| MISSION_DESCRIPTION |
Mission Overview
================
The Dawn spacecraft was successfully launched atop a Delta II rocket
on September 27, 2007. Spacecraft operations ceased on October 31,
2018. Dawn was an ion-propelled spacecraft capable of visiting
multiple targets in the main asteroid belt. Dawn flew to
and orbited the main belt asteroids 1 Ceres and 4 Vesta, orbiting
Vesta for about 15 months and Ceres for 3.5 years. The spacecraft flew
by Mars in a gravity assist maneuver in 2009 en route to Vesta.
Dawn carried three science instruments whose data is used to
characterize the target bodies. The instrument suite consisted 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
was used to determine the gravity field. The Dawn mission was 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 imaged the surfaces of the minor planets Vesta
and Ceres to determine their bombardment, thermal, tectonic, and possible
volcanic history. Dawn determined 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 determined 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). Dawn also used 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 collected the data needed to map
the elemental composition of the surfaces of 4 Vesta and 1 Ceres
(Prettyman et al. 2003B). GRaND measured 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
==============
Phase Name (Phase ID) Start time End time
------------------------- --------------- ----------------
INITIAL CHECKOUT (ICO) 2007-09-27 2007-12-17T19:45
EARTH-MARS CRUISE (EMC) 2007-12-17T19:45 2009-02-16T00:00
MARS GRAVITY ASSIST (MGA) 2009-02-16T00:00 2010-03-23T00:00
MARS-VESTA CRUISE (MVC) 2010-03-23T00:00 2011-05-03T10:54
VESTA ENCOUNTER 2011-05-03T10:54 2012-09-10T21:49
VESTA SCIENCE APPROACH (VSA) 2011-05-03T10:54 2011-08-11T12:05
VESTA SCIENCE SURVEY (VSS) 2011-08-11T12:05 2011-08-31T20:00
VESTA TRANSFER TO HAMO (VTH) 2011-08-31T20:00 2011-09-29T09:59
VESTA SCIENCE HAMO (VSH) 2011-09-29T09:59 2011-11-02T10:42
VESTA TRANSFER TO LAMO (VTL) 2011-11-02T10:42 2011-12-12T22:45
VESTA SCIENCE LAMO (VSL) 2011-12-12T22:45 2012-05-01T11:50
VESTA TRANSFER TO HAMO 2 (VT2) 2012-05-01T11:50 2012-06-24T01:00
VESTA SCIENCE HAMO 2 (VH2) 2012-06-24T01:00 2012-07-25T15:08
VESTA TRANSFER TO CERES (VTC) 2012-07-25T15:08 2012-09-10T21:49
VESTA-CERES CRUISE (VCC) 2012-09-10T21:49 2014-12-26T02:50
CERES ENCOUNTER 2014-12-27T02:44 2018-10-31
CERES SCIENCE APPROACH (CSA) 2014-12-27T02:44 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) 2015-10-23T20:30 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 1 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 2016-10-07T10:00
CERES EXTENDED JULING (CXJ) 2016-10-07T10:00 2016-11-04T08:00
CERES TRANSFER TO GRAND (CTG) 2016-11-04T08:00 2016-12-10T05:59
CERES EXTENDED GRAND (CXG) 2016-12-10T05:59 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 2017-06-03T16:50
CERES TRANSFER TO HOLDING (CXH) 2017-06-03T16:50 2017-06-28T02:30
CERES X2 HOLDING (CX2) 2017-06-28T02:30 2017-07-01T00:00
END OF CERES EXTENDED MISSION 1 2017-07-01T00:00
CERES EXTENDED MISSION 2 2017-07-01T00:00 2018-10-31
CERES X2 HOLDING (CX2) 2017-06-28T02:30 2018-04-16T21:00
CERES TRANSFER TO INTERMEDIATE (CTI)2018-04-16T21:00 2018-05-15T11:00
CERES X2 INTERMEDIATE (C2I) 2018-05-15T11:00 2018-05-31T19:30
CERES TRANSFER TO ELLIPTICAL (CTE) 2018-05-31T19:30 2018-06-09T07:30
CERES X2 ELLIPTICAL (C2E) 2018-06-09T07:30 2018-10-31
END OF CERES EXTENDED MISSION 2 2018-10-31
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.
Vesta and Ceres
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 quick-look 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 altitude 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 (XMO4) 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 Ceres extended mission phase is called CXO
(Opposition). Since there would only be one chance to make opposition
observations, Dawn acquired images with both FC1 and FC2 to protect
against complete data loss in the event that FC2 reset during
the observation. The FC1 images were slightly offset in time from the
FC2 images thereby increasing the range of observed phase angles if data
from both cameras were returned. Neither camera reset during this
activity and all images acquired by the two cameras were returned.
Ceres Extended Mission 2
Dawn was allowed to extend its mission at Ceres for roughly one year
(XM2)in order to acquire key data that were not previously acquired at
Ceres. The primary objective of this extension was to acquire the highest
possible resolution imaging and spectra (VIR and GRaND) over Occator
crater. In order to provide the team with time to develop the orbit
transfer and science observation plans, the extended mission began
in a high altitude elliptical orbit (XMO5) below XMO4 to conserve fuel.
This orbit is called the Ceres X2 Holding and the mission phase is CX2.
As soon as it was possible, the spacecraft began to move into an
elliptical orbit (XMO6) with a periapsis altitude slightly above the
previous LAMO orbits. This orbit was designed to provide VIR with
additional observations and provide some FC2 color imaging of targets
of opportunity at HAMO resolution or better. This altitude is called
Ceres X2 Intermediate and the mission phase is C2I and it included
10 orbits. The orbit was designed to target Juling near HAMO altitude
in orbit 6.
After the Intermediate observations were complete, the spacecraft
altitude was changed into the final Elliptical orbit (XMO7) as quickly
as possible for the final orbit's science objectives. This orbit is
called Ceres X2 Elliptical and the phase is referred to as C2E. This
orbit was designed to be resonant with the Occator longitude in order
to maximize the likelihood of acquiring the desired high resolution
data. The orbit was specifically designed to have the 35 km periapsis
over Cerealia Facula in Occator crater during orbit 14. Due to the
high ellipticity of this orbit, the latitude of periapsis drifted south
at roughly 1.7 degrees per orbit. Later in this mission phase, periapsis
drifted across the south pole and on to the dark side of Ceres. The GRaND
and gravity experiments continued to make observations until the end of
mission on Oct 31, 2018 but there were limited observation opportunities
for FC and VIR after Sept 1, 2018.
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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