User:Xession/Draft4

 Dawn is a 1250 kg robotic space probe launched by NASA on September 27, 2007, to study and characterize asteroid 4 Vesta and dwarf planet Ceres. The mission will be the first to study objects in the asteroid belt and first to enter into orbit around multiple celestial bodies; all previous missions with multiple targets—such as the Voyager program—have been planetary flybys.

History
Dawn is intended to study two large bodies in the asteroid belt in order to answer questions about the formation of the solar system.

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 our understanding between the formation of rocky planets and the icy bodies of our solar system, and under what conditions a rocky planet can hold water.

The IAU adopted a new definition of planet on August 24, 2006, and thus, 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 New Horizons 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. Smaller Vesta, a 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.

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).

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 found meteoritic samples on Earth, the Howardite Eucrite Diogenite ("HED") meteorites, are thought to be the result of a collision or collisions with Vesta.

The mission's goal is to characterize the conditions and processes of the solar system's earliest epoch by investigating in detail two of the largest protoplanets remaining intact since their formation. Ceres and Vesta have many contrasting characteristics that are thought to have resulted from them forming in two different regions of the early solar system; Peter Thomas of Cornell University has proposed that Ceres has a differentiated interior; its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle. 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.

To cruise from Earth to its targets, Dawn will travel in a long outward spiral trajectory. The estimated chronology is as follows:
 * September 27, 2007: launch
 * February 17, 2009: Mars gravity assist
 * July 16, 2011: Vesta arrival
 * est. July 2012: Vesta departure
 * est. February 2015: Ceres arrival
 * est. July 2015: End of primary operations

NASA posts and continually updates the current location and status of Dawn online.

An extended mission following the completion of the Ceres study is also possible, although unlikely, as greater return is expected by spending the available time at Vesta and Ceres. 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 also not specifically allocated to break orbit from Ceres, and will depend upon the details of the flight reaching Ceres.

Mission cancellations and reinstatements The status of the Dawn mission has changed several times. In December 2003, the project was first cancelled, 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. On March 2, 2006, Dawn was publicly, but not formally canceled by NASA headquarters.

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, and on March 27, 2006, it was announced that the mission would not be canceled after all. In the last week of September 2006, the Dawn mission instrument payload integration reached a full functional status.

The Dawn mission team is led by UCLA space scientist and Dawn Principal Investigator Christopher T. Russell. 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 DOE Los Alamos National Laboratory provided the gamma ray and neutron  spectrometer.

The Dawn mission is managed by NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, for NASA's Science Mission Directorate in Washington, D.C..

The Dawn microchip
Onboard Dawn is a small computer microchip bearing the names of more than 360,000 space enthusiasts. The names were submitted online as part of a public outreach effort between September 2005 and November 4, 2006. The microchip (about the size of a nickel) was installed above the forward ion thruster, underneath the spacecraft's High Gain Antenna, on May 17, 2007. 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.

Attitude control and propulsion
The Dawn spacecraft is propelled by three DS1 heritage xenon ion thrusters (firing only one at a time). They have a specific impulse of 3,100 s and produce a thrust of 90 mN. The whole spacecraft, including the ion propulsion thrusters, is powered by a 10 kW triple-junction photovoltaic solar array manufactured by Dutch Space. To get to Vesta, Dawn is allocated 275 kg Xe, with another 110 kg to reach Ceres, out of a total capacity of 425 kg (937 pounds) of on-board propellant. 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. Dawn is NASA's first purely exploratory mission to use ion propulsion engines.

Scientific instruments
{| The camera uses an f/7.9 refractive optical system with a focal length of 150 mm. A frame-transfer charge-coupled device (CCD) at the focal plane has 1024 × 1024 sensitive 93-μrad pixels, yielding a wide field of view. An 8-position filter wheel permits panchromatic and spectrally selective imaging. The broadest filter allows imaging from about 450 to 920 nm. In addition, FC will acquire images for optical navigation in the vicinities of Vesta and Ceres. Because FC is so vital for both science and navigation, the payload actually includes two identical and physically separate cameras for redundancy, each with its own optics, electronics, and structure.
 * colspan="2" style="background: #e5e5e5" | Framing camera (FC)
 * colspan="2" style="background: #e5e5e5" | Framing camera (FC)
 * Principal investigator: Horst Uwe Keller / Max-Planck-Institut fur Aeronomie


 * colspan="2" style="background: #e5e5e5" | Visual and infrared spectrometer (VIR)
 * colspan="2" style="background: #e5e5e5" | Visual and infrared spectrometer (VIR)

The instrument is a modification of the visible and infrared thermal imaging spectrometer flying on Rosetta and Venus Express. It also draws heritage from Cassini's visible and infrared mapping spectrometer. VIR spectral frames are 256 (spatial) × 432 (spectral). 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. An array of HgCdTe photodiodes cooled to about 70K spans the spectrum from 0.95 to 5.0 μm.
 * Principal investigator: Angioletta Coradini / Instituto di Fisica dello Spazio Interplanetario

This instrument has heritage from similar instruments flown on Lunar Prospector and Mars Odyssey. It will be used to measure the abundances of the major rock-forming elements (O, Mg, Al, Si, Ca, Ti, and Fe) as well as K, Th, and U, and water (inferred from hydrogen content). Dawn will take pictures and measure the chemical composition of Ceres and Vesta.
 * colspan="2" style="background: #e5e5e5" | Gamma Ray and Neutron Detector (GRaND)
 * colspan="2" style="background: #e5e5e5" | Gamma Ray and Neutron Detector (GRaND)
 * Principal investigator: William C. Feldman / Los Alamos National Laboratory


 * colspan="2" style="background: #e5e5e5" | Magnetometer (Mag)
 * colspan="2" style="background: #e5e5e5" | Magnetometer (Mag)

--Description.--
 * Principal investigator: Christopher T. Russell / University of California, Los Angeles


 * }

Launch and trajectory
Dawn was scheduled to launch from pad 17-B at the Cape Canaveral Air Force Station on a Delta 7925-H rocket. On April 10, 2007, Dawn arrived at the Astrotech Space Operations subsidiary of SPACEHAB, Inc. in Titusville, Florida, where it was prepared for launch. Launch was originally scheduled for June 20, but was delayed until June 30 due to delays with part deliveries. A broken crane at the launch pad, used to raise the solid rocket boosters, delayed the launch for a week, until July 7, but on June 15 the second stage was successfully hoisted into position. 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, before the launch was delayed further to avoid knock-on delays with the Phoenix mission to Mars, which was successfully launched on August 4.

Launch of Dawn was then rescheduled for September 26, 2007, 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). During the final built-in hold at T-4 minutes, a ship entered the exclusion area offshore, the sea strip where the rocket boosters were likely to fall after separation. The ship was commanded to leave the area. The launch then had to wait for the end of a collision avoidance window with the International Space Station. The spacecraft launched at 07:34 EDT from pad 17-B on a Delta II launch vehicle.

The launch rocket propelled Dawn to 11.46 kilometers per second (25,600 miles per hour) relative to Earth. Thereafter, Dawn's ion thrusters took over.

Current Status
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. 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 not necessary.

Dawn made its closest approach (549 km) to Mars on February 17, 2009, during a successful gravity assist. 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. On May 3, 2011, Dawn photographed Vesta for the first time, and began its approach phase to the asteroid.