Dawn Observations in Low Altitude Orbit Continue Smoothly
Dawn spends most of its time measuring gamma rays and neutrons from Vesta so scientists can determine the abundances of elements in the material near the surface. The team is continuing to track the spacecraft in order to map Vesta's gravity field. Taking advantage of the low altitude, the imaging campaign continues to reveal new details of the surface.
The spacecraft performed another small adjustment to its orbit on December 24. These last two maneuvers were performed so accurately and the actual orbit is matching the predictions so well that the next two weekly adjustments have been canceled.
As the activities in the low altitude mapping orbit are intended to be highly repetitive, these mission status updates will be less frequent (thus saving your correspondent the effort of finding ways to restate what he wrote last week and this week). Updates will continue to be posted when events warrant and occasionally to reassure readers that the mission is continuing smoothly. More information about Dawn's work in this phase of the mission can be found in the Dawn Journals.
December 21, 2011
Dawn Continues Gathering Science Data
Dawn is continuing to gather gamma ray spectra and neutron spectra in its low altitude orbit. In addition, scientists and engineers are making accurate measurements of the spacecraft's orbit to determine Vesta's gravity field.
The bonus imaging is yielding pictures more than three times better than those acquired in the high altitude mapping orbit.
Every week at this low altitude, Dawn will use its ion propulsion system to fine tune its orbit. (The reasons for this will be explained in an upcoming Dawn Journal.) The first of these weekly orbit adjustments was performed on December 17.
December 13, 2011
Dawn Begins New Science Phase at Lowest Altitude
Dawn began a new set of science observations on schedule on Dec. 12 in its low altitude mapping orbit (LAMO) at an average altitude of 210 kilometers (130 miles). This phase will be the longest of the science campaigns at Vesta, lasting at least 10 weeks.
Dawn's investigations in this orbit will focus on measuring the elemental composition of the surface and subsurface material with the gamma ray and neutron detector and on mapping the interior structure by measuring Vesta's gravity field. In addition, the science camera and the visible and infrared mapping spectrometer will be used for some bonus observations. An overview of the plan for LAMO is in the Dawn Journal from Dec. 30, 2010, and further details will be in upcoming Dawn Journals.
The Dawn "image of the day" will take a break until Jan. 9. When it returns, there will be more spectacular views of this exotic world.
December 8, 2011
Dawn Reaches Planned Orbit Altitude
Dawn has reached its target altitude, averaging 210 kilometers (130 miles) above Vesta.
On Dec. 3, while the spacecraft was turning in preparation for a final brief period of ion thrusting to refine its orbit, protective software detected a discrepancy in the orientation and put the spacecraft into safe mode. The discrepancy was a result of the spacecraft computing and then using a turn rate that was slightly too high. The operations team quickly established that the spacecraft was healthy, and on Dec. 5 they commanded it out of safe mode.
Mission planners already had several more windows planned for ion thrusting in order to make further fine adjustments to the orbit. Lengthening one of those windows allows sufficient time to compensate for the safe mode.
Science observations in the new orbit are scheduled to begin on Dec. 12.
December 1, 2011
Dawn Continues Toward New Orbit
Dawn is getting close to its targeted altitude of 210 kilometers (135 miles). Today it is at an altitude of about 245 kilometers (145 miles). In addition to lowering its altitude, the spacecraft has changed the plane of its orbit most of the way to the new target as well, and it is now less than one degree from the intended value. (See the latest Dawn Journal for an explanation of the shift in the orbital plane.)
The spacecraft acquired more images of Vesta on Nov. 27.
Science observations in the new orbit are scheduled to begin on Dec. 12.
November 23, 2011
Dawn Progressing to New Orbit
Dawn is continuing to make good progress toward its next planned orbit. The spacecraft thrusts occasionally with its ion propulsion system, but most of the time it coasts, letting Vesta's gravity reorient its orbit.
The spacecraft acquired some images of Vesta on November 20 and 23.
November 17, 2011
Dawn Thrusting to a Lower Orbit
Dawn has been continuing to change its orbit with its ion propulsion system. Today the spacecraft reaches down to an altitude of about 265 kilometers (165 miles). While more ion thrusting lies ahead to reduce the altitude, the craft will spend most of the time before its next science phase begins on December 12 using Vesta's gravity to change the orientation of its orbit. (The target altitude and orientation and the method of achieving them will be explained in the next Dawn Journal.)
November 10, 2011
Dawn Lowering Orbital Altitude
Dawn is using its ion propulsion system to lower its orbital altitude. Today the spacecraft is at an altitude of about 400 kilometers (250 miles).
On November 7 thrusting was halted for the spacecraft to collect some images of Vesta.
November 2, 2011
Dawn Concludes Intensive Mapping
On October 31, Dawn completed the sixth and final mapping cycle. It spent another two days transmitting to Earth the last of the science data it had gathered. The successful completion of this mapping marks an extraordinary milestone in the exploration of Vesta. More information about this remarkable phase of the mission is in the October 31 Dawn Journal.
Today Dawn began thrusting with its ion propulsion system. It will take more than five weeks of maneuvering to reach the next science orbit.
October 26, 2011
Dawn Concludes Topographical Mapping
Today Dawn successfully concluded its fifth mapping cycle and the final one devoted to topographical mapping. It is now beginning the last mapping cycle of the high altitude mapping orbit. This sixth cycle is nearly identical to the first and was included in the plan in case there were problems with the the first. As it was successful, most of these observations will provide views very similar to the ones already returned.
October 21, 2011
Dawn Completes More Topographical Mapping
With its fourth mapping cycle complete, today Dawn began its fifth mapping cycle. This is the last one devoted to acquiring images for topography. The camera will mostly be pointed to the side as the spacecraft orbits Vesta.
October 16, 2011
Dawn Reaches Halfway Point in HAMO Mapping
Dawn has completed its third mapping cycle in the high altitude mapping orbit (HAMO). Now it is beginning the fourth cycle, in which the spacecraft points its camera ahead at a different angle from the second cycle. The images from the different viewing angles will be combined to create topographic maps.
Observations with the visible and infrared mapping spectrometer and the gamma-ray and neutron spectrometer continue as well.
The mapping cycles are conducted in the order of their importance, so the most valuable measurements in HAMO have now been completed successfully.
October 11, 2011
Dawn Gathers Images for Topographic Mapping
Dawn has completed its second mapping cycle. Instead of pointing the instruments straight down, as in the first cycle, the spacecraft aimed them at an angle, simultaneously ahead and to the side as it orbited Vesta. The third cycle will acquire images at a different angle, with the craft pointing both back and to the side. During this topographic mapping, spectra continue to be acquired with the visible and infrared mapping spectrometer.
Controllers working on the gamma-ray and neutron detector (GRaND) have eliminated the noise from the gamma-ray sensor. The instrument is healthy and operating normally.
October 5, 2011
First Mapping Cycle Completed
Dawn's mapping of Vesta from an altitude of about 680 kilometers (420 miles) is going very smoothly. It has completed its first cycle of 10 orbits in which the principal objective was to acquire images looking straight down. Today the spacecraft will begin a new cycle of observing the surface at an angle to provide images scientists will use to create topographic maps and stereo images. This mapping strategy is described in more detail in the September 27, 2010 Dawn Journal.
Scientists and engineers are investigating increased noise in one of the gamma-ray sensors in the gamma-ray and neutron detector (GRaND).
September 29, 2011
Dawn began its new phase of Vesta observations today. Circling Vesta every 12.3 hours at an average altitude of 680 kilometers (420 miles), it will take pictures and acquire visible and infrared spectra when it passes over the day side and transmit the data to Earth while over the night side. This intensive phase of the mission will last about a month. Additional details on this high altitude mapping orbit are in the September 27, 2010 Dawn Journal.
The maneuvers to adjust the orbit earlier this week were executed very smoothly. In between them, on Sept. 27, Dawn celebrated its fourth anniversary of being in space. Additional anniversary details are in the Dawn Journal for September 27, 2011.
September 26, 2011
Controllers have designed a pair of maneuvers with the ion propulsion system to adjust Dawn's orbit around Vesta to match it to the conditions needed for the planned observations in its new high altitude mapping orbit (HAMO). The first maneuver will be performed tonight and the second will be the next night.
The operations team has taken advantage of the period between the end of thrusting on Sept. 18 and the beginning of intensive science observations in HAMO to perform other activities. They have conducted bonus science measurements with all the instruments. In addition, they used the backup science camera to verify that it continues to be healthy. They have also performed further tests with the visible and infrared mapping spectrometer.
On Sept. 21, when the spacecraft was reconfiguring its memory in preparation for collecting science data in HAMO, a software function took longer than expected and caused the computer to reboot. As a result, Dawn entered safe mode. Controllers detected the condition that night, diagnosed the cause, and returned the spacecraft to its normal operating condition less than two days later.
September 20, 2011
On Sept. 18, Dawn successfully completed the majority of the planned ion thrusting needed to reach its new science orbit, the high altitude mapping orbit. Now navigators are measuring its orbital parameters precisely so they can design a final maneuver to ensure the spacecraft is in just the orbit needed to begin its intensive mapping observations next week. (An explanation of why the orbit needs adjustment can be found in the February 27, 2011 Dawn Journal.)
Dawn is orbiting at an altitude of about 680 kilometers (420 miles).
September 15, 2011
Dawn remains on course and on schedule as it uses its ion propulsion system to spiral to its next science orbit. The spacecraft begins the day today at an altitude of about 880 kilometers (550 miles) and ends it at 790 kilometers (490 miles).
September 8, 2011
Dawn's spiral flight from survey orbit to its next science orbit is going very smoothly. As it continues to lower its altitude, the spacecraft begins the day today at about 1550 kilometers (960 miles) and ends it at 1330 kilometers (830 miles). For more on the unusual nature of Dawn's method of changing orbits, visit the February 27, 2011 Dawn Journal.
September 1, 2011
Dawn completed survey orbit, returning more than 2,800 pictures covering the entire illuminated surface and over three million visible and infrared spectra, exceeding the objectives for this first scientific phase of the mission. The spectacular results will keep scientists busy (and happy!) for years. For more information on what Dawn accomplished in survey orbit, visit the September 1 Dawn Journal .
On August 31, the spacecraft resumed ion thrusting, and it will spend most of September spiraling down to the next science orbit, which is four times closer to Vesta than survey orbit.
August 24, 2011
Dawn has now acquired images and spectra through a total of five passes over the lit side of Vesta, and the quality of the data remains excellent. The visible and infrared mapping spectrometer (VIR) experienced an anomaly on the third orbit very similar to the one on the first orbit, and engineers and scientists from the instrument team in Italy and the operations team at JPL are continuing to work to understand it.
The principal scientific objective of survey orbit is to acquire 5000 sets of spectra with VIR. VIR has already returned well in excess of 8000. All other objectives for survey orbit have also been met, so this phase of the mission is set to conclude on schedule on August 31.
August 16, 2011
Dawn has observed Vesta during two passes over its illuminated face, each lasting about 34 hours. It acquired all the images planned with the camera. After the visible and infrared mapping spectrometer (VIR) had successfully completed about 40% of its observations during the first orbit, its internal computer detected a temporary problem, so the rest were not performed. As explained in the latest Dawn Journal, plans included collecting more data than needed, so missing the observations is easily tolerated. During the subsequent communications session (while Dawn was over the night side), controllers verified the instrument was healthy and reconfigured it. VIR acquired all of the planned measurements during the second orbit. Scientists have confirmed that the data are of very high quality.
August 11, 2011
Having successfully completed its approach phase, Dawn began its survey orbit observations today. (See the August 11, 2011 Dawn Journal for more on this. This is the first major science phase of the mission. During seven revolutions of 69 hours each, it will acquire images and spectra when it passes over the day side of Vesta and radio the results back to Earth over the night side.
During survey orbit, a new image will be posted here every day.
Dawn is in a circular polar orbit about 2700 kilometers (1700 miles) from Vesta.
August 7, 2011
On August 6, Dawn acquired the last of its images and spectra in the approach phase. Meanwhile, mission controllers are completing the adjustments to the command sequences to be used for survey orbit observations beginning on August 11 when the spacecraft passes from the night side of Vesta to the day side. For an overview of survey orbit plans, see the Dawn Journal from May 27, 2010.
Dawn is in a circular polar orbit about 2700 kilometers (1700 miles) from Vesta.
August 2, 2011
Today Dawn completed the ion thrusting needed to reach its first science orbit. Now the operations team will refine measurements of its orbit and use that information to make final adjustments to the timing and other details of the commands the spacecraft will follow when it begins survey orbit observations on August 11.
On July 31, it paused thrusting to collect more images of Vesta.
Dawn is about 2700 kilometers (1700 miles) from Vesta.
July 26, 2011
Dawn stopped ion thrusting on July 22 at an altitude of about 5200 kilometers (3200 miles) to begin its most intensive observations of the entire approach phase. Over the subsequent three days, the camera and the visible and infrared mapping spectrometer observed Vesta from northern latitudes to the south pole. Spectacular results will be unveiled on August 1.
Thrusting to spiral down to survey orbit is scheduled to resume on July 28.
July 21, 2011
During its arc over the night side of Vesta, Dawn passed over the equator on July 20, still thrusting with its ion propulsion system. The angle of its orbit prevents the spacecraft from entering the shadow of Vesta.
During a communications session on July 20, engineers powered on the control unit that stopped operating valves on June 27. It had been off since then, with a different unit controlling the flow of xenon to the ion thruster. As they anticipated, the unit is now perfectly healthy, and it flawlessly completed all the tests they gave it. All evidence points to its temporary inability to control the valves as having been a result of a high energy particle of space radiation striking it. By turning it off and on again, the glitch was cleared. See the July 18, 2011 Dawn Journal for more details.
Dawn is about 6000 kilometers (3700 miles) from Vesta today and approaching it at 10 meters per second (22 mph).
July 19, 2011
Even as it orbits around Vesta, Dawn is continuing to thrust with its ion propulsion system just as it did during its years of interplanetary cruise around the sun. The spacecraft is gradually spiraling down toward its survey orbit. As it began arcing over the south pole and heading toward the dark side of Vesta, Dawn acquired another set of images on July 18.
Dawn is about 9000 kilometers (6200 miles) from Vesta today and approaching it at 19 meters per second (43 mph).
July 17, 2011
As Dawn continued thrusting, it was gently captured in orbit by Vesta around 10:00 PM PDT on July 15. Navigational analysis will be required to determine the exact time of capture. On July 16 it observed Vesta again.
Dawn is about 13,000 kilometers (8,000 miles) from Vesta today and approaching it at 23 meters per second (51 mph).
July 13, 2011
Dawn remains on course as it continues ion thrusting to reach its first science orbit in August. On July 12 it collected another set of Vesta images for navigation.
Dawn is about 23,000 kilometers (14,000 miles) from Vesta today and approaching it at 37 meters per second (83 mph).
July 10, 2011
After thrusting all week with its ion propulsion system, on July 9-10 Dawn acquired more images of Vesta. For the second time, it observed Vesta during one complete rotation of 5 hours, 20 minutes. The camera also acquired images of the space around Vesta to allow scientists to search for moons of the protoplanet.
Dawn is about 35,000 kilometers (22,000 miles) from Vesta today and approaching it at 50 meters per second (110 mph).
July 4, 2011
During a routine communications session on July 3, engineers verified that Dawn resumed thrusting, as instructed, on June 30. The spacecraft's approach phase is continuing, as the spacecraft spends most of the time thrusting with its ion propulsion system to bring its orbit closer to that of Vesta. It successfully acquired another set of navigation images on July 3.
Dawn is 65,000 kilometers (40,000 miles) from Vesta today and approaching it at 75 meters per second (170 mph).
July 1, 2011
On June 29-30, Dawn observed Vesta throughout one full rotation of the protoplanet on its axis, which takes 5 hours 20 minutes. Both the camera and the visible and infrared mapping spectrometer (VIR) were used. After VIR had successfully completed some of its observations, its internal computer detected a temporary problem with the sensor, so the rest of its observations were not executed. Mission controllers subsequently verified that VIR is otherwise healthy.
On June 27, an electrical circuit that controls valves in the system that feeds xenon propellant to the ion thruster stopped operating. Software detected the anomaly, halted the thrust, and canceled other planned activities to point the main antenna to Earth. Engineers discovered it during a routine communications session on June 28. After assessing the state of the spacecraft, they sent instructions to switch to a different valve control circuit and resume thrusting on June 30 following the receipt of the data from the Vesta observations. The change in the thrust schedule does not affect the plan for the intensive Vesta science observation phase, which begins in August.
Dawn is 85,000 kilometers (53,000 miles) from Vesta today and approaching it at 90 meters per second (200 mph).
June 24, 2011
Now more than halfway through the approach phase, Dawn completed another week of ion thrusting. It stopped thrusting twice this week to collect images for navigation.
Dawn is 145,000 kilometers (90,000 miles) from Vesta today and approaching it at 110 meters per second (250 mph).
June 17, 2011
While still devoting most of its time in the approach phase to ion thrusting, Dawn now is observing Vesta more often. It halted thrusting on June 14 for navigation images and more test observations with the visible and infrared mapping spectrometer to prepare for later science observations. Today, it acquired another set of navigation images with the camera.
Dawn is 225,000 kilometers (140,000 miles) from Vesta today and approaching it at 140 meters per second (310 mph)..
Continuing its weekly pattern, Dawn used its camera to image Vesta again today. In addition, the visible and infrared mapping spectrometer observed Vesta to help scientists choose proper instrument settings for when Dawn is close enough to begin its scientific measurements.
On June 6, Dawn's distance to Vesta was the same as Earth's distance to the moon. With the gradual nature of the approach inherent in the use of ion propulsion, the distance will continue to decrease slowly.
Dawn is 350,000 kilometers (220,000 miles) from Vesta today and approaching it at 190 meters per second (420 mph).
June 1, 2011
Dawn continues to devote most of its time to thrusting with its ion propulsion system. Today it conducted its fifth session of acquiring images of Vesta for use in navigation.
Dawn is 470,000 kilometers (290,000 miles) from Vesta today and approaching it at 220 meters per second (490 mph).
May 24, 2011
Dawn imaged Vesta again today, providing more data for navigators to use in refining the approach trajectory. As in interplanetary cruise, the majority of the week is devoted to ion thrusting.
Dawn is 640,000 kilometers (400,000 miles) from Vesta today and approaching it at 260 meters per second (580 mph).
May 17, 2011
Dawn acquired another set of images of Vesta today. Navigators use the pictures of Vesta and the background stars to refine their determination of the spacecraft's trajectory. The results will be incorporated into updates to the plan for thrusting with the ion propulsion system. Meanwhile, the spacecraft is continuing to make good progress, spending most of its time thrusting.
Dawn is 810,000 kilometers (500,000 miles) from Vesta today and approaching it at 300 meters per second (670 mph).
May 10, 2011
The approach phase is going smoothly. Dawn stopped thrusting today for its second opportunity to acquire images of Vesta for use in navigation. In addition, the visible and infrared mapping spectrometer observed Vesta for the first time. The results will be used to set instrument parameters for subsequent measurements when the spacecraft is closer to its target.
Dawn is one million kilometers (620,000 miles) from Vesta today and approaching it at 340 meters per second (760 mph).
May 3, 2011
Today Dawn concluded its interplanetary cruise phase and started the Vesta mission phase. The beginning of the approach phase is marked by the spacecraft's first pictures of Vesta. While Vesta appears very small at this distance (3.2 times the average distance between the Earth and moon), measuring its location against the background stars will help navigators pin down the position of Dawn relative to Vesta. Navigation images will be taken weekly until the middle of June, when the rate will increase.
The gamma-ray and neutron detector (GRaND) was powered on today, and it will remain on until Dawn departs Vesta in July 2012.
Other reconfigurations were made on the spacecraft as well, including turning the reaction wheels on.During the approach phase, Dawn will continue to thrust most of the time with its ion propulsion system. It will enter orbit on July 16 and continue thrusting as it spirals to its first science orbit. The phase will end in the second week of August when survey orbit begins.
April 30, 2011
As usual, Dawn thrust for most of the month, bringing its orbit around the sun still closer to Vesta's orbit. April 11 to 19 was planned as a coast period so controllers could load a new version of software to the spacecraft's main computer. The new software would allow the spacecraft to operate with only two reaction wheels in the event one of the three healthy wheels becomes inoperable. This was the fourth time the software has been upgraded in flight, and the entire procedure was executed flawlessly. The complex work was completed in only three days.
Dawn is now only 3.4 times farther from Vesta than the moon is from Earth. The spacecraft is approaching Vesta at less than 0.39 kilometers per second (870 mph).
March 31, 2011
Dawn spent most of the month thrusting toward its Vesta rendezvous but stopped for the week of March 14 in order to perform several final tests. All of the science instruments were powered on and checked out, and each one proved to be in excellent condition. In addition, controllers updated the software in the two identical science cameras. One of the gamma-ray sensors was carefully heated for five days to restore its sensitivity after years of exposure to radiation in space.
To refine their measurements of the thrust from the ion propulsion system, engineers commanded Dawn to align a thruster with Earth. They stepped the thruster through 13 of the throttle levels the spacecraft will use at Vesta while measuring the Doppler shift of the spacecraft's radio signal. These measurements show the change in the radio frequency caused by the change in the spacecraft's velocity, thus revealing the thrust. This procedure was performed with two of the thrusters to allow navigators to develop accurate flight profiles for maneuvering from one science orbit at Vesta to another.
February 28, 2011
Mission controllers conducted another test of the spacecraft's capability to operate in orbit around the giant asteroid. It executed some portions of the maneuvers it will perform in order to thrust along a spiral path from its high altitude mapping orbit to its low altitude mapping orbit. The probe's performance was very good and provided a valuable verification of its capabilities as engineers continue to prepare for Vesta operations.
January 31, 2011
In addition to thrusting most of this month, the spacecraft executed a rehearsal of one of the activities it will need to perform in its low altitude mapping orbit at Vesta. The test successfully demonstrated that even during the times that Vesta blocks the starlight used by a sensor to help orient the ship in the zero-gravity of spaceflight, an alternate kind of sensor can be used.
The operations team spent a week handling simulated problems during the approach phase to Vesta. The exercise included a wide variety of creative complications, including hypothesized damage to the spacecraft, unexpected characteristics of Vesta, loss of some planned Deep Space Network coverage, and unavailability of two members of the team. This "operational readiness test" is an important component of preparing for real operations.
After more than 2.2 years of powered flight, Dawn has now expended more than half of its original supply of 425 kilograms (937 pounds) of xenon propellant for its ion propulsion system. That has been enough to change the velocity by about 5.7 kilometers per second (nearly 13,000 mph), far more than any other spacecraft's propulsion system has achieved.