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Marc Rayman

Dawn Journal: Measuring Light

Posted by Marc Rayman

01-02-2016 13:20 CST

Topics: mission status, asteroids, Dawn, asteroid 1 Ceres

Dear Spellbindawngs,

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.

Kupalo crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Kupalo crater
Dawn captured this view of Kupalo crater on Dec. 20, shortly after beginning the observations from its current low altitude mapping orbit at 240 miles (385 kilometers). (Kupalo is a Slavic harvest deity associated with love and fertility.) This is a relatively young crater, as seen by its sharp, clear features and the paucity of overlying smaller impact craters which would have formed later. Bright material on the rim and walls may be salts, as explained last month. The crater is 16 miles (26 kilometers) across. Full image and caption.

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.

Ceres in false-color

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Ceres in false-color
This is an excerpt from a much more extensive animation providing a colorful tour of some of the highlights on Ceres. It is made with the color and stereo pictures Dawn collected in its third mapping orbit 915 miles (1,470 kilometers) above the dwarf planet. Here we see Occator crater, with its famous bright regions. The full animation (in which both color and sound are exaggerated) also shows the strange, conical mountain Ahuna Mons plus UrvaraHaulani and Dantu (seen in more detail below) craters and more. The colors indicate different compositions, which may include salts and phyllosilicates, as explained last monthFull animation and caption.

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.

Dantu crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Dantu crater
On Dec. 21, in its lowest orbit at about 240 miles (385 kilometers), Dawn flew over Dantu crater and obtained pictures with four times the clarity of the third mapping orbit, where we saw the entire crater. (Dantu is a timekeeper god who initiates the cycle of planting rites among the Ga people of the Accra Plains of southeastern Ghana.) The bright material here is at the 4:00 position, half way from the center to the rim, in the picture shown in November. The network of fractures may have formed when the ground cooled after being heated by the crater-forming impact, or perhaps later when other geological processes caused the crater floor to be uplifted. The crater is about 78 miles (126 kilometers) in diameter. The next picture below shows detail of another part of Dantu. The animation above includes Dantu (as seen from farther away). Full image and caption.

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.

Dantu crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Dantu crater
On Dec. 19, Dawn’s orbit took it over a different part of Dantu crater, showing more reflective material on the walls and floor. (This scene is from the right side of the crater as pictured in November.) More of the fractures visible in the picture above are in the upper left of this picture. Full image and caption.

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!

Unnamed crater

NASA /JPL-Caltech / UCLA / MPS / DLR / IDA

Unnamed crater
Dawn photographed this unnamed crater on Dec. 23. It is 20 miles (32 kilometers) in diameter and is between Dantu and Rao craters. (See the map here.) Part of this crater is shown at the bottom left of the photo of Dantu we saw in November. The many ridges and steep slopes here may be the result of the crater partially collapsing during its formation. The complex geology evokes an image of a flower (at least for this writer). Full image and caption.

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.

Victa crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Victa crater
Dawn observed Victa crater on Dec. 19. (Victa was a Roman goddess of food and nourishment.) The crater is 20 miles (32 kilometers) in diameter and so is the same size as the unnamed one shown above. Full image and caption.

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.

Dr. Marc D. Rayman
4:30 p.m. PST January 31, 2016

See other posts from February 2016


Or read more blog entries about: mission status, asteroids, Dawn, asteroid 1 Ceres


Jonathan Chone: 02/02/2016 06:19 CST

Hi Dr Marc, thanks for all your information and sorry for out of topic this time. Can I know did Ceres has an atmosphere? If yes, can I have some information about its composition and formation?

pietro: 02/03/2016 03:19 CST

Hi Dr. Rayman, thanks for this exciting update. How much of Ceres' surface has Dawn imaged since it reached LAMO, and how much do you plan to cover before the end of the mission? If you could travel all the way to Ceres and explore only one of its geological features, which one would it be? Thank you!

Marc Rayman: 02/03/2016 01:01 CST

Jonathan: Dawn is not designed to search for or study atmospheres but rather to explore airless worlds. In Dec 2014 (search for "Pardawn"), I described findings from the Herschel Space Observatory that showed an extremely thin veil of water vapor. In my second Dawn Journal of March 2015 (search for "Dawnticipating"), I explained our plans to conduct bonus measurements of it. No evidence of an atmosphere was found (which is not a surprise). Some members of the team report observing haze over Occator crater. I reserve judgment on that.

Marc Rayman: 02/03/2016 01:04 CST

Pietro: Dawn has imaged about 90% with its camera in this low orbit (and, as I explained above, far less with the infrared mapping spectrometer). We should have photographed essentially 100% well before the end of the primary mission, although it requires patience for the orbit to take Dawn over the small areas we have yet seen. (That's OK. I'm patient.) If I could explore only one geological feature, I'd chose the one that thoughtful assessment by the scientific community concluded would be the most productive for understanding this fascinating alien world based on all the data from Dawn (some of which has not yet been collected). Yes, I know that's not a satisfying answer, but I'm very committed to the power of science. For pure coolness, however, I think Occator crater would be the first choice. As an avid hiker, a low gravity expedition to explore Ahuna Mons would be pretty rewarding too. Both of those areas surely would also yield important geological insights. Where would you go?

seyss: 02/03/2016 06:00 CST

Hey Dr Rayman, when can we expect to see pics and other science data of the Occator Crater from LAMO? Thanks.

Marc Rayman: 02/03/2016 08:48 CST

Seyss: As I wrote above, we have photographed about 90% of Ceres from this low altitude orbit, and yet we have neither photographed the central region of Occator yet nor seen it with the infrared spectrometer. That is just the way Dawn's orbit has worked out. We did not design the orbit or the schedule of observations and telecommunications to view any specific targets. Rather, we designed them so that in six weeks, we would see most of the surface. For reasons I will explain in my next Dawn Journal, the first few weeks of LAMO don't contribute to this pattern, so the beginning of the six week period was January 10. By simple coincidence, Dawn will not have the opportunity to see Occator until the very end of that period! Therefore, we all will wait patiently. After Occator is photographed, we will follow our usual process for releasing the images that I've mentioned in previous blog comments. The need for accuracy and scientific review of the data sometimes slows the release of some products, but all of the data are released to the public after the science team has performed the necessary analysis and interpretation for scientific publication. I am confident it will be worth the wait!

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