Dawn (spacecraft)

Dawn

Artist's rendering of the Dawn spacecraft.
Mission type Multi-target orbiter
Operator NASA
COSPAR ID 2007-043A
Website NASA
NASA JPL
Mission duration ~9 years[1]
Spacecraft properties
Manufacturer Orbital Sciences · JPL(Jet Propulsion Laboratory) · UCLA(University of California, Los Angeles)
BOL mass 1,240 kg (2,730 lb) (wet)[2]
Power 1300 W (Solar array) at 3 AU[2]
Start of mission
Launch date September 27, 2007 (2007-09-27) 11:34:00 UTC[3]
(8 years, 7 months and 9 days ago)
Rocket Delta II 7925H
Launch site Space Launch Complex 17B
Cape Canaveral Air Force Station, Florida, United States
Flyby of Mars (Gravity assist)
Closest approach February 4, 2009 (2009-02-04)
(7 years, 3 months and 2 days ago)
Distance 549 km (341 mi)
4 Vesta orbiter
Orbital insertion July 16, 2011 (2011-07-16) 04:47 UTC[4]
(4 years, 9 months and 20 days ago)
Departed orbit September 5, 2012 (2012-09-05)
(3 years, 8 months and 1 day ago)
1 Ceres orbiter
Orbital insertion March 6, 2015 (2015-03-06)[5]


Dawn mission patch

Discovery program
← Deep Impact Kepler →

Dawn is a space probe launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt, Vesta and Ceres. It is currently in orbit about its second target, the dwarf planet Ceres. Dawn is the first spacecraft to orbit two extraterrestrial bodies,[6] the first spacecraft to visit either Vesta or Ceres, and also the first to visit a dwarf planet, arriving at Ceres in March 2015, a few months before New Horizons flew by Pluto in July 2015.

Dawn entered Vesta orbit on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012.[7][8] Dawn entered Ceres orbit on March 6, 2015,[9][10] and while originally predicted to remain in orbit perpetually after the conclusion of its mission, NASA is now considering a third target.[11][12]

The Dawn mission is managed by NASA's Jet Propulsion Laboratory, with spacecraft components contributed by European partners from the Netherlands, Italy and Germany. It is the first NASA exploratory mission to use ion propulsion, which enabled it to enter and leave the orbit of multiple celestial bodies. Previous multi-target missions using conventional drives, such as the Voyager program, were restricted to flybys.[2]

Project history

Technological background

SERT-1: first ion engine NASA spacecraft;[13] launched on July 20, 1964.[14]

The first working ion thruster 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 fuel. 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.[15] 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 an ion thruster on a science mission,[16] and validated a number of technologies, including the NSTAR electrostatic ion thruster, as well as performing a flyby of an asteroid and a comet.[17] Among the other technologies validated by the DS1 was the Small Deep Space Transponder, which is used on Dawn for long-range communication.[17]

Initial cancellations

The status of the Dawn mission changed several times. The project was cancelled in December 2003,[18] 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.[19] On March 2, 2006, Dawn was again cancelled by NASA.[20]

Reinstatement

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,[21] and on March 27, 2006, it was announced that the mission would not be cancelled after all.[22][23] 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.[24] Christopher T. Russell was chosen to lead the Dawn mission team.

Scientific background

Scale comparison of Vesta, Ceres, and Earth's moon

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 drive in deep space. 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.[25]

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.

Dawn image of Ceres from 13,600 km, 4 May 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.[26] 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.[27]

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).[27]

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;[28] its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[28] There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 1,400 HED meteorites,[29] 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.[30][31][32]

The first color map of Ceres by Dawn (exaggerated color, March 2015) 

Objectives

Dawn's approximate flight trajectory

The Dawn mission's goal is 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.[33] The primary question that the mission addresses is the role of size and water in determining the evolution of the planets.[33] 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.[33]

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.[33]

Specifications

Dawn prior to encapsulation at its launch pad on July 1, 2007

Dimensions

With its solar array in the retracted launch position, the Dawn spacecraft is 2.36 meters (7.7 ft) long. With its solar arrays fully extended, Dawn is 19.7 m (65 ft) long.[34] The solar arrays have a total area of 36.4 m2 (392 sq ft).[35] The main antenna is five feet in diameter.[6]

Propulsion system

The Dawn spacecraft is propelled by three xenon ion thrusters based on an evolution of the NSTAR technology used by the Deep Space 1 spacecraft,[36] and uses only one at a time. They have a specific impulse of 3,100 s and produce a thrust of 90 mN.[37] The whole spacecraft, including the ion propulsion thrusters, is powered by a 10 kW (at 1 AU) triple-junction gallium arsenide photovoltaic solar array manufactured by Dutch Space.[38][39] Dawn was allocated 275 kg (606 lb) of xenon for its Vesta approach, and carried another 110 kg (243 lb) to reach Ceres,[40] out of a total capacity of 425 kg (937 lb) of on-board propellant.[41] With the propellant it carries, Dawn can perform a velocity change of more than 10 km/s over the course of its mission, far more than any previous spacecraft achieved with onboard propellant after separation from its launch rocket.[40] Dawn is NASA's first purely exploratory mission to use ion propulsion engines.[42] The spacecraft also has twelve 0.9 N hydrazine thrusters for attitude control, which are designed to assist in orbital insertion.[43]

Outreach microchip

Dawn carries a memory chip bearing the names of more than 360,000 space enthusiasts.[44] The names were submitted online as part of a public outreach effort between September 2005 and November 4, 2006.[45] 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.[46] 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.

Payload

Framing camera view of the Ceres bright spots

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.[2]

A magnetometer and laser altimeter were considered for the mission, but were not ultimately flown.[57]

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.[58][59] The launch was originally scheduled for June 20, but was delayed until June 30 due to delays with part deliveries.[60] 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.[61] 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

Dawn launching on a Delta II rocket from Cape Canaveral Air Force Station Space Launch Complex 17 on September 27, 2007

The launch of Dawn was rescheduled for September 26, 2007,[62][63][64] 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 EDT (11:20–11:49 GMT).[65] 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.[66] Dawn finally launched from pad 17-B at the Cape Canaveral Air Force Station on a Delta 7925-H rocket[67] at 07:34 EDT,[68][69][70] reaching escape velocity with the help of a spin-stabilized solid-fueled third stage.[71][72] Thereafter, Dawn's ion thrusters took over.

Transit (Earth to Vesta)

After initial testing, during which the ion thrusters accumulated more than 11 days 14 hours of thrust, Dawn began long-term cruise propulsion on December 17, 2007.[73] 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 kilometers per second. On November 20, 2008, Dawn performed its first trajectory correction maneuver (TCM1), firing its number 1 thruster for 2 hours, 11 minutes.

Greyscale NIR image of Mars (northwest Tempe Terra), taken by Dawn during its 2009 flyby

Dawn made its closest approach (549 km) to Mars on February 17, 2009 during a successful gravity assist.[74][75] This flyby slowed Mars' orbital speed by about 2.3 cm over 180 million years.[6] 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.[76]

To cruise from Earth to its targets, Dawn traveled in an elongated outward spiral trajectory. NASA posts and continually updates the current location and status of Dawn online.[77] The actual Vesta chronology and estimated Ceres chronology are as follows:[1]

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.

On May 3, 2011, Dawn acquired its first targeting image, 1,200,000 km from Vesta, and began its approach phase to the asteroid.[78] On June 12, Dawn's speed relative to Vesta was slowed in preparation for its orbital insertion 34 days later.[79][80]

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.[81] NASA later confirmed that it received telemetry from Dawn indicating that the spacecraft successfully entered orbit around Vesta.[82] 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.[83]

Vesta orbit

After being captured by Vesta's gravity and entering its orbit on July 16, 2011,[84] 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. It assumed a 12.3-hour high-altitude mapping orbit at 680 km on September 27, and finally entered a 4.3-hour low-altitude mapping orbit at 210 km on December 8.[85][86][87]

In May 2012, NASA released the preliminary results of Dawn's study of Vesta, including estimates of the size of Vesta's metal-rich core, which is theorized to be 220 km across. NASA scientists furthermore 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.[84][88][89] In October 2012, NASA stated that data from Dawn had revealed the origin of anomalous dark spots and streaks on Vesta's surface, which were likely deposited by ancient asteroid impacts.[90][91][92] 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.[93][94] 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.[8] 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.[7][95][96][97][98]

Geologic map of Vesta based on Dawn data[99]
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)[100] and light cyan (the Divalia Fossae Formation, equatorial),[99] 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;[101][102] areas modified by more recent impacts or mass wasting are yellow/orange or green, respectively.

Transit (Vesta to Ceres)

Imaging dates (2014–2015) and resolution[103]
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.[104]

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 a high-energy particle. Upon exiting the safe mode on September 15, 2014, the probe's ion thruster resumed normal operation.[105]

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.[105]

Ceres approach

Dawn began photographing an extended disk of Ceres on December 1, 2014,[106] 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.[107]

Progression of images of Ceres taken by Dawn between January and March 2015

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 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 Dawn's vantage point to take pictures safely.[108]

Ceres orbit

Mapping orbits

Mapping orbits (2015) and resolution[109] – Photos of Ceres by Dawn on Commons
Orbit phase No.Dates[110] Altitude
(km; mi) 		Orbital period Resolution
(km/px) 		Improvement
over Hubble
RC3 1stApril 23, 2015 – May 9, 2015 13,500 km (8,400 mi) 15 days 1.3 24×
Survey 2ndJune 6, 2015 – June 30, 2015 4,400 km (2,700 mi) 3.1 days 0.41 73×
HAMO 3rdAugust 17, 2015 – October 23, 2015 1,450 km (900 mi) 19 hours 0.14 (140 m) 217×
LAMO 4thDecember 16, 2015 – end of mission 375 km (233 mi) 5.5 hours 0.035 (35 m) 850×

Dawn entered Ceres orbit on March 6, 2015,[5] 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.[111][112] 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.[113]

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 which Dawn alternated taking pictures and sensor measurements and then relayed the resulting data back to Earth.[114] 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.[115] The Survey phase lasted 22 days (7 orbits), and was designed to obtain a global view of Ceres with Dawn'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.[116]

On August 17, 2015, Dawn entered the HAMO orbit ("High-Altitude Mapping Orbit").[117] 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 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 375 km (233 mi). Since reaching this fourth and final orbit in December, Dawn was scheduled to acquire data for the next three months with its gamma-ray and neutron detector (GRaND) and other instruments that will determine the chemical elements at the surface.[118]

Mission conclusion

It is expected that Dawn will become a perpetual satellite of Ceres when the mission is over, due to its highly stable orbit.[12] 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' orbit relative to Ceres.[119]

On April 20, 2016, New Scientist announced that the Dawn team had sent NASA a proposal for an extended mission. According to the report, the spacecraft retains enough xenon fuel to break Ceres orbit and reach a third asteroid. The mission team has not yet revealed the proposed destination.[120]

Media

Examples of images from Dawn at Ceres

Ceres - Survey Maps (June 2015)
Overall
Kerwan section
(PDF version)
Asari-Zadeni section
(PDF version)
Occator section
(PDF version)
Ceres - polar regions (released November 20, 2015)
Official Nomenclature of Ceres (August 2015)
The Dawn spacecraft's color map of Ceres (September 2015)[121]
Topographic maps of Ceres - east and west hemispheres (July 2015)

Flyover video

Ceres flyover animations
Surface features exaggerated
(simulated; 01:15; June 8, 2015)[122]
Focus on Occator Crater
(false colors; 01:12; December 9, 2015)
Flight over dwarf planet Ceres
(color; 03:43; January 29, 2016)

Selected discoveries by Dawn at Ceres

Bright "Spot 5"
imaged by Dawn
from 385 km (239 mi) LAMO
Bright "Spot 5"
imaged by Dawn
from 1,450 km (900 mi) HAMO
Bright "Spot 5"
in Occator crater
(red=high; blue=low)
(28 July 2015)[121]
Ahuna Mons,
a mountain on Ceres
(red=high; blue=low)
(28 July 2015)
Ahuna Mons,
a mountain on Ceres
5 km (3.1 mi) high
(6 June 2015)[123]

See also

Features on Ceres
Other asteroid missions

References

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  2. 1 2 3 4 5 6 Rayman, Marc; Fraschetti, Thomas C.; Raymond, Carol A.; Russell, Christopher T. (April 5, 2006). "Dawn: A mission in development for exploration of main belt asteroids Vesta and Ceres" (PDF). Acta Astronautica 58 (11): 605–616. Bibcode:2006AcAau..58..605R. doi:10.1016/j.actaastro.2006.01.014. Retrieved April 14, 2011.
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  4. ↑ "NASA's Dawn Spacecraft Begins Science Orbits of Vesta". NASA. August 1, 2011. Retrieved August 6, 2011.
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