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Dawn (spacecraft)

                   
Dawn
Dawn Flight Configuration 2.jpg
Artist's concept of Dawn with Vesta (left) and Ceres (right). The proximity of Vesta to Ceres is not to scale.
Operator NASA
Major contractors Orbital Sciences, JPL, UCLA
Mission type Flyby / Orbiter
Flyby of Mars (February 4, 2009)
Satellite of Vesta (current), Ceres (planned)
Orbital insertion date Vesta: July 16, 2011, 04:47 UTC[1]
(11 months and 17 days ago)
Ceres: February 2015 (projected)
Launch date 2007-09-27 11:34:00 UTC[2]
(&100000000000000040000004 years, &10000000000000311000000311 days ago)
Launch vehicle Delta II 7925H
Launch site Space Launch Complex 17B
Cape Canaveral Air Force Station
Mission duration through July 2015
COSPAR ID 2007-043A
Homepage dawn.jpl.nasa.gov
Mass 1,240 kg (2,700 lb) (wet) [3]
Power 1300 W (Solar array) at 3 AU [3]
Orbital elements
Eccentricity ~ circular
Inclination Polar

Dawn is a robotic NASA spacecraft tasked with the exploration and study of the asteroid Vesta and the dwarf planet Ceres, the two largest members of the asteroid belt. The spacecraft was constructed with some European cooperation, with components contributed by partners in Germany, Italy, and the Netherlands. The Dawn mission is managed by NASA's Jet Propulsion Laboratory.

Dawn was the first spacecraft to visit Vesta, and will be the first to visit Ceres. It may also be the first spacecraft to orbit two separate extraterrestrial bodies, using ion thrusters to travel between its targets. Previous multi-target missions using conventional drives, such as the Voyager program, were restricted to flybys.[3]

Launched on September 27, 2007, Dawn entered orbit around Vesta on July 16, 2011, and will explore it until August 26, 2012.[4] Thereafter, the spacecraft will head to Ceres, which it is scheduled to reach in February 2015.[5] Dawn is NASA’s first purely exploratory mission to use ion propulsion.

Contents

  Project history

  Initial cancellation

  One of Dawn's solar arrays pictured in 2007, showing slight damage.

The status of the Dawn mission has changed several times. In December 2003, the project was first cancelled,[6] and then reinstated in February 2004. In October 2005, work on Dawn was placed in "stand down" mode. In January 2006, Dawn's "stand down" was discussed in the press as "indefinitely postponed", even though NASA had announced no new decisions regarding the mission's status.[7] On March 2, 2006, Dawn was cancelled by NASA.[8]

  Reinstatement

The spacecraft's manufacturer, Orbital Sciences Corporation, appealed the decision and offered 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,[9] and on March 27, 2006, it was announced that the mission would not be cancelled after all.[10][11] In the last week of September 2006, the Dawn mission 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.[12] The Dawn mission team is led by Christopher T. Russell.

  Scientific background

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

Dawn is intended 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 feasibility of its ion drive.

Ceres and Vesta were chosen as two contrasting protoplanets, the first one apparently "wet" (that is, icy) and the other "dry" (or rocky), whose accretion was terminated by the formation of Jupiter. They 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.[13]

The 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 neighborhood". If the IAU's definition stands and the spacecraft experiences no delays, Dawn will become the first mission to study a dwarf planet, arriving at Ceres five months prior to the arrival of the New Horizons probe at Pluto.

Ceres is a dwarf planet whose mass comprises about one-third of the total mass of the bodies in the asteroid belt, and whose spectral characteristics suggest a composition similar to that of a water-rich carbonaceous chondrite.[14] Vesta, a smaller, water-poor achondritic asteroid, has experienced significant heating and differentiation. It shows signs of a metallic core, a Mars-like density and lunar-like basaltic flows.[15]

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 only three million years. Thermal evolution studies suggest that Ceres must have formed a little later, more than three million years after the formation of CAIs (the oldest known objects of Solar System origin).[16]

Moreover, Vesta is 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.

Peter Thomas of Cornell University has proposed that Ceres has a differentiated interior;[17] its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[17] There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 200 HED meteorites, giving insight into Vestian geologic history and structure. Vesta is thought to consist of a metallic iron–nickel core, an overlying rocky olivine mantle, with a surface crust.[18][19][20]

  Objectives

  A Dawn image of Vesta from orbit, taken on July 17, 2011.
  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.[21] The primary question that the mission addresses is the role of size and water in determining the evolution of the planets.[21] Ceres and Vesta are highly suitable bodies with which to address this question, as they are 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.[21]

There are three principal scientific drivers for the mission. Firstly, the Dawn mission can capture the earliest moments in the origin of the solar system, enabling us to understand the conditions under which these objects formed. Secondly, Dawn determines the nature of the building blocks from which the terrestrial planets formed, improving our understanding of this formation. Finally, it contrasts the formation and evolution of two small planets that followed very different evolutionary paths, so that we can understand what controls that evolution.[21]

An extended mission following the completion of the Ceres study is also possible, although unlikely, as greater scientific returns may be attained by spending more time at Vesta and Ceres.[22] Although 2 Pallas would have been a feasible extended target for the originally scheduled launch date, launch delays have meant that this may no longer be the case. Fuel was not specifically allocated to break orbit from Ceres, so doing this depends upon the details of the flight to Ceres.

  Specifications

  Dimensions

With its solar array in the retracted launch position, the Dawn spacecraft is 2.36 meters (7 feet, 9 inches) long. With its solar arrays fully extended, Dawn is 19.7 meters (65 ft) long.

  Propulsion system

  Dawn's solar array at full extension.

The Dawn spacecraft is propelled by three xenon ion thrusters which inherited NSTAR engineering technology from the Deep Space 1 spacecraft.[23] They have a specific impulse of 3,100 s and produce a thrust of 90 mN.[24] 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.[25][26] To get to Vesta, Dawn is allocated 275 kg (606 lb) of xenon, with another 110 kg (243 lb) to reach Ceres,[27] out of a total capacity of 425 kg (937 pounds) of on-board propellant.[28] With the propellant it carries, it can perform a velocity change of over 10 km/s, far more than any other spacecraft has done with onboard propellant after separation from the launch rocket.[27] Dawn is NASA’s first purely exploratory mission to use ion propulsion engines.[29]

  Microchip

Dawn carries a memory chip bearing the names of more than 360,000 space enthusiasts.[30] The names were submitted online as part of a public outreach effort between September 2005 and November 4, 2006.[31] The microchip (about the size of a United States nickel coin) was installed on 17 May 2007 above the forward ion thruster, underneath the spacecraft's high-gain antenna.[32] More than one microchip was made, with a back-up copy on display at the 2007 Open House at the Jet Propulsion Laboratory in Pasadena, California.

  Payload

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.[3]

  • Framing camera (FC) — The framing camera uses 20 mm aperture, f/7.5 refractive optical system with a focal length of 150 mm.[33] A frame-transfer charge-coupled device (CCD), a Thomson TH7888A,[33] at the focal plane has 1024 × 1024 sensitive 93-μrad pixels, yielding a wide 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 from about 400 to 1050 nm. In addition, the framing camera will acquire images for optical navigation in the vicinities of Vesta and Ceres. The FC computer is a custom radiation-hardened Xilinx with a Leon core and 8 GiB of memory.[33] The camera will offer resolutions of 17 m/pixel for Vesta and 66 m/pixel for Ceres.[33] Because the framing camera is vital for both science and navigation, the payload has two identical and physically separate cameras (FC1 & FC2) for redundancy, each with its own optics, electronics, and structure.[3][34]
  • Visual 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 also draws its heritage from the Saturn orbiter Cassini'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 70K spans the spectrum from 0.95 to 5.0 μm.[3][35]
  • Gamma Ray and Neutron Detector (GRaND) — This instrument is based on similar instruments flown on the Lunar Prospector and Mars Odyssey space missions. It will be used to measure the abundances of the major rock-forming elements (oxygen, magnesium, aluminium, silicon, calcium, titanium, and iron) on Vesta and Ceres, as well as potassium, thorium, uranium, and water (inferred from hydrogen content).[36][37][38][39][40][41]

A magnetometer and laser altimeter were considered for the mission, but were not ultimately flown.[42]

  Mission summary

  Launch

Dawn was launched on September 27, 2007, from pad 17-B at the Cape Canaveral Air Force Station on a Delta 7925-H rocket.[43] Its original planned launch date was three months earlier on June 20, but the launch schedule suffered many interruptions and delays.

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.[44][45] The launch was originally scheduled for June 20, but was delayed until June 30 due to delays with part deliveries.[46] A broken crane at the launch pad, used to raise the solid rocket boosters, further delayed the launch for a week, until July 7, but on June 15 the second stage was successfully hoisted into position.[47][48] 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.

The launch of Dawn was first rescheduled for September 26, 2007,[49][50] then September 27, due to bad weather delaying fueling of the second stage, the same problem which had earlier delayed the July 7 launch attempt. The launch window extended from 07:20–07:49 EDT (11:20–11:49 GMT).[51] 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.[52] The spacecraft finally launched at 07:34 EDT from pad 17-B on a Delta II launch vehicle,[53][54][55] propelling Dawn to a velocity of 11.46 kilometers per second (25,600 miles per hour) relative to Earth.[56] Thereafter, Dawn's ion thrusters took over.

  Cruise

After initial checkout, during which the ion thrusters accumulated more than 11 days of thrust, Dawn began long-term cruise propulsion on December 17, 2007.[57] 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 (158 pounds) of xenon propellant for a total change in velocity of 1.81 kilometers per second (4050 miles per hour). On November 20, 2008, Dawn performed its first trajectory correction maneuver (TCM1), firing its number 1 thruster for 2 hours, 11 minutes. Following Dawn's solar conjunction, a course correction maneuver originally scheduled for January 2009 was determined to not be necessary.[citation needed]

  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.[58][59] 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.[60]

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.[61] The estimated chronology is as follows:[62]

  • September 27, 2007: launch
  • February 17, 2009: Mars gravity assist
  • July 16, 2011: Vesta arrival (initial orbit)
  • December 13, 2011: Vesta lowest orbit
  • August 26, 2012: Vesta departure
  • c. February 2015: Ceres arrival
  • c. July 2015: End of primary operations

  Vesta approach

As Dawn approached Vesta, it could take progressively higher-resolution images. The images were taken by the Framing Camera instrument, and published online and at news conferences by NASA and MPI.

On May 3, 2011, Dawn acquired its first targeting image, 1,200,000 kilometres (750,000 mi) from Vesta, and began its approach phase to the asteroid.[63] On June 12, Dawn's speed relative to Vesta was slowed in preparation for its orbital insertion 34 days later.[64]

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.[65] NASA later confirmed that it received telemetry from Dawn indicating that the spacecraft successfully entered orbit around Vesta.[66] The exact time of insertion could not be confirmed, as it depended on Vesta's mass distribution, which has only been estimated.[67]

On December 8, 2011, Dawn reached its low-altitude mapping orbit, 210 kilometres (130 mi) above the surface of Vesta.[68]

  Vesta orbit

After being captured by Vesta's gravity and entering its orbit on July 16, 2011, Dawn moved itself to a lower, closer orbit by running its xenon ion rocket engine via solar power. On August 2, it paused its spiralling approach to enter a 69-hour survey orbit at an altitude of 2,750 kilometres (1,710 mi). It assumed a 12.3-hour high-altitude mapping orbit at 680 kilometres (420 mi) on September 27, and finally entered a 4.3-hour low-altitude mapping orbit at 210 kilometres (130 mi) on December 8, 2011.[68][69][70] 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 kilometres (140 mi) across. NASA scientists furthermore stated that they believed Vesta to be the "last of its kind" – the only remaining example of the large planetoids that came together to form the rocky planets during the solar system's genesis.[71][72]

  Ceres

Dawn is planned to depart Vesta and begin its journey to Ceres on August 26, 2012.[4] It is scheduled to arrive at Ceres in 2015, five months prior to the arrival of New Horizons at Pluto; Dawn will thus be the first mission to study a dwarf planet at close range. Dawn's mission profile calls for it to enter orbit around Ceres at an initial altitude of 5,900 kilometres (3,700 mi). The spacecraft will reduce its orbital distance to 1,300 kilometres (810 mi) after five months of study, then to 700 kilometres (430 mi) after another five months.[73]

  See also

Comparable missions
Other related articles

  References

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  59. ^ Malik, Tariq (February 18, 2009). "Asteroid-Bound Probe Zooms Past Mars". Space.com. http://www.space.com/missionlaunches/090218-dawn-asteroid-mars.html. Retrieved 2010-03-21. 
  60. ^ "Dawn Receives Gravity Assist from Mars". NASA/JPL. 2009-02-28. http://dawn.jpl.nasa.gov/mission/status.asp. Retrieved 2009-08-04. 
  61. ^ "Dawn Spacecraft Current Location". http://neo.jpl.nasa.gov/orbits/fulltraj.jpg. Retrieved July 18, 2011. 
  62. ^ "GSpace Topics: Dawn". http://planetary.org/explore/topics/dawn/. Retrieved 2010-03-21. 
  63. ^ http://www.nasa.gov/mission_pages/dawn/news/dawn20110511.html NASA
  64. ^ http://neo.jpl.nasa.gov/orbits/fullview4.jpg
  65. ^ Wall, Mike (July 16, 2011). "NASA Spacecraft Now Orbiting Huge Asteroid Vesta ... Hopefully". Space.com. http://www.space.com/12306-dawn-vesta-asteroid-orbit-arrival.html. Retrieved July 17, 2011. 
  66. ^ Jonathan Amos (10:50, 17 July 2011). "Dawn probe orbits asteroid Vesta". BBC News. http://www.bbc.co.uk/news/science-environment-14160135. 
  67. ^ Vega, Priscilla; Brown, Dwayne (July 16, 2011). "NASA's Dawn Spacecraft Enters Orbit Around Asteroid Vesta". NASA. http://www.nasa.gov/mission_pages/dawn/news/dawn20110716.html. Retrieved July 17, 2011. 
  68. ^ a b Dawn Mission > Mission Status > 2011, JPL
  69. ^ SpringerLink.
  70. ^ Spacegrant.org (PDF).
  71. ^ "Asteroid Vesta is 'last of a kind' rock". BBC, 11 May 2012.
  72. ^ "Incredible video 'fly-over' by Nasa's Dawn probe reveals huge rippled asteroid Vesta is more like a small planet". Daily Mail, 15 May 2012. Retrieved 23 May 2012.
  73. ^ Rayman, Marc (2006-07-13). "Dawn: mission description". UCLA—IGPP Space Physics Center. Archived from the original on 2010-01-18. http://www-ssc.igpp.ucla.edu/dawn/mission.html. Retrieved 2007-04-27. 

  External links

  Dawn mission patch

Media related to Dawn (spacecraft) at Wikimedia Commons

   
               

 

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