Dawn MissionMars Gravity AssistPreliminary Report for GRaND, v1.1

Prepared by Tom Prettyman

3-Mar-2009

Overview

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GRaNDacquired neutron and gamma ray counting data during Mars Gravity Assist
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Data acquired up to S/C entry into safe-mode (occurred after MCA at an altitude of 3000 km)
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GRaNDgot close enough to Mars to see neutrons and gamma rays:
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Mars closest approach (MCA) was about 550 km
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About 16 min were spent below 1000 km during which time the S/C was nadir-pointing to within about 10°
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The space background environment was quiet, ideal for science data acquisition
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GRaNDwas nominal during the encounter
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Preliminary results with comparisons to the Odyssey NS are presented

Operations and Data Analysis

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Instrument settings
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35s measurement intervals below 16000 km
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Timestamp regression was not observed
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Data Volume
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295 science data records
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Level 1a data delivered to DSC
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Higher order data products under development
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Instrument was powered on successfully following MGA yesterday (2-Mar-09)
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Continued data acquisition followed by power-off in DC024
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Anneal operations deferred
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Lessons learned (Should HV sequence be separate from Power ON and parameter set?)

Mars Closest Approach

18-Feb-2008, LS=212˚ Mid Autumn in the Northern Hemisphere
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Thermal

Epithermal

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°

°

°

Trajectory (below 1000 km)

The seasonal cap in the northern hemisphere was thin. At high northern latitudes, the neutron spectrometer was primarily sensitive to the water-rich regolith.

GRaNDsubsatellitepoints and Field of View

Counting Data

test61.png

Frame from QuickTime movie: GRaNDData MCA.mov

6Li(n,t)

Background

Signal

Background

Neutron Counting Data

Nadir-pointing Li-glass

CAT1, 6Li(n,t) peak

Fast neutrons CAT4, nadir+sides

Time relative to MCA (s)

Time relative to MCA (s)

Net counts per 35s

Net counts per 35s

The counting data were averaged using a 5-point sliding window

Analysis Method: Simple Approach

Simulated, footprint-averaged Odyssey data prepared prior to Dawn/MGA

Procedure:
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Correct GRaNDtime series data for altitude and S/C orientation
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Divide by the solid angle subtended by Mars at the S/C
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Divide by the cosine of the S/C angle relative to nadir
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Compare the corrected GRaNDcounting rate for each measurement to the Odyssey counting rate averaged over GRaND’sfootprint

Expected counting rate for GRaND, based on Odyssey measurements

The corrected counting rate is sensitive to footprint averaged variations in surface composition

Comparison Measured GRaNDcounting data to Odyssey

Odyssey, footprint-averaged thermal (counts/s)

Odyssey, footprint-averaged fast (counts/s)

GRaNDcorrected (counts/35s)

GRaNDcorrected (counts/35s)

6Li(n,t) peak

Fast, nadir+sides

Sub-satellite latitude (degrees)

Sub-satellite latitude (degrees)

GRaNDcorrected (counts/35s)

Fast, nadir+sides

GRaNDcorrected (counts/35s)

6Li(n,t) peak

GRaND

Odyssey

(linear stretch)

GRaND

Odyssey

(linear stretch)

R2= 0.85

R2= 0.94

Detailed Analysis

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Neutrons
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Compare modeled counting rates based on neutron leakage spectra to GRaNDdata
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Accounts for neutron decay, ballistic trajectories and relative motion of the S/C (ram effect)
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Gamma rays
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Compare O and Fe counting rates to mapped ODY/GRS data available in the PDS
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Analysis will include implications for science at Vestaand Ceres
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Results to be presented at EGU
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Manuscript on GRaNDperformance during cruise and MGA in preparation
Power law

Conclusions

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Mars Gravity Assist was a tremendous success from the standpoint of acquiring useful GRaNDdata
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GRaNDmeasured gamma rays and neutrons originating from Mars
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GRaNDneutron counting rates are strongly correlated with footprint-averaged Odyssey data
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The GRaNDdata will be useful for calibration and mission planning
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Results presented here should be regarded as preliminary -addition work on data reduction and analysis methods is needed in order to provide accurate comparisons to Odyssey