Deep Impact
First Look Inside a Comet
Michael F. A’Hearn
Outline
•
•
•
•
Scientific Objectives, Mission Overview, Context
Cratering Physics
The Target and Environment
Measurements and Observations
EMCSC 22 June 2001
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Fundamental Goal
• Explore the interior of a cometary
nucleus
• Recreate a natural phenomenon
under controlled circumstances
• Excavate a football field 7 stories
deep in a true, controlled
experiment
• Conceptually simple!
• Technically challenging!
EMCSC 22 June 2001
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Simple But Challenging
Even 33 Years Ago
"It [an asteroid] was racing past them at almost thirty miles a second;
they had only a few frantic minutes in which to observe it closely. The automatic cameras
took dozens of photographs, the navigation radar's returning echoes were carefully recorded
for future analysis - and there was just time for a single impact probe.
The probe carried no instruments; none could survive a collision at such cosmic speeds.
It was merely a small slug of metal, shot out from Discovery on a course which should
intersect that of the asteroid.
.....They were aiming at a hundred-foot-diameter target, from a distance of thousands of
miles...
Against the darkened portion of the asteroid there was a sudden,
dazzling explosion of light. ...”
____________________
Arthur C. Clarke, 1968. In 2001: A Space Odyssey. Chapter 18
EMCSC 22 June 2001
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Science Team
M. F. A’Hearn, PI
Management, Emission
Spectra, Coma relation to
nucleus, PDS Archiving
M. J. S. Belton, Deputy PI
Imaging, Rotation
A. Delamere
Instrumentation
J. Kissel
Dust, Ejecta from crater
K. Klaasen
Mission operations,
Geomorphology
L. A. McFadden, EPO Dir.
Outreach, Reflection
Spectroscopy, geology
K. J. Meech
Earth-based observing
program
H. J. Melosh
Cratering - numerical
simulations
P. Schultz
Cratering - experiments
J. Sunshine
Reflection spectroscopy,
analysis
J. Veverka
Relation among comets &
asteroids, Data processing
pipeline
D. K. Yeomans
Dynamics, Radio science
EMCSC 22 June 2001
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Scientific Objectives
•
Primary Scientific Theme
– Understand the differences between interior and surface
– Determine basic cometary properties
– Search for pristine material below surface
•
Secondary Scientific Theme
– Distinguish extinction from dormancy
•
Additional Science Addressed
– Address terrestrial hazard from cometary impacts
– Search for heterogeneity at scale of cometesimals
– Calibration of cratering record
EMCSC 22 June 2001
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Mission Overview
•
•
2 spacecraft – Smart Impactor + Flyby
Fly together until 1 day before impact
– 1-year heliocentric orbit with Earth return to provide lunar calibration
of instruments and test of targeting
– 6-month Earth-to-comet trajectory
•
Smart Impactor
– Impactor Targeting Sensor (ITS)
• Scale 10 microrad/pixel
• Used for active navigation to target site
• Images relayed via flyby to Earth for analysis
– Cratering mass (≥350 kg at 10.2 km/s)
• Excavates 100-meter crater in few*100 seconds
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Mission Overview (continued)
•
Flyby Spacecraft
– Diverts to miss by 500 km
– Slows down to observe for 800 seconds
– Instruments body-mounted – spacecraft rotates to follow comet
during flyby
•
Instruments on Flyby Spacecraft
– High Resolution Imager (HRI)
• CCD imaging at 2 microrad/pixel
• 1-5 micron spectroscopy
– Medium Resolution Imager (MRI)
• CCD imaging at 10 microrad/pixel
• Identical to ITS but with filter wheel added
•
Major Earth-based Observing Campaign
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Spacecraft Overview
Instruments
MRI, ITS, HRI
EMCSC 22 June 2001
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Inter-Planetary Trajectory
Mars at
Encounter
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Interplanetary Trajectory
Earth-Relative Interplanetary Trajectory
Opening Launch Day - January 2, 2004
Rotating Coordinates
Distance from Sun in AU
Encounter
1.6 AU
Earth
One-year
Earth-to-Earth
Loop
t
30 days
1.4 AU
1.2 AU
1.0 AU
Sun
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
0.8 AU
View from North Ecliptic
mfa - 11
Encounter Schematic
Impactor Release
E-24 hours
AutoNav Enabled
E-2 hr
ITM-1 Start
E-88 min
ITM-2
E-48 min
ITM-3
E-15 min
Tempel-1
Nucleus
64
kbps
2-way
S-band
Crosslink
500 km
Flyby S/C
Deflection Maneuver
E-23.5 hr
Science and
Autonav Imaging to
Impact + 800 sec
Flyby S/C Science
And Impactor Data
at 175 kbps*
Flyby Science
Realtime Data
at 175 kbps*
Shield Mode
Attitude through
Inner Coma
TCA +
TBD sec
Flyby S/C Science
Data Playback at 175 kbps*
to 70-meter DSS
* data rates without Reed-Solomon encoding
EMCSC 22 June 2001
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Look-back
Imaging
mfa - 12
Flyby Geometry Varies With Launch
Date
A.
The spacecraft’s position with respect
to the Moon for A) Opening, B) Middle,
and C) Closing Launch Dates.
Sun
Earth/Moon Flyby on
December 31, 2004 for
Opening Launch Date
(January 2, 2004)
CALIBRATION SEQUENCES WILL BE VERY
LAUNCH DATE DEPENDENT!
S/C
Inbound
t
6 hrs
Moon
Orbit
t
1 day
S/C
Outbound
Earth
Launch
Date
(2004)
2-Jan
6-Jan
11-Jan
14-Jan
21-Jan
Perigee
Moon at S/C
Closest Approach
Moon at
S/C Perigee
North Ecliptic View
B.
Sun
Earth/Moon Flyby on
January 9, 2005 for
Middle Launch Date
(January 11, 2004)
Moon
Orbit
t
1 day
Moon at S/C Perigee
Moon at S/C Closest Approach
North Ecliptic View
Sun
Earth/Moon Flyby on
January 20, 2005 for
Closing Launch Date
(January 22, 2004)
Earth
S/C
Outbound
Moon
Orbit
t
1 day
Moon at S/C
Closest Approach
(near Perigee)
EMCSC 22 June 2001
Earth
Flyby
Earth Flyby Altitude
Date/Time
(km)
31-Dec-04 1,350
04-Jan-05 3,447
09-Jan-05 6,357
12-Jan-05 8,235
19-Jan-05 13,146
S/C
Inbound
t
6 hrs
S/C
Inbound
t
6 hrs
Perigee
C.
Lunar
Flyby
Lunar Flyby Distance
Date/Time
(km)
1-Jan-05
245,319
5-Jan-05
108,605
8-Jan-05
207,715
11-Jan-05 118,829
19-Jan-05 379,207
North Ecliptic View
Deep Impact - First Look Inside a Comet
Earth
Perigee
S/C
Outbound
mfa - 13
Context – Comets Unknown
•
Mass – no data
– Density and Surface Gravity uncertainty >10x
•
Strength
– Tensile strength < 103dyn/cm2 at km scale
– Nothing else known
•
Stratification
– Know only irradiated mantle on new comets
– Ice to rock ratio unknown
•
Shape
– Data only for 1P/Halley
•
Photometric Properties very uncertain
•
Coma dust and rocks very uncertain
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Cometary Dichotomies
•
•
•
•
•
•
Comets have the most primitive,
accessible material in the SS
Comets must become dormant
There must be many dormant
comets masquerading as NEAs
We know more chemical and
physical details than for other
small bodies in the SS
Abundances in the coma are
used to infer ices in the protoplanetary disk
Comets break apart under small
stresses
EMCSC 22 June 2001
•
•
•
•
•
•
We do not know what is hidden
below the evolved surface layers
Is the ice exhausted or sealed in?
We can not recognize dormant
comets among NEAs
We do not know how to use these
details to constrain models of
nuclei
Abundances in the coma differ
significantly but in unknown
ways from nuclear abundances
Variation of strength with scale is
totally unknown
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Everything We Know Directly
H. U. Keller
EMCSC 22 June 2001
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What We Won’t Know
•
•
•
•
•
•
•
Shape Details and Topography
Phase Function
Density
Mass
Dust environment
Rotational axis
Cratering Physics
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Nuclear Models
EMCSC 22 June 2001
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Interior Model?
EMCSC 22 June 2001
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Context – Other Small-Body
Missions
Mission
Launch
Encounter
Encounter
Encounter End Mission
NEAR
96/02/17
97/06/27
98/12/23
00/02/14
( 253)Mathilde (433) Eros
(rend)
DS-1
98/10/25 99/07/28
01/09/23
(9969) Braille 19P/Borrelly
Stardust
99/02/12
04/01/02
81P/Wild 2
Muses C
02/12/xx 05/09/xx
1998 SF36
CONTOUR 02/07/04 03/11/12
06/06/18
(08/08/18)
2P/Encke 73P/S-W 3 (6P/d'Arrest)
Rosetta
03/01/20 06/07/10
08/07/23
11/07/27
( 4979)Otawara (140)Siwa 46P/Wirtanen
Deep Impact 04/01/06 05/07/04
9P/Tempel 1
Goal
01/02/12 surface
surface
06/01/15 sample
return
07/06/xx sample
return
diversity
14/xx/xx ~1m deep
+ tomog.
~25m deep
Dates are yy/mm/dd
EMCSC 22 June 2001
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Cratering Physics
Possible Scenarios
•
Crater formation on an intact nucleus
– Gravity controlled crater
– Compression controlled crater
•
•
•
Aerogel-like capture of the impactor
Split nucleus
Crater formation on an intact nucleus
– Strength controlled crater
•
•
Shattered nucleus
Transit through the nucleus
•
Above are roughly in order of decreasing probability (as
guessed by the PI)
N.B.: K.E. of Impactor << Gravitational Binding Energy of
Cometary Nucleus
•
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
D. K. Yeomans
CSR page 1-12
mfa - 22
Cratering Physics
•
Gravity control expected
–
–
–
–
Size and time sensitive to cometary properties
Size ~ (impactor mass)1/3
Size insensitive to other properties
Details of early ejecta (speed, jets) sensitive to shape and density
Strength control possible
Size depends on impactor density (as does speed of early ejecta) –
much smaller than under gravity control;
greater depth/diameter than under gravity control;
details sensitive to shape of impactor
Compression control possible
Scaling relationships not known
Mechanism newly proposed to explain Mathilde’s craters
Distinguish mode by ejecta morphology and crater size
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Ejecta Cone
Figures are for gravity
controlled situations.
If strength controlled,
cone detaches from
surface.
If volatiles exist under
inert material,
vaporization drives
ejecta that tend to fill
in cone.
If compression
controlled, much less
total ejecta in cone
and no final rim.
EMCSC 22 June 2001
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Cratering Flow Pattern
EMCSC 22 June 2001
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EMCSC 22 June 2001
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Crater Scaling
Different Cometary Bulk Densities
Different Target Materials
(Affects Gravitational Acceleration)
Crater Depths
300
50
200
40
200
100
50
150
Depth (m)
Crater Diameter (m)
Crater Diameter (m)
30
20
100
Surface Density = 0.3 g/cc
Cometary Bulk Density = 0.8 g/cc
Surface Density = 0.3 g/cc
Cometary Bulk Density = 0.8 g/cc
Surface Density = 0.3 g/cc
10
100
200
400
600
1000
Impactor Mass (kg)
70
30
100
100
200
400
600
Impactor Mass (kg)
1000
200
400
600
1000
Impactor Mass (kg)
Crater depth combines excavation with compression
and displacement. Varies with target material
D ~ m1/3
D ~ rc-1/6
D ~ Rc-1/6
EMCSC 22 June 2001
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Formation Time Scaling
Different Cometary Bulk Densities
m1/6
T~
T ~ rc-2/3
T ~ Rc-2/3
(Affects Gravitational Acceleration)
550
50
800-sec observing
window provides large
margin for extreme
cometary properties,
even down to bulk
density 0.1 g/cc
Crater Formation Time (s)
450
400
350
300
250
200
Surface Density = 0.3 g/cc
150
100
200
400
600
1000
Impactor Mass (kg)
Most important thing is to know impactor properties
EMCSC 22 June 2001
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Impactor Designed to Optimize
Cratering
Debris
Shields
note:
radiator not
shown is
debris shield
too
Launch vehicle adaptor not shown
Design simplifies adding mass at start of I&T
Stacked plates can easily be made porous
Science traded less copper for more front mass
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
Radiator shown in
translucent blue
mfa - 29
Speed of Early Ejecta
Ejecta Velocities
Comparison
PLATE porous
~ 0.5 to 1.5
km/s
CAP solid
~ up to 5 km/s ,
high initial
temperatures
CAN hollow
~ 2 - 3.5 km/s
Solid Copper
Porous Copper
Porosity of
plate reduces
ejecta velocity!
Easier to track
ejecta!
J. D. O’Keefe
EMCSC 22 June 2001
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Baseline Prediction
•
•
Assumes Gravitationally Controlled Crater
Crater
– Diameter ~110m
– Depth ~ 27m
– Formation Time ~ 200 s
•
Ejecta
– Max velocity ~ 2 km/s
– Negligible quantity of “boulders”
– Clumping of ejecta to allow tracking
•
Long-term changes
– New “active area”
– Outgassing jet that may last days to months
– Increased ratio of CO & CO2 to H2O
EMCSC 22 June 2001
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Target Properties - 9P/Tempel 1
What Can We Know Before Impact?
Wilhelm Tempel
Target Requirements Easily Met
Property
CSR Requirement
Current Value
Radius
> 2 km
2.6 km
Approach Phase
< 70°
63°
Solar Elongation
> 70°
104°
Earth Range
< 1.3 AU
0.89 AU
Rotation Period
“long”
42 h
Dust
Prediction in CSR
Fig 1.2-8
Prediction validated
EMCSC 22 June 2001
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Target Update – Nucleus
•
Size and albedo
– Keck #1 + UH 88”, 2000 August 21; poor weather
– <R> = 2.6±0.5 km, pR = 0.04
•
Rotation
– UH 88” – many runs, HST – 1 run, several runs at Lowell and ESO and La
Palma, since Jan 1999
– Partially analyzed – P ~ 42 hours
– Axis orientation and sense of rotation MAY be determinable well before
impact, but not yet confident
•
Shape
– Axial ratio > 1.3, probably < 2
EMCSC 22 June 2001
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Nuclear Radius Determined
Keck, thermal IR (10.7 mm), 21 Aug 2000 composite
Radial profiles of thermal IR image
separate dust from nucleus
UH 88”, optical (0.7 mm), 21 Aug 2000
EMCSC 22 June 2001
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Slow Rotation of 9P/Tempel 1
P/Tempel 1 Mar 1999 data (Meech et al)
Model: P=1.7478; a=5.8,b=c=3.28km; A=0.04; C-type Hapke; Phi=180@JD2451206.94898
16.0
15.9
15.8
15.7
M(1,1,a)
15.6
15.5
15.4
15.3
15.2
15.1
15.0
2451253
2451254
2451255
2451256
2451257
2451258
2451259
2451260
2451261
2451262
2451263
Julian Days
P/Tempel 1 All 1999 data (Meech et al)
Model: P=1.7478 d; a=5.8,b=c=3.28km; A=0.04; C-type Hapke; Phi=180@JD2451206.94898
16.0
15.9
15.8
15.7
15.6
15.5
M(1,1,a)
15.4
15.3
15.2
15.1
15.0
14.9
14.8
14.7
14.6
14.5
2451197
2451207
2451217
2451227
2451237
2451247
2451257
Julian Days
EMCSC 22 June 2001
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Target Update – Dust
•
IRAS survey remapped and re-calibrated (January 2000), data from
5 days post-perihelion to several months post-perihelion in 1983;
preliminary models fitted
•
IRAS pointed observations to be recalibrated (one pre-perihelion
observation included)
•
Keck run – August 2000 (7 1/2 months post-perihelion)
EMCSC 22 June 2001
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IRAS Survey Image
Tempel 1
Dust Trail in Orbit Plane
Best image of dust
trail from comet
Tempel 1. 1983
Zodiacal
Dust
Density of old dust in
orbit plane is low
compared to dust
currently released near
nucleus!
EMCSC 22 June 2001
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Revised IRAS Results Limit Dust
R. Walker
HCON 421
R=1.56
=1.26
Trail visible
but very faint
EMCSC 22 June 2001
Improved spatial resolution and better flux
calibration (10-50% fainter; 25K hotter).
These are the only data sensitive to large
(> 10 mm) particles in inner coma.
HCON 339 1983 Jul 13, T+5 days
HCON 421 1983 Aug 24, T+46 days
Deep Impact - First Look Inside a Comet
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IRAS and Keck Results
Recalibration of IRAS data
constrains size distribution
of large dust 5 days after
perihelion.
EMCSC 22 June 2001
Light curve implies dust production is dropping at
perihelion. Allows interpolation or extrapolation of
other data to time of our impact. (Scaling of IR data
to optical data is done empirically.)
Deep Impact - First Look Inside a Comet
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Design Models
•
•
Due to uncertainties, must assume specific models to which
system is designed
Models needed include
–
–
–
–
•
Photometric behavior of dust and nucleus,
Shape and topography of nucleus,
Dust environment,
Cratering process
Must consider worst-case models while designing to a nominal
model
Design models are conservative to encompass cases that are
worse, in whatever sense, than best prediction!
Design models allow flight system design to be mature now!
EMCSC 22 June 2001
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Design Model – Photometry
•
Phase function for nucleus derived from recent observations of 2P/Encke
at large phase
• Bi-directional reflectance assumed
• Dust phase function from observations at 1P/Halley
• Dust brightness near opposition scaled from optical observations of
Tempel 1 in 1983
• Nuclear brightness near opposition from HST and UH-88” observations of
Tempel 1 in 1997-1999
_________________
> Average nuclear pixel is brighter than maximum plausible jet brightness at
limb
> Phase function for nucleus is assumed at dark end of range
> Targeting is straightforward (unlike at Halley)
> To be confirmed with data from Borrelly (DS 1) and Encke (CONTOUR)
EMCSC 22 June 2001
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Phase Functions
Design Case Selected from Many
Adopted Phase Laws for Nucleus and Dust
<<<<<
Nuclei
Ga
-0.252
Phase
[deg]
0
5
10
15
20
25
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
Normalized Phase Fcn
Dust
Nucleus
3
6
<=index 0.030m/deg 0.035m/deg 0.040m/deg
1.000
1.000
1.000
1.000
1.000
0.933
0.755
0.871
0.851
0.832
0.900
0.570
0.759
0.724
0.692
0.847
0.431
0.661
0.617
0.575
0.813
0.325
0.575
0.525
0.479
0.793
0.245
0.501
0.447
0.398
0.793
0.185
0.437
0.380
0.331
0.747
0.106
0.331
0.275
0.229
0.720
0.060
0.251
0.200
0.158
0.710
0.034
0.191
0.145
0.110
0.690
0.020
0.145
0.105
0.076
0.667
0.011
0.110
0.076
0.052
0.747
0.006
0.083
0.055
0.036
0.000
0.004
0.063
0.040
0.025
0.000
0.002
0.048
0.029
0.017
0.000
0.001
0.036
0.021
0.012
0.000
0.001
0.028
0.015
0.008
0.000
0.000
0.021
0.011
0.006
0.000
0.000
0.016
0.008
0.004
0.000
0.000
0.012
0.006
0.003
0.000
0.000
0.009
0.004
0.002
0.000
0.000
0.007
0.003
0.001
index=>
0
1
2
EMCSC 22 June 2001
Encke
IAU
#NUM!
0.54602
0.38189
0.27957
0.20897
0.15784
0.11976
0.06892
0.03913
0.02190
0.01245
0.00781
0.00597
0.00551
0.00535
0.00479
0.00355
0.00190
0.00058
0.00006
0.00000
0.00000
3
>>>>>
Q
-0.092
Encke
Orig L-B
#NUM!
0.59631
0.44261
0.34287
0.27133
0.21752
0.17597
0.11771
0.08148
0.05951
0.04707
0.04091
0.03858
0.03818
0.03823
0.03765
0.03571
0.03202
0.02646
0.01907
0.01004
0.00000
4
<<<<<
Dust
>>>>>
Gb
0.15
typ ast
IAU
0.061m/deg
#NUM!
1.000
0.67889
0.755
0.55105
0.570
0.46407
0.431
0.39824
0.325
0.34559
0.245
0.30198
0.185
0.23298
0.106
0.18018
0.060
0.13821
0.034
0.10414
0.020
0.07625
0.011
0.05354
0.006
0.03541
0.004
0.02151
0.002
0.01156
0.001
0.00520
0.001
0.00181
0.000
0.00042
0.000
0.00004
0.000
0.00000
0.000
0.00000
0.000
5
6
Deep Impact - First Look Inside a Comet
Ney &
Kiselev &
Merrill
Chernova
C/West P/Ash-Jack C/Meier
1.00
1.0000
1.0000
0.92
0.7447
0.7798
0.85
0.5649
0.6607
0.77
0.5248
0.6138
0.70
0.5346
0.65
0.6026
0.60
0.50
0.45
0.45
0.45
0.45
0.45
0.55
0.80
1.50
2.50
4.00
7.00
10.00
15.00
20.00
0
1
2
Krasno.
P/Halley
1.0000
0.9333
0.9000
0.8467
0.8133
0.7933
0.7933
0.7467
0.7200
0.7100
0.6900
0.6667
0.7467
Schleich.
P/Halley
1.0000
0.9376
0.8872
0.8318
0.7834
0.7447
0.7244
0.6730
0.6486
0.6607
0.6918
3
mfa - 43
4
Design Model – Shape
Stooke’s Halley Model
(fit to data)
Everything we know!
Gaskell’s Accretion Model
(Theoretical)
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 44
Shape Model Partly Confirmed
•
•
•
Rotational light curve suggests axial ratio < original specification
and < in Gaskell’s theoretical model
No comets have light curves suggesting dumbbell structure
(whereas some asteroids do)
Will evaluate data from Borrelly (DS 1 – Sept 2001) and Encke
(CONTOUR – Nov 2003) to determine if large-scale concavity is
likely to exist
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 45
Design Model – Dust
Best power-law fit
to IRAS spectral
energy distribution
Design Model
CSR Model
CSR model confirmed as good prediction. Design model, defined before
IRAS data were available, is conservative to allow for asymmetries and
other factors.
EMCSC 22 June 2001
Conservative design model has good
margin for uncertainties
Deep Impact - First Look Inside a Comet
mfa - 46
Dust Model Validated
Steeper power laws inconsistent with IRAS data - lack of 10-mm silicate emission
Shallow power laws, such as m-0.2, inconsistent with optical data
No data very sensitive to particles with m > 0.1 g
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 47
Dust Model Validated
•
IRAS data confirm that design model was conservative for mid-range
of dust particles
•
Improved optical scaling also confirms conservatism for smaller particles
•
Distribution by mass not well constrained but definitely different than
for P/Halley
•
Extensive search shows no evidence for significant asymmetries
within coma
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 48
Instruments and Measurements
Instrument Platform Assembly for Flyby
Spacecraft Maintains Instrument and ACS
Sensors in Alignment
Star
Trackers
Debris
Shielding
HRI
Instrument
IRU
Low Gain
Antenna
Instrument
Platform
EMCSC 22 June 2001
MRI
Instrument
Deep Impact - First Look Inside a Comet
mfa - 50
ITS Optics and Electronics Fit Into
Allocated Impactor Volumes
ITS
Instrument
Thermal
Radiator
ITS
Electronics
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
ITS
Thermal
Strap
mfa - 51
Instrument Functional Block
Diagrams
HRI
Dichroic Beamsplitter
Filter Wheel
Shutter
CCD
Telescope
IR FPA
Radiative
Cooler
SIM Bench
IR Spectrometer
CCD
Electronics
LVDS to S/C (Vis)
IR
Electronics
LVDS to S/C (IR)
Electronics
Controller
HRI Electronics
1553 Bus
MRI & ITS
Filter Wheel (MRI only)
Shutter
CCD
1553 Bus
CCD
Electronics
LVDS
Telescope
Controller
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 52
Visible Imagers
Parameter
HRI
MRI
ITS
FOV [mrad]
2.05
10.2
10.2
IFOV [mrad]
2
10
10
l [mm]
0.3-1.0
0.3-1.0
0.3-1.0
PSF FWHM
[pix@0.7mm]
<1.3
<0.6
<0.6
Full Frame
Rate [s-1]
1/1.7
1/1.7
1/1.7
Radiometric
Sensitivity
Stars to
m~11.3 in 0.1
s
Stars to
m~11.3 in 0.1
s
Stars to
m~11.3 in 0.1
s
Boresight
Alignment
<1 mrad
<1 mrad
N/A
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
CCDs
1024x1024 active
area
Bilateral frame
transfer
(2 1024x512
shielded areas)
mfa - 53
IR Spectrometer
Parameter
Capability
Units
Slit FOV
2.6
Mrad
IFOV
10
mrad
l
1.05-4.8
mm
PSF FWHM
<1
Pix@2.5mm
Resolving
Power, l/dl
744@1.04mm
209@2.6mm
385@4.8mm
Radiometric
Sensitivity
CO 300 kR/dl
Full Frame
Rate
1/1.75
EMCSC 22 June 2001
Detector
Rockwell HgCdTe with
“Hawaii” Mux
1024x512 with 2x2
readout binning
s-1 for 1.75s
exposure
Deep Impact - First Look Inside a Comet
mfa - 54
CO Lines Drive HRI IR Sensitivity
Background
1300
32,000
H2O
28,000
Surface Brightness (kR)
1100
24,000
1000
20,000
Pre-Impact Emission Brightnesses
CO Requirement
900
16,000
800
12,000
700
8,000
SNR=1
600
4,000
500
H2CO
400
0
CO2
150 K -4,000
145 K
140 K -8,000
300
200
-12,000
CO
100
-16,000
0
-20,000
2.5
3
3.5
4
4.5
Background (photons/pix/sec/nm)
1200
5
Wavelength (microns)
Removing background suppression (band-limit) filters and reducing bench
temperature to 135K improves limits to 200 kR/dl
Additional measures under consideration
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 55
Geometric Constraints on Data
•
•
Phase angle on approach 63°
Impactor Release
– 1 day before impact
– Range ~ 870,000 km
•
Flyby at impact
– Range ~ 8600 km
•
Flyby at last image
– 800s after impact
– Range ~ 700 km
– Rotation 45°
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 56
Impactor Measurements
•
•
Images for navigation as needed
Images for science at intervals of sqrt(d), where d is distance from
impact
– Early images are full frame
– Later images are sub-frames, down to 128x128, due to limitations of Sband link from impactor to flyby
– Best resolution if no dust hits - 20 cm
– Best resolution if dust hits are major problem - 1.5 m
•
Largest challenge
– Knowing time of impact in order to know when to switch image sizes
– A priori time ±30s 3-s
– Determine to ±5s 3-s from flyby rotation and uplink to shift image
sequence in time
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 57
Flyby Measurements
•
Before impact
–
–
–
•
At time of impact
–
–
•
Images of ejecta cone
Spectra of down-range ejecta
Track ejecta with images
After crater complete
–
–
–
•
High speed imaging subframes (1282)for light curve, initially dt < 0.17s
Shift to full frame at slower rate as time increases
Shortly after impact until crater completely formed
–
–
–
•
Monitor rotation of nucleus (brightness) & coma activity for weeks
Map coma with narrow-band filters
Map nucleus & innermost coma in filters and with spectrometer
Map nucleus & crater in filters and spectrometer
Spectra off limb for changes in outgassing
Final crater image with resolution ~ 3-4m
Look-back imaging
–
–
Minutes to hours after flyby
Images and spectra to study changes in activity and map other side of nucleus
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 58
Analysis Approach
•
•
•
•
•
•
•
•
Determine cratering regime debris cone detachment, lack of
ejecta
Confirm regime from scaling
relations
IF GRAVITY DOMINATED (i.e., one
possible analysis scenario)
Estimate porosity from half-angle
of debris cone
Estimate subsurface structure
from blockiness of crater walls
Estimate density ratio of impactor
to target from shape of
expanding plume
Determine buried ices from gasdriven jet pushing through ejecta
Determine layering of regolith
from crater walls
EMCSC 22 June 2001
•
•
•
•
•
Determine coefficients for scaling
laws applicable to small bodies in
the solar system
Determine composition of ejected
debris from downrange near-IR
spectra
Estimate differentiation of ices by
comparing pre- and post- spectra
of outgassing
Test for amorphous ice by
searching for exothermic reaction
driving outgassing above
sublimation rate
Determine composition of cool
debris and cometary surface from
spectra
Deep Impact - First Look Inside a Comet
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Earth-Based Observing Program
How all astronomers can participate
Earth-Based Geometry
•
•
•
Geocentric Distance ~ 0.89 AU
Solar Elongation ~ 104°
Declination ~ -10°
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 61
Earth-Based Elevations
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 62
Impact Time
80
70
CTIO
Paranal
Elevation Angle (deg)
60
50
Goldstone
40
30
La Palma
20
Goldstone
Madrid
Paranal
Paranal (Nautical)
Paranal (Day)
CTIO
CTIO (Nautical)
CTIO (Day)
La Palma
La Palma (Nautical)
Teide
Teide (Nautical)
DSN Redundancy
10
Madrid
0
Current baseline
70
2100
2300
0100
Canberra
60
IMPACT!
Baseline at CSR orals
Elevation Angle (deg)
3/4 July 2005 (UT)
Mauna Kea
Mauna Kea (Nautical)
Mauna Kea (Day)
Palomar
Goldstone
Canberra
50
Mauna Kea
40
30
Palomar
20
Goldstone
10
IMPACT!
0
EMCSC 22 June 2001
0500
0600
Deep Impact - First Look Inside a Comet
0700
0800
4 July 2005 (UT)
mfa - 63
Earth-Based Observing
Feature
CSR Orals Baseline
PDR Baseline
Chile: CTIO, La
Hawai’i:- Mauna
Prime Region
Silla, Campanas,
Kea, Haleakala
Paranal
CTIO - .44, .75
Weather MKO - .58, .83
La Silla - .42, est .8
photometric, usable
Haleakala - ?, ?
Paranal - .72, est >.9
Canaries
S. California
Backup Region
La Palma - ?, .96 Palomar - ?, est >.9
HST window
EMCSC 22 June 2001
±45 min
Deep Impact - First Look Inside a Comet
±25 min
mfa - 64
Earth-Based Goals
•
•
Thermal and scattered light curves
Emission-line spectroscopy at all wavelengths - euv to radio
– Temporal resolution - 1s allowed by photon statistics for strongest
lines
– Spatial resolution - limited e.g. to 1 arcsec ~ 700 km, i.e. a point source
•
•
•
X-ray emission
Long-term monitoring
Imaging & morphology at all wavelengths
– Spatial & temporal resolution - significant ejecta to > 1 arcsec takes
tens of minutes
– Long-term existence of jets?
– Long-term astrometry for non-gravitational accelerations
EMCSC 22 June 2001
Deep Impact - First Look Inside a Comet
mfa - 65