Dawn (spacecraft)

The Dawn Mission is a NASA mission that will send a robotic space probe to the asteroid belt. The Dawn spacecraft will orbit and examine the asteroid belt's two most massive members, the dwarf planet Ceres and the asteroid Vesta. Dawn will be the first spacecraft to enter into orbit around two different planetary bodies other than the Earth and Moon, and the first to visit the largest asteroid. Launch is scheduled for June 30, 2007.

Launch

On April 10, 2007, Dawn arrived at the Astrotech Space Operations subsidiary of SPACEHAB, Inc. in Titusville, Florida, where it will be prepared for launch. Dawn will launch from Cape Canaveral Air Force Station Launch Complex 17 on June 30, 2007 at 15:13:15 p.m. EDT, aboard a Delta 7925H rocket.

Mission

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 vulcanism.

Planned flight trajectory

To cruise from Earth to its targets it will use three DS1 heritage Xenon ion thrusters (firing only one at a time) to take it in a long outward spiral. The current estimated chronology is as follows:

• June 2007: launch
• March 2009: Mars gravity assist
• October 2011: Vesta arrival
• April 2012: Vesta departure
• February 2015: Ceres arrival
• July 2015: End of operations

An extended mission in which Dawn explores other asteroids after Ceres is also possible.

Mission team

The Dawn mission team is led by UCLA space scientist Christopher T. Russell. By March 2006, Orbital Sciences Corporation has completed more than 90% of the spacecraft, and NASA's Jet Propulsion Laboratory is providing the Ion Propulsion System and management of the overall flight system development. The German Aerospace Center (DLR) and the Max Planck Institute for Solar System Research is providing the framing camera, the Italian Space Agency is providing the mapping spectrometer, and the DOE Los Alamos National Laboratory is providing the gamma ray and neutron spectrometer.

Motivation

Dawn is intended to study two large asteroids in order to answer questions about the formation of the solar system.

Ceres and Vesta were chosen as two contrasting protoplanets, 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 encompasses about one-fourth of the total mass of the asteroids in the asteroid belt and whose spectral characteristics suggest a composition like (but not exactly) a water-rich carbonaceous chondrite. Smaller Vesta, a water-poor achondrite, 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 similar spectra 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.

Mission cancellations and reinstatements

Dawn in the middle of 2006

The status of the Dawn mission has changed dramatically. In December, 2003, the project was first cancelled, and then reinstated in February, 2004. In October, 2005, work on Dawn was placed into "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 cancelled by NASA headquarters.

In an unusual step, the cancellation was placed under review, and on 27 March, 2006, it was announced that the mission would not be cancelled after all.

In the last week of September 2006, the Dawn mission instrument payload integration reached a full functional status. Until Dawn's launch on (approximately) June 30 2007, there will be necessary environmental and other tests, but a launch is no longer in doubt.

See also

• Delta II
• Ceres (dwarf planet)
• 4 Vesta
• List of spaceflights (2007)

External links

• Dawn Mission Home Page at JPL
• Dawn Website at UCLA Space Physics Center
• Visual and Infrared Spectrometer Instrument at INAF (Istituto Nazionale di Astrofisica)
• Dawn Framing Camera at Max Planck Institute for Solar System Research
• Gamma Ray and Neutron Spectrometer for Dawn, short paper on the instrument, from 37th Lunar and Planetary Science Conference