Artist's concept of Dawn with Vesta (left) & Ceres (right) (the proximity of Vesta to Ceres is not to scale.)
|Major contractors||Orbital Sciences, JPL, UCLA|
|Satellite of||Mars flyby; Vesta; Ceres|
|Launch date||September 27, 2007, 11:34:00 UTC |
|Launch vehicle||Delta II 7925H|
|Mission duration||8 years
elapsed: 2 years, 5 months, and 19 days
|Home page||Dawn Home|
|Mass||1,250 kg (2,800 lb)|
Dawn is a robotic spacecraft sent by NASA on a space exploration mission to the two most massive members of the asteroid belt: the asteroid Vesta and the dwarf planet Ceres. Launched on September 27, 2007, Dawn is scheduled to explore Vesta between 2011 and 2012, and Ceres in 2015. It will be the first spacecraft to visit either body.
Dawn is innovative in that it will be the first spacecraft to enter into orbit around a celestial body, study it, and then re-embark under powered flight to proceed to a second target. All previous multi-target study missions—such as the Voyager program—have involved rapid planetary flybys.
The Dawn mission to Vesta and Ceres is managed by NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, for NASA's Science Mission Directorate, Washington.
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 7:20 a.m. – 7:49 a.m. 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, then the launch had to wait for the end of a collision avoidance window with the ISS. The spacecraft launched at 7:34 a.m. EDT from pad 17-B on a Delta II launch vehicle.
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, an originally scheduled course correction maneuver in 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 to the subsequent mission. The root cause of the event was reported to be a software programming error.
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; Ceres is theorized to have experienced a "cool and wet" formation that may have left it with subsurface water, and Vesta is theorized to have experienced a "hot and dry" formation that resulted in a differentiated interior and surface volcanism.
Using two redundant framing cameras, a visual and infared spectrometer, and a Gamma Ray and Neutron Spectrometer, Dawn will take pictures and measure the chemical composition of Ceres and Vesta.
NASA posts the current location of Dawn on the web.
An extended mission in which Dawn explores other asteroids after Ceres is also possible, although unlikely, as greater return is expected by spending the available time at Vesta and Ceres.
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 German Aerospace Center (DLR) and the Max Planck Institute for Solar System Research 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.
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 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 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 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. To get to Vesta, Dawn will use 275 kg (606 lb) Xe and another 110 kg (243 lb) to get to Ceres, out of a total of 425 kg (937 pounds) of on-board propellant. All in all, it will perform a velocity change of over 10 km/s, far more than any other spacecraft has done. Dawn is NASA’s first purely exploratory mission to use ion propulsion engines.
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.