Dawn Journal: Maneuvering for Science
Posted by Marc Rayman
29-11-2016 10:24 CST
Dear Decadawnt Readers,
Blue rope lights adorn Dawn mission control at JPL, but not because the flight team is in the holiday spirit (although they are in the holiday spirit). The felicitous display is more than decorative. The illumination indicates that the interplanetary spacecraft is thrusting with one of its ion engines, which emit a lovely, soft bluish glow in the forbidding depths of space. Dawn is completing another elegant spiral around dwarf planet Ceres, maneuvering to its sixth science orbit.
Dawn’s ion propulsion system has allowed the probe to accomplish a mission unlike any other, orbiting two distant extraterrestrial destinations. Even more than that, Dawn has taken advantage of the exceptional efficiency of its ion engines to fly to orbits at different altitudes and orientations while at Vesta and at Ceres, gaining the best perspectives for its photography and other scientific investigations.
Dawn has thrust for a total of 5.7 years during its deep-space adventure. All that powered flight has imparted a change in the ship’s velocity of 25,000 mph (40,000 kilometers per hour). As we have seen, this is not the spacecraft’s actual speed, but it is a convenient measure of the effect of its propulsive work. Reaching Earth orbit requires only about 17,000 mph (less than 28,000 kilometers per hour). In fact, Dawn’s gentle ion engines have delivered almost 98 percent of the change in speed that its powerful Delta 7925H-9.5 rocket provided. With nine external rocket engines and a core consisting of a first stage, a second stage and a third stage, the Delta boosted Dawn by 25,640 mph (41,260 kilometers per hour) from Cape Canaveral out of Earth orbit and onto its interplanetary trajectory, after which the remarkable ion engines took over. No other spacecraft has accomplished such a large velocity change under its own power. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)
Early this year, we were highly confident Dawn would conclude its operational lifetime in its fourth orbit at Ceres (and remain there long after). But unexpectedly healthy and with an extension from NASA, Dawn is continuing its ambitious mission. After completing all of its tasks in its fifth scientific phase at Ceres, Dawn is pursuing new objectives by flying to another orbit for still more discoveries. Although we never anticipated adding a row to the table of Dawn’s orbits, last presented in December 2015, we now have an updated version.
in ft (m)
distance of a soccer ball
|1||RC3||04.23.15 – 05.09.15||8,400
|3||HAMO||08.17.15 – 10.23.15||915
|12.16.15 – 09.02.16||240
|5||XMO2||10.16.16 – 11.04.16||920
As with the obscure Dawn code names for other orbits, this fifth orbit’s name requires some explanation. The extended mission is devoted to undertaking activities not envisioned in the prime mission. That began with two extra months in the fourth mapping orbit performing many new observations, but because it was then the extended mission, that orbit was designated extended mission orbit 1, or XMO1. (It should have been EMO1, of course, but the team’s spellchecker was offline on July 1, the day the extended mission started.) Therefore, the next orbit was XMO2. Dawn left XMO2 on Nov. 4, and we leave it to readers’ imaginations to devise a name for the orbit the spacecraft is now maneuvering to.
Surprisingly, Dawn is flying higher to enhance part of the scientific investigation that motivated going to the lowest orbit. We have explained before that Dawn’s objective in powering its way down to the fourth mapping orbit was to make the most accurate measurements possible of gravity and of nuclear radiation emitted by the dwarf planet.
For more than eight months, the explorer orbited closer to the alien world than the International Space Station is to Earth, and the gamma ray spectra and neutron spectra it acquired are outstanding, significantly exceeding all expectations. But ever-creative scientists have recognized that even with that tremendous wealth of data, Dawn can do still better. Let’s look at this more carefully and consider an example to resolve the paradox of how going higher can yield an improvement.
The gamma ray and neutron detector (GRaND) reveals some of Ceres’ atomic constituents down to about a yard (meter) underground. The principal limitation in analyzing these spectra is "noise." In fact, noise limits the achievable accuracy of many scientific measurements. It isn’t necessarily the kind of noise that you hear from loud machinery (nor from the mouth of your unhelpful parent, inattentive progeny or boring and verbose coworker), but all natural systems have something similar. Physical processes other than the ones of interest make unwanted contributions to the measurements. The part of a measurement scientists want is called the "signal." The part of a measurement scientists don’t want is called the "noise." The quality of a measurement may be characterized by comparing the strength of the signal to the strength of the noise. (This metric is called the "signal to noise ratio" by people who like to use jargon like "signal to noise ratio.")
We have discussed that cosmic rays, radiation that pervades space, strike atomic nuclei on Ceres, creating the signals that GRaND measures. Remaining at low altitude would have allowed Dawn to enhance its measurement of the Cerean nuclear signal. But scientists determined that an even better way to improve the spectra than to increase the signal is to decrease the noise. GRaND’s noise is a result of cosmic rays impinging directly on the instrument itself and on nearby parts of the spacecraft. With a more thorough measurement of the noise from cosmic rays, scientists will be able to mathematically remove that component of the low altitude measurements, leaving a clearer signal.
For an illustration of all this, suppose you want to hear the words of a song. The words are the signal and the instruments are the noise. (This is a scientific discussion, not a musical one.) It could be that the instruments are so loud and distracting that you can’t make the words out easily.
You might try turning up the volume, because that increases the signal, but it increases the noise as well. If the performance is live, you might even try to position yourself closer to the singer, perhaps making the signal stronger without increasing the noise too much. (Other alternatives are simply to Google the song or ask the singer for a copy of the lyrics, but those methods would ruin this example.)
If you’re doing this in the 21st century (or later), there’s another trick you can employ, taking advantage of computer processing. Suppose you had a recording of the singing with the instruments and then obtained separate recordings of the instruments. You could subtract the muscial sounds that constitute the noise, removing the contributions from both guitars, the drums, the harp, both ukuleles, the kazoo and all the theremins. And when you eliminate the noise of the instruments, what remains is the signal of the words, making them much more intelligible.
To obtain a better measure of the noise, Dawn needs to go to higher altitude, where GRaND will no longer detect Ceres. It will make detailed measurements of cosmic ray noise, which scientists then will subtract from their measurements at low altitude, where GRaND observed Ceres signal plus cosmic ray noise. The powerful capability to raise its orbit so much affords Dawn the valuable opportunity to gain greater insight into the atomic composition. Of course, it’s not quite that simple, but essentially this method will help Dawn hear Ceres’ nuclear song more clearly.
To travel from one orbit to another, the sophisticated explorer has followed complex spiral routes. We have discussed the nature of these trajectories quite a bit, including how the operations team designs and flies them. But now they are using a slightly different method.
Those of you at Ceres who monitor the ship’s progress probably wouldn’t notice a difference in the type of trajectory. And the rest of you on Earth and elsewhere who keep track through our mission status updates also would not detect anything unusual in the ascent profile (to the extent that a spacecraft using ion propulsion to spiral around a dwarf planet is usual). But celestial navigators are now enjoying their use of a method they whimsically call local maximal energy spiral feedback control.
The details of the new technique are not as important for our discussion here as one of the consequences: Dawn’s next orbit will not be nearly as circular as any of its other orbits at Ceres (or at Vesta). Following the conclusion of this spiral ascent on Dec. 5, navigators will refine their computations of the orbit, and we will describe the details near the end of the month. We will see that as the spacecraft follows its elliptical loops around Ceres, each taking about a week, the altitude will vary smoothly, dipping below 4,700 miles (7,600 kilometers) and going above 5,700 miles (9,200 kilometers). Such a profile meets the mission’s needs, because as long as the craft stays higher than about 4,500 miles (7,200 kilometers), it can make the planned recordings of the cacophonous cosmic rays. We will present other plans for this next phase of the mission as well, including photography, in an upcoming Dawn Journal.
As Dawn continues its work at Ceres, the dwarf planet continues its stately 4.6-year-long orbit around the sun, carrying Earth’s robotic ambassador with it. Ceres follows an elliptical path around the sun (see, for example, this discussion, including the table). In fact, all orbits, including Earth’s, are ellipses. Ceres’ orbit is more elliptical than Earth’s but not as much as some of the other planets. The shape of Ceres’ orbit is between that of Saturn (which is more circular) and Mars (which is more elliptical). (Of course, Ceres’ orbit is larger than Mars’ and smaller than Saturn’s, but here we are describing how much each orbit deviates from a perfect circle.)
When Ceres tenderly took Dawn into its gravitational embrace in March 2015, they were 2.87 AU (267 million miles, or 429 million kilometers) from the sun. In January 2016, we mentioned that Ceres had reached its aphelion, or greatest distance from the sun, at 2.98 AU (277 million miles, or 445 million kilometers). Today at 2.85 AU (265 million miles, or 427 million kilometers), Ceres is closer to the sun than at any time since Dawn arrived, and the heliocentric distance will gradually decrease further throughout the extended mission. (If the number of numbers is overwhelming here, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles or kilometers. Ignore the other two scales so you can focus on the relative distances.)
Another consequence of orbiting the sun is the progression of seasons. Right on schedule, as we boldly predicted in August 2015, Nov. 13 was the equinox on Ceres, marking the beginning of northern hemisphere autumn and southern hemisphere spring. Although it is celebrated on Ceres with less zeal than on Earth, it is fundamentally the same: the sun was directly over the equator that day, and now it is moving farther south. It takes Ceres so long to orbit the sun that this season will last until Dec. 22, 2017.
A celebration that might occur on Ceres (and which you, loyal Dawnophile, are welcome to attend) would honor Dawn itself. Although the spacecraft completed its ninth terrestrial year of spaceflight in September, on Dec. 12, it will have been two Cerean years since Dawn left Earth for its interplanetary journey. Be sure to attend in order to learn how a dawnniversary is commemorated in that part of the solar system.
Although a year on Ceres lasts much longer than on Earth, 2016 is an unusually long year on our home planet. Not only was a leap day included, but a leap second will be added at the very end of the year to keep celestial navigators’ clocks in sync with nature. (Clocks used for other purposes will not have the added second. The extra second applies in the specialized timekeeping system known as "ephemeris time.") The Dawn team already has accounted for the leap second in the intricate plans formulated for the spacecraft. And at that second, on Dec. 31 at 23:59:60, we will be able to look back on 366 days and one second, an especially full and gratifying year in this remarkable deep-space expedition. But we needn’t wait. Even now, as mission control is bathed in a lovely glow, the members of the team as well as space enthusiasts everywhere are aglow with the thrill of new knowledge, the excitement of a daring, noble adventure and the anticipation of more to come.
Dawn is 3,150 miles (5,070 kilometers) from Ceres. It is also 2.08 AU (194 million miles, or 312 million kilometers) from Earth, or 770 times as far as the moon and 2.11 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 35 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PST November 28, 2016