Mission Overview
  ================
 
The Dawn spacecraft was successfully launched atop a Delta II rocket
on September 27, 2007.  Dawn is an ion-propelled spacecraft capable
of visiting multiple targets in the main asteroid belt.  In the baseline
mission, Dawn flies to and orbits the main belt asteroids 1 Ceres and
4 Vesta, orbiting Vesta for a period of not less than seven months
and Ceres for not less than five months.  The spacecraft flies by
Mars in a gravity assist maneuver in 2009 en route to Vesta.
 
Dawn carries three science instruments whose data is used to
characterize the target bodies.  The instrument suite consists of
redundant Framing Cameras (FC1 and FC2), a Visible and Infrared mapping
spectrometer (VIR) and a Gamma Ray and Neutron Detector (GRaND).  In
addition to these instruments, radiometric and optical navigation data
is used to determine the gravity field.  The Dawn mission is an
international cooperation with instrument teams located in Germany, Italy,
and the United States.
 
 
  Science Goals
  =============
 
In order to achieve the overall scientific goal of understanding
conditions and processes acting at the solar system's earliest epoch,
the Dawn spacecraft images the surfaces of the minor planets Vesta
and Ceres to determine their bombardment, thermal, tectonic, and possible
volcanic history. It determines the topography and internal structure
of these two complementary protoplanets that have remained intact since
their formation, by measuring their mass, shape, volume, and spin rate
with navigation data and imagery. Dawn determines mineral and elemental
composition from infrared, gamma ray, and neutron spectroscopy to
constrain the thermal history and compositional evolution of Ceres and
Vesta, and in addition provides context for meteorites (asteroid samples
already in hand). It also uses the spectral information to search for
water-bearing minerals.
 
 
  Instruments
  ===========
 
Framing Camera (FC):
 
The Framing Camera is a multispectral imager that also serves as
an optical navigation camera.  The detector is a 1024x1024 pixel
Atmel/Thomson TH7888A CCD with 14 micron pixels.  It has eight filters
numbered F1 through F8, including a broadband (clear) filter and
narrow band filters ranging from 438 nm to 965 nm.  The Framing camera
instrument includes two redundant cameras of identical design, referred
to as FC1 and FC2.  For full information about the FC instrument, see
Schroeder and Gutierrez-Marques (2011).
 
Visible and Infrared Mapping Spectrometer (VIR):
 
VIR is an imaging spectrometer with an optical design derived from
the visible channel of the Cassini Visible Infrared Mapping
Spectrometer (VIMS-V) and from the Rosetta Visible Infrared Thermal
Imaging Spectrometer (VIRTIS).  It has moderate resolution and
combines two data channels in one instrument.  The two data channels,
Visible (spectral range 0.25-1 micron) and Infrared (spectral range
0.95-5 micron), are committed to spectral mapping and are housed
in the same optical subsystem.  The spectrometer has the ability to
point and scan along the direction perpendicular to the slit.  A
complete description of the instrument and its performance can be
found in De Sanctis et al. (2010) and Coradini et al. (2011).
 
Gamma Ray and Neutron Detector (GRaND):
 
GRaND is a nuclear spectrometer that will collect data needed to map
the elemental composition of the surfaces of 4 Vesta and 1 Ceres
(Prettyman et al. 2003B).  GRaND measures the spectrum of planetary
gamma rays and neutrons, which originate from cosmic ray interactions
and radioactive decay within the surface, while the spacecraft is in
orbit around each body.  The instrument, which is mounted on the
+Z deck of the spacecraft, consists of 21 sensors designed to
separately measure radiation originating from the surface of each
asteroid and background sources, including the space energetic
particle environment and cosmic ray interactions with the spacecraft.
A complete description of GRaND is given in the GRaND instrument
paper, Prettyman et al. (2011). Instrument performance during cruise
and Mars Flyby is given by Prettyman et al. (2012).
 
 
  Mission Phases
  ==============
 
(Dates in parentheses are projected at the time of writing.)
 
  Phase Name (Phase ID)              Start time        End time
  -------------------------          ----------------  ----------------
  INITIAL CHECKOUT (ICO)             2007-09-27        2008-01-19T00:00
  EARTH-MARS CRUISE (EMC)            2008-01-19T00:00  2009-02-16T00:00
  MARS GRAVITY ASSIST (MGA)          2009-02-16T00:00  2010-03-23T00:00
  MARS-VESTA CRUISE (MVC)            2010-03-23T00:00  2011-05-03T10:49
  VESTA ENCOUNTER                    2011-05-03T10:49  2012-09-10T21:50
    VESTA SCIENCE APPROACH (VSA)     2011-05-03T10:49  2011-08-11T12:00
    VESTA SCIENCE SURVEY (VSS)       2011-08-11T12:00  2011-08-31T20:26
    VESTA TRANSFER TO HAMO (VTH)     2011-08-31T20:26  2011-09-29T09:59
    VESTA SCIENCE HAMO (VSH)         2011-09-29T09:59  2011-11-02T10:40
    VESTA TRANSFER TO LAMO (VTL)     2011-11-02T10:40  2011-12-12T22:44
    VESTA SCIENCE LAMO (VSL)         2011-12-12T22:44  2012-05-01T11:50
    VESTA TRANSFER TO HAMO 2 (VT2)   2012-05-01T11:50  2012-06-15T10:00
    VESTA SCIENCE HAMO 2 (VH2)       2012-06-15T10:00  2012-07-25T15:10
    VESTA TRANSFER TO CERES (VTC)    2012-07-25T15:10  2012-09-10T21:50
  VESTA-CERES CRUISE (VCC)           2012-09-10T21:50  2014-12-26T02:50
  CERES ENCOUNTER                    2014-12-26T02:50 (2016-07-01)
    CERES SCIENCE APPROACH (CSA)     2014-12-26T02:50  2015-04-24T00:00
    CERES SCIENCE RC3 (CSR)          2015-04-24T00:00  2015-05-09T10:00
    CERES TRANSFER TO SURVEY (CTS)   2015-05-09T10:00  2015-06-04T12:00
    CERES SCIENCE SURVEY (CSS)       2015-06-04T12:00  2015-07-01T00:00
    CERES TRANSFER TO HAMO (CTH)     2015-07-01T00:00  2015-08-16T23:59
    CERES SCIENCE HAMO (CSH)         2015-08-16T23:59  2015-10-23T20:30
    CERES TRANSFER TO LAMO (CTL)     2015-10-23:20:30  2015-12-16T01:00
    CERES SCIENCE LAMO (CSL)         2015-12-16T01:00 (2016-07-01)
  END OF MISSION                    (2016-04-21)       TBD
 
The following mission phase activities are summarized from the Dawn
Dawn Science Plan (Raymond 2007).
 
Initial Checkout (ICO) - ICO covers the 60-day period following launch
and is used to turn on and perform initial checkout of the instruments.
Only a minimal set of instrument checkout activities are performed
during ICO to minimize interference with critical spacecraft checkouts.
 
Cruise Phases - Seven days of non-thrusting periods per year are
designated for science calibration activities.  These periods are
used to perform functional, performance, and calibration tests of the
instruments using stellar and planetary targets.  During cruise,
GRaND measures the response to galactic cosmic rays and energetic
particles in the space environment, characterizing the background
sources.
 
Mars Gravity Assist (MGA) - The purpose of MGA is to add energy to the
spacecraft trajectory to ensure adequate mass and power margins for
the designated trajectory.   In addition, the MGA provides an
opportunity for instrument calibration, a readiness exercise for
Vesta operations, an absolute calibration of GRaND, and an
extended source for calibrating VIR and FC.  VIR obtains
scientifically valuable spectroscopy.  GRaND acquires data for direct
comparison with data from 2001 Mars Odyssey, enabling cross calibration
during flight.  None of the data gathered at Mars are critical to
achieving the goals of the mission.
 
Approach Phases - During the Vesta Approach phase the instruments
go through complete calibration, repeating some of the activities
that were done during the post-launch checkout calibration period,
including annealing GRaND if necessary.  For both Vesta and Ceres
Approach phases, the FC collects rotation characterization (RC) maps
and VIR obtains full-disc spectra coincident with the RCs.  Data
obtained in the Approach phases provide a range of illumination angles
to initialize the topographic model, and data to aid in finalizing
the plans for HAMO and LAMO.  During the approach phases several
searches for hazards (dust, moons) are performed in the near-asteroid
environment.  An additional activity in the Vesta Approach phase is
to exercise the processing streams for the instruments' data, mainly
the FC and VIR, to verify that quicklook products can be produced on
the required timelines, and to check and improve the calibration
parameters.
 
Survey Orbits - The goals for the Vesta and Ceres survey orbits are
to obtain global coverage with VIR, and to create overlapping global
images with the FC in multiple filters.  The VIR map constitutes the
primary (and perhaps only) global reference set.  The VIR and FC global
maps will be used for defining targets to be investigated at lower
altitudes, and the FC data will contribute significantly to the
topographic model.  Cross-calibration of the VIR and FC will be
facilitated by concurrent imaging during this phase.
 
High Altitude Mapping Orbit (HAMO) - The HAMO is used primarily to
create global FC maps of the lighted surface of the body in multiple
filters from a nadir attitude, and two clear-filter maps from two
different off-nadir viewing angles.  These maps will be used to
create a topographic model.  VIR will also collect at least 5000
frames to sample the spectral variability at smaller scales than the
global survey map, and to build up high-resolution coverage of
areas of interest.  Additional imaging is planned near the end of
the encounter (after LAMO), as the spacecraft is spiraling out from
the asteroid, to capture different lighting conditions and fill in
gaps in the coverage.  These activities will not constitute a
separate mission phase, but will supplement the HAMO objectives in
order to complete the global FC mosaics in multiple filters, and
obtain supplemental VIR coverage of selected targets.
 
Low Altitude Mapping Orbit (LAMO) - The purpose of LAMO is to obtain
spatially resolved neutron and gamma ray spectra of the asteroid, and
global tracking coverage to determine the gravity field.  In addition,
as much FC and VIR nadir imaging as can be fit into the data buffers
will be obtained.  Data lost or not acquired in HAMO that are needed
to complete the global mapping may be acquired in LAMO using very
short FC exposures or using target motion compensation.  Off-nadir
targeted acquisition is anticipated for a small fraction of the time,
including some very high resolution imaging of selected targets.  A
dedicated gravity campaign (several days to one week) will be used for
resolving the wobble in rotation to measure the moment of inertia of
the asteroid.
 
 
  References
  ==========
 
De Sanctis, M. C., A. Coradini, E. Ammannito, G. Filacchione, M.T. Capria,
S. Fonte, G. Magni, A. Barbis, A. Bini, M. Dami, I. Ficai-Veltroni, and
G. Preti, VIR Team, The VIR Spectrometer, Space Sci Rev,
doi:10.1007/s11214-010-9668-5, 2010.
 
A. Coradini, D. Turrini, C. Federico, G. Magni, Vesta and Ceres: crossing
the history of the Solar system. Space Sci. Rev., 2011.
 
Prettyman, T.H. and W.C. Feldman, PDS Data Processing:  Gamma Ray and
Neutron Detector, version 5.0, Feb. 1, 2012.  [Archived as a document
in the Dawn GRaND Calibrated Mars Flyby data set,
DAWN-M-GRAND-2-RDR-MARS-COUNTS-V1.0.]
 
Prettyman, T.H., W.C. Feldman, F.P. Ameduri, B.L. Barraclough, E.W.
Cascio, K.R. Fuller, H.O. Funsten, D.J. Lawrence, G.W. McKinney,
C.T. Russell, S.A. Soldner, S.A. Storms, C. Szeles, and R.L. Tokar,
Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and
4 Vesta, IEEE Transactions on Nuclear Science Volume: 50, Issue: 4, 1,
August 2003B, pp. 1190-1197.
 
Rayman, M.D., T.C. Fraschetti, C.A. Raymond, and C.T. Russell, Dawn:
A mission in development for exploration of main belt asteroids
Vesta and Ceres, Acta Astronautica 58, 605-616, 2006.
 
Raymond, C.A., Dawn Science Plan, JPL D-31827, 2007. [A copy of this
document is included in the /DOCUMENT directory of each of the Dawn
archive volumes.]
 
Schroeder, S.E. and P. Gutierrez-Marques, Calibration Pipeline, MPS
report DA-FC-MPAE-RP-272, Issue 2, Rev. a, 20 July 2011. [A copy of
this document is included in the /DOCUMENT directory of the Dawn FC1
and FC2 archive archive volumes.]