Starship Asterisk*

http://en.wikipedia.org/wiki/TIE_fighter wrote:
TIE fighters are starfighters in the Star Wars universe. Propelled by Twin Ion Engines (hence the TIE acronym),
TIE fighters are depicted as fast, fragile starfighters produced by Sienar Fleet Systems for the Galactic Empire.

http://www.planetary.org/blog/article/00002868/ wrote:
Dawn Journal: LAMO (Low Altitude Mapping Orbit)
By Marc Rayman, Jan. 5, 2011 | 14:35 PST | 22:35 UTC

<<Dawn finishes 2010 much as it began the year, thrusting with its ion propulsion system in steady pursuit of a distant world. During the next year, the probe will arrive there and begin its scrutiny. In the meantime, it continues thrusting patiently, but now with a difference.

[b][color=#FF00FF]Dawn is outfitted with three thrusters[/color][/b]

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Dawn is outfitted with three ion thrusters, although it is designed to use only one at a time. (The locations and whimsical names [i.e., #1, #2 & #3] of the thrusters were divulged once the spacecraft was too far from Earth for anyone to see it.) Thruster #3 was the first to see action in the mission, and it propelled the spacecraft until June 16, 2008, after which thruster #1 took over. On January 4, 2010, the ship switched to thruster #2. Prior to this year, #2 had accumulated little more than a day of operation for some tests. But in 2010, it operated flawlessly for 304 days, and Dawn accomplished nearly all of its thrusting this year with only that thruster. While #2 is ready for much more, on December 6 mission controllers assigned thrusting to #3 as part of the strategy of balancing the work for this long journey among the three units.

During its extensive service this year, thruster #2 expended less than 79 kilograms of xenon propellant. With its uniquely efficient but exquisitely gentle touch, the thruster accelerated the ship by more than 2.2 kilometers per second.

Thruster #3 had remained idle for 2.5 years, waiting with the same patience it displays when thrusting. Earlier this month, the long hiatus ended; it came to life again and operated as smoothly as ever. It is once more powering the ship ahead, impelling the probe toward its July rendezvous with the mysterious and alien world Vesta.

In addition to ensuring Dawn's journey to Vesta continues to go so well, the operations team has been making good progress formulating the detailed instructions the robotic explorer will follow when it is there. Earlier this year, we examined what the ship will do as it approaches the distant port and how it will sail into orbit under ion thrust as well as what it will do in the first two principal science phases, survey orbit and the high altitude mapping orbit (HAMO). Engineers are working now on the timeline of commands, known as sequences, for the third major science phase.

From HAMO, at an altitude of about 660 kilometers, Dawn will have an excellent view of Vesta, close enough to see plenty of fascinating details and yet far enough to allow its science camera to cover most of the surface of this uncharted world during the month of mapping. In addition to using the camera to develop the global maps, the visible and infrared mapping spectrometer (VIR) will be trained on many regions, providing even better resolution on the minerals that compose the surface than it could achieve from the higher survey orbit. When its work in HAMO is complete, the craft will fly in for an even closer look.

We saw in April that just as the interplanetary traveler has spiraled around the sun from Earth on its way to Vesta (and will do so en route from Vesta to Ceres), it will spiral around Vesta as its ion propulsion system takes it from one orbital altitude to another. Although operation of the ion propulsion system itself is independent of whether it is in orbit around the sun or around Vesta, there is much more to thrusting than that one system. There will be several important differences in how the mission control team plans the flight profile and operates the craft when transiting from one orbit to another at Vesta, and we will consider those in a future log. We also will see that as-yet uncharacterized physical properties of Vesta itself will affect the nature of the trip. The operations team has been working hard to prepare for the many possibilities that might be found at this unexplored world.

Dawn's target after HAMO will be an altitude of around 180 kilometers, closer to the surface beneath it than most satellites are that orbit Earth. It may take six to eight weeks to follow the winding path from HAMO to this low altitude mapping orbit (LAMO) under the delicate push of the ion thrust. Dawn will spend about two months in LAMO, revolving around the rocky body once every four hours. The science camera will acquire many exciting pictures from this new vantage point, and while they certainly will afford a better view of some regions than could be obtained in HAMO, that is not the primary objective of LAMO. Images and observations with VIR will be a valuable bonus, but the primary science data will be in two other areas.


Dawn's gamma ray & neutron detectors will be __ Vesta compared with all the asteroids and comets ever
operating continuously during approach phase __ visited by spacecraft, all to the same scale (2 km/pixel)

The gamma ray and neutron detector (GRaND) will have been operating continuously since the approach phase, but it will be in LAMO that it is most effective. The instrument is designed to detect the byproducts of cosmic rays hitting Vesta. Cosmic rays are energetic radiation, composed of a variety of particles, that pervades space. As Vesta's surface is exposed directly to space, cosmic rays strike the nuclei of atoms in the uppermost meter. Some of the gamma rays and neutrons produced by these impacts make their way back into space, carrying the signatures of the atoms. When GRaND is in LAMO, it will sense enough of the emitted particles to reveal the identities of many kinds of atoms in the surface. It also will record some radioactive decays of atoms there.

GRaND can detect some of the cosmic rays directly, and it has done so whenever it has been tested in flight, far from a planetary body. It also observed gamma rays and neutrons from Mars during the spacecraft's brief visit there last year.

Unlike the relatively bright light reflected from Vesta's surface that the camera and VIR detect, the subatomic particles GRaND measures provide a very faint signal. Just as taking a picture of a dim object requires a longer exposure than for a bright one, to make GRaND's "picture" of Vesta will require a very long exposure. So, much of Dawn's time in LAMO will be devoted to pointing GRaND at Vesta and letting it measure the energy and other properties of the particles that come its way.

Dawn was designed with all instruments pointing in the same direction. Even when GRaND is the principal instrument, simultaneous bonus observations with the others will greatly enhance the data returned from LAMO.

In addition to providing GRaND's measurements of the elemental composition, LAMO is designed to enable another important method of characterizing Vesta. As Dawn travels in its orbits, its motion is dictated by the combined gravitational attraction of all of the matter within the giant protoplanet. By making ultrasensitive measurements of the probe's orbit (more accurate even than for the normal needs of its deep-space navigation), scientists can calculate the arrangement of Vesta's constituent masses. If, for example, there is a volume far below the surface filled with rock of greater density than the surrounding regions, even though it is hidden from the instruments, its stronger gravitational pull will reveal it. Dawn will accelerate just a little as its orbit brings it closer to this feature and decelerate just a little when it has passed by. These effects are miniscule and the measurements very challenging, but the view of the interior of Vesta, from crust to core, will be rewarding.

There is good reason to believe Vesta has a complex internal structure, as do the other large rocky residents of the inner solar system, one of which is immediately beneath your correspondent as he writes this and some of you as you read it. In addition to Earth and Vesta, Mercury, Venus, the moon, and Mars all are thought to have grown very hot as they were forming, and that caused the minerals within them to separate into layers of different composition. In this process, known to planetary geologists as differentiation, the denser materials tend to sink while the lighter materials rise to the top, and when the body cools, the layers are frozen in place. Other processes during the history of the planet may create pockets of higher or lower density rock as well.

Vesta may be the smallest relic from the solar system's formation to have experienced planetary differentiation, and the information scientists glean from studying the interior structure will contribute to understanding the process by which planets formed. Even though it is Lilliputian compared to the planets, Vesta is Brobdingnagian compared to most asteroids. In the context of planetary formation mechanisms, its closer brethren are the rocky worlds named above.

To make a good map of the interior structure, measurements of Dawn's orbital motion need to occur above all parts of Vesta. In essence, scientists use the spacecraft to map variations in Vesta's gravitational field. Several different kinds of data are required to do this, but the principal type is the Doppler shift of a radio signal transmitted from one of the giant antennas of NASA's Deep Space Network to Dawn, which then sends a signal back to the same antenna. This technique was applied to calibrate the gentle thrust of Dawn's ion thrusters early in the mission, and it will be performed so exquisitely at Vesta that changes in the distant craft's speed of about 0.1 millimeters per second (1 foot per hour) will be evident. So with the ship sailing over the alien world, mapping the gravity field is accomplished not with an instrument pointed at the surface but rather with the main antenna aimed at distant Earth.

It is likely that the irregularities in the gravity field not only will permit insights into the interior of Vesta, but they also will perturb Dawn's path enough that the probe will have to maneuver to maintain the orbit within the parameters needed for operations. Therefore, the ion propulsion system will be used about once a week for a few hours to adjust the orbit. The specifics of these maneuvers will depend on the details of the gravity field, which won't be known until Dawn measures them. Nevertheless, engineers plan windows for the orbital corrections now and will work out those specifics when they measure the craft's orbit.

In order to obtain the data needed for the map of the atomic constituents and the map of the interior mass of Vesta, Dawn will spend more time in LAMO than any of the other parts of the orbital mission. In both of the preceding science phases, survey orbit and HAMO, as well as the fourth phase, which will be the subject of a future log, Dawn will devote most of its time over the illuminated side of Vesta to acquiring data with the science camera and VIR and most of its time over the unilluminated side to radioing that precious information to Earth. Neither the GRaND nor gravity measurements depend on the sun shining on Vesta, however, so that regular schedule will not be followed in LAMO. Completing one revolution every four hours, it would be inconvenient and inefficient anyway to switch back and forth so often between directing the antenna to Earth or the instruments to the surface. Rather, the timing is determined by strategies to ensure good global coverage for the two primary scientific investigations, the need to transmit data when memory is full, and other considerations.

Even though LAMO is the lowest altitude from which Dawn will observe Vesta, it is not the final phase of the Vesta mission. Nevertheless, by the time the spacecraft is ready to climb back above LAMO, it will have returned a wealth of breathtaking information that will allow scientists to begin transforming this unfamiliar world into one we know. Dawn's removal of the veil that shrouds Vesta in secrets will nourish everyone who hungers for the exhilaration of new knowledge and new understanding, the excitement of adventure and exploration, and the thrill of discovery.

Dawn is 0.060 AU from Vesta, its next destination. It is also 2.96 AU from Earth, or 1,175 times as far as the Moon. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.>>
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Scientists from the Max Planck Institute for Solar System Research discover a cosmic chunk in space that orginiated from deep inside the third largest asteroid

On its southern side the asteroid Vesta shows a huge crater. This picture shows the asteroid in an image taken by the Hubble Space Telescope (top, left), as a reconstruction based on theoretical calculations (top, right), and as a topological map (bottom). [i](Image: Ben Zellner (Georgia Southern University) / Peter Thomas (Cornell University) / NASA)[/i]

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Researchers from the University of North Dakota and from the Max Planck Institute for Solar System Research in Germany have discovered a new kind of asteroid using NASA’s Infrared Telescope Facility on Mauna Kea, Hawaii. The mineralogical composition of 1999 TA10 suggests that unlike many other asteroids it did not originate from the outer rocky crust of its parent asteroid Vesta, but from deeper layers. Until now, no asteroid with this composition was known. With the help of this new discovery it is now possible to determine the thickness of Vesta’s crust and study its internal structure. In this summer Vesta will be the first destination of NASA's mission DAWN. In addition, the body with a diameter of approximately 525 kilometers is believed to be the only remaining protoplanet from the early phase of our solar system. (Icarus, in press, published online on December 5th, 2010)

The asteroid Vesta is unique: Unlike all other minor planets, that orbit the Sun within the main belt between the orbits of Mars and Jupiter, Vesta has a differentiated inner structure: A crust of cooled lava covers a rocky mantle and a core made of iron and nickel - quite similar to the terrestrial planets Mercury, Venus, Earth, and Mars. Scientists therefore believe this onion-like built asteroid to be a protoplanet, a relict from an early phase of planet formation more than four and half billion years ago. All other protoplanets either accumulated to form planets or broke apart due to violent collisions.

Vesta seems to have witnessed a huge impact, as can be seen from a large crater on its southern hemisphere. The so-called Vestoids - a group of asteroids with a composition similar to that of Vesta - were most probably created due to this impact. Since some meteorites that were found on Earth consist of similar rock as does Vesta’s mantle, scientists believe that this collision also hurled material from deep within the asteroid into space. But till now there was no source in the form of near-Earth Vestoids for these meteorites with Vesta’s mantle composition.

Near-Earth asteroid 1999 AT10 fills this gap. Using the NASA IRTF, the scientists were now able to analyze the infrared-radiation that 1999 AT10 reflected into space and compared its characteristic spectral fingerprints with those of Vesta. Apart from calcium-rich wollastonite, the measurements mainly point to iron-rich ferrosillite. ‘‘These materials can be found in Vesta’s mantle and crust," explains Dr. Andreas Nathues from MPS. ‘‘However, the ratio is decisive." In the case of 1999 AT10 the concentration of iron is clearly lower than in any known Vestoids. "This all points to 1999 TA10 having originated from the interior of Vesta," says Nathues.

The newly discovered body now allows important inferences about its parent asteroid. Models of Vesta's surface based on observations made by the Hubble Space Telescope render a depth of the South Pole crater of ~25 kilometers at the most. The new discovery now suggests that this would be the maximum possible thickness of the outer crust.

In order to reconstruct the processes that led to the formation of planets more than 4.5 billion years ago, scientists need to determine the thickness of Vesta's layers as precisely as possible. Only this makes it possible to calculate from which material mixture the protoplanet was made - and thus which materials were present when the solar system formed and in what ratio.

The scientists now hope for more information about Vesta's structure from NASA's mission Dawn. In August 2011, the probe that has been traveling through space since 2007, will rendezvous with Vesta and orbit the asteroid for a year. On board Dawn there are two cameras that were designed and built under the leadership of the Max Planck Institute for Solar System Research in cooperation with the German Space Agency (DLR) and the Institut für Datentechnik und Kommunikationsnetze of the Technical University of Braunschweig.

• Incarus (in press 2010 Dec 05) DOI: 10.1016/j.icarus.2010.11.032

http://www.planetary.org/blog/ wrote:

In a paper published in 2011 in Geophysical Research Letters, Martin Jutzi and Eric Asphaug modeled the effects of a 50-kilometer body striking a spherical, rotating asteroid the size of Vesta at an oblique angle. This animation shows the splash of the impact and the motion of the ejecta that spread out from it. Because of Vesta's rotation the ejecta is asymmetric in its distribution, piling up in a steep wall on one side of the asteroid and spreading out in a gradually shallowing deposit on the opposite side. Credit: Martin Jutzi

Click to play embedded YouTube video.

After 3 ½ years of thrusting silently through the void, NASA's Dawn spacecraft is on the threshold of a new world. It's deep in the asteroid belt, less than 4 months from giant asteroid Vesta.

"We're closing in," says Marc Rayman, Dawn's chief engineer and mission manager. "And I'm getting more excited every day!"

Dawn will enter orbit around Vesta in July 2011, becoming the first spacecraft ever to orbit a body in the asteroid belt. After conducting a detailed study of the uncharted alien world for a year, the spacecraft will pull off an even more impressive first. It will leave Vesta, fly to dwarf planet Ceres, and enter orbit there.

"This is unprecedented," says Rayman. "No spacecraft has ever orbited two target bodies, much less worlds in the asteroid belt. A few probes have passed through this vast region of space, but not one could stop and develop an intimate portrait of its residents."

A conventional spacecraft gets a boost from a big rocket, then coasts to its target. Carrying enough fuel for making significant changes in speed or direction along the way would make it too heavy to launch.

Dawn is far more fuel efficient. Spanning 65 feet, its solar arrays collect power from the sun to ionize atoms of xenon gas. These ions are expelled silently out the back of the spacecraft by a strong electric field, producing a gentle thrust. The weightless, frictionless conditions of space flight allow this gossamer force effect to build up, so the spacecraft continuously gains speed.

"This spacecraft ultimately achieves fantastically high velocity while consuming very little propellant -- using only a kilogram of xenon every 4 days, though its engines are almost constantly active."

With this system Dawn has been quietly, gradually reshaping its orbit around the sun, slowly spiraling out to its target, getting closer and closer as it loops around.

"By the time the spacecraft is in the vicinity of Vesta, its orbit will be very much like the asteroid's," explains Rayman. "So upon arrival, Dawn can slip into orbit as gently as it's been moving for 3 ½ years."*

A conventional spacecraft screeches into orbit in a single dramatic, nail biting instant. The mission team is usually gathered in the mission control room with their eyes riveted on the telemetry to see that the final critical maneuver goes smoothly.

"With Dawn, there is no one big maneuver, no fiery burn, no single critical moment. Dawn's entry into orbit will be no different from what the spacecraft does almost all the time, what it's doing as you read this article. In fact, when Dawn sidles into orbit, I might be asleep. Or if it's Friday night I'll be dancing, or if it's Saturday I might be out taking pictures of dragonflies."

But you can bet he'll be in mission control when the pictures start coming in.

"It will be incredibly exciting to watch Dawn close in on Vesta. We'll witness the uninteresting smudge in the first distant images grow into a full-sized world as we loop closer and closer, ending up just 110 miles above the surface. That's closer than the ISS is to Earth! We'll be right there, and if there are no tall trees we'll be safe."

After exploring Vesta for a year, Dawn will take leave of the rocky world as softly as it arrived there, climbing out along a spiral, gradually getting farther and farther away, the loops getting longer and longer, until the asteroid's gravity gently releases the spacecraft. Dawn will again be orbiting the sun on its own, just as it is now. It will complete about two thirds of a lap before arriving at Ceres.

There it will once again slide gently into orbit around a new world, guided by ion thrusters as silent as space itself.

"Even if we imagined a sound, it would be the faintest of whispers, the softest of sighs. Yet it tells us the secret of making an interplanetary spaceship that can travel to and explore distant, alien worlds, carrying with it the dreams of those on Earth who long to know the cosmos."

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NASA's Dawn spacecraft has reached its official approach phase to the asteroid Vesta and will begin using cameras for the first time to aid navigation for an expected July 16 orbital encounter. The large asteroid is known as a protoplanet – a celestial body that almost formed into a planet.

At the start of this three-month final approach to this massive body in the asteroid belt, Dawn is 1.21 million kilometers (752,000 miles) from Vesta, or about three times the distance between Earth and the moon. During the approach phase, the spacecraft's main activity will be thrusting with a special, hyper-efficient ion engine that uses electricity to ionize and accelerate xenon. The 12-inch-wide ion thrusters provide less thrust than conventional engines, but will provide propulsion for years during the mission and provide far greater capability to change velocity.

"We feel a little like Columbus approaching the shores of the New World," said Christopher Russell, Dawn principal investigator, based at the University of California in Los Angeles (UCLA). "The Dawn team can't wait to start mapping this Terra Incognita."

Dawn previously navigated by measuring the radio signal between the spacecraft and Earth, and used other methods that did not involve Vesta. But as the spacecraft closes in on its target, navigation requires more precise measurements. By analyzing where Vesta appears relative to stars, navigators will pin down its location and enable engineers to refine the spacecraft's trajectory. Using its ion engine to match Vesta's orbit around the sun, the spacecraft will spiral gently into orbit around the asteroid. When Dawn gets approximately 16,000 kilometers (9,900 miles) from Vesta, the asteroid's gravity will capture the spacecraft in orbit.

"After more than three-and-a-half years of interplanetary travel, we are finally closing in on our first destination," said Marc Rayman, Dawn's chief engineer, at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We're not there yet, but Dawn will soon bring into focus an entire world that has been, for most of the two centuries scientists have been studying it, little more than a pinpoint of light."

Scientists will search the framing camera images for possible moons around Vesta. None of the images from ground-based and Earth-orbiting telescopes have seen any moons, but Dawn will give scientists much more detailed images to determine whether small objects have gone undiscovered.

The gamma ray and neutron detector instrument also will gather information on cosmic rays during the approach phase, providing a baseline for comparison when Dawn is much closer to Vesta. Simultaneously, Dawn's visible and infrared mapping spectrometer will take early measurements to ensure it is calibrated and ready when the spacecraft enters orbit around Vesta.

Dawn's odyssey, which will take it on a journey of 4.8-billion kilometers (3-billion miles), began on Sept. 27, 2007, with its launch from Cape Canaveral Air Force Station in Florida. It will stay in orbit around Vesta for one year. After another long cruise phase, Dawn will arrive at its second destination, an even more massive body in the asteroid belt, called Ceres, in 2015.

These two icons of the asteroid belt will help scientists unlock the secrets of our solar system's early history. The mission will compare and contrast the two giant bodies, which were shaped by different forces. Dawn's science instrument suite will measure surface composition, topography and texture. In addition, the Dawn spacecraft will measure the tug of gravity from Vesta and Ceres to learn more about their internal structures.