Dawn (spacecraft)
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Dawn is a retired space probe that was launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt: Vesta and Ceres.<ref name="NASA-20180907" /> In the fulfillment of that mission—the ninth in NASA's Discovery Program—Dawn entered orbit around Vesta on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012.<ref name=5SepDepart>Template:Cite news</ref><ref name=Aug262012>Template:Cite web</ref> It entered orbit around Ceres on March 6, 2015.<ref name="NASA-20150306">Template:Cite web</ref><ref name="Rayman2015.03.06">Template:Cite web</ref> In 2017, NASA announced that the planned nine-year mission would be extended until the probe's hydrazine fuel supply was depleted.<ref name="NASA-20171019">Template:Cite web</ref> On November 1, 2018, NASA announced that Dawn had depleted its hydrazine, and the mission was ended. The derelict probe remains in a stable orbit around Ceres.<ref>Template:Cite news</ref>
Dawn is the first spacecraft to have orbited two extraterrestrial bodies,<ref name=siliconvalleyastrolecture>Template:Cite speech</ref> the first spacecraft to have visited either Vesta or Ceres, and the first to have orbited a dwarf planet.<ref>Template:Cite book</ref>
The Dawn mission was managed by NASA's Jet Propulsion Laboratory, with spacecraft components contributed by European partners from Italy, Germany, France, and the Netherlands.<ref name="amspace20171008">Template:Cite news</ref> It was the first NASA exploratory mission to use ion propulsion, which enabled it to enter and leave the orbit of two celestial bodies. Previous multi-target missions using rockets powered by chemical engines, such as the Voyager program, were restricted to flybys.<ref name="Rayman 2006 605–616">Template:Cite journal</ref>
Project history
Technological background
The first working ion thruster in the US was built by Harold R. Kaufman in 1959 at NASA's Glenn Research Center in Ohio. The thruster was similar to the general design of a gridded electrostatic ion thruster with mercury as its propellant. Suborbital tests of the engine followed during the 1960s, and in 1964 the engine was tested on a suborbital flight aboard the Space Electric Rocket Test 1 (SERT 1). It successfully operated for the planned 31 minutes before falling back to Earth.<ref name="Glenn">Template:Cite web</ref> This test was followed by an orbital test, SERT-2, in 1970.
Deep Space 1 (DS1), which NASA launched in 1998, demonstrated the long-duration use of a xenon-propelled ion thruster on a science mission,<ref name="MD">Template:Cite journal</ref> and validated a number of technologies, including the NSTAR electrostatic ion thruster, as well as performing a flyby of an asteroid and a comet.<ref name="Dawn">Template:Cite web</ref> In addition to the ion thruster, among the other technologies validated by the DS1 was the Small Deep Space Transponder, which is used on Dawn for long-range communication.<ref name="Dawn" />
Discovery Program selection
Twenty-six proposals were submitted to the Discovery Program solicitation, with budget initially targeted at US$300 million.<ref name="autogenerated3"/> Three semi-finalists were downselected in January 2001 for a phase-A design study: Dawn, Kepler, and INSIDE Jupiter.<ref>Template:Cite web</ref> In December 2001 NASA selected the Kepler and the Dawn mission for the Discovery program.<ref name="autogenerated3">Template:Cite web</ref> Both missions were initially selected for a launch in 2006.<ref name="autogenerated3"/>
Cancellation and reinstatement
The status of the Dawn mission changed several times. The project was cancelled in December 2003,<ref>Template:Cite news</ref> and then reinstated in February 2004. In October 2005, work on Dawn was placed in "stand down" mode, and in January 2006, the mission was discussed in the press as "indefinitely postponed", even though NASA had made no new announcements regarding its status.<ref>Template:Cite news</ref> On March 2, 2006, Dawn was again cancelled by NASA.<ref>Template:Cite web</ref>
The spacecraft's manufacturer, Orbital Sciences Corporation, appealed NASA's decision, offering to build the spacecraft at cost, forgoing any profit in order to gain experience in a new market field. NASA then put the cancellation under review,<ref>Template:Cite news</ref> and on March 27, 2006, it was announced that the mission would not be cancelled after all.<ref>Template:Cite web</ref><ref>Template:Cite news</ref> In the last week of September 2006, the Dawn mission's instrument payload integration reached full functionality. Although originally projected to cost US$373 million, cost overruns inflated the final cost of the mission to US$446 million in 2007.<ref>Template:Cite web</ref> Christopher T. Russell was chosen to lead the Dawn mission team.
Scientific background
The Dawn mission was designed to study two large bodies in the asteroid belt in order to answer questions about the formation of the Solar System, as well as to test the performance of its ion thrusters in deep space.<ref name="NASA-20180907">Template:Cite web</ref> Ceres and Vesta were chosen as two contrasting protoplanets, the first one apparently "wet" (i.e. icy and cold) and the other "dry" (i.e. rocky), whose accretion was terminated by the formation of Jupiter. The two bodies provide a bridge in scientific understanding between the formation of rocky planets and the icy bodies of the Solar System, and under what conditions a rocky planet can hold water.<ref>Template:Cite web</ref>
The International Astronomical Union (IAU) adopted a new definition of planet on August 24, 2006, which introduced the term "dwarf planet" for ellipsoidal worlds that were too small to qualify for planetary status by "clearing their orbital neighborhood" of other orbiting matter. Dawn is the first mission to study a dwarf planet, arriving at Ceres a few months before the arrival of the New Horizons probe at Pluto in July 2015.
Ceres comprises a third of the total mass of the asteroid belt. Its spectral characteristics suggest a composition similar to that of a water-rich carbonaceous chondrite.<ref>Template:Cite journal</ref> Vesta, a smaller, water-poor achondritic asteroid comprising a tenth of the mass of the asteroid belt, has experienced significant heating and differentiation. It shows signs of a metallic core, a Mars-like density and lunar-like basaltic flows.<ref name=SolarViewsCalvin>Template:Cite web</ref>
Available evidence indicates that both bodies formed very early in the history of the Solar System, thereby retaining a record of events and processes from the time of the formation of the terrestrial planets. Radionuclide dating of pieces of meteorites thought to come from Vesta suggests that Vesta differentiated quickly, in three million years or less. Thermal evolution studies suggest that Ceres must have formed some time later, more than three million years after the formation of CAIs (the oldest known objects of Solar System origin).<ref name=SolarViewsCalvin/>
Moreover, Vesta appears to be the source of many smaller objects in the Solar System. Most (but not all) V-type near-Earth asteroids, and some outer main-belt asteroids, have spectra similar to Vesta, and are thus known as vestoids. Five percent of the meteoritic samples found on Earth, the howardite–eucrite–diogenite (HED) meteorites, are thought to be the result of a collision or collisions with Vesta.
It is thought that Ceres may have a differentiated interior;<ref name="Thomas">Template:Cite journal</ref> its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.<ref name=Thomas/> There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 1,400 HED meteorites,<ref name="Meteoritical Bulletin Database">Template:Cite web</ref> giving insight into Vesta geologic history and structure. Vesta is thought to consist of a metallic iron–nickel core, an overlying rocky olivine mantle and crust.<ref>Template:Cite journal</ref><ref name="Sahijpal2007">Template:Cite journal</ref><ref name="Gupta2010">Template:Cite journal</ref> Template:Gallery
Objectives
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The Dawn mission's goal was to characterize the conditions and processes of the Solar System's earliest eon by investigating in detail two of the largest protoplanets remaining intact since their formation.<ref name="NASA-20180907" /><ref name="Objectives">Template:Cite news</ref>
Although the mission has finished, the data analyses and interpretations will continue for many years. The primary question that the mission addresses is the role of size and water in determining the evolution of the planets.<ref name="Objectives"/> Ceres and Vesta are highly suitable bodies with which to address this question, as they are two of the most massive of the protoplanets. Ceres is geologically very primitive and icy, while Vesta is evolved and rocky. Their contrasting characteristics are thought to have resulted from them forming in two different regions of the early Solar System.<ref name="Objectives"/>
There are three principal scientific drivers for the mission. First, the Dawn mission can capture the earliest moments in the origin of the Solar System, granting an insight into the conditions under which these objects formed. Second, Dawn determines the nature of the building blocks from which the terrestrial planets formed, improving scientific understanding of this formation. Finally, it contrasts the formation and evolution of two small planets that followed very different evolutionary paths, allowing scientists to determine what factors control that evolution.<ref name="Objectives"/>
Instruments
NASA's Jet Propulsion Laboratory provided overall planning and management of the mission, the flight system and scientific payload development, and provided the ion propulsion system. Orbital Sciences Corporation provided the spacecraft, which constituted the company's first interplanetary mission. The Max Planck Institute for Solar System Research and the German Aerospace Center (DLR) provided the framing cameras, the Italian Space Agency provided the mapping spectrometer, and the Los Alamos National Laboratory provided the gamma ray and neutron spectrometer.<ref name="Rayman 2006 605–616" />
- Framing camera (FC) – Two redundant framing cameras were flown. Each used a f/7.9 refractive optical system with a focal length of 150 mm.<ref name="Dawn Spacecraft and Instruments">Template:Cite web</ref><ref name=sierks>Template:Cite conference</ref> A frame-transfer charge-coupled device (CCD), a Thomson TH7888A,<ref name=sierks /> at the focal plane has 1024 × 1024 sensitive 93-μrad pixels, imaging a 5.5° x 5.5° field of view. An 8-position filter wheel permits panchromatic (clear filter) and spectrally selective imaging (7 narrow band filters). The broadest filter allows imaging at wavelengths from 400 to 1050 nm. The FC computer is a custom radiation-hardened Xilinx system with a LEON2 core and 8 GiB of memory.<ref name=sierks /> The camera offered resolutions of 17 m/pixel for Vesta and 66 m/pixel for Ceres.<ref name=sierks /> Because the framing camera was vital for both science and navigation, the payload had two identical and physically separate cameras (FC1 & FC2) for redundancy, each with its own optics, electronics, and structure.<ref name="Rayman 2006 605–616" /><ref>Template:Cite journal</ref>
- Visible and infrared spectrometer (VIR) – This instrument is a modification of the visible and infrared thermal-imaging spectrometer used on the Rosetta and Venus Express spacecraft. It draws its heritage from the Saturn orbiter [[Cassini–Huygens|CassiniTemplate:'s]] visible and infrared mapping spectrometer. The spectrometer's VIR spectral frames are 256 (spatial) × 432 (spectral), and the slit length is 64 mrad. The mapping spectrometer incorporates two channels, both fed by a single grating. A CCD yields frames from 0.25 to 1.0 μm, while an array of HgCdTe photodiodes cooled to about 70 K spans the spectrum from 0.95 to 5.0 μm.<ref name="Rayman 2006 605–616" /><ref>Template:Cite journal</ref>
- Gamma Ray and Neutron Detector (GRaND)<ref>T.H. Prettyman Gamma Ray and Neutron Spectrometer for Dawn, Lunar and Planetary Science XXXVII (2006), abstract 2231</ref> – This instrument is based on similar instruments flown on the Lunar Prospector and Mars Odyssey space missions. It had 21 sensors with a very wide field of view.<ref name="Dawn Spacecraft and Instruments" /> It was used to measure the abundances of the major rock-forming elements (oxygen, magnesium, aluminium, silicon, calcium, titanium, and iron) and potassium, thorium, uranium, and water (inferred from hydrogen content) in the top 1 m of the surface of Vesta and Ceres.<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
A magnetometer and laser altimeter were considered for the mission, but were not ultimately flown.<ref>Template:Cite news</ref>
Specifications
Dimensions
With its solar array in the retracted launch position, the Dawn spacecraft is Template:Convert wide. With its solar arrays fully extended, Dawn is Template:Convert wide.<ref>Template:Cite web</ref> The solar arrays have a total area of Template:Convert.<ref>Template:Cite web</ref> The main antenna is five feet (1.24 metres) in diameter.<ref name=siliconvalleyastrolecture />
Propulsion system
The Dawn spacecraft was propelled by three xenon ion thrusters derived from NSTAR technology used by the Deep Space 1 spacecraft,<ref>Template:Cite web</ref> using one at a time. They have a specific impulse of 3,100 s and produce a thrust of 90 mN.<ref>Template:Cite web</ref> The whole spacecraft, including the ion propulsion thrusters, was powered by a 10 kW (at 1 AU) triple-junction gallium arsenide photovoltaic solar array manufactured by Dutch Space.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> Dawn was allocated Template:Convert of xenon for its Vesta approach, and carried another Template:Convert to reach Ceres,<ref name="GarnerEtAl">Template:Cite conference</ref> out of a total capacity of Template:Convert of on-board propellant.<ref>Template:Cite web</ref> With the propellant it carried, Dawn was able to perform a velocity change of approximately 11 km/s over the course of its mission, far more than any previous spacecraft achieved with onboard propellant after separation from its launch rocket.<ref name="GarnerEtAl"/> However, the thrust was very gentle; it took four days at full throttle to accelerate Dawn from zero to 60 mph (96 km/h).<ref name=siliconvalleyastrolecture /> Dawn is NASA's first purely exploratory mission to use ion propulsion engines.<ref>Template:Cite web</ref> The spacecraft also has twelve 0.9 N hydrazine thrusters for attitude control (orientation), which were also used to assist in orbital insertion.<ref>Template:Cite web</ref>
The Dawn spacecraft was able to achieve a record-breaking level of propulsion from its ion engine.<ref>Template:Cite web</ref> NASA noted three specific areas of excellence:<ref>Template:Cite web</ref>
- First to orbit two different astronomical bodies (not including Earth).
- Solar-electric propulsion record, including a velocity change in space of 25,700 mph (11.49 km/s). This is 2.7 times the velocity change by solar-electric ion drive than the past record.
- Achieved 5.9 years of ion engine runtime by September 7, 2018. This amount of runtime equates to 54% of Dawn's time in outer space.
Outreach microchip
Dawn carries a memory chip bearing the names of more than 360,000 space enthusiasts.<ref>Template:Cite web</ref> The names were submitted online as part of a public outreach effort between September 2005 and November 4, 2006.<ref>Template:Cite web</ref> The microchip, which is two centimetres in diameter, was installed on May 17, 2007, above the spacecraft's forward ion thruster, underneath its high-gain antenna.<ref>Template:Cite web</ref> More than one microchip was made, with a back-up copy put on display at the 2007 Open House event at the Jet Propulsion Laboratory in Pasadena, California.
Mission summary
Launch preparations
On April 10, 2007, the spacecraft arrived at the Astrotech Space Operations subsidiary of SPACEHAB, Inc. in Titusville, Florida, where it was prepared for launch.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> The launch was originally scheduled for June 20, but was delayed until June 30 due to delays with part deliveries.<ref>Template:Cite web</ref> A broken crane at the launch pad, used to raise the solid rocket boosters, further delayed the launch for a week, until July 7; prior to this, on June 15, the second stage was successfully hoisted into position.<ref>Template:Cite web</ref> A mishap at the Astrotech Space Operations facility, involving slight damage to one of the solar arrays, did not have an effect on the launch date; however, bad weather caused the launch to slip to July 8. Range tracking problems then delayed the launch to July 9, and then July 15. Launch planning was then suspended in order to avoid conflicts with the Phoenix mission to Mars, which was successfully launched on August 4.
Launch
The launch of Dawn was rescheduled for September 26, 2007,<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite book</ref> then September 27, due to bad weather delaying fueling of the second stage, the same problem that delayed the July 7 launch attempt. The launch window extended from 07:20–07:49 EDY (11:20–14:49 GMT).<ref>Template:Cite web</ref> During the final built-in hold at T−4 minutes, a ship entered the exclusion area offshore, the strip of ocean where the rocket boosters were likely to fall after separation. After commanding the ship to leave the area, the launch was required to wait for the end of a collision avoidance window with the International Space Station.<ref>Template:Cite web</ref> Dawn finally launched from Space Launch Complex 17B at the Cape Canaveral Air Force Station on a Delta 7925-H rocket<ref>Template:Cite web</ref> at 07:34 EDT,<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref> reaching escape velocity with the help of a spin-stabilized solid-fueled third stage.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> Thereafter, Dawn's ion thrusters took over.
Transit to Vesta
After initial testing, during which the ion thrusters accumulated more than 11 days 14 hours of operation, Dawn began long-term cruise propulsion on December 17, 2007.<ref>Template:Cite web</ref> On October 31, 2008, Dawn completed its first thrusting phase to send it on to Mars for a gravity assist flyby in February 2009. During this first interplanetary cruise phase, Dawn spent 270 days, or 85% of this phase, using its thrusters. It expended less than 72 kilograms of xenon propellant for a total change in velocity of 1.81 km/s. On November 20, 2008, Dawn performed its first trajectory correction maneuver (TCM1), firing its number 1 thruster for 2 hours, 11 minutes.
Dawn made its closest approach (549 km) to Mars on February 17, 2009, during a successful gravity assist.<ref>Template:Cite web</ref><ref>Template:Cite news</ref> This flyby slowed Mars's orbital velocity by about Template:Convert per 180 million years.<ref name=siliconvalleyastrolecture /> On this day, the spacecraft placed itself in safe mode, resulting in some data acquisition loss. The spacecraft was reported to be back in full operation two days later, with no impact on the subsequent mission identified. The root cause of the event was reported to be a software programming error.<ref name=jplstatus>Template:Cite web</ref>
To cruise from Earth to its targets, Dawn travelled in an elongated outward spiral trajectory. The actual Vesta chronology and estimatedTemplate:Update inline Ceres chronology are as follows:<ref name=GSpaceTimeline>Template:Cite web</ref>
- September 27, 2007: launch
- February 17, 2009: Mars gravity assist
- July 16, 2011: Vesta arrival and capture
- August 11–31, 2011: Vesta survey orbit
- September 29 – November 2, 2011: Vesta first high altitude orbit
- December 12, 2011 – May 1, 2012: Vesta low altitude orbit
- June 15 – July 25, 2012: Vesta second high altitude orbit
- September 5, 2012: Vesta departure
- March 6, 2015: Ceres arrival
- June 30, 2016: End of primary Ceres operations
- July 1, 2016: Beginning of Ceres extended mission<ref name="sfnow20160702">Template:Cite news</ref>
- November 1, 2018: End of Mission
Vesta approach
As Dawn approached Vesta, the Framing Camera instrument took progressively higher-resolution images, which were published online and at news conferences by NASA and MPI.
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On May 3, 2011, Dawn acquired its first targeting image, 1,200,000 km from Vesta, and began its approach phase to the asteroid.<ref>"NASA's Dawn Captures First Image of Nearing Asteroid" Template:Webarchive. NASA. May 11, 2011. Retrieved September 1, 2012.</ref> On June 12, Dawn's speed relative to Vesta was slowed in preparation for its orbital insertion 34 days later.<ref>Template:Cite web</ref><ref>"View of Vesta from Dawn". NASA/JPL MYSTIC simulator (updated periodically). Retrieved September 1, 2012.</ref>
Dawn was scheduled to be inserted into orbit at 05:00 UTC on July 16 after a period of thrusting with its ion engines. Because its antenna was pointed away from the Earth during thrusting, scientists were not able to immediately confirm whether or not Dawn successfully made the maneuver. The spacecraft would then reorient itself, and was scheduled to check in at 06:30 UTC on July 17.<ref>Template:Cite news</ref> NASA later confirmed that it received telemetry from Dawn indicating that the spacecraft successfully entered orbit around Vesta, making it the first spacecraft to orbit an object in the asteroid belt.<ref>Template:Cite news</ref><ref>Template:Cite book</ref> The exact time of insertion could not be confirmed, since it depended on Vesta's mass distribution, which was not precisely known and at that time had only been estimated.<ref>Template:Cite news</ref>
Vesta orbit
After being captured by Vesta's gravity and entering its orbit on July 16, 2011,<ref name="sciencemag.org">Template:Cite journal</ref> Dawn moved to a lower, closer orbit by running its xenon-ion engine using solar power. On August 2, it paused its spiralling approach to enter a 69-hour survey orbit at an altitude of Template:Convert. It assumed a 12.3-hour high-altitude mapping orbit at Template:Convert on September 27, and finally entered a 4.3-hour low-altitude mapping orbit at Template:Convert on December 8.<ref name="status2011">Template:Cite web</ref><ref>Template:Cite journal</ref><ref>Template:Cite conference</ref>
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In May 2012, the Dawn team published preliminary results of their study of Vesta, including estimates of the size of Vesta's metal-rich core, which is theorized to be Template:Convert across. The scientists stated that they think that Vesta is the "last of its kind" – the only remaining example of the large planetoids that came together to form the rocky planets during the formation of the Solar System.<ref name="sciencemag.org" /><ref>Template:Cite news</ref> In October 2012, further Dawn results were published, on the origin of anomalous dark spots and streaks on Vesta's surface, which were likely deposited by ancient asteroid impacts.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite news</ref> In December 2012, it was reported that Dawn had observed gullies on the surface of Vesta that were interpreted to have been eroded by transiently flowing liquid water.<ref>Template:Cite conference</ref><ref>Template:Cite news</ref> More details about the Dawn mission's scientific discoveries at Vesta are included on the Vesta page.
Dawn was originally scheduled to depart Vesta and begin its two and a half year journey to Ceres on August 26, 2012.<ref name="Aug262012" /> However, a problem with one of the spacecraft's reaction wheels forced Dawn to delay its departure from Vesta's gravity until September 5, 2012.<ref name="5SepDepart" /><ref name="NewSci2012">Template:Cite news</ref><ref>Template:Cite web</ref><ref name="DawnDepartedVesta">Template:Cite web</ref><ref name="Yahoo Dawn Departs Vesta">Template:Cite news</ref>
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Central Mound at the South Pole on the asteroid Vesta on August 12, 2011
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The snowman shaped craters on Vesta
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Craters and ridges of Vesta
| Geologic map of Vesta based on Dawn data<ref name="Williams2014">Template:Cite journal</ref> |
|---|
| File:PIA18788-VestaAsteroid-GeologicMap-DawnMission-20141117.jpg
The most ancient and heavily cratered regions are brown; areas modified by the Veneneia and Rheasilvia impacts are purple (the Saturnalia Fossae Formation, in the north)<ref name="Scully2014">Template:Cite journal</ref> and light cyan (the Divalia Fossae Formation, equatorial),<ref name = "Williams2014"/> respectively; the Rheasilvia impact basin interior (in the south) is dark blue, and neighboring areas of Rheasilvia ejecta (including an area within Veneneia) are light purple-blue;<ref name="Schäfer2014">Template:Cite journal</ref><ref name="Kneissl2014">Template:Cite journal</ref> areas modified by more recent impacts or mass wasting are yellow/orange or green, respectively. |
Transit to Ceres
| Date | distance (km) |
diameter (px) |
resolution (km/px) |
portion of disk illuminated |
|---|---|---|---|---|
| December 1 | 1,200,000 | 9 | 112 | 94% |
| January 13 | 383,000 | 27 | 36 | 95% |
| January 25 | 237,000 | 43 | 22 | 96% |
| February 3 | 146,000 | 70 | 14 | 97% |
| February 12 | 83,000 | 122 | 7.8 | 98% |
| February 19 | 46,000 | 222 | 4.3 | 87% |
| February 25 | 40,000 | 255 | 3.7 | 44% |
| March 1 | 49,000 | 207 | 4.6 | 23% |
| April 10 | 33,000 | 306 | 3.1 | 17% |
| April 15 | 22,000 | 453 | 2.1 | 49% |
During its time in orbit around Vesta, the probe experienced several failures of its reaction wheels. Investigators planned to modify their activities upon arrival at Ceres for close range geographical survey mapping. The Dawn team stated that they would orient the probe using a "hybrid" mode utilizing both reaction wheels and ion thrusters. Engineers determined that this hybrid mode would conserve fuel. On November 13, 2013, during the transit, in a test preparation, Dawn engineers completed a 27-hour-long series of exercises of said hybrid mode.<ref>Template:Cite web</ref>
On September 11, 2014, DawnTemplate:'s ion thruster unexpectedly ceased firing and the probe began operating in a triggered safe mode. To avoid a lapse in propulsion, the mission team hastily exchanged the active ion engine and electrical controller with another. The team stated that they had a plan in place to revive this disabled component later in 2014. The controller in the ion propulsion system may have been damaged by a high-energy particle. Upon exiting the safe mode on September 15, 2014, the probe's ion thruster resumed normal operation.<ref name=SafeMode2014>Template:Cite web</ref>
Furthermore, the Dawn investigators also found that, after the propulsion issue, Dawn could not aim its main communications antenna towards Earth. Another antenna of weaker capacity was instead temporarily retasked. To correct the problem, the probe's computer was reset and the aiming mechanism of the main antenna was restored.<ref name=SafeMode2014/>
Ceres approach
Dawn began photographing an extended disk of Ceres on December 1, 2014,<ref name=dawnjournal1214>Template:Cite web</ref> with images of partial rotations on January 13 and 25, 2015 released as animations. Images taken from Dawn of Ceres after January 26, 2015, exceeded the resolution of comparable images from the Hubble Space Telescope.<ref>Template:Cite web</ref>
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Because of the failure of two reaction wheels, Dawn made fewer camera observations of Ceres during its approach phase than it did during its Vesta approach. Camera observations required turning the spacecraft, which consumed precious hydrazine fuel. Seven optical navigation photo sessions (OpNav 1–7, on January 13 and 25, February 3 and 25, March 1, and April 10 and 15) and two full rotation observation sessions (RC1–2, on February 12 and 19) were plannedTemplate:Update inline before full observation begins with orbital capture. The gap in March and early April was due to a period when Ceres appears too close to the Sun from DawnTemplate:'s vantage point to take pictures safely.<ref>Template:Cite web</ref>
Ceres orbit
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| Orbit phase | No. | Dates<ref name="PS-20150730">Template:Cite web</ref> | Altitude (km; mi)<ref>Distance from surface, not radius of the orbit</ref> |
Orbital period | Resolution (km/px) |
Improvement over Hubble |
Notes |
|---|---|---|---|---|---|---|---|
| RC3 | 1st | April 23 – May 9, 2015 | Template:Convert | 15 days | 1.3 | 24× | |
| Survey | 2nd | June 6–30, 2015 | Template:Convert | 3.1 days | 0.41 | 73× | |
| HAMO | 3rd | August 17 – October 23, 2015 | Template:Convert | 19 hours | 0.14 (140 m) | 217× | |
| LAMO/XMO1 | 4th | December 16, 2015 – September 2, 2016 | Template:Convert | 5.5 hours | 0.035 (35 m) | 850× | |
| XMO2 | 5th | October 5 – November 4, 2016 | Template:Convert | 19 hours | 0.14 (140 m) | 217× | <ref>Template:Cite web</ref><ref name="Dawn Mission Status Updates" /><ref>Template:Cite web</ref> |
| XMO3 | 6th | December 5, 2016 – February 22, 2017 | Template:Convert | ≈8 days | 0.9 (est) | 34× (est) | <ref name="Dawn Mission Status Updates"/><ref>Template:Cite web</ref> |
| XMO4 | 7th | April 22 – June 22, 2017 | Template:Convert | ≈29 days | <ref name="status2017">Template:Cite web</ref> | ||
| XMO5 | 8th | June 30, 2017 – April 16, 2018 | Template:Convert | 30 days | <ref name="status2017" /><ref name="plansoc20180525">Template:Cite web</ref><ref name="nasa20180320">Template:Cite web</ref> | ||
| XMO6 | 9th | May 14–31, 2018 | Template:Convert | 37 hours | <ref name="status2018">Template:Cite web</ref> | ||
| XMO7 (FINAL) | 10th | June 6, 2018 – present | Template:Convert | 27.2 hours | <ref name="MissionStatus2018">Template:Cite web</ref><ref name="SFN-20180615">Template:Cite web</ref><ref name="sfi20180602">Template:Cite news</ref><ref name="nasa20180429">Template:Cite web</ref> |
Dawn entered Ceres orbit on March 6, 2015,<ref name="nbcnews3615">Template:Cite news</ref> four months prior to the arrival of New Horizons at Pluto. Dawn thus became the first mission to study a dwarf planet at close range.<ref name="NasaDawn">Template:Cite web</ref><ref name="Ceresintroduction">Template:Cite book</ref> Dawn initially entered a polar orbit around Ceres, and continued to refine its orbit. It obtained its first full topographic map of Ceres during this period.<ref>Template:Cite web</ref>
From April 23 to May 9, 2015, Dawn entered an RC3 orbit (Rotation Characterization 3) at an altitude of Template:Convert. The RC3 orbit lasted 15 days, during which Dawn alternated taking pictures and sensor measurements and then relayed the resulting data back to Earth.<ref>Template:Cite web</ref> On May 9, 2015, Dawn powered its ion engines and began a month-long spiral descent down to its second mapping point, a Survey orbit, three times closer to Ceres than the previous orbit. The spacecraft stopped twice to take images of Ceres during its spiral descent into the new orbit.
On June 6, 2015, Dawn entered the new Survey orbit at an altitude of Template:Convert. In the new Survey orbit, Dawn circled Ceres every three Earth days.<ref>Template:Cite web</ref> The Survey phase lasted 22 days (7 orbits), and was designed to obtain a global view of Ceres with DawnTemplate:'s framing camera, and generate detailed global maps with the visible and infrared mapping spectrometer (VIR).
On June 30, 2015, Dawn experienced a software glitch when an anomaly in its orientation system occurred. It responded by going into safe mode and sending a signal to engineers, who fixed the error on July 2, 2015. Engineers determined the cause of the anomaly to be related to the mechanical gimbal system associated with one of Dawn's ion engines. After switching to a separate ion engine and conducting tests from July 14 through July 16, 2015, engineers certified the ability to continue the mission.<ref>Template:Cite web</ref>
On August 17, 2015, Dawn entered the HAMO orbit (High-Altitude Mapping Orbit).<ref>Template:Cite web</ref> Dawn descended to an altitude of Template:Convert, where in August 2015 it began the two-month HAMO phase. During this phase, Dawn continued to acquire near-global maps with the VIR and framing camera at higher resolution than in the Survey phase. It also imaged in stereo to resolve the surface in 3D.
On October 23, 2015, Dawn began a two-month spiral toward Ceres to achieve a LAMO orbit (Low-Altitude Mapping Orbit) at a distance of Template:Convert. Since reaching this fourth orbit in December 2015, Dawn was scheduled to acquire data for the next three months with its gamma-ray and neutron detector (GRaND) and other instruments that identified the composition at the surface.<ref name="Dawn Mission Status Updates">Template:Cite web</ref>
Having surpassed its mapping objectives, Dawn climbed to its fifth science orbit of Template:Convert beginning on September 2, 2016, to complete additional observations from a different angle.<ref name="nasa20160831">Template:Cite web</ref> Dawn began raising its altitude to its sixth science orbit of Template:Convert on November 4, 2016, with a goal of reaching it by December 2016. The return to a higher altitude allowed for a second set of data at this altitude, which improves the overall science quality when added to the first batch. However, this time the spacecraft was placed where it was not spiraling and was orbiting in the same direction as Ceres, which reduced propellant consumption.<ref name="nasa20161118">Template:Cite web</ref>
Mission conclusion
A flyby of the asteroid 2 Pallas after the completion of the Ceres mission was suggested but never formally considered; orbiting Pallas would not have been possible for Dawn due to the high inclination of Pallas's orbit relative to Ceres.<ref>Template:Cite web</ref>
In April 2016, the Dawn project team submitted a proposal to NASA for an extended mission that would have seen the spacecraft break orbit from Ceres and perform a flyby of the asteroid 145 Adeona in May 2019,<ref name="ExtendedMission">Template:Cite news</ref> arguing that the science gained from visiting a third asteroid might outweigh the returns from staying at Ceres.<ref name="sfnow20160702" /> NASA's Planetary Mission Senior Review Panel, however, declined the proposal in May 2016.<ref name="pmsr2016">Template:Cite web</ref><ref name="verge20160701">Template:Cite news</ref> A one-year mission extension was approved, but the review panel ordered that Dawn remain at Ceres, stating that the long-term observations of the dwarf planet, particularly as it approached perihelion, would potentially yield better science.<ref name="sfnow20160702" />
The one-year extension expired on June 30, 2017.<ref name="sfnow20170617">Template:Cite news</ref><ref name="nasadawn20170927">Template:Cite web</ref> The spacecraft was placed in an uncontrolled but relatively stable orbit around Ceres, where it ran out of hydrazine propellant by October 31, 2018,<ref name="NYT-20181101" /> and where it will remain as a "monument" for at least 20 years.<ref name="NASA-20180906">Template:Cite web</ref><ref name="NASA-20181101-EndOfDawnMission">Template:Cite news</ref><ref name="NYT-20181101">Template:Cite news</ref> Template:Multiple image
Media
High-resolution image
Ceres atlas images
Maps of Ceres
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Map of Ceres feature names
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Topographical map of Ceres
Flyover videos
See also
- Features on Ceres
- Ahuna Mons, a mountain on Ceres
- Bright spots on Ceres
- List of geological features on Ceres
- Other asteroid missions
- List of minor planets and comets visited by spacecraft
- Chang'e 2 Template:Ndash 4179 Toutatis flyby
- Galileo probe Template:Ndash 951 Gaspra and 243 Ida flybys
- Hayabusa Template:Ndash 25143 Itokawa rendezvous and sample return
- Hayabusa2 Template:Ndash 162173 Ryugu rendezvous and sample return
- NEAR Shoemaker Template:Ndash 253 Mathilde flyby, orbited 433 Eros from 2000 to 2001
- OSIRIS-REx Template:Ndash 101955 Bennu sample return mission
- Rosetta Template:Ndash 2867 Šteins and 21 Lutetia flyby
References
External links
- Dawn website by Jet Propulsion Laboratory
- Dawn website by NASA
- Dawn public data archives Template:Webarchive by UCLA
- Dawn Legacy – Video (03:04) by NASA (September 7, 2018).
- Dawn Mission Archive at the NASA Planetary Data System, Small Bodies Node
- Instruments
- Framing Cameras by the Max Planck Institute for Solar System Research
- Visual and Infrared Spectrometer Template:Webarchive by the Istituto Nazionale di Astrofisica
- Gamma Ray and Neutron Spectrometer by NASA (journal paper)
Template:Ceres Template:4 Vesta Template:Planetary Missions Program Office Template:Asteroid spacecraft Template:Mars spacecraft Template:Planetary defense Template:Solar System probes Template:Jet Propulsion Laboratory Template:Orbital launches in 2007 Template:2011 in space Template:2015 in space Template:2018 in space Template:NASA navbox
- Pages with broken file links
- Discovery Program
- Missions to main-belt asteroids
- Missions to dwarf planets
- Orbiters (space probe)
- NASA space probes
- 4 Vesta
- Ceres (dwarf planet)
- Spacecraft launched by Delta II rockets
- Space probes launched in 2007
- Derelict space probes
- Articles containing video clips
- Space probes decommissioned in 2018