[meteorite-list] Soltmany is first official fall of 2011

From: Galactic Stone & Ironworks <*meteoritemike_at_meteoritecentral.com*> Date: Mon, 18 Jul 2011 20:53:52 -0400 Message-ID: <CAKBPJW-ffY4EJCh3hBeNGbx-pjDzW0=JNeb8L40kMbpDOdTeVw_at_mail.gmail.com> · meteoritemike_at_meteoritecentral.com

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On 7/18/11, Ron Baalke <baalke at zagami.jpl.nasa.gov> wrote:
>
> http://dawn.jpl.nasa.gov/mission/journal_07_18_11.asp
>
> Dawn Journal
> Dr. Marc Rayman
> July 18, 2011
>
> Dear Residawnts of Vesta,
>
> Dawn has arrived!!
>
> After covering 2.8 billion kilometers (1.7 billion miles) on its own,
> after traveling for nearly four years through the lonely emptiness of
> interplanetary space, after being bound by the gravity only of the sun,
> Dawn is finally in orbit around Vesta. To get here, it gently propelled
> itself with its ion propulsion system for 70% of its journey, or more than
> 2.6 years. Deep in the asteroid belt, far from its planet of origin, well
> beyond Mars (which it visited ever so briefly more than two years ago),
> where no spacecraft has ever been before, Dawn now resides with a giant.
>
> While more detailed navigational analyses will be required to determine
> the exact time, around 10:00 pm PDT on July 15, as the spacecraft performed
> its familiar routine of ion thrusting, its orbit around the sun finally was
> so close to that of Vesta that the protoplanet's gravity could take hold of
> it. Dawn was only about 16,000 kilometers (9,900 miles) above the ancient,
> scarred surface of the alien world. Traveling together around the sun at
> more
> than 20.5 kilometers per second (46,000 mph), their orbits were so
> similar that the cosmic craft was closing in at the leisurely speed of
> only 27 meters per second (60 mph). The last time it approached a nearby
> destination so slowly was in April 2007. At that time, it used more
> conventional propulsion technology: it rode on a truck from Washington, DC
> to Cape Canaveral, Florida.
>
> That may be too many numbers for some readers (and too few for our good
> friends the Numerivores). But it all reduces to one cool fact: humankind has
> succeeded in delivering an interplanetary spaceship to orbit around one of
> the
> largest objects in the main asteroid belt between Mars and Jupiter. Indeed,
> Dawn is the first spacecraft to orbit any object in the main belt.
>
> The probe slipped gently into orbit with the same grace it has displayed
> during its nearly 1000 days of ion thrusting through the solar system.
> Although the unusual nature of the spiral capture has been explained in
> detail before, there is one important difference (in addition to some minor
> ones)
> from previous descriptions: now it is history.
>
> Dawn has orbited two other bodies. Shortly after it left Cape Canaveral
> atop a fiery rocket, the spacecraft spent about 45 minutes in Earth orbit,
> waiting
> for the proper orbital alignment to begin its ambitious deep-space voyage.
> Once
> the rocket gave it enough energy to leave the planet behind, Dawn orbited
> the sun
> as surely as Earth and the other planets do, although, of course, it spent
> most of
> its time reshaping that orbit. Now it is orbiting Vesta, as surely as
> the moon orbits Earth.
>
> Entering orbit around the protoplanet is essential to Dawn's plans for
> comprehensive studies of this exotic world, but simply being in orbit is
> not adequate. The craft did not miss a beat as it flew into Vesta's
> grasp; it is spiraling around its new master as it aims for its first
> science orbit at an altitude of 2700 kilometers (1700 miles). The
> intensive scrutiny of Vesta from survey orbit will begin in the second week
> of
> August.
>
> It's a noteworthy coincidence that Earth and Vesta will happen to be very
> well
> aligned then. As they follow their independent orbits around the sun,
> occasionally
> their paths bring them relatively near to each other. So just as Dawn begins
> looking closely at Vesta, so too can residents of Earth. The protoplanet is
> the
> brightest object in the asteroid belt, and the only one ever visible to
> unaided
> terrestrial eyes, although binoculars or a telescope make it much easier
> to spot, especially under skies that are brightened by the lights of
> cities.
>
> Even when their separation is at its minimum, Earth and Vesta will come
> only to within about 1.23 AU (184 million kilometers or 114 million miles)
> of
> each other. While their closest approach is late at night on July 31, the
> geometry
> changes slowly enough that there are good viewing opportunities well before
> and after.
> Go here <http://dawn.jpl.nasa.gov/news/observing_vesta.asp> for guidance on
> how to
> find Vesta in the constellation Capricornus. And if you are fortunate enough
> to
> glimpse that distant point of light, let your imagination add to the scene
> the
> recent immigrant from Earth, representing you and the rest of humankind
> on its mission of exploration. There, far from its erstwhile home and
> the beings who urge it on, this ambitious adventurer is translating that
> dot of light among the myriad stars into an exciting and fascinating
> account of the dawn of the solar system.
>
> Dawn has spent most of its time since the last log thrusting as usual. The
> thrusting
> even at the time it was captured by Vesta's gravity was no different. We
> have seen
> before that, in stark contrast to the tension when other missions enter
> orbit, with
> ion propulsion, the process is very calm indeed. For that matter, since May
> 2010,
> Dawn has thrust with its radio transmitter turned off, devoting that
> precious power to
> accelerating xenon ions rather than generating radio waves. The ship
> continued in
> silence when it went into orbit on Friday night. Mission control was empty,
> there
> being no need to monitor the probe's operation. In fact, your
> correspondent was dancing, confident that the pas de deux being
> performed 188 million kilometers (117 million miles) away would be
> executed with graceful beauty and flawless precision.
>
> Confirmation that Dawn was in orbit came shortly before 11:30 pm PDT on
> July 16 (more than 24 hours after it glided into orbit) when its radio
> signals were received at the Deep Space Network.
> Following its preprogrammed sequence of instructions, the spacecraft
> acquired more images of Vesta earlier in the evening and then initiated
> communications with Earth right on schedule. Observing that it was in
> good health and continuing to perform all of its functions demonstrated
> that it had achieved orbit. The choreography was beautiful!
>
> Reliable as Dawn is, it did experience an unexpected interruption in
> thrust recently. On June 27, a cosmic ray, a high energy subatomic
> particle traveling through space, apparently managed to strike an
> electrical component on the spacecraft in an especially effective way.
> The component is used by the ion propulsion system computer controller
> to operate valves in the complex
> plumbing that transports xenon from the main tank to the operating
> thruster. The propellant needs to be delivered at just the right rate
> for optimal performance. When the cosmic ray deposited its energy in
> that device, it deprived the circuit of the ability to send signals to
> the valves, even when directed to do so by the computer. (A cosmic ray
> is the most likely culprit, but other explanations for the circuit's
> inaction are still being considered.) As a result, when it was time to
> open valves to feed a little more xenon into the thruster, the
> controller was unable to comply. The computer detected the problem,
> followed the appropriate procedure for terminating thrust, and alerted
> the main spacecraft computer. That computer correctly responded by
> canceling other planned activities and commanding the ship into one of
> its safe modes. In this case, because all other systems were healthy, it
> was not necessary to invoke the normal safe mode. Rather, the robot
> properly chose to make fewer reconfigurations. It pointed its main
> antenna to Earth and transmitted its status, awaiting a response from
> controllers.
>
> The Deep Space Network began a routine communications session early on
> June 28, and the Dawn team immediately understood the spacecraft
> condition. Before the end of the day, they had restored it to its normal
> flight mode and made preparations to activate the other controller.
>
> Dawn had been using controller #1 and ion thruster #3 since December.
> With the controller unable to operate valves, engineers instructed the ship
> to switch to controller #2, which was in command for most of the thrusting
> in
> 2010.  Its ability to operate the valves was
> not compromised. That unit can be used with thruster #2 and #3, but it
> was faster to formulate commands to use thruster #2, so in the interest
> of time, that was the choice.
>
> Later this summer, engineers will conduct tests with controller #1 to
> assess its health and determine whether its valve signals can be
> restored. That controller operates thruster #1 and #3. Mission planners
> had long ago decided not to use venerable #1 for the rest of the
> mission, as it requires slightly more power than its siblings, so
> whether controller #1 will be fully functional or not, Dawn's
> extraterrestrial expedition can be completed as planned with controller #2.
>
> Once the spacecraft had deviated from its intended flight plan by not
> thrusting, navigators had to devise a new plan to fly to Vesta. To
> ensure there would be enough time to make up for the lost thrust, they
> removed one of the navigation imaging
> sessions (and the communications period that followed it) from the
> schedule and another routine communications session. Of course, as
> experienced interplanetary explorers, Dawn's mission team had always
> recognized that glitches could interfere with any activity, so more
> imaging and more communications had been planned than truly were
> required. Doing without a few to allow time for some compensatory
> thrusting was easily accommodated.
>
> In order to resume thrusting quickly, controllers chose not to optimize
> the plan but rather simply to devise a plan that was adequate. The
> consequence was that they ended up giving Dawn /more/ time to thrust
> than it really even needed. The entire episode beginning with the balky
> controller cost 1.2 days of thrust, and the revised plan added 1.8 days
> of thrust at other times. As a result, the insertion into orbit shifted
> 15 hours earlier. Such flexibility is another of the many differences
> between missions that use ion propulsion and those that use conventional
> propulsion.
>
> Before restarting its powered flight, however, the team was eager to
> allow Dawn to conduct its first planned observation of Vesta throughout
> one full rotation of the protoplanet on its axis, a Vestian day of 5
> hours 20 minutes. (This and other activities during the approach phase
> were described last year.) Thanks to the fleet and flawless work of the
> team, that was carried out on schedule on June 29-30, and all the planned
> images were acquired. The visible and infrared mapping spectrometer (VIR)
> also peered at Vesta to provide additional information for use in setting
> instrument parameters for the science observations in survey orbit.
> After it acquired two excellent sets of data, its internal computer
> detected an unexpected condition, so it did not complete the rest of its
> activities. As the camera's images were beaming back to Earth on June
> 30, engineers verified that VIR was in good condition, and they will
> study its telemetry further as they continue to plan for its important
> measurements of the minerals that compose Vesta's surface.
>
> In the original itinerary, ion thrusting would recommence after the
> communications session on June 30. And that is exactly what occurred,
> even with the unplanned thrusting hiatus in the preceding days. Dawn
> continued closing in on Vesta with the gentle pressure of thruster #2,
> just as it still is today.
>
> As a reminder, an easy way you can have the same otherworldly view of Vesta
> as Dawn is to visit here
> <http://dawn.jpl.nasa.gov//multimedia/vesta_dawn_gallery.asp>.
> These logs generally will not provide interpretations of the rich bounty of
> images
> (but they are fantastic, aren't they?) or other fascinating measurements. As
> the
> data are assessed by Dawn's team of planetary scientists from four
> countries,
> news of the results will be distributed by NASA's and JPL's news
> organizations. And for more frequent updates on the progress of the
> mission than are provided in these logs, readers may want to go here
> <http://dawn.jpl.nasa.gov/mission/status.asp>, where your correspondent
> abandons
> his idiolect to provide extremely brief reports much more often (with much
> less,
> ahem, color).
>
> On July 9-10, the spacecraft's agenda included another pause in
> thrusting. This time, in addition to acquiring its second set of images
> while Vesta completed a full rotation, Dawn photographed the space
> around Vesta in search of moons. Remote observations with the Hubble
> Space Telescope and other observatories on Earth had not found any, but
> that did not rule out their presence. As no moons had been detected yet,
> however, they would have to be small and therefore faint. In order to
> try to discover whether there might be any, the camera used different
> exposures, some as long as 4.5 minutes. (For photographers, the
> effective shutter speed for the pictures of Vesta that reveal its
> surface features is 1/125 of a second.) The spacecraft pointed its
> camera around Vesta and acquired 72 images. Three hours later, it imaged
> the same locations, and then another nine hours after that, it repeated
> the sequence once again. The pictures are being scrutinized for points
> of light that shift position from one set of images to another,
> betraying the orbital motion of natural satellites of Vesta.
>
> Although those results are not yet available, we now know with certainty
> that Vesta does have a moon. Its name is Dawn!
>
> Dawn is 11,000 kilometers (6,800 miles) from Vesta, closer than many
> terrestrial satellites are to Earth. It is also 1.25 AU (187 million
> kilometers or 116 million miles) from Earth, or 470 times as far as the
> moon and 1.23 times as far as the sun today. Radio signals, traveling at
> the universal limit of the speed of light, take 21 minutes to make the
> round trip.
>
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