# Dawn Mission 

## 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.                                                       

## Mission Objectives Overview                                                
                                                                              
The specific Dawn science objectives by instrument are as follows.            
                                                                              
### Framing Camera                                                            
                                                                              
- To determine the origin and evolution of Vesta and Ceres by mapping
the extent of geologic processes on the asteroid surfaces, and by using
the cratering record to establish a relative chronology of the crustal
units and population of impactors in the early solar system.

- To map the shape, determine the spin state, and establish the degree       
of cratering of the asteroids visited.                                        
                                                                              
- To map the topography of Vesta and Ceres.                                  
                                                                              
- To search for dust and satellites in the environment of the                
asteroids visited.                                                            
                                                                              
- To provide a geologic, compositional and geophysical context for           
the HED meteorites.                                                           
                                                                              
- To provide an opportunity to identify Ceres-derived meteorites             
in their geologic context.                                                    
                                                                              
                                                                              
### Visible and Infrared Mapping Spectrometer                                     
                                                                              
- To provide a geologic, compositional and geophysical context for           
the HED meteorites.                                                           
                                                                              
-  To provide an opportunity to identify Ceres-derived meteorites in          
their geologic context.                                                       
                                                                              
- To map the thermophysical properties of Vesta and Ceres.                   
                                                                              
- To determine the origin and evolution of Vesta and Ceres by                
mapping the mineralogical composition and its spatial variation               
across the asteroidal surface.                                                
                                                                              
                                                                              
### Gamma Ray and Neutron Detector                                               
                                                                              
- To map the major elemental composition of O, Si, Fe, Mg, Ti, Al,           
Ca, and H on Vesta and Ceres.                                                 
                                                                              
- To map the trace elements U, Th, K, Gd and Sm on Vesta and Ceres.          
                                                                              
- To provide a geologic, compositional and geophysical context for           
the HED meteorites.                                                           
                                                                              
                                                                              
### Gravity science                                                             
                                                                              
- To determine the masses of the asteroids visited.                          
                                                                              
- To measure the bulk density of Vesta and Ceres, in conjunction             
with topography, and determine its heterogeneity.                             
                                                                              
- To determine the gravitational fields of Vesta and Ceres.   
                                                                              
```
The above science goals are extracted from the Dawn Science Plan              
(Raymond 2007).  Specific science measurement requirements necessary          
to meet the stated science goals are outlined in the same document,           
as well as in Rayman et al. (2006)
```

                                                                              
## 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 X2 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 X2 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.]                                             
