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Marc Rayman

Dawn Journal: Getting Down to Science at Ceres

Posted by Marc Rayman

01-05-2015 15:02 CDT

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

Let's get Dawn to business, Dear Readers,

Dawn's assignment when it embarked on its extraordinary extraterrestrial expedition in 2007 can be described quite simply: explore the two most massive uncharted worlds in the inner solar system. It conducted a spectacular mission at Vesta, orbiting the giant protoplanet for 14 months in 2011-2012, providing a wonderfully rich and detailed view. Now the sophisticated probe is performing its first intensive investigation of dwarf planet Ceres. Dawn is slowly circling the alien world of rock and ice, far from Earth and far from the sun, executing its complex operations with the prowess it has demonstrated throughout its ambitious journey.

Ceres anaglyph from Dawn RC3 orbit

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

Ceres anaglyph from Dawn RC3 orbit
This anaglyph of Ceres is part of a sequence of images taken by NASA's Dawn spacecraft April 24 to 26, 2015, from a distance of 8,500 miles (13,500 kilometers). A non-anaglyph version can be viewed here.

Following an interplanetary trek of 7.5 years and 3.1 billion miles (4.9 billion kilometers), Earth's ambassador arrived in orbit on March 6, answering Ceres' two-century-old celestial invitation. With its advanced ion propulsion system and ace piloting skills, it has maneuvered extensively in orbit. Traveling mostly high over the night side of Ceres, arcing and banking, thrusting and coasting, accelerating and decelerating, climbing and diving, the spaceship flew to its first targeted orbital altitude, which it reached on April 23.

Dawn is at an altitude of about 8,400 miles (13,600 kilometers) above the mysterious terrain. This first mapping orbit is designated RC3 by the Dawn team and is a finalist in the stiff competition for the coveted title of Most Confusing Name for a Ceres Mapping Orbit. (See this table for the other contestants.) Last month we described some of the many observations Dawn will perform here, including comprehensive photography of the alien landscapes, spectra in infrared and visible wavelengths, a search for an extremely tenuous veil of water vapor and precise tracking of the orbit to measure Ceres' mass.

Dawn’s mapping orbits

NASA / JPL-Caltech

Dawn’s mapping orbits
Dawn’s four mapping orbits, shown to scale in altitude with the size of Ceres, which is about 590 miles (950 kilometers) in diameter. See this table for descriptions of the orbits and links to the activities scheduled for each.

On the way down to this orbit, the spacecraft paused ion thrusting twice earlier this month to take pictures of Ceres, as it had seven times before in the preceding three months. (We presented and explained the schedule for photography during the three months leading up to RC3 here.) Navigators used the pictures to measure the position of Ceres against the background of stars, providing crucial data to guide the ship to its intended orbit. The Dawn team also used the pictures to learn about Ceres to aid in preparing for the more detailed observations.

We described last month, for example, adjusting the camera settings for upcoming pictures to ensure good exposures for the captivating bright spots, places that reflect significantly more sunlight than most of the dark ground. Scientists have also examined all the pictures for moons of Ceres (and many extra pictures were taken specifically for that purpose). And thanks to Dawn's pictures, everyone who longs for a perspective on the universe unavailable from our terrestrial home has been transported to a world one million times farther away than the International Space Station.

The final pictures before reaching RC3 certainly provide a unique perspective. (You can see Dawn's pictures of Ceres here.) On April 10 and April 14-15, Dawn peered down over the northern hemisphere and watched for two hours each time as Ceres turned on its axis, part of the unfamiliar cratered terrain bathed in sunlight, part in the deep dark of night. This afforded a very different view from what we are accustomed to in looking at other planets, as most depictions of planetary rotations are from nearer the equator to show more of the surface. (Indeed, Dawn acquired views like that in its February "rotation characterizations.") The latest animations of Ceres rotating beneath Dawn are powerful visual reminders that this capable interplanetary explorer really is soaring around in orbit about a distant, alien world. Following the complex flight high above the dark hemisphere, where there was nothing to see, the pictures also show us that the long night's journey into day has ended.

Ceres' northern hemisphere

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

Ceres' northern hemisphere
This animated sequence of images from the Dawn spacecraft shows northern terrain on the sunlit side of dwarf planet Ceres. Dawn took these images on April 14–15, 2015 from a vantage point 14,000 miles (22,000 kilometers) above Ceres' northern hemisphere as the spacecraft was settling into its first circular orbit, called RC3 (for "rotation characterization 3"). Image scale on Ceres is about 1.3 miles (2.1 kilometers) per pixel.

Gradually descending atop its blue-green beam of high velocity xenon ions, Dawn crossed over the terminator -- the boundary between the dark side and the lit side -- on April 15 almost directly over the north pole. On April 20, on final approach to RC3, it flew over the equator at an altitude of about 8,800 miles (14,000 kilometers).

The spacecraft completed its ion thrusting shortly after 1:00 a.m. PDT on April 23. What an accomplishment this was! From the time Dawn left its final mapping orbit at Vesta in July 2012this is where it has been headed. The escape from Vesta's gravitational clutches in September 2012, the subsequent two and a half years of interplanetary travel and entering into orbit around Ceres on March 6, as genuinely exciting and important as it was, all really occurred as consequences of targeting this particular orbit.

In September 2014, the aftereffects of being struck by cosmic radiation compelled the operations team to rapidly develop a complex new approach trajectory because they still wanted to achieve this very orbit, where Dawn is now. And the eidetic reader will note that even when the innovative flight profile was presented five months ago (with many further details in subsequent months), we explained that it would conclude on April 23. And it did! Here we are! All the descriptions and figures plus a cool video elucidated a pretty neat idea, but it's also much more than an idea: it's real!! A probe from Earth is in a mapping orbit around a faraway dwarf planet.

When it had accomplished the needed ion thrusting, the veteran space traveler turned to point its main antenna to Earth so mission controllers could prepare it for the intensive mapping observations. The first task was to measure the orbital parameters so they could be transmitted to the spacecraft.

A few readers (you and I both know who you are) may have noted that in Dawn Journals during the last year, we have described the altitude of RC3 as 8,400 miles and 13,500 kilometers. Above, however, it is 13,600 kilometers. This is not a mistake. (It would be a mistake if the previous sentence were written, "Above, howevr, it is 13,600 kilometers.") This subtle difference belies several important issues about the orbits at Ceres. Let's take a further look.

As we explained when Dawn resided at Vesta, the orbital altitude we present is always an average (and rounded off, to avoid burdening readers with too many unhelpful digits). Vesta, Ceres, Earth and other planetary bodies are not perfect spheres, so even if the spacecraft traveled in a perfect circle, its altitude would change. They all are somewhat oblate, being wider across the equator than from pole to pole. In addition, they have more localized topography. Think of flying in a plane over your planet. If the pilot maintains a constant altitude above sea level, the distance above the ground changes because the elevation of the ground itself varies, coming closer to the aircraft on mountains and farther in valleys. In addition, as it turns out, orbits are not perfect circles but tend to be slightly elliptical, as if the plane flies slightly up and down occasionally, so the altitude changes even more.

In their exquisitely detailed planning, the Dawn team has had to account for the unknown nature of Ceres itself, including its mass and hence the strength of its gravitational pull. Dawn is the only spacecraft to orbit large, massive planetary bodies that were not previously visited by flyby spacecraft. Mercury, Venus, the moon, Mars, Jupiter and Saturn all were studied by spacecraft that flew past them before subsequent missions were sent to orbit them. The first probes to each provided an initial measurement of the mass and other properties that were helpful for the arrival of the first orbiters. At Vesta and Ceres, Dawn has had to discover the essential characteristics as it spirals in closer and closer. For each phase, engineers make the best measurements they can and then use them to update the plans for the subsequent phases. As a result, however, plans are based on impressive but nevertheless imperfect knowledge of what will be encountered at lower altitudes. So even if the spacecraft executes an ion thrust flight profile perfectly, it might not wind up exactly where the plan had specified.

There are other reasons as well for small differences between the predicted and the actual orbit. One is minor variations in the thrust of the ion propulsion system, as we discussed here. Another is that every time the spacecraft fires one of its small rocket thrusters to rotate or to stabilize its orientation in the zero-gravity conditions of spaceflight, that also nudges the spacecraft, changing its orbit a little. (See here for a related example of the effect of the thrusters on the trajectory.)

The Dawn flight team has a deep understanding of all the sources of orbit discrepancies, and they always ensure that their intricate plans account for them. Even if the RC3 altitude ended up more than 300 miles (500 kilometers) higher or lower than the specified value, everything would still work just fine to yield the desired pictures and other data. In fact, the actual RC3 orbit is within 25 miles (40 kilometers), or less than one tenth what the plan was designed to accommodate, so the spacecraft achieved a virtual cosmic bullseye!

In the complex preparations on April 23, one file was not radioed to Dawn on time, so late that afternoon when the robot tried to use this file, it could not find it. It responded appropriately by running protective software, stopping its activities, entering "safe mode" and beaming a signal back to distant Earth to indicate it needed further instructions. After the request arrived in mission control at JPL, engineers quickly recognized what had occurred. That night they reconfigured the spacecraft out of safe mode and back to its normal operational configuration, and they finished off the supply of ice cream in the freezer just outside the mission control room. Although Dawn was not ready to begin its intensive observation campaign in the morning of April 24, it started later that same day and has continued to be very productive.

Dawn is a mission of exploration. And rather than be constrained by a fast flight by a target for a brief glimpse, Dawn has the capability to linger in orbit for a very long time at close range. The probe will spend more than a year conducting detailed investigations to reveal as much as possible about the nature of the first dwarf planet discovered, which we had seen only with telescopes since it was first glimpsed in 1801. The pictures Dawn has sent us so far are intriguing and entrancing, but they are only the introduction to this exotic world. They started transforming it from a smudge of light into a real, physical place and one that a sophisticated, intrepid spacecraft can even reach. Being in the first mapping orbit represents the opportunity now to begin developing a richly detailed, intimate portrait of a world most people never even knew existed. Now, finally, we are ready to start uncovering the secrets Ceres has held since the dawn of the solar system.

Dawn is 8,400 miles (13,600 kilometers) from Ceres. It is also 2.66 AU (247 million miles, or 398 million kilometers) from Earth, or 985 times as far as the moon and 2.64 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 44 minutes to make the round trip.

Dr. Marc D. Rayman
10:30 p.m. PDT April 29, 2015

 
See other posts from May 2015

 

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

Comments:

igibud: 05/01/2015 08:22 CDT

This website, and Marc Rayman's articles, are a wonderful indispensible resource. Again, Rayman's coverage of the Dawn mission is informed and fascinating. I have to nit-pick that it made no mention of the mysterious lights on Ceres. Planetary.org fills a vacuum of English language poplular science journalism. If anybody knows where else I can find this info written for the layman, please let me know. JPL published a fascinating page online a few days ago, inviting users to take a poll about the mysterious lights on Ceres. That was newsworthy in itself. I thought the poll was fun, amazing and not the least bit sensational. I can't say as much about the news headlines I saw about NASA's poll. Why does this article make no mention of the newsworthy poll, or the newsworthy mysterious lights? There are plenty of anti-intellectual minded folks who have already decided NASA is keeping everything a secret. It seems to me that this omission by planetary.org gives more fuel to those anti-intellectuals.

Marc Rayman: 05/01/2015 08:31 CDT

Thank you for your comment, igibud. I made no mention of the bright spots this time, because I wrote about them in my three previous Dawn Journals (posted here on Feb. 25, March 6, and April 1), and we have no new information. I'm glad you enjoyed the poll. I don't mention every outreach activity in my Dawn Journals.

Dawn Ceres: 05/02/2015 01:21 CDT

It’s just dawned on me why I find the beginning of your posts so irritating. Please stop being so dawnting. God-dawn it!

StephenB: 05/02/2015 05:26 CDT

This is the best blog on this topic. The enthusiasm and wonder at having a spacecraft orbiting this distant object is so appropriate yet missing from most articles. Thank you for these posts. You did fail to close one loop, or perhaps it was your intent to keep your readers hanging until the next post. You made no mention of replenishment plans for the ice cream supply that was recently consumed.

DaGuz: 05/02/2015 06:07 CDT

Why no color pictures? The picture taken by Hubble showed the terrain was brown. The pictures released so far make Ceres look like our moon.

Solon: 05/03/2015 01:31 CDT

"cratered terrain bathed in sunlight" Bathed in sunlight? What kind of reading would my exposure meter get if it was looking at Ceres? Anything at all?

Gareth: 05/03/2015 03:57 CDT

Another great informative blog. You are certainly keeping your growing fan base across of everything that is happening and Spock would be proud of you........fascinating. I have also the enjoyed the fact that NASA has asked the public to vote on what the spots are. You know my crazy left field answer on that one but it is wonderful that we can all be so involved in this mission and getting direct feedback as well. Of course the key question is what date do you take your next photos of the bright spots at rc3 and what date they get released to the Public?

Gareth: 05/03/2015 04:12 CDT

Sorry marc having reread your 31st blog I see that the 3rd and 4th and 6th and 7th of this coming week are when you photograph the spots again. Would they get released on the 10th?

Ellis: 05/03/2015 03:07 CDT

Marc, it's a priviledge to have a daily follow up of this Dawn great adventure-thank you-following from The Dominican Republic.

Rick: 05/03/2015 05:07 CDT

OK, Heads up! Those "two" bright spots, alas, are going to resolve into many, MANY such spots; but NONE of them are going to span any more than about five pixels across, thereby perplexing the scientists still further and further!

Marc Rayman: 05/04/2015 11:49 CDT

I appreciate everyone's interest and kind comments. StephenB: Already replenished, in plenty of time for celebrating the receipt of cool Ceres data! DaGuz: Most of our releases so far are only grayscale, but we have released maps based on our color data. Like the Hubble views, they are false color. We will release true color images as well. Ceres would look mostly gray (although darker than the moon). Solon: The intensity of the incident sunlight is 12% of what it is on Earth. As I explained on March 31 (posted here on April 1), Ceres reflects about 9% of the light. Cameras typically assume a reflection of twice that, so your camera would read about 6% of what you might have expected on Earth.

Robert Clark: 05/20/2015 07:58 CDT

With all the interest in the Ceres "bright spots", how about a countdown to when DAWN reaches its closest orbit phases? Bob Clark

Marc Rayman: 05/21/2015 12:02 CDT

Bob, I included a table with the timeline for Dawn’s orbit phases in a previous Dawn Journal. You can jump to it from the link in the caption for the figure above that shows the orbits.

Jonathan Lovejoy: 05/25/2015 08:39 CDT

Will the "Survey orbit of Dawn start from June 6 close enough to take true color photo of Ceres? By the way did Ceres has a magnetic field?

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