolves in a polar orbit, completing a loop in slightly under 5.5 hours. It flies from the north pole to the south over the side of Ceres facing the sun. Then when it heads north, the ground beneath it is cloaked in the deep dark of night on a world without a moon (save Dawn itself). As we discussed last month, Dawn's primary measurements do not depend on illumination. It can sense the nuclear radiation (specifically, gamma rays and neutrons) and the gravity field regardless of the lighting. This month, let's take a look at the other measurements our explorer is performing, most of which do depend on sunlight.
Of course the photographs do. Dawn had already mapped Ceres quite thoroughly from higher altitudes. The spacecraft acquired an extensive set of stereo and color pictures in its third mapping orbit. But now that Dawn is only about 240 miles (385 kilometers) high, its images are four times as sharp, revealing new details of the strange and beautiful landscapes.
Our spaceship is closer to Ceres than the International Space Station is to Earth. At that short range, it takes a long time to capture all of the vast territory, because each picture covers a relatively small area. Dawn’s camera sees a square about 23 miles (37 kilometers) on a side, less than one twentieth of one percent of the more than one million square miles (nearly 2.8 million square kilometers). In an ideal world (which is not the one Dawn is in or at), it would take just over two thousand photos from this altitude to see all the sights. However, as we will discuss in more detail next month, it is not possible to control the orbital motion and the pointing of the camera accurately enough to manage without more photos than that.
Most of the time, Dawn is programmed to turn at just the right rate to keep looking at the ground beneath it as it travels, synchronizing its rotation with its revolution around Ceres. It photographs the passing scenery, storing the pictures for later transmission to Earth. But some of the time, it cannot take pictures, because to send its bounty of data, it needs to point its main antenna at that distant planet, home not only to its controllers but also to many others (including you, loyal reader) who share in the thrill of a bold cosmic adventure. Dawn spends about three and a half days (nine Cerean days) with its camera and other sensors pointed at Ceres. Then it radios its findings home for a little more than one day (almost three Cerean days). During these communications sessions, even when it soars over lit terrain, it does not observe the sights below.
Mission planners have devised an intricate plan that should allow nearly complete coverage in about six weeks. To accomplish this, they guided Dawn to a carefully chosen orbit, and it has been doing an exceptionally good job there executing its complex activities.
Last month, we marveled at a stunning view that was not the typical perspective of peering straight down from orbit. Sometimes controllers now program Dawn to take a few more pictures after it stops aiming its instruments down, while it starts to turn to aim its antenna to Earth. This clever idea provides bonus views of whatever happens to be in the camera's sights as it slowly rotates from the point beneath the spacecraft off to the horizon. Who doesn't feel the attraction of the horizon and long to know what lies beyond?
Another of Dawn's scientific devices is two different sensors combined into one instrument. Like the camera, the visible and infrared mapping spectrometers (VIR) look at the sunlight reflected from the ground. (As we'll see below, however, VIR also can detect something more.) A spectrometer breaks up light into its constituent colors, just as a prism or a droplet of water does when revealing, quite literally, all the colors of the rainbow. Dawn's visible spectrometer would have a view very much like that. The infrared spectrometer, of course, looks at wavelengths of light our limited eyes cannot see, just as there are wavelengths of sound our limited ears cannot hear (consult with your dog for details).
A spectrometer does more than simply disperse the light into its components, however. It measures the intensity of that light at the different wavelengths. The materials on the surface leave their signature in the sunlight they reflect, making some wavelengths relatively brighter and some dimmer. That characteristic pattern is called a spectrum. By comparing these spectra with spectra measured in laboratories, scientists can infer the nature of the minerals on the ground. We described some of the intriguing conclusions last month.
VIR does still more. Rather than record the visible spectrum and the infrared spectrum from a single region, it takes spectra at 256 adjacent locations simultaneously. This would be like taking one column of 256 pixels in a picture and having a separate spectrum for each. By stitching columns together, you could construct the two dimensional picture but with the added dimension of an extensive spectrum at every location. (Because the extra information provides a sort of depth that flat pictures don't have, the result is sometimes called an “image cube.”) This capability to build up an image with spectra everywhere is what makes it a mapping spectrometer. VIR produces a remarkably rich view of its targets!
VIR's spectra contain much finer measurements of the colors and a wider range of wavelengths than the camera's images. In exchange, the camera has sharper vision and so can discern smaller geological features. In more technical terms, VIR achieves better spectral resolution and the camera achieves better spatial resolution. Fortunately, it is not a competition, because Dawn has both, and the instruments yield complementary measurements.
VIR generates a very large volume of data in each snapshot. As a result, Dawn can only capture and store relatively small areas of the dwarf planet with the mapping spectrometers, especially at this low altitude. Scientists have recognized from the first design of the mission that it would not be possible to cover all of Ceres (or Vesta) with VIR from the closer orbits. Nevertheless, Dawn has far exceeded expectations, returning a great many more spectra than anticipated. Still, as long as the spacecraft operates in this final mapping orbit, there will continue to be interesting targets to study with VIR.
Based on the nearly 20 million spectra of Ceres that VIR acquired from higher altitudes, the team has determined that new infrared spectra will provide more insight into the dwarf planet's character than the visible spectra. Because of their composition, the minerals display more salient signatures in infrared wavelengths than visible. The excellent visible spectra from the first three mapping orbits are deemed more than sufficient. Therefore, to make the best use of our faithful probe and to dedicate the resources to what is most likely to yield new knowledge about Ceres, VIR is devoting its share of the mission data in this final orbit to its infrared mapping spectrometer. We have many more exciting discoveries to look forward to!
The infrared light Ceres reflects from the sun can tell scientists a great deal about the composition, but they can learn even more from analyzing VIR's measurements. The sun isn't the only source of infrared. Ceres itself is. Many people correctly associate infrared with heat, because warm objects emit infrared light, and the strength at different wavelengths depends on the temperature. That calls for measuring the spectrum! Distant from the sun though it is, Ceres is warmed slightly by the brilliant star, so it has a very faint infrared glow of its own. Scientists can distinguish in VIR's observations between the reflected infrared sunlight and the infrared light Ceres radiates. In essence, VIR can function as a remote thermometer.
Last month, in one of Dawn's best photos yet of Ceres, we considered planning a hike across a breathtaking landscape. In case we do, VIR has shown we should be prepared for chilly conditions. Observed temperatures (all rounded to the nearest multiple of five) during the day on the dwarf planet range from -135 degrees Fahrenheit (-95 degrees Celsius) to -30 degrees Fahrenheit (-35 degrees Celsius). (It is so cold in some locations and times, especially at night, that Ceres produces too little infrared light for VIR to measure. Temperatures below the coldest reported here actually don't register.) This finding provides compelling support for this writer's frequent claim that Ceres is really cool. In addition, knowing the temperatures will be very important for understanding geological processes on this icy, rocky world, just as we know the movement of terrestrial glaciers depends on temperature.
Your loyal correspondent can't -- or, at least, won't -- help but indulge his nerdiness with a brief tangent. The range of temperatures above represent the warmest on Ceres, given that VIR cannot measure lower values. It's amusing, if you have a similar weird sense of humor, that Ceres' average temperature apparently is not that far from what it would be for a black hole of the same mass. We won't delve into the physics here, but such a black hole would be -225 degrees Fahrenheit (-140 degrees Celsius). OK, enough hilarity. Back to Dawn and Ceres...
Ever creative, scientists are attempting another clever method to gain insight into the nature of this exotic orb. When Dawn is at just the right position in its orbit on the far side of Ceres, so that a straight line to Earth passes very close to the limb of Ceres itself, the spacecraft's radio signal will actually hit the dwarf planet. The radio waves interact with the materials on the surface, which can induce an exquisitely subtle distortion. After bouncing off the ground at a grazing angle, the radio signal continues on its way, heading toward Earth. The effect on the signal is much too small to affect the normal communications at all, but specialized equipment at NASA's Deep Space Network designed for this purpose might still be able to detect the tiny changes. The fantastically sensitive antennas measure the properties of the radio waves, and by studying the details, scientists may be able to learn more about the properties of the surface of the distant world. For example, this could help them distinguish between different types of materials (such as ice, rocks, sand, etc.) as well as reveal how rough or smooth the ground is at scales far, far smaller than the camera can discern. This is an extremely challenging measurement, and no small distortions have been detected so far, but always making the best possible use of the resources, scientists continue to look for them.
In addition to those bonus measurements, Dawn remains very productive in acquiring infrared spectra, photographs, gamma ray spectra and neutron spectra plus conducting measurements of the massive body's gravitational field, all of which contribute to unlocking the mysteries of the first dwarf planet ever discovered or explored. The venerable adventurer is in good condition and is operating flawlessly.
We have discussed extensively the failures of two of the four reaction wheels, devices Dawn used to depend on to control its orientation in space. Without three healthy reaction wheels, the probe has had to rely instead on hydrazine propellant expelled from the small jets of the reaction control system. (When Dawn uses its ion engine, that remarkable system does double duty, reducing the need for the hydrazine.)
For most of the time since escaping from Vesta's gravitational clutches in 2012, Dawn has kept the other two reaction wheels in reserve so any remaining lifetime from those devices could offset the high cost of hydrazine propellant to turn and point in this current tight orbit. Those two wheels have been on and functioning flawlessly since Dec. 14, 2015, and every day they operate, they keep the expenditure of the dwindling supply of hydrazine to half of what it would be without them. (Next month we will offer some estimates of how long Dawn might continue to operate.) But the ever-diligent team recognizes another wheel could falter at any moment, and they remain ready to continue the mission with pure hydrazine control after only a short recovery operation. If a third failure is at all like the two that have occurred already, the hapless wheel won't give an indication of a problem until it's too late. A reaction wheel failure evidently is entirely unpredictable. We'll know about it only after it occurs in the remote depths of space where Dawn resides at an alien world.
Earth and Ceres are so far from each other that their motions are essentially independent. The planet and the dwarf planet follow their own separate repetitive paths around the sun. And each carries its own retinue: Earth has thousands of artificial satellites and one prominent natural one, the moon. Ceres has one known satellite. It arrived there in March 2015, and its name is Dawn.
Coincidentally, both reached extremes earlier this month in their elliptical heliocentric orbits. Earth, in its annual journey around our star, was at perihelion, or the closest point to the sun, on Jan. 2, when it was 0.98 AU (91.4 million miles, or 147 million kilometers) away. Ceres, which takes 4.6 years (one Cerean year) for each loop, attained its aphelion, or greatest distance from the sun, on Jan. 6. On that day, it was 2.98 AU (277 million miles, or 445 million kilometers) from the gravitational master of the solar system.
Far, far from the planet where its deep-space voyage began, Dawn is now bound to Ceres, held in a firm but gentle gravitational embrace. The spacecraft continues to unveil new and fascinating secrets there for the benefit of all those who remain with Earth but who still look to the sky with wonder, who feel the lure of the unknown, who are thrilled by new knowledge, and who yearn to know the cosmos.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.87 AU (360 million miles, or 580 million kilometers) from Earth, or 1,440 times as far as the moon and 3.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and four minutes to make the round trip.
This atomic clock was used at the Goldstone Time Standards Laboratory in 1970, to synchronize clocks at Deep Space Network stations around the world. This master clock was accurate to plus or minus two millionths of a second, when compared to clocks maintained by the National Bureau of Standards and the U.S. Naval Observatory. In the late 1960s, JPL had developed a moon bounce technique to transmit signals from one deep space antenna to another. Experiments included periodic measurement of timing signals that were reflected from the surface of the moon, to find out if the station clocks were within allowable limits for accuracy.
“The moment the satellite separated from the rocket got me feeling emotional,” Dr. Josh Willis, lead project scientist for the Jason-3 mission, told me. I imagined the satellite emerging from the nosecone of SpaceX’s Falcon 9 rocket and unfurling its solar panels 830 miles above where we were standing near the bar at the Jason-3 launch after-party. Seeing a NASA science dude with a crisp shirt, black suit jacket and—can you believe it—cufflinks was heartwarming. I recognized his dad, his wife, his in-laws nearby. My husband was there, too, along with most of our peers, all part of an odd little NASA ocean sciences extended family.
When Willis told me he “had affection” for the Jason-3 satellite, I felt relief; glad that I wasn’t the only one who’d been anthropomorphizing. He said that the French engineers from CNES, the French Space Agency, who were responsible for connecting the satellite to the rocket, had drawn a pair of eyes on the nitrogen storage bags used for sealing the satellite to prevent rust. “It looked like it was alive,” he said.
Unless you’re a total whack, your affection for flight hardware builds up over time. And Willis’ work with satellites that measure sea surface height goes back to TOPEX/Poseidon, the great granddaddy of ocean surface topography, which launched in 1992 when he was a graduate student. “Back then, the data was cool and interesting and was really accurate. It did what it was supposed to do, which was amazing to me.” TOPEX/Poseidon was originally designed as a 5-year mission to measure currents. “In the beginning, it wasn’t obvious that these satellites would measure climate change. It took years to ensure that the satellites were accurate enough to measure global sea level change, and, of course, now they’re the most important tool for measuring global warming.”
After 23 years of data, we’re continuing the series with the launch of Jason-3, the fourth member of the family. “That’s a huge triumph of science and engineering,” he explained. “NASA always wants to do new things, but for climate science, we really need to do the same thing over and over. That’s a different type of job.” I looked around at our spouses and thought about how I explain marriage to my single friends: You can get a lot of interesting things from a long-term commitment. Willis agreed. It’s a whole career, going the distance, not just one conquest after the other.
“It took years and years for the entire science team, which is a couple hundred people looking at this data year in and year out, to feel confident that we were measuring more than currents. Everything has to be perfect to measure global sea level rise.” And over that 23-year period, while the scientists’ abilities to use the data improved, global sea level rose an inch or two, which, sad but true, made it easier to measure.
Jason-3 launched just in time to observe the 2016 El Niño with its many extreme sea levels, storms and high winds in the ocean. The Jason-2 and Jason-3 satellites will fly right next to each other, separated by 60 seconds, and the calibration will happen over a wide range of different conditions. When I asked Willis if this year’s El Niño is bigger than the one in 1997-98, he said, “The water at its peak temperature in the Pacific this time is warmer than the peak temperature in 97-98. But what most people care about is rainfall, and by that measure, we’ll just have to wait and see. We’ve got a few more months before El Niño clobbers us here in the U.S. Plus, we’ve had another 18 years of global warming.”
“Let’s face it, the ocean dominates everything,” he continued. “Two-thirds of the planet’s surface is rising. That’s the story of global warming. You have to have a satellite to see that, and the Jasons do what nothing else can.”
As always, I welcome your comments.
TOPEX/Poseidon and Jason-1 were cooperative missions between NASA and the French space agency, CNES. Additional partners in the Jason-2 mission included NOAA and Eumetsat. Jason-3 continues the international cooperation, with NOAA and Eumetsat leading the efforts, along with partners NASA and CNES.
Dawn is now performing the final act of its remarkable celestial choreography, held close in Ceres’ firm gravitational embrace. The distant explorer is developing humankind’s most intimate portrait ever of a dwarf planet, and it likely will be a long, long time before the level of detail is surpassed.
The spacecraft is concluding an outstandingly successful year 1,500 times nearer to Ceres than it began. More important, it is more than 1.4 million times closer to Ceres than Earth is today. From its uniquely favorable vantage point, Dawn can relay to us spectacular views that would otherwise be unattainable. At an average altitude of only 240 miles (385 kilometers), the spacecraft is closer to Ceres than the International Space Station is to Earth. From that tight orbit, the dwarf planet looks the same size as a soccer ball seen from only 3.5 inches (9.0 centimeters) away. This is in-your-face exploration.
The spacecraft has returned more than 16,000 pictures of Ceres this year (including more than 2,000 since descending to its low orbit this month). One of your correspondent’s favorites (below) was taken on Dec. 10 when Dawn was verifying the condition of its backup camera. Not only did the camera pass its tests, but it yielded a wonderful, dramatic view not far from the south pole. It is southern hemisphere winter on Ceres now, with the sun north of the equator. From the perspective of the photographed location, the sun is near the horizon, creating the long shadows that add depth and character to the scene. And usually in close-in orbits, we look nearly straight down. Unlike such overhead pictures typical of planetary spacecraft (including Dawn), this view is mostly forward and shows a richly detailed landscape ahead, one you can imagine being in — a real place, albeit an exotic one. This may be like the breathtaking panorama you could enjoy with your face pressed to the porthole of your spaceship as you are approaching your landing sight. You are right there. It looks — it feels! — so real and physical. You might actually plan a hike across some of the terrain. And it may be that a visiting explorer or even a colonist someday will have this same view before setting off on a trek through the Cerean countryside.
Of course, Dawn's objectives include much more than taking incredibly neat pictures, a task at which it excels. It is designed to collect scientifically meaningful photos and other valuable measurements. We'll see more below about what some of the images and spectra from higher altitudes have revealed about Ceres, but first let's take a look at the three highest priority investigations Dawn is conducting now in its final orbit, sometimes known as the low altitude mapping orbit (LAMO). While the camera, visible mapping spectrometer and infrared mapping spectrometer show the surface, these other measurements probe beneath.
With the spacecraft this close to the ground, it can measure two kinds of nuclear radiation that come from as much as a yard (meter) deep. The radiation carries the signatures of the atoms there, allowing scientists to inventory some of the key chemical elements of geological interest. One component of this radiation is gamma ray photons, a high energy form of electromagnetic radiation with a frequency beyond visible light, beyond ultraviolet, even beyond X-rays. Neutrons in the radiation are entirely different from gamma rays. They are particles usually found in the nuclei of atoms (for those of you who happen to look there). Indeed, outweighing protons, and outnumbering them in most kinds of atoms, they constitute most of the mass of atoms other than hydrogen in Ceres (and everywhere else in the universe, including in your correspondent).
To tell us what members of the periodic table of the elements are present, Dawn's gamma ray and neutron detector (GRaND) does more than detect those two kinds of radiation. Despite its name, GRaND is not at all pretentious, but its capabilities are quite impressive. Consisting of 21 sensors, the device measures the energy of each gamma ray photon and of each neutron. (That doesn't lend itself to as engaging an acronym.) It is these gamma ray spectra and neutron spectra that reveal the identities of the atomic species in the ground.
Some of the gamma rays are produced by radioactive elements, but most of them and the neutrons are generated as byproducts of cosmic rays impinging on Ceres. Space is pervaded by cosmic radiation, composed of a variety of subatomic particles that originate outside our solar system. Earth's atmosphere and magnetic field protect the surface (and those who dwell there) from cosmic rays, but Ceres lacks such defenses. The cosmic rays interact with nuclei of atoms, and some of the gamma rays and neutrons that are released escape back into space where they are intercepted by GRaND on the orbiting Dawn.
Unlike the relatively bright light reflected from Ceres's surface that the camera, infrared spectrometer and visible spectrometer record, the radiation GRaND measures is very faint. Just as a picture of a dim object requires a longer exposure than for a bright subject, GRaND's "pictures" of Ceres require very long exposures, lasting weeks, but mission planners have provided Dawn with the necessary time. Because the equivalent of the illumination for the gamma ray and neutron pictures is cosmic rays, not sunlight, regions in darkness are no fainter than those illuminated by the sun. GRaND works on both the day side and the night side of Ceres.
In addition to the gamma ray spectra and neutron spectra, Dawn's other top priority now is measuring Ceres' gravity field. The results will help scientists infer the interior structure of the dwarf planet. The measurements made in the higher altitude orbits turned out to be even more accurate than the team had expected, but now that the probe is as close to Ceres as it will ever go, and so the gravitational pull is the strongest, they can obtain still better measurements.
Gravity is one of four fundamental forces in nature, and its extreme weakness is one of the fascinating mysteries of how the universe works. It feels strong to us (well, most of us) because we don't so easily sense the two kinds of nuclear forces, both of which extend only over extremely short distances, and we generally don't recognize the electromagnetic force. With both positive and negative electrical charges, attractive and repulsive electromagnetic forces often cancel. Not so with gravity. All matter exerts attractive gravity, and it can all add up. The reason gravity -- by far the weakest of the four forces -- is so salient for those of you on or near Earth is that there is such a vast amount of matter in the planet and it all pulls together to hold you down. Dawn overcame that pull with its powerful Delta rocket. Now the principal gravitational force acting on it is the cumulative effect of all the matter in Ceres, and that is what determines its orbital motion.
The spacecraft experiences a changing force both as the inhomogeneous dwarf planet beneath it rotates on its axis and as the craft circles that massive orb. When Dawn is closer to locations within Ceres with greater density (i.e., more matter), the ship feels a stronger tug, and when it is near regions with lower density, and hence less powerful gravity, the attraction is weaker. The spacecraft accelerates and decelerates very slightly as its orbit carries it closer to and farther from the volumes of different density. By carefully and systematically plotting the exquisitely small variations in the probe's motion, navigators can calculate how the mass is distributed inside Ceres, essentially creating an interior map. This technique allowed scientists to establish that Vesta, the protoplanet Dawn explored in 2011-2012, has a dense core (composed principally of iron and nickel) surrounded by a less dense mantle and crust. (That is one of the reasons scientists now consider Vesta to be more closely related to Earth and the other terrestrial planets than to typical asteroids.)
Mapping the orbit requires systems both on Dawn and on Earth. Using the large and exquisitely sensitive antennas of NASA's Deep Space Network (DSN), navigators measure tiny changes in the frequency, or pitch, of the spacecraft's radio signal, and that reveals changes in the craft's velocity. This technique relies on the Doppler effect, which is familiar to most terrestrial readers as they hear the pitch of a siren rise as it approaches and fall as it recedes. Other readers who more commonly travel at speeds closer to that of light recognize that the well-known blueshift and redshift are manifestations of the same principle, applied to light waves rather than sound waves. Even as Dawn orbits Ceres at 610 mph (980 kilometers per hour), engineers can detect changes in its speed of only one foot (0.3 meters) per hour, or one five-thousandth of a mph (one three-thousandth of a kilometer per hour). Another way to track the spacecraft is to measure the distance very accurately as it revolves around Ceres. The DSN times a radio signal that goes from Earth to Dawn and back. As you are reminded at the end of every Dawn Journal, those signals travel at the universal limit of the speed of light, which is known with exceptional accuracy. Combining the speed of light with the time allows the distance to be pinpointed. These measurements with Dawn's radio, along with other data, enable scientists to peer deep into the dwarf planet
Although it is not among the highest scientific priorities, the flight team is every bit as interested in the photography as you are. We are visual creatures, so photographs have a special appeal. They transport us to mysterious, faraway worlds more effectively than any propulsion system. Even as Dawn is bringing the alien surface into sharper focus now, the pictures taken in higher orbits have allowed scientists to gain new insights into this ancient world. Geologists have located more than 130 bright regions, none being more striking than the mesmerizing luster in Occator crater. The pictures taken in visible and infrared wavelengths have helped them determine that the highly reflective material is a kind of salt.
It is very difficult to pin down the specific composition with the measurements that have been analyzed so far. Scientists compare how reflective the scene is at different wavelengths with the reflective properties of likely candidate materials studied in laboratories. So far, magnesium sulfate yields the best match (although it is not definitive). That isn't the type of salt you normally put on your food (or if it is, I'll be wary about accepting the kind invitation to dine in your home), but it is very similar (albeit not identical) to Epsom salts, which have many other familiar uses.
Scientists' best explanation now for the deposits of salt is that when asteroids crash into Ceres, they excavate underground briny water-ice. Once on the surface and exposed to the vacuum of space, even in the freezing cold so far from the sun, the ice sublimes, the water molecules going directly from the solid ice to gas without an intermediate liquid stage. Left behind are the materials that had been dissolved in the water. The size and brightness of the different regions depend in part on how long ago the impact occurred. A very preliminary estimate is that Occator was formed by a powerful collision around 80 million years ago, which is relatively recent in geological times. (We will see in a future Dawn Journal how scientists estimate the age and why the pictures in this low altitude mapping orbit will help refine the value.)
As soon as Dawn's pictures of Ceres arrived early this year, many people referred to the bright regions as "white spots," although as we opined then, such a description was premature. The black and white pictures revealed nothing about the color, only the brightness. Now we know that most have a very slight blue tint. For reasons not yet clear, the central bright area of Occator is tinged with more red. Nevertheless, the coloration is subtle, and our eyes would register white.
Measurements with both finer wavelength discrimination and broader wavelength coverage in the infrared have revealed still more about the nature of Ceres. Scientists using data from one of the two spectrometers in the visible and infrared mapping spectrometer instrument (VIR) have found that a class of minerals known as phyllosilicates is common on Ceres. As with the magnesium sulfate, the identification is made by comparing Dawn's detailed spectral measurements with laboratory spectra of a great many different kinds of minerals. This technique is a mainstay of astronomy (with both spacecraft and telescopic observations) and has a solid foundation of research that dates to the nineteenth century, but given the tremendous variety of minerals that occur in nature, the results generally are neither absolutely conclusive nor extremely specific.
There are dozens of phyllosilicates on Earth (one well known group is mica). Ceres too likely contains a mixture of at least several. Other compounds are evident as well, but what is most striking is the signature of ammonia in the minerals. This chemical is manufactured extensively on Earth, but few industries have invested in production plants so far from their home offices. (Any corporations considering establishing Cerean chemical plants are invited to contact the Dawn project. Perhaps, however, mining would be a more appropriate first step in a long-term business plan.)
Ammonia's presence on Ceres is important. This simple molecule would have been common in the material swirling around the young sun almost 4.6 billion years ago when planets were forming. (Last year we discussed this period at the dawn of the solar system.) But at Ceres' present distance from the sun, it would have been too warm for ammonia to be caught up in the planet-forming process, just as it was even closer to the sun where Earth resides. There are at least two possible explanations for how Ceres acquired its large inventory of ammonia. One is that it formed much farther from the sun, perhaps even beyond Neptune, where conditions were cool enough for ammonia to condense. In that case, it could easily have incorporated ammonia. Subsequent gravitational jostling among the new residents of the solar system could have propelled Ceres into its present orbit between Mars and Jupiter. Another possibility is that Ceres formed closer to where it is now but that debris containing ammonia from the outer solar system drifted inward and some of it ultimately fell onto the dwarf planet. If enough made its way to Ceres, the ground would be covered with the chemical, just as VIR observed.
Scientists continue to analyze the thousands of photos and millions of infrared and visible spectra even as Dawn is now collecting more precious data. Next month, we will summarize the intricate plan that apportions time among pointing the spacecraft's sensors at Ceres to perform measurements, its main antenna at Earth to transmit its findings and receive new instructions and its ion engine in the direction needed to adjust its orbit.
The plans described last month for getting started in this fourth and final mapping orbit worked out extremely well. You can follow Dawn's activities with the status reports posted at least twice a week here. And you can see new pictures regularly in the Ceres image gallery.
We will be treated to many more marvelous sights on Ceres now that Dawn's pictures will display four times the detail of the views from its third mapping orbit. The mapping orbits are summarized in the following table, updated from what we have presented before. (This fourth orbit is listed here as beginning on Dec. 16. In fact, the highest priority work, which is obtaining the gamma ray spectra, neutron spectra and gravity measurements, began on Dec. 7, as explained last month. But Dec. 16 is when the spacecraft started its bonus campaign of measuring infrared spectra and taking pictures. Recognizing that what most readers care about is the photography, regardless of the scientific priorities, that is the date we use here.
|Mapping orbit||Dawn code name||Dates||Altitude in miles (kilometers)||Resolution in feet (meters) per pixel||Resolution compared to Hubble||Orbit period||Equivalent distance of a soccer ball|
|1||RC3||April 23 - May 9||8,400 (13,600)||4,200 (1,300)||24||15 days||10 feet (3.2 meters)|
|2||Survey||June 6-30||2,700 (4,400)||1,400 (410)||73||3.1 days||3.4 feet (1.0 meters)|
|3||HAMO||Aug 17 - Oct 23||915 (1,470)||450 (140)||217||19 hours||14 inches (34 cm)|
|4||LAMO||Dec 16 - end of mission||240 (385)||120 (35)||830||5.4 hours||3.5 inches (9.0 cm)|
Dawn is now well-positioned to make many more discoveries on the first dwarf planet discovered. Jan. 1 will be the 215th anniversary of Giuseppe Piazzi's first glimpse of that dot of light from his observatory in Sicily. Even to that experienced astronomer, Ceres looked like nothing other than a star, except that it moved a little bit from night to night like a planet, whereas the stars were stationary. (For more than a generation after, it was called a planet.) He could not imagine that more than two centuries later, humankind would dispatch a machine on a cosmic journey of more than seven years and three billion miles (five billion kilometers) to reach the distant, uncharted world he descried. Dawn can resolve details more than 60 thousand times finer than Piazzi's telescope would allow. Our knowledge, our capabilities, our reach and even our ambition all are far beyond what he could have conceived, and yet we can apply them to his discovery to learn more, not only about Ceres itself, but also about the dawn of the solar system.
On a personal note, I first saw Ceres through a telescope even smaller than Piazzi's when I was 12 years old. As a much less experienced observer of the stars than he was, and with the benefit of nearly two centuries of astronomical studies between us, I was thrilled! I knew that what I was seeing was the behemoth of the main asteroid belt. But it never occurred to me when I was only a starry-eyed youth that I would be lucky enough to follow up on Piazzi's discovery as a starry-eyed adult, responsible for humankind's first visitor to that fascinating alien world, answering a celestial invitation that was more than 200 years old.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.66 AU (340 million miles, or 547 million kilometers) from Earth, or 1,360 times as far as the moon and 3.72 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.
Professor James Van Allen of the University of Iowa designed the cosmic ray detector experiment on JPL’s Explorer satellite, launched in 1958. He was also the principal investigator for the radiation experiment that was part of the Pioneer III and IV payloads. In this photo, Dr. Van Allen is looking at the cone-shaped Pioneer probe, before it was gold plated and painted with stripes (to maintain a temperature of 10-50 degrees C during flight).
After the launch of Pioneer IV on March 3, 1959 the experiment successfully measured radiation found around the Earth. It was also designed to measure lunar radiation, but the flyby distance of 37,000 miles was not close enough for the optical trigger to work. The instrument used two Geiger-Mueller tubes to detect and measure radiation and a small battery-powered radio transmitter to send the data to Earth. The low-power signal was received by the 85-foot antenna at Goldstone, California -- part of what became known as the Deep Space Network in 1963. The probe also tested technology that would be needed for future lunar photographic missions. After passing by the moon, Pioneer IV went into a heliocentric orbit.
Why go to Antarctica to fly balloons? The answer is the anticyclone that sets up over the continent in December. The anticyclone is a weather system in the upper atmosphere in which the winds flow counter-clockwise around the continent. The wind flow can keep balloons afloat for long flights and allow for recovery of the payload so it can be flown again.
Each rotation around the continent is approximately 14 days. These wind patterns typically set up around December 15, although they can be ready as early as December 5 or as late as December 25. This year, the winds are expected to be in place around December 18, so in just a couple of days. The anticyclone pattern usually lasts one to two months, and some payloads teams will often try for two or three trips around the continent. The longest flight was 55 days.
Since the Antarctic summer season falls over a number of holidays including, Thanksgiving, Hanukkah, Christmas, and New Year's, people are interested in how the holidays are celebrated down here. I have written a post on Thanksgiving. The next holiday to be celebrated is Hanukkah. It is not the most important holiday on the Jewish calendar, but has grown to prominence in the United States because it falls around the same time as Christmas. Part of the holiday is to light a candle for every night of the holiday such that on the first night, one candle is lit, on the second night, two candles are lit, and so on and so forth. Technically, lighting candles is strictly forbidden at McMurdo. For this reason, the base requested special permission to allow a celebration. Permission was granted for one menorah to be lit only in the McMurdo galley, and the fire marshal had to be present. (The menorah is the base that holds all of the candles. There is a special one for Hanukkah with space for nine candles -- one candle for each night of Hanukkah and one candle to light all of the others.)
The most interesting thing about celebrating Jewish holidays in Antarctica is deciding when they actually start. Under the Jewish calendar, all days start at sundown, but during the Antarctic summer, the sun never sets. And there was no actual consensus about when the candle lighting should take place. I had heard that it was celebrated based on the closest land mass where the sun actually sets (i.e., New Zealand). Someone else said we should celebrate with Jerusalem, and yet a third said we should celebrate based on our home time back in the US. Ultimately, though, we just had to go with the time that the fire marshal was available, which was 7:15 p.m. There was also a nice party with latkes, matzah ball soup, the retelling of the Hanukkah story, and dreidel on the fifth night. Overall, I had a very nice holiday! Happy Hanukkah!!!
Think back to when you were a kid imagining what you were going to be when you grew up. You dreamt that someday, somehow, you would make a difference, a contribution, that your work would be meaningful in the world. If you accomplished this today, how pumped would you be?
"This is going to sound really cheesy and lame," NASA oceanographer Michelle Gierach told me over a Skype call from COP21 in Paris, "but I just get a sense of pride being from the U.S. and being a cool NASA representative and seeing people get excited about what we do. In my day-to-day job, I sometimes forget how much Americans and international people from everywhere love to know what we're doing. It reinvigorates a sense of pride in NASA's work."
Because of the nine-hour time difference, I was barely awake for our call, and through my morning mental blur I wondered for a moment if the glee in her voice had something to do with the fact that I'm a fantastic person and she was thrilled to be speaking with me, or perhaps she was hopped up on chocolate. "They give you chocolate bars every day!" she squealed. "I'm not lying, and it's really good chocolate."
But it was the conference, COP21, the 21st Conference of the Parties of the U.N. Framework Convention on Climate Change, that had her all giddy. You see, after so many years of stagnation, resistance and even moving backwards, finally, finally there seems to be movement toward global action against climate change. Yes, it's baby steps, and yes, there's more work to do, "but some movement is significant," she stressed. "I'll take it."
All of us are hungry for something positive. Always. Especially now, since most of the news lately has been such a total bummer. A positive message around climate could be that bump in optimism that we all need right now.
The U.N. COP21 meeting in Paris began on Nov. 30, and by this Friday, Dec. 11, 195 member nations hope to reach a unanimous agreement to cut greenhouse gas emissions, hold global warming to 2 degrees Celsius or even lower and provide financial support to developing nations so they can bypass fossil fuels.
It's hard not to feel optimistic. Part of you wants to get your hopes up, but you also don't want to be disappointed, because for so many years there's been so much disappointment. Then there's that part of you that says, This time is different. This time we can do it. Gierach told me that she felt an energy about reaching an outcome at COP21. The overall vibe is "completely optimistic, everybody wants to do something, everybody knows we have to do something. There's a 'let's do it' kind of attitude."
Hyper about the Hyperwall
Last week, we'd spoken about her upcoming trip. She had conflicted feelings due to the recent events in Paris and was concerned about a heightened state of worry and icky vibes. "As a NASA representative," she explained, "my role is to show what NASA is doing with regards to climate change, even though I'm not a delegate or a policy maker. I was so excited to go, and now I'm just not so excited about it anymore." But what a difference one week and a few thousand miles made. From the conference her voice sounded triumphant: "Everybody here wants to show that it's not going to stop what they're trying to do here. It hasn't stopped it at all."
Gierach also told me she was, "super excited that this time around it finally seems people are listening. People see that the oceans are part of a massive system and actually are a significant reason we haven't had a more extreme temperature rise. That message seems to be getting out there." She's been talking about the oceans every day on NASA's hyperwall, an ultra-high resolution visualization that combines nine computer monitors into a giant screen that plays animations in tandem.
On Dec. 3, she joined a panel called "Oceans under pressure" to discuss the following main points of consensus that we can see from satellites:
- The sea surface temperature record shows that the ocean is warming, which clearly impacts Arctic sea ice reduction, the different types of sea ice, and ice sheet reduction.
- Sea level rise is not equal around the globe; for example, the western tropical Pacific has much higher sea level rise than the eastern equatorial Pacific.
And because a significant portion of the conference is dedicated to carbon emissions, she's also talking about the interaction between the ocean and the atmosphere and how carbon dioxide transfers between the two.
Just before we hung up, she added, with power in her voice like a chant or rally call, "Yeah, we're here and we're going to do something. We're not just speaking; we're actually acting and showing that we're acting."
Watch the live stream from the U.S. Center at COP21 in Paris here.
Watch the "Oceans Under Pressure" panel with panelists Jean-Pierre Gattuso, IDDRI/CNRS; Jean-Pierre Gattuso, IDDRI/CNRS; Alexander MacDonald, NOAA; Michelle Gierach, NASA; Cassandra deYoung, FAO here.
Thank you so much for reading,
P.S. Michelle was totally inspired by President Obama's speech and said, "Regardless of what people may think, he is trying to make the world a better place. It made me extremely proud to be part of the United States and have him as our president." Watch the speech here.
The project I came to Antarctica to work on, the Stratospheric Terahertz Observatory II, or STO-2, is a balloon-borne mission designed to trace the phases of the interstellar medium, which is the gas and dust between the stars. This gas and dust follows a life-cycle that can be traced with atoms such as carbon, nitrogen, and oxygen. These elements radiate in the terahertz (THz) region of the electromagnetic spectrum, which is the region between microwaves and infrared, hence the name Stratospheric Terahertz Observatory (it is our second flight, hence the II).
The STO-2 instrument is specifically designed to look at the star forming phase of the interstellar medium. Each atomic tracer transmits its own spectral signature, which can be thought of as its own radio station. We build radio receivers specifically tuned to look at each of the frequencies being emitted by the tracers. The technique used is called heterodyne detection and it mixes two signals near the same frequency to provide extremely high spectral resolution (the process is described below). The high spectral resolution is important because it enables us to look at the Doppler shift of the sources, which can be used to calculate the physical properties of the cloud in order to provide a better understanding of the star-formation process.
A block diagram of a heterodyne receiver. The sky signal is mixed with a local oscillator signal generated in the lab. In astronomy, we often use superconducting mixers for their extremely high sensitivity. The intermediate frequency is amplified using a low-noise amplifier (LNA) and then filtered and amplified as needed to match the power desired by the spectrometer. Image credit: Jenna Kloosterman, PhD thesis.
Our instrument has three frequency bands to observe nitrogen, carbon, and oxygen in the Milky Way. The nitrogen and carbon bands each have two pixels and use multiplier chains as local oscillators. The oxygen band has one pixel and uses a quantum cascade laser as a local oscillator. A local oscillator is a lab-generated signal near the target frequency. It is then mixed with the incoming sky signal. The mixers (aka the detectors) are bridges of niobium nitride called hot electron bolometers (HEBs), which when cooled to 4 K (-452 degrees F) are a superconducting material. Generally, a mixer can be thought of as a switch. When the sky signal and the local oscillator are in phase, the mixer is in the "on" position. When they are out of phase, the mixer is in the "off" position. This creates a beat frequency, like playing an instrument out of tune (sound waves instead of light waves, but the effect is the same) referred to as an intermediate frequency.
In reality, the process with a superconductor is a little more complicated. A superconductor has infinite electrical current at 0 V because electrons are moving with no resistance. When a local oscillator signal is applied to an HEB, it heats some of the electrons in the superconducting bridge so that they no longer have infinite currents, i.e., they are no longer superconducting. The incoming sky signal modulates the size of the "hot spot" on the bridge, which modulates the resistance of the bridge in a manner much like the on/off switch analogy.
This past week, we have been very busy aligning the local oscillators. The local oscillator signal is quasi-optically injected via a beam splitter, which reflects a small percentage of the local oscillator power in the direction of the hot electron bolometers (see the diagram above). This alignment is very hard since the beams are not optical and we cannot see them with our own eyes. Since the local oscillator power is heating the superconducting bridge, we instead monitor the hot electron bolometer current to find the local oscillator beam. It requires a lot of patience and a bit of luck.
After successfully aligning all five pixels, we mounted the local oscillator plate to the cryostat and fixed it in place. Once the alignment was complete, it was time to integrate the instrument to the gondola and telescope. This successful integration occurred on Tuesday and is now almost complete. The control and back-end electronics all had to be mounted and connected. Furthermore, when flying in the stratosphere, the instrument and gondola can get very, very hot and zap the electronics on-board the balloon. Therefore, most of the funny looking gold loops you see on the gondola in the picture above are cooling loops running cold water to keep the room temperature instrument and electronics cool at altitude. This system also needs to be connected.
All in all, the project is going very well down here. There is still a lot of work to be done, but we hope to start with systems-level observatory testing tomorrow. Unfortunately, we are still waiting on the upper atmospheric winds to set up over the continent, so a launch is still a few weeks away.