NASA’s Dawn Fills Out its Ceres Dance Card

November 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
This graphic shows the planned trek of NASA’s Dawn spacecraft from its launch in 2007 through its arrival at the dwarf planet Ceres in early 2015. Image credit: NASA/JPL-Caltech
› Full image and caption

Dear Hand-Me-Dawns,

Gliding smoothly through the main asteroid belt between Mars and Jupiter, Dawn continues to make good progress on its ambitious mission of exploration. It is patiently but persistently pursuing Ceres, the second destination on its interplanetary itinerary.

Protoplanets Ceres and Vesta, the two most massive residents of the asteroid belt, were discovered at the beginning of the 19th century, and they have tantalized astronomers and others curious about the nature of the universe ever since. (Indeed, Ceres was the first dwarf planet discovered, having been found 129 years before Pluto.) They have waited patiently for a visitor from Earth since the dawn of the solar system. Dawn’s objective is to turn these uncharted orbs from tiny smudges of light amidst the stars into richly detailed places. It succeeded spectacularly at Vesta in 2011 - 2012, and it remains on course and on schedule for doing so at Ceres in 2015.

Next month, the adventurer will pass an invisible milestone on its celestial journey. On Dec. 27, it will be equidistant from these behemoths of the asteroid belt as all three follow their own independent heliocentric paths. The spacecraft will be 0.21 AU (19.4 million miles, or 31.3 million kilometers) away from each world, the one already visited and the one yet to be reached. And as the indefatigable ship sails on the cosmic seas with its sights set on Ceres, our anticipation for glimpsing the alien landscape ahead grows and grows, while the now-familiar scenery of Vesta shrinks into the distance, fading over the horizon.

The next day, Dawn will be equidistant from two other solar system bodies, both of which have been known (to our human readers, at least) for somewhat longer than Ceres and Vesta have. On Dec. 28, our celestial ambassador will be 2.46 AU (229 million miles, or 368 million kilometers) from Earth and the sun. (We cannot specify in which century either of them was discovered.)

Its complex route through the solar system has already taken the spacecraft farther from each of these bodies before. In the current phase of the mission, it is receding from the sun again, climbing the solar system hill from Vesta to Ceres. (It approached the sun in late 2012 and 2013 as part of the strategy for arriving at Ceres’s orbit when Ceres itself was there.) Having attained its greatest distance from Earth for the year in August, the spacecraft is temporarily getting closer to its planet of origin. (More precisely, as we discussed then, Earth is currently moving toward Dawn, because Earth travels faster in its solar orbit than Dawn does in its much more remote orbit.)

Dawn will reach two more impressive milestones in December, although neither pertains to its location. Soon the craft will surpass four years of ion thrust. While most spacecraft rely on conventional propulsion and hence coast most of the time (just as planets, moons, and asteroids do), Dawn’s mission would be impossible if it did that. In order to orbit and explore two distant destinations, the only terrestrial probe ever to attempt such a feat, it must accomplish a great deal of maneuvering. It spends the majority of its time using its uniquely efficient and capable ion propulsion system, constantly putting a gentle pressure on its trajectory to gradually reshape it. Although the spacecraft has already accumulated far more time in powered flight than any other mission, it still has a great deal more ahead.

And in December, that thrusting will push the craft’s speedometer past an extraordinary 20,000 mph (8.94 kilometers per second). (As we have seen in many previous logs, such as this one, this measure of the speed does not represent the actual spacecraft velocity. Nevertheless, it is a useful metric that avoids the complicating effects of orbital mechanics.) That is more than twice the previous record for propulsive velocity change set by Deep Space 1, the first interplanetary mission to use ion propulsion.

Dawn spends most of its time emitting a lovely blue-green beam of high-velocity xenon ions to propel itself. As foretold in the prophecy commonly known as the October log, however, we are now in one of just two periods of the long mission in which coasting is better for the trajectory than thrusting. Mission controllers took advantage of this time to instruct the robot to perform some special activities that would have been less convenient during routine ion thrusting. The reliable ship completed all of them flawlessly.

For more than 27 hours on Nov. 12 and 13, Dawn operated in a mode that had not even been conceived of when it was designed and built. It controlled its orientation in the frictionless, zero-gravity conditions of spaceflight using a scheme that was developed long after it left Earth. This “hybrid” control method operated perfectly, validating the extensive work engineers have invested in it and verifying its readiness for use at Ceres.

When it embarked on its bold journey more than six years ago, the ship was outfitted with four reaction wheels. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can turn or stabilize itself. It generally used three at a time, with a fourth kept in reserve. For such a long and complex expedition, extending to well over one million times farther from Earth than the International Space Station, backup systems are essential.

One of the wheels experienced increased friction in June 2010, but the mission continued with the other three. A second met the same fate in August 2012, as Dawn was climbing away from Vesta. Other spacecraft have encountered similar issues with their reaction wheels as well, and the consequences can be dire.

We have described the operations team’s swift and productive responses to the regrettable behavior of the reaction wheels in a number of logs (see, as one example, here). As soon as the first wheel faltered, JPL and Orbital Sciences Corporation began working on a method to operate with fewer than three in case another one had difficulty. They developed software to operate in a hybrid mode of two wheels plus the small hydrazine-powered jets of the reaction control system and installed it in the craft’s main computer in April 2011 so it would be available at Vesta if needed.

Given the problems with reaction wheels on Dawn and other spacecraft, engineers do not have high confidence that the two remaining units will operate for long (although it certainly is possible they will). Thanks to their remarkable ingenuity and resourcefulness, the team has devised a detailed plan that should allow Dawn to complete its extraordinary mission using only the hydrazine thrusters, achieving all of its objectives in exploring Ceres regardless of the condition of its wheels. (Note that it is not even obvious that doing so is possible, but then again, it isn’t obvious that sending probes so far from our home planet is possible either. Part of the thrill of a solar system adventure is overcoming the extremely daunting challenges.) So now, hybrid control would provide an enhancement, extending the supply of precious hydrazine propellant and giving the spacecraft the opportunity to operate even longer at Ceres than it would without the two functioning wheels. When the hydrazine is exhausted, the mission will conclude.

Dawn will use hybrid control only in its lowest altitude orbits at Ceres, the final phase of the mission. (Beginning in December and continuing in 2014, we will describe all phases of the Ceres plan in detail.) Hybrid control will be called upon to perform three kinds of tasks for the spacecraft: train the suite of sophisticated sensors at the mysterious world beneath it, point the main antenna to distant Earth to transmit its findings and receive updated instructions, and rotate from one orientation to another. The innovative system has now unerringly demonstrated its capability to accomplish all three by executing exactly those functions earlier this month.

The confirmation that hybrid control works as intended is not the only task Dawn is carrying out during this coasting period. All of its scientific instruments (including even the backup camera) are being powered on and given thorough health checks, verifying that they remain fully functional and ready to reveal Ceres’s secrets. Engineers also conducted some tests with the ion engine that has operated the longest of the three to confirm expectations of how it will perform at Ceres.

On Dec. 9, Dawn’s four-week coast period will end. Once again it will turn to point an ion engine in the direction needed to push forward to its rendezvous with the distant and exotic world ahead. As the probe nears and then passes the halfway point on its remarkable journey from Vesta to Ceres, it is pulled by forces even more powerful than ion propulsion: the attraction of discovery, the lure of the unknown, and the draw of tremendous new insights and profound new understandings to be gained in a daring adventure far, far from home.

Dawn is 18 million miles (29 million kilometers) from Vesta and 22 million miles (35 million kilometers) from Ceres. It is also 2.78 AU (258 million miles, or 415 million kilometers) from Earth, or 1,125 times as far as the moon and 2.82 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.

– Dr. Marc D. Rayman

› Read more entries from Marc Rayman’s Dawn Journal


Inside the United Nations Climate Change Conference

November 20th, 2013
NASA-Generated Damage Map To Assist With Typhoon Haiyan Disaster ResponseWhen Super Typhoon Haiyan, one of the most powerful storms ever recorded on Earth, struck the Philippines Nov. 8, 2013, it tore a wide swath of destruction across large parts of the island nation. Image Credit: ASI/NASA/JPL-Caltech

Over on My Big Fat Planet, Carmen Boening, a scientist in the Climate Physics Group at NASA’s Jet Propulsion Laboratory, is sharing news from the United Nations Climate Change Conference in Poland. Read her reports on the discussions shaping climate change policy and the emotional speech delivered in the wake of Typhoon Haiyan.


Slice of History: Ranger Impact Limiter

November 4th, 2013

By Julie Cooper

Each month in “Slice of History” we feature a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at https://beacon.jpl.nasa.gov/.

Ranger Impact Limiter
Ranger Impact Limiter — Photograph number 292-41A

This photo was taken in November 1960 to show the lightweight balsa wood impact limiter that was to be used in the NASA Jet Propulsion Laboratory’s Ranger Block II spacecraft design (Rangers 3, 4, and 5). The woman holding the sphere is Systems Design secretary Pat McKibben. The sphere was 65 cm in diameter, and it surrounded a transmitter and a seismometer instrument that was designed by the Caltech Seismological Laboratory. The sphere would separate from the spacecraft shortly before impact and survive the rough landing on the moon. The capsule was also vacuum-filled with a protective fluid to reduce movement during impact. After landing, the instrument was to float to an upright position, then the fluid would be drained out so it could settle and switch on.

Due to a series of malfunctions in 1962, these three Ranger spacecraft either crashed without returning data or missed the moon. In July 1964, the first successful Ranger spacecraft, Ranger 7, reached the moon and transmitted more than 4,000 images to Earth.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.


For Dawn, a Time to Thrust and and a Time to Coast

October 31st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear All Hallows’ Dawns,

Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.

The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn’s heliocentric orbit was the same as the rocky behemoth’s. Now they are very far apart, and by early 2015, the robotic explorer’s path will be close enough to Ceres’s that they will become locked in a gravitational embrace.

Without ion propulsion, Dawn’s unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA’s Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.

During its years of interplanetary travel, Dawn has reliably followed a carefully formulated flight plan from Earth past Mars to Vesta and now from Vesta to Ceres. We discussed some of the principles underlying the development of the complex itinerary in a log written when Dawn was still gravitationally anchored to Earth. To carry out its ambitious adventure, Dawn should thrust most of the time, but not all of the time. Indeed, at some times, thrusting would be unproductive.

We will not delve into the details here, but remember that Dawn is doing more than ascending the solar system hill, climbing away from the sun. More challenging than that is making its orbit match the orbit of its targets so that it does not fly past them for a brief encounter as some other missions do. Performing its intricate interplanetary choreography requires exquisite timing with the grace and delicacy of the subtly powerful ion propulsion.

Of course Dawn does not thrust much of the time it is in orbit at Vesta and Ceres; rather, its focus there is on acquiring the precious pictures and other measurements that reveal the detailed nature of these mysterious protoplanets. But even during the interplanetary flight, there are two periods in the mission in which it is preferable to coast. Sophisticated analysis is required to compute the thrusting direction and schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the mass of the sun and the masses and orbits of Earth, Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when mission planners will not allow thrusting (e.g., during spacecraft maintenance periods).

The first interval that interplanetary trajectory designers designated as “optimal coast” was well over four years and 1.8 billion miles (2.8 billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8, 2009. During that time, the ship took some of Mars’s orbital energy to help propel itself toward Vesta. (In exchange for boosting Dawn, Mars slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters, in 180 million years.)

The second and final interval when coasting is better than thrusting begins next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit around the sun without modifying it. The timing of this coast period is nearly as important to keeping the appointment with Ceres as is the timing of the thrusting. In next month’s log, we will describe some of the special assignments the sophisticated robot will perform instead of its usual quiet cruise routine of accelerating and emitting xenon ions. We also will look ahead to some interesting celestial milestones and alignments in December.

While the spacecraft courses through the asteroid belt, the flight team continues refining the plans for Ceres. In logs in December and several months in 2014, we will present extensive details of those plans so that by the time Dawn begins its mission there, you will be ready to ride along and share in the experience.

In the meantime, as the stalwart ship sails on, it is propelled not only by ions but also by the promise of exciting new knowledge and the prospects of a thrilling new adventure in exploring an uncharted alien world.

Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million miles, or 460 million kilometers) from Earth, or 1,200 times as far as the moon and 3.10 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

– Dr. Marc D. Rayman

P.S. This log is posted early enough to allow time for your correspondent to don his Halloween costume. In contrast to last year’s simple (yet outlandish) costume, this year’s will be more complex. He is going in double costume, disguised as someone who is only pretending to be passionate about the exploration of the cosmos and the rewards of scientific insight.

› Read more entries from Marc Rayman’s Dawn Journal


NASA’s Dawn Spacecraft Celebrates Six Years in Space

September 27th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft
Artist’s concept of NASA’s Dawn spacecraft between the giant asteroid Vesta and the dwarf planet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawnniversaries,

On the sixth anniversary of leaving Earth to embark on a daring deep-space expedition, Dawn is very, very far from its erstwhile planetary residence. Now humankind’s only permanent resident of the main asteroid belt between Mars and Jupiter, the seasoned explorer is making good progress toward the largest object in that part of the solar system, the mysterious dwarf planet Ceres. The voyage is long, and the intrepid but patient traveler will not reach its next destination until half a year after its seventh anniversary of departing Earth.

On its fifth anniversary, Dawn was still relatively close to Vesta, the giant protoplanet that had so recently held the craft in its gravitational grip. The only probe ever to orbit a main belt asteroid, Dawn spent 14 months (including its fourth anniversary) accompanying Vesta on its way around the sun. After more than two centuries of appearing to astronomers as little more than a fuzzy blob of light among the stars, the second most massive body in the asteroid belt has been revealed as a fascinating, complex, alien world more closely related to terrestrial planets (including Earth) than to typical asteroids.

Most of the ship’s first four years of spaceflight were devoted to using its ion propulsion system to spiral away from the sun, ascending the solar system hill from Earth to Vesta. Now it is working to climb still higher up that hill to Ceres.

For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the sixth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the logs from its first, second, third, fourth, and fifth anniversaries.

In its six years of interplanetary travels, the spacecraft has thrust for a total of 1,410 days, or 64 percent of the time (and about 0.000000028 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 318 kilograms (701 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 8.7 kilometers per second (19,500 mph). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished about three-quarters of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have completed six revolutions around the sun, covering about 37.7 AU (5.6 billion kilometers or 3.5 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 27.4 AU (4.1 billion kilometers or 2.5 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It will have to slow down still more to rendezvous with Ceres. Since Dawn’s launch, Vesta has traveled only 24.2 AU (3.6 billion kilometers or 2.2 billion miles), and the even more sedate Ceres has gone 22.8 AU (3.4 billion kilometers or 2.1 billion miles).

Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.

› Continue reading Marc Rayman’s Dawn Journal


Slice of History: Mariner 4 Television Experiment Team

September 3rd, 2013

By Julie Cooper

Each month in “Slice of History” we feature a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at https://beacon.jpl.nasa.gov/.

Mariner 4 Television Experiment Team
Mariner 4 Television Experiment Team — Photograph number P-5005B

Because the data return rate from Mariner 4 was very low, the Mariner 4 Television Experiment Team spent hours waiting for each new image to appear. In this photo they are waiting for the first picture from Mars. Mariner eventually returned 22 images. From left to right: Robert Nathan (NASA’s Jet Propulsion Laboratory), Bruce Murray (associate professor of planetary science), Robert Sharp (Caltech), Robert Leighton (principal investigator), and Clayton La Baw (JPL).

Murray had been a member of the Caltech faculty for about five years when this photo was taken in July 1965. He went on to replace William Pickering as Director of JPL in 1976, retired from that position in 1982, and returned to Caltech.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.


Mariner 4 Taught Us to See

August 30th, 2013
The first 'image' of Mars from NASA's Mariner 4
Mission team members for NASA’s Mariner 4 spacecraft, incredibly anxious to see the first up-close photograph of Mars, devised a way to see the image before it made its way to Earth by color-coding binary code on strips of ticker tape. The resulting collage became known as “the first image of Mars.” Image credit: NASA/JPL-Caltech

In today’s universe, it seems unimaginable that a planetary spacecraft would leave the comfort of its terrestrial perch without some kind of imaging system on board. But in the early 1960s, as NASA’s Jet Propulsion Laboratory was reveling in the success of its first planetary mission to Venus and setting its sights on Mars — a destination whose challenges would unfurl themselves much more readily than they had with Venus — for some scientists, the question of camera or none was still just that, a question.

Bud Schurmeier, project manager for NASA’s Ranger missions, a few years ago recalled, “There were a lot of scientists who said, ‘Pictures, that’s not science. That’s just public information.’ Over the years, that attitude has changed so markedly, and so much information has been obtained just from the photographs.”

The recent passing of former JPL Director and career-long planetary imaging advocate Bruce C. Murray, 81, is a reminder of how different our understanding of the planets — and our appreciation of them — would be without space-based cameras.

This truth was evident as early as 1965, when NASA’s Mariner 4, carrying an imaging system designed by a young Murray and his colleagues, arrived at Mars. It marked the world’s first encounter with the Red Planet, a remarkable achievement in itself. But for an anxious press, public and mission team, the Holy Grail lay in catching that first glimpse of Mars up-close.

It was a waiting game that was too much for some. For everyone, in fact:


This is a clip from the JPL-produced film The Changing Face of Mars about the laboratory’s early attempts to explore the Red Planet. Credit: NASA’s Jet Propulsion Laboratory

What resulted became known as “The first image of Mars.” And in many ways it symbolizes — more than any of the actual 22 photographs captured by Mariner 4 — how significant this opportunity to truly “see” Mars had been.

Now, nearly 50 years after Mariner 4’s arrival at Mars, imaging systems are an integral piece of our quest to understand the planets and the universe beyond, playing key roles in scientific investigations, spacecraft navigation and public support for missions. It’s because of that first image that we can now look at that red dot in the night sky and picture what has become our new reality of Mars:

Curiosity's first billion pixel panorama
This image is a portion of a billion-pixel panorama from NASA’s Mars rover Curiosity that combines 900 images taken by the rover from Oct. 5 through Nov. 16, 2012 from its “Rocknest” site on Mars. Image credit: NASA/JPL-Caltech
› Explore the full panorama

Earth and Dawn on Opposite Sides Now

August 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear Antecedawnts,

Traveling confidently and alone, Dawn continues to make its way through the silent depths of the main asteroid belt. The only spacecraft ever to have orbited a resident of the vast territory between Mars and Jupiter, Dawn conducted a spectacular exploration of gigantic Vesta, revealing a complex place that resembles the terrestrial planets more than typical asteroids. Now the interplanetary adventurer is on its long journey to the uncharted dwarf planet Ceres, by far the largest of all asteroids (975 kilometers, or more than 600 miles, in equatorial diameter). In 2015, the mysterious world of rock and ice will begin to give up its ancient secrets to the immigrant from distant Earth.

Earth, Vesta, Dawn, and Ceres are following their own separate paths around the sun. The spacecraft is patiently reshaping its orbit, using its uniquely efficient ion propulsion system to accomplish a deep-space expedition that would be impossible with conventional propulsion.

As we have seen in many previous logs (including, for example, here), the higher an object’s orbit, the slower it needs to move in order to balance the gravitational pull, which diminishes with distance. Blistering Mercury orbits the sun faster than Venus, Venus goes faster than Earth, Earth goes faster than Mars, and Mars goes faster than the residents of the asteroid belt and the cold planets of the outer solar system. In the same way, satellites that orbit close to Earth, including the International Space Station, move faster than those at greater altitudes, and the moon travels even more slowly in its very high orbit.

Dawn is now a permanent inhabitant of the main asteroid belt. Therefore, the massive sun, the gravitational master of the solar system, has a weaker grip on it than on Earth. So as Dawn maneuvers from Vesta to Ceres, Earth revolves more rapidly around the sun. This month, their independent motions have taken them to their greatest separation of the year, as they are on opposite sides of the sun. How truly remarkable that humankind can accomplish such a feat!

On August 4, the planet and its robotic ambassador to the cosmos were an extraordinary 3.47 AU (519 million kilometers, or 322 million miles) apart. (To recapture the feeling of your position in the universe then, it may be helpful to know that the maximum range was attained at 4:16 a.m. PDT.) From the perspective of terrestrial observers, had they possessed the superhuman (and even supertelescopic) vision needed to descry the tiny ship far beyond the blindingly bright star, Dawn would have appeared to be very close to the sun but not directly behind it. To rendezvous with Vesta and then with Ceres, the spacecraft has tilted the plane of its solar orbit. Some of the time it is north of Earth’s orbital plane, sometimes it is south. August 4 was during the northern segment, so Dawn would have been a little north of the sun.

It’s time to refer to one of those novel clocks available in the Dawn gift shop on your planet (although if you already have such a clock, it probably doesn’t tell you that it’s time — we stand by our policy of full refunds within 24 hours, as measured by our Dawn clocks). With the sun at the center of the clock, Earth’s motion would be like that of a short minute hand. Dawn, both farther from the sun and moving more slowly, would be following the path of a longer hour hand. If we ignore the effect of the ion thrust, which is constantly changing the orbit, and the slight misalignment of the hour hand representing Dawn’s being in a different plane, the conditions on August 4 were like those at 6:00.

As time progresses and Earth continues circling the sun, it will come closer to Dawn until April 2014 (like 12:00). Even then, however, they will be over 1.55 AU (232 million kilometers, or 144 million miles) apart, and they will never be that close again. The spacecraft will continue climbing higher and higher from the sun toward Ceres, so by the time Earth loops around once more, Dawn will be even farther from it. In the meantime, when next the arrangement is like 6:00, in December 2014, the separation will be more than 3.78 AU (565 million kilometers, or 351 million miles), even greater than the remarkable range a few weeks ago.

› Continue reading Marc Rayman’s Dawn Journal


Slice of History: Hadamard Matrix

August 1st, 2013

By Julie Cooper

Each month in “Slice of History” we feature a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at https://beacon.jpl.nasa.gov/.

Hadamard Matrix
Hadamard Matrix — Photograph Number 331-3717Ac

In 1961, mathematicians from NASA’s Jet Propulsion Laboratory and Caltech worked together to construct a Hadamard Matrix containing 92 rows and columns, with combinations of positive and negative signs. In a Hadamard Matrix, if you placed all the potential rows or columns next to each other, half of the adjacent cells would be the same sign, and half would be the opposite sign. This mathematical problem had been studied since about 1893, but the solution to the 92 by 92 matrix was unproven until 1961 because it required extensive computation.

From left to right, holding a framed representation of the matrix, are Solomon Golomb, assistant chief of the Communications Systems Research Section; Leonard Baumert, a postdoc student at Caltech; and Marshall Hall, Jr., a Caltech mathematics professor. In a JPL press release, Sol Golomb pointed out the possible significance of the discovery in creating codes for communicating with spacecraft.

The team used JPL’s IBM 7090 computer, programmed by Baumert, to perform the computations.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.


Dawn’s Journey: A Power Trip

July 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Dawn's solar arrays are folded to fit inside the nose cone in preparation for launch
The Dawn spacecraft’s solar array wings — pictured here in a folded position in preparation for launch — span 19.7 meters (nearly 65 feet) and are designed to keep the spacecraft powered even as it ventures further from the sun into the remote asteroid belt. Image credit: NASA/JPL-Caltech

Dear Megalodawniacs,

Powering its way through the main asteroid belt between Mars and Jupiter, Dawn continues on course and on schedule for its 2015 appointment with dwarf planet Ceres. After spending more than a year orbiting and scrutinizing Vesta, the second most massive object in the asteroid belt, the robotic explorer has its sights set on the largest object between the sun and Neptune that a spacecraft has not yet visited. This exotic expedition to unveil mysterious alien worlds would be impossible without the probe’s ion propulsion system.

Ion propulsion is not a source of power for this interplanetary spaceship. Rather, the craft needs a great deal of power to operate its ion propulsion system and all other systems. It needs so much that…

We crave power!!

The ion propulsion system is power-hungry. The process of ionizing xenon and then accelerating it to high velocity consumes a significant amount of electrical power, all of which is provided by the spacecraft’s huge solar arrays. With these two wings and its ion tail, Dawn resembles a celestial dragonfly. But this extraterrestrial odonate is a giant, with a wingspan of 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest tip-to-tip length of any probe NASA had ever dispatched on an interplanetary voyage. (Some such spacecraft have had flexible wire-like antennas that reach to greater lengths.) The large area of solar cells is needed to capture feeble sunlight in the remote asteroid belt to meet all of the electrical needs. Each solar array wing is the width of a singles tennis court, and the entire structure would extend from a pitcher’s mound to home plate on a professional baseball field, although Dawn is engaged in activities considerably more inspiring and rewarding than competitive sports.

To sail the ship to its intended destination, navigators plot a complex course on the solar system sea. The thrust delivered by the ion engine depends on the power level; higher power translates into higher (but still ever so gentle) thrust. The farther Dawn is from the luminous sun, the less power is available, so the thrust is lower. Therefore, to keep it on its itinerary, mission planners need to know the thrust at all times in the future. It would not be a recipe for success to propel the spacecraft to a position in space from which it could not achieve enough thrust to accomplish the rest of the carefully designed journey to Ceres.

To formulate the flight plan then requires knowing how much power will be available even as the probe ventures farther from the sun. Engineers make mathematical predictions of the power the solar arrays will generate, but these calculations are surprisingly difficult. Well, perhaps some readers would not be surprised, but it is more complicated than simply reducing the power in proportion to the intensity of the sunlight. As one example, at greater distances from the sun, the temperature of the arrays in the cold depths of space would be even lower, and the efficiency of the solar cells depends on their temperature. In 2008, the operations team devised and implemented a method to refine their estimates of the solar array performance, and that work enabled the deep-space traveler to arrive at Vesta earlier and depart later. Now they have developed a related but superior technique, which the faithful spacecraft executed flawlessly on June 24.

The only way to measure the power generation capability of the arrays is to draw power from them. With the ion thrust off, even with all other systems turned on, the spacecraft cannot consume as much power as the arrays can provide, so no meaningful measurement would be possible.

In typical operations, Dawn keeps its solar arrays pointed directly at the sun. For this special calibration, it rotated them so the incident sunlight came at a different angle. This reduced the total amount of light falling on the cells, effectively creating the conditions the spacecraft will experience when it has receded from the sun. As the angle increased, corresponding to greater distances from the brilliant star, the arrays produced less power, so the ion engine had to be throttled down. (The engines can be operated at 112 different throttle levels, each with a different input power and different thrust level.)

Engineers estimated what the maximum throttle level would be at each of the angles as well as the total power all other systems would consume during the test and then programmed it so the ion propulsion system would throttle down appropriately as the solar array angle increased. Of course, they could not know exactly what the highest throttle level at each angle would be; if they did, then they would already know the solar array characteristics well enough that the calibration would be unnecessary. Fortunately, however, they did not need to determine the perfect levels in advance. The sophisticated robot is smart enough to reduce by a few throttle levels if it detects that all systems combined are drawing more power than the solar arrays generate.

Under normal circumstances, the spacecraft doesn’t need to adjust the ion throttle level on its own. Engineers know the solar array performance well enough that they can predict the correct setting with high accuracy for a typical four-week sequence of commands stored onboard. It is only for the much greater distances from the sun in the years ahead that the uncertainty becomes important. In addition, during regular operations, if the spacecraft temporarily needs to use more heaters than usual (more than 140 heaters are distributed around the ship, each turning on and off as needed), thereby increasing the power demand, its battery can make up for the difference. That avoids unnecessary throttle changes.

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