| MISSION_DESCRIPTION |
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 2016-06-01
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 2016-06-19T12:00
END OF PRIME MISSION 2016-06-19T12:00
CERES EXTENDED MISSION 2016-06-19T12:00 2017-07-01T00:00
CERES EXTENDED LAMO (CXL) 2016-06-19T12:00 2016-09-02T12:00
CERES TRANSFER TO JULING (CTJ) 2016-09-02T12:00 2016-10-10T00:00
CERES EXTENDED JULING (CXJ) 2016-10-10T00:00 2016-11-03T12:00
CERES TRANSFER TO GRAND (CTG) 2016-11-03T12:00 2016-12-14T00:00
CERES EXTENDED GRAND (CXG) 2016-12-14T00:00 2017-02-23T00:00
CERES TRANSFER TO OPPOSITION (CTO) 2017-02-23T00:00 2017-04-28T00:00
CERES EXTENDED OPPOSITION (CXO) 2017-04-28T00:00 2017-07-01T00:00
END OF CERES EXTENDED MISSION (2017-06-30T23:59)
The following mission phase activities are summarized from the Dawn
Dawn Science Plan (Raymond 2007).
Initial Checkout (ICO) - ICO covered the 60-day period following launch
and was used to turn on and perform initial checkout of the instruments.
Only a minimal set of instrument checkout activities were performed
during ICO to minimize interference with critical spacecraft checkouts.
Cruise Phases - Seven days of non-thrusting periods per year were
designated for science calibration activities. These periods were
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 was to add energy to the
spacecraft trajectory to ensure adequate mass and power margins for
the designated trajectory. In addition, the MGA provided 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 could have obtained
scientifically valuable spectroscopy. GRaND acquired data for direct
comparison with data from 2001 Mars Odyssey, enabling cross calibration
during flight. Fortunately, none of the data gathered at Mars were critical
to achieving the goals of the mission. The spacecraft safed shortly after
Mars closest approach. Only a number of images and a few minutes of resolved
GRaND data were recoverable - no VIR spectra were recovered.
Both Vesta are Ceres were intentionally mapped in very similar fashion. This
both reduced planning efforts and results in similar scientific products that
hopefully facilitates comparison of the two bodies.
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. The design of the Vesta and Ceres
approach activities were similar, although scaled to the different
body sizes. For both Vesta and Ceres approach phases, the FC collected
rotation characterization (RC) maps and VIR obtained full-disc spectra
coincident with the RCs. The RC maps were used to accurately determine the
pole positions of the bodies in order to get into nearly polar orbits.
Data obtained in both approach phases provided a range of illumination angles
to initialize the topographic model, and data to aid in finalizing
the plans for HAMO and LAMO. For both bodies, the final RC (RC3) was targeted
at a radius where the full disk just fit within the FC2 FOV. At Vesta, this
occurred at a radius of ~5500 km and at Ceres it was ~14,000 km. During both
approach phases several searches for hazards (dust, moons) were performed in
the near-asteroid environment. An additional activity in the Vesta Approach
phase was to exercise the processing streams for the instruments' data, mainly
the FC and VIR, to verify that quicklook products could 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 were
to obtain global coverage with VIR, and to create overlapping global
images with the FC2 in multiple filters. The VIR Survey maps constitute the
primary global reference set. The VIR and FC2 global maps were used for
defining targets to be investigated at lower altitudes, and the FC data
contribute significantly to the topographic models. Cross-calibration of the
VIR and FC was facilitated by concurrent imaging during this phase.
High Altitude Mapping Orbits (HAMO) - HAMO was used primarily to
create global FC2 maps of the illuminated surface of the body. HAMO altitudes
were selected to provide full global maps in a small number of orbits with
sufficient resolution of meet our Level 1 requirements for topography in both
horizontal and vertical dimensions. For Vesta, a full mapping (Cycle) was
completed in 10 orbits at a radius of ~950 km. At Ceres, the Dawn resolution
requirements were half the values for Vesta so the orbit radius was increased
to ~1950 km and 12 orbits were required to complete a cycle. Color filter data
were acquired at or near nadir for two complete mapping cycles. This provided
redundancy so that it was not necessary to recover individual lost images or
orbits. Clear filter data were acquired in both nadir and off-nadir attitudes
to meet the topography requirements. Fixed off-nadir attitudes were flown for
complete mapping cycles. Different off-nadir angles were selected for each of
the cycles in order to support both SPG (stereo) and SPC (clinometry)
topographic analysis. VIR also collected as much data as could be supported by
our downlink ability during HAMO. The various off-nadir angles allowed
different latitude bands to be efficiently mapped at both Vesta and Ceres.
VIR collected several times the minimum requirement of at least 5000 frames
in the HAMO orbits where it sampled the spectral variability at smaller scales
than the global survey map. At the HAMO altitudes, the GRaND instrument begins
to see particles originating from the target body, in addition to the cosmic
background.
Low Altitude Mapping Orbit (LAMO) - The purpose of LAMO was to obtain
spatially resolved neutron and gamma ray spectra of each asteroid, and get
global tracking coverage to determine the gravity field. There were no
Level-1 requirements to collect any images or VIR spectra at the LAMO
altitudes at either Vesta or Ceres. However, Dawn collected as much FC2
and VIR nadir imaging as could be fit into the data buffers. In general,
during LAMO, the spacecraft needed to be pointed at nadir to meet the
GRaND requirements. There were no off-nadir images, and very few color
filter images acquired at Vesta in the LAMO orbit. At Ceres, Dawn was able
to extend the duration of LAMO by conserving fuel. Once GRaND had met its
Level-1 requirements and FC completed a clear filter map at nadir, Dawn began
to acquire some targeted color images and eventually some off-nadir mapping
cycles. The orbit of Dawn was extremely difficult to predict so most of the
Ceres color imaging and targeted VIR cubes did not fully cover the planned
targets in LAMO. Off-nadir coverage in LAMO was insuffient to allow high
resolution global shape models to be produced but a few regional models
can be created for selected targets (Occator, etc.).
HAMO-2 (Vesta) Dawn arrived at Vesta just before the southern summer and the
obliquity of the orbit prevented the illumination of the northern hemisphere
above about 30 degrees latitude. A short extended mission at Vesta was
negotiated with NASA that allowed Dawn to delay its Ceres arrival date and
expected end-of-mission. Dawn used this time at Vesta to extend the LAMO phase
and add a second HAMO during the spiral out to Ceres. During the 2nd HAMO
the subsolar latitude had moved nearly to the equator and Dawn was able to
map nearly all of the northern hemisphere with the FC2 and greatly extend
the VIR coverage at HAMO resolution. HAMO-2 was flown at the same radius as
HAMO.
Ceres Extended Mission
Dawn was allowed to extend its mission at Ceres for roughly one year in
order to acquire key data that were not acquired during the prime mission.
The extended mission included three additional mapping cycles at the LAMO
altitude in order to collect VIR spectra over high value targets Occator and
Juling that were unsuccessfully observed in the prime mission. In addition,
off-nadir clear filter images were acquired to add to the high resolution
topography and persistently shadowed region data sets. Additional GRaND and
gravity data were also acquired. This first extended mission phase is referred
to as eXtended Mission Orbit 1 (XMO1) or Ceres eXtended LAMO (CXL).
As soon as it was possible, the spacecraft was moved to a higher altitude
(XMO2) as quickly as possible to conserve fuel. Dawn maintained an altitude
that was very similar to the Prime Mission HAMO altitude for about three
weeks. At this altitude, the VIR instrument observed Juling under a variety of
local time and illumination conditions while the camera acquired additional
clear and color filter data and data in the persistently shadowed regions.
This phase is also referred to as Ceres eXtended Juling or CXJ.
After the Juling observations were complete, the spacecraft altitude was
raised again as quickly as possible so that GRaND could acquire the long
duration background data necessary to properly calibrate the LAMO data.
This orbit is called XMO3 and the phase is referred to as CXG (for GRaND). At
this radius (~8000 - 9500 km, elliptical), Dawn acquired several full
rotation observations to look for surface changes since RC3. In the prime
mission when the orbit altitude was lowered, it was done in a very controlled
fashion in order to maintain a circular orbit with a desired period. During
the extended mission ascent, the 'fast as possible' raising of the altitude
in order to conserve fuel led to elliptical mapping orbits.
Finally, the orbit altitude was raised into a very elliptical orbit with
apoapsis high enough to allow the orbit plane to be changed by 90 degrees
(~55,000 km). This maneuver was performed so that Ceres could be observed at
opposition (zero phase) on the inbound leg at an altitude near 20,000 km.
This last orbit is called XMO4 and the mission phase is CXO (Opposition).
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.]
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