Posts Tagged ‘propulsion’

For Dawn, a Little Push Goes a Long Way

Monday, July 1st, 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.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Dawnamic Readers,

The indefatigable Dawn spacecraft is continuing its extraordinary interplanetary flight on behalf of inquisitive creatures on distant Earth. Progressing ever farther from Vesta, the rocky and rugged world it so recently explored, the ship is making good progress toward its second port of call, dwarf planet Ceres.

We have seen in many logs that this adventure would be quite impossible without its advanced ion propulsion system. Even a mission only to orbit Vesta, which Dawn has accomplished with such stunning success, would have been unaffordable in NASA’s Discovery Program without ion propulsion. This is the only probe ever to orbit an object in the main asteroid belt between Mars and Jupiter. But now, thanks to this sophisticated technology, it is going beyond even that accomplishment to do something no other spacecraft has attempted. Dawn is the only mission ever targeted to orbit two extraterrestrial destinations, making it truly an interplanetary spaceship.

Ion Propulsion System Hot Fire Test for Deep Space 1
Ion Propulsion System Hot Fire Test for the Deep Space 1 spacecraft. Image credit: NASA/JPL-Caltech

Ion propulsion is 10 times more efficient than conventional chemical propulsion, so it enables much more ambitious missions. It uses its xenon propellant so parsimoniously, however, that the thrust is also exceptionally gentle. Indeed, the ion engine exerts about as much force on the spacecraft as you would feel if you held a single sheet of paper in your hand. At today’s thrust level, it would take more than five days to accelerate from zero to 60 mph. While that won’t rattle your bones, in the frictionless, zero-gravity conditions of spaceflight, the effect of the thrust gradually accumulates. Instead of thrusting for five days, Dawn thrusts for years. Ion propulsion delivers acceleration with patience, and patience is among this explorer’s many virtues.

To accomplish its mission, Dawn is outfitted with three ion engines. In the irreverent spirit with which this project has always been conducted, the units are fancifully known as #1, #2, and #3. (The locations of the thrusters were disclosed in a log shortly after launch, once the spacecraft was too far from Earth for the information to be exploited for tawdry sensationalism.) For comparison, the Star Wars TIE fighters were Twin Ion Engine ships, so now science fact does one better than science fiction. On the other hand, the TIE fighters employed a design that did seem to provide greater agility, perhaps at the expense of fuel efficiency. Your correspondent would concur that when you are trying to destroy your enemy while dodging blasts from his laser cannons, economy of propellant consumption probably isn’t the most important consideration.

At any rate, Dawn only uses one ion engine at a time. Since August 31, 2011, it has accomplished all of its thrusting with thruster #3. That thruster propelled Dawn along its complex spiral path down from an altitude of 2,700 kilometers (1,700 miles) to 210 kilometers (130 miles) above Vesta’s dramatic landscape and then back up again. Eventually, the engine pushed Dawn out of orbit, and it has continued to work to reshape the spacecraft’s heliocentric course so that it ultimately will match Ceres’s orbit around the sun.

Although any of the thrusters can accomplish the needed propulsion, and all three are still healthy, engineers consider many factors in deciding which to use at different times in the mission. Now they have decided to put #2 back to work. So on June 24, after its regular monthly hiatus in thrusting to point the main antenna to Earth for a communications session, the robotic explorer turned to aim that thruster, rather than thruster #3, in the direction needed to continue the journey to Ceres. Despite not being operated in nearly two years, #2 came to life as smoothly as ever. It is now emitting a blue-green beam of xenon ions as the craft has its sights set on the mysterious alien world ahead.

› Continue reading Marc Rayman’s Dawn Journal


Short Puffs Keep Dawn Chugging Along

Tuesday, December 4th, 2012

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 at Ceres
Artist’s concept of NASA’s Dawn spacecraft at its next target, the protoplanet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawndroids,

Dawn is continuing to gently and patiently change its orbit around the sun. In September, it left Vesta, a complex and fascinating world it had accompanied for 14 months, and now the bold explorer is traveling to the largest world in the main asteroid belt, dwarf planet Ceres.

Dawn has spent most of its time since leaving Earth powering its way through the solar system atop a column of blue-green xenon ions emitted by its advanced ion propulsion system. Mission controllers have made some changes to Dawn’s operating profile in order to conserve its supply of a conventional rocket propellant known as hydrazine. Firing it through the small jets of the reaction control system helps the ship rotate or maintain its orientation in the zero-gravity of spaceflight. The flight team had already taken some special steps to preserve this precious propellant, and now they have taken further measures. If you remain awake after the description of what the changes are, you can read about the motivation for such frugality.

Dawn’s typical week of interplanetary travel used to include ion thrusting for almost six and two-thirds days. Then it would stop and slowly pirouette to point its main antenna to Earth for about eight hours. That would allow it to send to the giant antennas of NASA’s Deep Space Network a full report on its health from the preceding week, including currents, voltages, temperatures, pressures, instructions it had executed, decisions it had made, and almost everything else save its wonderment at operating in the forbidding depths of space so fantastically far from its planet of origin. Engineers also used these communications sessions to radio updated commands to the craft before it turned once again to fire its ion thruster in the required direction.

Now operators have changed the pace of activities. Every turn consumes hydrazine, as the spacecraft expels a few puffs of propellant through some of its jets to start rotating and through opposing jets to stop. Instead of turning weekly, Dawn has been maintaining thrust for two weeks at a time, and beginning in January it will only turn to Earth once every four weeks. After more than five years of reliable performance, controllers have sufficient confidence in the ship to let it sail longer on its own. They have refined the number and frequency of measurements it records so that even with longer intervals of independence, the spacecraft can store the information engineers deem the most important to monitor.

Although contact is established through the main antenna less often, Dawn uses one of its three auxiliary antennas twice a week. Each of these smaller antennas produces a much broader signal so that even when one cannot be aimed directly at Earth, the Deep Space Network can detect its weak transmission. Only brief messages can be communicated this way, but they are sufficient to confirm that the distant ship remains healthy.

In addition to turning less often, Dawn now turns more slowly. Its standard used to be the same blinding pace at which the minute hand races around a clock (fasten your seat belt!). Engineers cut that in half two years ago but returned to the original value at the beginning of the Vesta approach phase. Now they have lowered it to one quarter of a minute hand’s rate. Dawn is patient, however. There’s no hurry, and the leisurely turns are much more hydrazine-efficient.

With these two changes, the robotic adventurer will arrive at Ceres in 2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply it had when it rocketed away from Cape Canaveral on a lovely September dawn in 2007. Mission planners will be able to make excellent use of it as they guide the probe through its exploration of the giant of the main asteroid belt.

Any limited resource should be consumed responsibly, whether on a planet or on a spaceship. Hydrazine is not the only resource that Dawn’s controllers manage carefully, but let’s recall why this one has grown in importance recently.

› Continue reading Marc Rayman’s Dawn Journal


Dawn’s Stellar Anniversary

Thursday, September 27th, 2012

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. Image credit: NASA/JPL-Caltech

Dear Dawnniversaries,

On the fifth anniversary of the beginning of its ambitious interplanetary adventure, Dawn can look back with great satisfaction on its spectacular exploration of the giant protoplanet Vesta and forward with great eagerness to reaching dwarf planet Ceres. Today Earth’s robotic ambassador to the main asteroid belt is in quiet cruise, gradually reshaping its orbit around the sun so it can keep its appointment in 2015 with the mysterious alien world that lies ahead.

This anniversary resembles the first three more than the fourth. Its first years in space were devoted to spiraling away from the sun, ascending the solar system hill so it could gracefully slip into orbit around Vesta in time for its fourth anniversary. One year ago, Dawn was in the behemoth’s gravitational grip and preparing to map its surface in stereo and make other measurements. The subsequent year yielded stunning treasures as Dawn unveiled the wondrous secrets of a world that had only been glimpsed from afar for over two centuries. While at Vesta, it spiraled around the massive orb to position itself for the best possible perspectives. Its final spiral culminated in its departure from Vesta earlier this month. Now for its fifth anniversary, it is spiraling around the sun again, climbing beyond Vesta so that it can reach 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 fifth 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, and fourth anniversaries.

In its five years of interplanetary travels, the spacecraft has thrust for a total of 1060 days, or 58 percent of the time (and about 0.000000021 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 267 kilograms (587 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The fraction of time the ship has spent in powered flight is lower than last year (when it was 68 percent), because Dawn devoted relatively little of the past year to thrusting. Although it did change orbits extensively at Vesta, most of the time it was focused on exactly what it was designed and built to do: scrutinize the ancient world for clues about the dawn of the solar system.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 7.14 kilometers per second (16,000 miles per hour). 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 slightly more than half 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 five revolutions around the sun, covering about 31.4 AU (4.70 billion kilometers or 2.92 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 23.4 AU (3.50 billion kilometers or 2.18 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. Since Dawn’s launch, Vesta has traveled only 20.4 AU (3.05 billion kilometers or 1.90 billion miles) and the even more sedate Ceres has gone 18.9 AU (2.82 billion kilometers or 1.75 billion miles).

› Continue reading Marc Rayman’s Dawn Journal


Dawn’s Split from Asteroid Vesta - Mission Insider Explains

Wednesday, September 5th, 2012

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 dwarf planet Ceres as imaged by the Keck Observatory
NASA’s Dawn spacecraft departed the giant asteroid Vesta on Sept. 04, 2012 PDT to begin its journey to a second destination, the dwarf planet Ceres, which is seen in this image from the Keck Observatory on Mauna Kea, Hawaii. Image credit: NASA/JPL-Caltech, Keck Observatory, C. Dumas

Dear Marvestalous Readers,

An interplanetary spaceship left Earth in 2007. Propelling itself gently and patiently through the solar system with a blue-green beam of xenon ions, it gradually spiraled away from the sun. It sailed past Mars in 2009, its sights set on more distant and exotic destinations. In July 2011, it gracefully and elegantly entered orbit around the second most massive resident of the main asteroid belt, Vesta. It spent more than 13 months there scrutinizing the gigantic protoplanet with all of its sensors and maneuvering to different orbits to optimize its investigations, making myriad marvelous discoveries. After they traveled together around the sun for 685 million kilometers (426 million miles), the ship left orbit in September 2012 and is now headed for dwarf planet Ceres, the largest body between the sun and Neptune not yet visited by a spacecraft. No other probe has ever been capable of the amazing feats Dawn is performing, exploring two of the largest uncharted worlds in the inner solar system.

The population of the main asteroid belt numbers in the millions. Vesta is such a behemoth that Dawn has now single-handedly examined about eight percent of the mass of the entire belt. And by the time it finishes at the colossus Ceres, it will have investigated around 40 percent.

The expedition to Vesta has produced riches beyond everyone’s hopes. With 31,000 photos, 20 million visible and infrared spectra, and thousands of hours of neutron spectra, gamma ray spectra, and gravity measurements, Dawn has revealed to humankind a unique and fascinating member of the solar system family. More akin to Earth and the other terrestrial planets than to typical asteroids, Vesta is not just another chunk of rock. It displays complex geology and even has a dense iron-nickel core, a mantle, and a crust. Its heavily cratered northern hemisphere tells the story of more than 4.5 billion years of battering in the rough and tumble asteroid belt. Its southern hemisphere was wiped clean, resurfaced by an enormous impact at least two billion years ago and an even greater collision one billion years ago. These events excavated the 400-kilometer (250-mile) Veneneia and 500-kilometer (310-mile) Rheasilvia basins. The larger basin has a mountain at the center that towers more than twice the height of Mt. Everest; indeed, it soars higher than all but one of the mountains known in the solar system. The impacts were so forceful, they nearly destroyed Vesta. The fierce shock reverberated through the entire body and left as scars an extraordinary network of vast troughs near the equator, some hundreds of kilometers (miles) long and 15 kilometers (10 miles) wide.

The powerful impacts liberated tremendous amounts of material, flinging rocks far out into space, some of which eventually made it all the way to Earth. It is astonishing that more than one thousand meteorites found here came from Vesta. We have some meteorites from Mars, and we have some meteorites from the moon, but we have far, far more that originated in those impacts at Vesta, so distant in time and space. Vesta, Mars, and the moon are the only celestial bodies identified as the source of specific meteorites.

Scientists will spend years productively poring through Dawn’s fabulous findings and learning what secrets they hold about the dawn of the solar system, and many more people will continue to marvel at the spectacular sights of this alien world. But the emissary from Earth has completed its assignment there and moved on. It has spent most of its time since the previous log using its ion propulsion system to climb higher and higher above Vesta. This departure spiral is the mirror image of the approach spiral the robotic adventurer followed last year. The unique method of entering and leaving orbit is one of the many intriguing characteristics of a mission that uses ion propulsion. Without that advanced technology, this ambitious deep space adventure would be impossible.

As Dawn ascended, Vesta’s gravitational grip grew weaker and weaker. At some point along its spiral, the explorer was far enough and moving fast enough that Vesta could no longer hold it in orbit. As smoothly and tenderly as Vesta had taken Dawn in its embrace last year, it released its erstwhile companion, each to go its own way around the sun. The bond was severed at about 11:26 p.m. PDT yesterday, when they were 17,200 kilometers (10,700 miles) apart, separating at the remarkably leisurely speed of less than 33 meters per second (73 miles per hour). Many of our readers drove their cars that fast today (although we hope it was not in school zones).

Unlike missions that use conventional chemical propulsion, there was no sudden change on the spacecraft and no nail-biting on Earth. If you had been in space watching the action, you probably would have been hungry, cold, and hypoxic, but you would not have noticed anything unusual about the scene. Apart from a possible hint of self-satisfaction, Dawn would have looked just as it had for most of its interplanetary flight, a monument to humankind’s ingenuity and passionate drive to know the cosmos perched atop a blue-green pillar of xenon ions. If, instead, you had been in Dawn mission control watching the action, you would have been in the dark and all alone (until JPL Security arrived). There was no need to have radio contact with the reliable spaceship. It had already thrust for almost 2.9 years, or 58 percent of its time in space. Thrusting during escape was no different. No one was tense or anxious; rather, all the drama is in the spectacular results of the bold mission at Vesta and the promise of what is to come at Ceres. When Dawn entered orbit, your correspondent was dancing. When Dawn left orbit, he was sleeping serenely.

A month earlier, on August 8, with the craft more than 2,100 kilometers (1,300 miles) above the surface, patiently powering its way up through Vesta’s gravity field, one of the reaction wheels experienced an increase in internal friction. Reaction wheels are used to control a spacecraft’s orientation in the frictionless, zero-gravity conditions of spaceflight. By electrically changing a wheel’s spin rate, Dawn can rotate or stabilize itself. Protective software quickly detected the event and correctly responded by deactivating that wheel and the other two that were operating, switching to the small jets that are available for the same function, and reconfiguring other systems, including powering off the ion thrust and turning to point the main antenna to Earth.

A routine communications session the next day revealed to mission controllers what had occurred. They had planned long ago to turn the wheels off for the flight from Vesta to Ceres, so having them off a few weeks early was not a significant change. The team soon restored the spacecraft to normal operations and reformulated the departure plan, and on August 17 Dawn resumed its ascent. Because of the hiatus in thrusting, escape shifted from August 26 to September 4. The flexibility in the mission timeline provided by ion propulsion made this delay easy to accommodate.

In order to conserve the hydrazine propellant that the jets use, the bonus departure observations described before were curtailed, as they were not a high priority for the mission. Nevertheless, on August 25 and 26, at an altitude of around 6,000 kilometers (3,700 miles), the explorer did peer at Vesta once more with its camera and visible and infrared mapping spectrometer. The last time it had been this far away was July 21, 2011, during its descent to an unfamiliar destination. This time, 13 months later, the spacecraft turned back for a final gaze at the magnificent world it had unveiled during its remarkable time there, a world that prior to last year had appeared as little more than a tiny smudge among the stars for the two centuries it had been observed.

The delay in the departure schedule provided a convenient benefit. Vesta has seasons, just as Earth does, although they progress more slowly on that distant orb. August 20 was the equinox, when northern hemisphere spring began. Until then, the sun had been in Vesta’s southern hemisphere throughout Dawn’s residence there. While most of the northern hemisphere was revealed during the second high-altitude mapping orbit, the illumination of the landscape immediately around the north pole was even better for this last look. After radioing its parting shots to wistful mission controllers, the ship commenced its climb again.

And then, with an stunningly successful mission behind it, a newly explored world below it, and a mysterious dwarf planet ahead of it, the indomitable and indefatigable adventurer left Vesta forever.

Dawn is 18,500 kilometers (11,500 miles) from Vesta and 64 million kilometers (40 million miles) from Ceres. It is also 2.45 AU (367 million kilometers or 228 million miles) from Earth, or 910 times as far as the moon and 2.43 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 a.m. PDT September 5, 2012

› Read previous Dawn Journals by Marc Rayman