Dawn is a retiredspace probe that was launched byNASA in September 2007 with the mission of studying two of the three knownprotoplanets of theasteroid belt:Vesta andCeres.[1] In the fulfillment of that mission—the ninth in NASA'sDiscovery Program—Dawn entered orbit around Vesta on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012.[11][12] It entered orbit around Ceres on March 6, 2015.[13][14] In 2017, NASA announced that the planned nine-year mission would be extended until the probe'shydrazine fuel supply was depleted.[15] On November 1, 2018, NASA announced thatDawn had depleted its hydrazine, and the mission was ended. The derelict probe remains in a stable orbit around Ceres.[16]
Dawn is the first spacecraft to have orbited two extraterrestrial bodies,[17] the first spacecraft to have visited either Vesta or Ceres, and the first to have orbited a dwarf planet.[18]
TheDawn mission was managed by NASA'sJet Propulsion Laboratory, with spacecraft components contributed by European partners from Italy, Germany, France, and the Netherlands.[19] It was the first NASA exploratory mission to useion propulsion, which enabled it to enter and leave the orbit of two celestial bodies. Previous multi-target missions using rockets powered bychemical engines, such as theVoyager program, were restricted toflybys.[5]
SERT-1: firstion engine NASA spacecraft;[20] launched on July 20, 1964.[21]
The first working ion thruster in the US was built byHarold R. Kaufman in 1959 at NASA'sGlenn Research Center inOhio. The thruster was similar to the general design of a gridded electrostatic ion thruster withmercury as its propellant. Suborbital tests of the engine followed during the 1960s, and in 1964 the engine was tested on a suborbital flight aboard theSpace Electric Rocket Test 1 (SERT 1). It successfully operated for the planned 31 minutes before falling back to Earth.[22] 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 axenon-propelled ion thruster on a science mission,[23] and validated a number of technologies, including theNSTARelectrostatic ion thruster, as well as performing a flyby of an asteroid and a comet.[24] In addition to the ion thruster, among the other technologies validated by the DS1 was theSmall Deep Space Transponder, which is used onDawn for long-range communication.[24]
Twenty-six proposals were submitted to theDiscovery Program solicitation, with budget initially targeted at US$300 million.[25] Three semi-finalists were downselected in January 2001 for a phase-A design study: Dawn, Kepler, and INSIDE Jupiter.[26] In December 2001 NASA selected the Kepler and the Dawn mission for the Discovery program.[25] Both missions were initially selected for a launch in 2006.[25]
The status of theDawn mission changed several times. The project was cancelled in December 2003,[27] and then reinstated in February 2004. In October 2005, work onDawn 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.[28] On March 2, 2006,Dawn was again cancelled by NASA.[29]
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,[30] and on March 27, 2006, it was announced that the mission would not be cancelled after all.[31][32] In the last week of September 2006, theDawn 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.[33]Christopher T. Russell was chosen to lead theDawn mission team.
TheDawn mission was designed to study two large bodies in theasteroid belt in order to answer questions about the formation of theSolar System, as well as to test the performance of itsion thrusters in deep space.[1] Ceres and Vesta were chosen as two contrastingprotoplanets, the first one apparently "wet" (i.e. icy and cold) and the other "dry" (i.e. rocky), whoseaccretion was terminated by the formation ofJupiter. The two bodies provide a bridge in scientific understanding between the formation ofrocky planets and the icy bodies of the Solar System, and under what conditions a rocky planet can hold water.[34]
Ceres comprises a third of the total mass of the asteroid belt. Its spectral characteristics suggest a composition similar to that of a water-richcarbonaceous chondrite.[35] Vesta, a smaller, water-poorachondriticasteroid comprising a tenth of the mass of the asteroid belt, has experienced significant heating anddifferentiation. It shows signs of a metalliccore, a Mars-like density and lunar-like basaltic flows.[36]
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 ofCAIs (the oldest known objects of Solar System origin).[36]
Moreover, Vesta appears to be the source of many smaller objects in the Solar System. Most (but not all)V-typenear-Earth asteroids, and some outer main-belt asteroids, havespectra similar to Vesta, and are thus known asvestoids. Five percent of the meteoritic samples found on Earth, thehowardite–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;[37] its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icymantle.[37] There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 1,400 HED meteorites,[38] giving insight into Vesta geologic history and structure. Vesta is thought to consist of a metallic iron–nickel core, an overlying rockyolivine mantle and crust.[39][40][41]
The first color map of Ceres byDawn (exaggerated color, March 2015)
Animation of Dawn's trajectory from September 27, 2007, to October 5, 2018 Dawn·Earth·Mars·4 Vesta·1 CeresDawn's approximate flight trajectory
TheDawn 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.[1][42]
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.[42] 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.[42]
There are three principal scientific drivers for the mission. First, theDawn 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.[42]
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.[43][44] A frame-transfercharge-coupled device (CCD), a Thomson TH7888A,[44] 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 permitspanchromatic (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 customradiation-hardenedXilinx system with aLEON2 core and 8GiB of memory.[44] The camera offered resolutions of 17 m/pixel for Vesta and 66 m/pixel for Ceres.[44] 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.[5][45]
Visible andinfraredspectrometer (VIR) – This instrument is a modification of the visible and infrared thermal-imaging spectrometer used on theRosetta andVenus Express spacecraft. It draws its heritage from theSaturn orbiterCassini's visible and infrared mapping spectrometer. The spectrometer's VIR spectral frames are 256 (spatial) × 432 (spectral), and the slit length is 64mrad. 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.[5][46]
Gamma Ray and Neutron Detector (GRaND)[47] – This instrument is based on similar instruments flown on theLunar Prospector andMars Odyssey space missions. It had 21 sensors with a very wide field of view.[43] 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.[48][49][50][51][52][53]
Amagnetometer and laseraltimeter were considered for the mission, but were not ultimately flown.[54]
With itssolar array in the retracted launch position, theDawn spacecraft is 2.36 metres (7.7 ft) wide. With its solar arrays fully extended,Dawn is 19.7 m (65 ft) wide.[55] The solar arrays have a total area of 36.4 m2 (392 sq ft).[56] The main antenna is five feet (1.24 metres) in diameter.[17]
TheDawn spacecraft was propelled by threexenonion thrusters derived fromNSTAR technology used by theDeep Space 1 spacecraft,[57] using one at a time. They have aspecific impulse of 3,100 s and produce athrust of 90 mN.[58] The whole spacecraft, including the ion propulsion thrusters, was powered by a 10 kW (at 1 AU)triple-junctiongallium arsenidephotovoltaic solar array manufactured by Dutch Space.[59][60]Dawn was allocated 247 kg (545 lb) of xenon for its Vesta approach, and carried another 112 kg (247 lb) to reach Ceres,[61] out of a total capacity of 425 kg (937 lb) of on-boardpropellant.[62] With the propellant it carried,Dawn was able to perform avelocity 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.[61] However, the thrust was very gentle; it took four days at full throttle to accelerateDawn from zero to 60 mph (96 km/h).[17]Dawn is NASA's first purely exploratory mission to use ion propulsion engines.[63] The spacecraft also has twelve 0.9 Nhydrazine thrusters forattitude control (orientation), which were also used to assist in orbital insertion.[64]
The Dawn spacecraft was able to achieve a record-breaking level of propulsion from its ion engine.[65] NASA noted three specific areas of excellence:[66]
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.
Dawn carries a memorychip bearing the names of more than 360,000 space enthusiasts.[67] The names were submitted online as part of a public outreach effort between September 2005 and November 4, 2006.[68] The microchip, which is two centimetres in diameter, was installed on May 17, 2007, above the spacecraft's forward ion thruster, underneath itshigh-gain antenna.[69] More than one microchip was made, with a back-up copy put on display at the 2007 Open House event at theJet Propulsion Laboratory in Pasadena, California.
On April 10, 2007, the spacecraft arrived at the Astrotech Space Operations subsidiary ofSPACEHAB, Inc. inTitusville, Florida, where it was prepared for launch.[70][71] The launch was originally scheduled for June 20, but was delayed until June 30 due to delays with part deliveries.[72] A broken crane at the launch pad, used to raise thesolid 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.[73] 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 thePhoenix mission to Mars, which was successfully launched on August 4.
The launch ofDawn was rescheduled for September 26, 2007,[74][75][76] 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 (11:20–14:49GMT).[77] 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 theInternational Space Station.[78]Dawn finally launched fromSpace Launch Complex 17B at theCape Canaveral Air Force Station on aDelta 7925-H rocket[79] at 07:34 EDT,[80][81][82] reaching escape velocity with the help of aspin-stabilized solid-fueled third stage.[83][84] Thereafter,Dawn's ion thrusters took over.
After initial testing, during which theion thrusters accumulated more than 11 days 14 hours of operation,Dawn began long-term cruise propulsion on December 17, 2007.[85] On October 31, 2008,Dawn completed its first thrusting phase to send it on toMars for agravity 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 firsttrajectory correction maneuver (TCM1), firing its number 1 thruster for 2 hours, 11 minutes.
Greyscale NIR image of Mars (northwestTempe Terra), taken byDawn during its 2009 flyby
Dawn made its closest approach (549 km) toMars on February 17, 2009, during a successful gravity assist.[86][87] This flyby slowed Mars's orbital velocity by about 2.5 cm (1 in) per 180 million years.[17] On this day, the spacecraft placed itself insafe 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. Theroot cause of the event was reported to be a software programming error.[88]
To cruise from Earth to its targets,Dawn travelled in an elongated outward spiral trajectory. The actual Vesta chronology and estimated[needs update] Ceres chronology are as follows:[2]
AsDawn approached Vesta, the Framing Camera instrument took progressively higher-resolution images, which were published online and at news conferences by NASA and MPI.
June 14, 2011 265,000 km (165,000 mi)
June 24, 2011 152,000 km (94,000 mi)
July 1, 2011 100,000 km (62,000 mi)
July 9, 2011 41,000 km (25,000 mi)
On May 3, 2011,Dawn acquired its first targeting image, 1,200,000 km from Vesta, and began its approach phase to the asteroid.[90] On June 12,Dawn's speed relative to Vesta was slowed in preparation for its orbital insertion 34 days later.[91][92]
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 notDawn successfully made the maneuver. The spacecraft would then reorient itself, and was scheduled to check in at 06:30 UTC on July 17.[93] NASA later confirmed that it received telemetry fromDawn indicating that the spacecraft successfully entered orbit around Vesta, making it the first spacecraft to orbit an object in the asteroid belt.[94][95] 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.[96]
After being captured by Vesta's gravity and entering its orbit on July 16, 2011,[97]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 2,750 km (1,710 mi). It assumed a 12.3-hour high-altitude mapping orbit at 680 km (420 mi) on September 27, and finally entered a 4.3-hour low-altitude mapping orbit at 210 km (130 mi) on December 8.[98][99][100]
Animation of Dawn's trajectory around4 Vesta from July 15, 2011, to September 10, 2012 Dawn·4 Vesta
July 17, 2011 16,000 km (9,900 mi)
July 18, 2011 10,500 km (6,500 mi)
July 23, 2011 5,200 km (3,200 mi)
July 24, 2011 5,200 km (3,200 mi)
In May 2012, theDawn 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 220 km (140 mi) 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.[97][101] In October 2012, furtherDawn results were published, on the origin of anomalous dark spots and streaks on Vesta's surface, which were likely deposited by ancient asteroid impacts.[102][103][104] In December 2012, it was reported thatDawn had observed gullies on the surface of Vesta that were interpreted to have been eroded by transiently flowing liquid water.[105][106] More details about theDawn mission's scientific discoveries at Vesta are included on theVesta page.
Dawn was originally scheduled to depart Vesta and begin its two and a half year journey to Ceres on August 26, 2012.[12] However, a problem with one of the spacecraft'sreaction wheels forcedDawn to delay its departure from Vesta's gravity until September 5, 2012.[11][107][108][109][110]
Central Mound at the South Pole on the asteroid Vesta on August 12, 2011
The most ancient and heavily cratered regions are brown; areas modified by theVeneneia andRheasilvia impacts are purple (the Saturnalia Fossae Formation, in the north)[112] and light cyan (the Divalia Fossae Formation, equatorial),[111] 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;[113][114] areas modified by more recent impacts or mass wasting are yellow/orange or green, respectively.
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. TheDawn 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.[116]
On September 11, 2014,Dawn'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 ahigh-energy particle. Upon exiting the safe mode on September 15, 2014, the probe's ion thruster resumed normal operation.[117]
Furthermore, theDawn 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.[117]
Dawn began photographing an extended disk of Ceres on December 1, 2014,[118] with images of partial rotations on January 13 and 25, 2015 released as animations.Images taken fromDawn of Ceres after January 26, 2015, exceeded the resolution of comparable images from theHubble Space Telescope.[119]
Progression of images of Ceres taken byDawn between January and March 2015
January 25, 2015 237,000 km (147,000 mi)
February 4, 2015 145,000 km (90,000 mi)
February 12, 2015 80,000 km (50,000 mi)
February 19, 2015 46,000 km (29,000 mi)
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 planned[needs update] 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 fromDawn's vantage point to take pictures safely.[120]
April 23, 2015 1st Map Orbit – RC3 13,600 km (8,500 mi) (view on commons)
June 6, 2015 2nd Map Orbit – SRVY 4,400 km (2,700 mi) (view on commons)
August 17, 2015 3rd Map Orbit – HAMO 1,470 km (915 mi) (view on commons)
December 10, 2015 4th Map Orbit – LAMO 385 km (240 mi) (view on commons)
October 5, 2016 5th Map Orbit – XMO2 1,480 km (920 mi) (view on commons)
June 9, 2018 10th Map Orbit – XMO7 35 km (22 mi) (view on commons)
Dawn entered Ceres orbit on March 6, 2015,[136] four months prior to the arrival ofNew Horizons at Pluto.Dawn thus became the first mission to study a dwarf planet at close range.[137][138]Dawn initially entered apolar orbit around Ceres, and continued to refine its orbit. It obtained its first full topographic map of Ceres during this period.[139]
From April 23 to May 9, 2015,Dawn entered an RC3 orbit (Rotation Characterization 3) at an altitude of 13,500 km (8,400 mi). The RC3 orbit lasted 15 days, during whichDawn alternated taking pictures and sensor measurements and then relayed the resulting data back to Earth.[140] 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 4,430 km (2,750 mi). In the new Survey orbit,Dawn circled Ceres every three Earth days.[141] The Survey phase lasted 22 days (7 orbits), and was designed to obtain a global view of Ceres withDawn'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 intosafe 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 ofDawn'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.[142]
On August 17, 2015,Dawn entered the HAMO orbit (High-Altitude Mapping Orbit).[143]Dawn descended to an altitude of 1,480 km (920 mi), 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 instereo 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 375 km (233 mi). 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.[125]
Having surpassed its mapping objectives,Dawn climbed to its fifth science orbit of 1,460 km (910 mi) beginning on September 2, 2016, to complete additional observations from a different angle.[144]Dawn began raising its altitude to its sixth science orbit of 7,200 km (4,500 mi) 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.[145]
A flyby of the asteroid2 Pallas after the completion of the Ceres mission was suggested but never formally considered; orbiting Pallas would not have been possible forDawn due to the high inclination of Pallas's orbit relative to Ceres.[146]
In April 2016, theDawn 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 asteroid145 Adeona in May 2019,[147] arguing that the science gained from visiting a third asteroid might outweigh the returns from staying at Ceres.[89] NASA's Planetary Mission Senior Review Panel, however, declined the proposal in May 2016.[148][149] A one-year mission extension was approved, but the review panel ordered thatDawn remain at Ceres, stating that the long-term observations of the dwarf planet, particularly as it approachedperihelion, would potentially yield better science.[89]
The one-year extension expired on June 30, 2017.[150][151] The spacecraft was placed in an uncontrolled but relatively stable orbit around Ceres, where it ran out of hydrazine propellant by October 31, 2018,[7] and where it will remain as a "monument" for at least 20 years.[8][152][7]
Ceres – some of the last views by theDawn spacecraft (September 1, 2018)[8][152][7]
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^Garner, Charles E.; Rayman, Marc D.; Brophy, John R.; Mikes, Steven C. (2011).The Dawn of Vesta Science(PDF). 32nd International Electric Propulsion Conference. September 11–15, 2011. Wiesbaden, Germany. IEPC-2011-326. Archived fromthe original(PDF) on November 9, 2013. RetrievedMay 6, 2012.
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Missions are ordered by launch date. Sign† indicates failure en route or before intended mission data returned.‡ indicates use of the planet as agravity assist en route to another destination.
Launches are separated by dots ( • ), payloads by commas ( , ), multiple names for the same satellite by slashes ( / ). Crewed flights are underlined. Launch failures are marked with the † sign. Payloads deployed from other spacecraft are (enclosed in parentheses).
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