The Deep Impact Mission
Karen J. Meech, Astronomer
Institute for Astronomy
ESO, Feb 13, 2004
Photo: Olivier Hainaut (MKO, ESO)
Comets Inspire Terror




Sudden appearance in sky
Only a few bright naked-eye comets / century
Tail physically large  millions of km
Early composition: toxic chemicals
Historical Highlights
1066
1456
1531
1744
1858
1811
1861
1901
Halley
Halley
Halley
De Cheseaux
Donati
Flaugergeus
Tebbutt
Great S
Wm conqueror
Excommunicated
Obs by Kepler
6 tails
Most beautiful
comet wine
Naked eye, aurorae
Daytime visibility
Historical
Understanding

Tycho Brahe 1577


Edmund Halley




Parallax – outside atm.
1531, 1607, 1681
Orbit determination
Newton – Principia
1950’s – Models


Whipple  ‘Dirty Snowball’
Lyttleton  ‘Sandbank’
Physical Processes - Sublimation
Physical Processes


Sublimation of gases
Drags dust from nucleus






Gravity low
Most dust escapes
Solar radiation pressure
 coma  dust tail
photodissociation
Ionization  gas tail
Energy Balance
Sunlight  Scattered light + Heating/Sublimation + Conduction
Usually very small
Energy needed depends on ice
Inverse square law: 1/r2
A. Gomez
Comet Spectra



Reflected sunlight from dust
(blackbody radiation)
Emitted “heat”
Fluorescence
1P/Halley, 1910
Archaeological
Remnants




Icy debris left from
formation
Keys to chemistry &
physics in nebula
Preservation of interstellar material?
Sources of organics
 necessary for life
Comet Paradigms

“Comets are the most
pristine things in the
Solar System”

“Comets tell us about
the formation of the
Solar System
Comet Formation
Ice Physics




Ices condense T < 100K trap gasses
T < 30, trap @ solar abundance
Fractionation @ higher T
Annealing, 35K, 60K – gas release
Comet Formation Regions
• Oort:
• form in Jupiter-Neptune zone
• KBO:
• form in-situ
• hot population scattered out
• 1/3 scatter to Oort cloud
• Oort  LP comets, HF SP comets
• KBO  Centaurs  JF SP comets
Evolutionary Processes

Pre-Solar Nebula


Accretion phase


Sublimation/re-condense
Storage in Oort Cloud







CR bombardment
Radiation damage
Volatile loss
Chemical alteration
Heating from stars, SN
Radioactive Decay
Gardening / erosion
Active Phase



Loss of surface
Crystallization of ice
Build up of dust mantle
Aging Processes

Build up of surface dust




Lower albedo
Large grains cannot
leave
Uneven surface  jets
Non gravitational
acceleration
Observing Techniques





Sun-warmed ices
vaporize, drag dust
Ground-based telescopes
observe when bright
Complex processes &
chemistry
Primordial composition?
Comet surface evolves
over 4.5 Billion years
Comet Missions






Giotto Halley
1986
Flyby
Deep Space 1 9/01
Flyby
Stardust
1/04
Sample return
CONTOUR
3/12
Tour 3 comets
Deep Impact
4/05
Active Experiment
Rosetta(ESA) 2015
Orbit/Lander
ESA Giotto Mission

1P/Halley – March 1986






ESA – Giotto
USSR – Vega
Size 15.3 x 7.2 x 7.22 km
Sunward Jets (from
“craters”)
Mass spec: CHON
particles
Plasma experiments
Deep Space 1




Encounter with 19P/Borrelly 9/22/01
Flyby distance 3417 km
8 km long nucleus
Large albedo variations (0.009-0.03)
Stardust Results



Entered coma 12/31/03
Dust collection 1/2/04
Close approach



236 km
Comet diam 5 km
Pass through zero
phase
The Deep Impact Mission

Primary Goal



Differences between
interior and surface
Pristine Solar System
material
Secondary Goal




Cratering physics
Assess comet impact
hazard
Calibrate crater record
Comet evolution
Simple but Challenging, 33 yrs 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
Mission Overview

The Deep Impact mission will launch in 1/05 and arrive
at comet 9P/Tempel 1 7/4/05; impacting the comet with a
370 kg impactor @10.2 km/sec. The goals are


Uncover the primordial nature of the comet
Learn about impact cratering

The pre-encounter observations are used to understand
the nucleus properties (size, rotation, albedo, activity,
dust environment) to plan for the encounter, and to
establish a baseline for comparison post encounter

To date the observations include



> 200 nights of data
Participation by > 25 astronomers
Participation from 17 telescopes, world-wide
Interplanetary Trajectory
• Launch Dec 2004
• Encounter July 4, 2005
• Geocentric Dist
• Heliocentric Dist
• Approach phase
• Solar Elong
0.89 AU
1.49 AU (q)
63o
104o
Approach & Encounter
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
Spacecraft Overview
Instruments
MRI, ITS, HRI
Imagers
Parameter
HRI
MRI
ITS
FOV [mrad]
2.05
10.2
10.2
IFOV [mrad] 2
10
10
Dl [mm]
0.3-1.0
0.3-1.0
PSF FWHM <1.3
[@0.7mm]
<0.6
<0.6
Full Frame
Rate [s-1]
1/1.7
1/1.7
Radiometric Stars 0.1s
Sensitivity
m~11.3
Stars 0.1 s
m~11.3
Stars
m~11.3
Boresight
Alignment
<1 mrad
N/A
0.3-1.0
1/1.7
<1 mrad
HRI Spectrograph
Slit FOV
2.6Mrad
IFOV
10 mrad
Dl
1.05-4.8 mm
PSF FWHM
< 1 pix
l/dl
744 @ 1.04 mm
209 @ 2.6 mm
385 @ 4.8 mm
Cratering Physics

Gravity control expected




Strength control possible





Size (& ejecta speed) depends on impactor density
Smaller crater than gravity control
Greater depth/diameter
Details sensitive to impactor shape
Compression control possible



Size & time sensitive to comet properties
Size ~ (impactor mass)1/3; insensitive to other properties
Ejecta speed, jets – sensitive to other properties
Scaling relationships not known
Mechanism used to explain Mathilde’s craters
Distinguish mode by ejecta morphology and crater size
Formation Time Scaling
Different Cometary Bulk Densities
m1/6
T~
T ~ rc-2/3
T ~ Rc-2/3
550
50
450
Crater Formation Time (s)
800-sec observing
window provides large
margin for extreme
cometary properties,
even down to bulk
density 0.1 g/cc
(Affects Gravitational Acceleration)
400
350
300
250
200
Surface Density = 0.3 g/cc
150
100
200
400
600
Im pactor Mass (kg)
Most important thing is to know impactor properties
1000
Baseline Predictions


Gravity Controlled
Crater




Diameter – 110m
Depth – 27 m
Formation Time 200s
Ejecta



Max v = 2 km/s
Negligible boulders
Ejecta clumping -> tracking
(mass)

Long-term changes



New active area (dys to
months)
Increase ratio of CO and
CO2 to H2O
Simulations  Mass
determination




Dv = 1.09 x 10-3 mm/s
Below doppler limit
Need “sub-surface” flyby
Ejecta plume can get mass
HRI Spectroscopy
1400
H2O
Surface Brightness (kR)
1300
150 K
145 K
140 K
135 K
CO Requirement
Pre-Impact
3.5 um Requirement
1200
1100
1000
900
800
700
600
500
H2CO
CO 2
400
300
200
100

CO
0
2.5
3.0
3.5
4.0
Wavelength (microns)
4.5
5.0
Halley spectra @ 42000 km
Ames Vertical Gun Facility





Experiments: P. Schultz
Cu sphere @ 4.5 km/s
Target: porous pumice
(1 g/cc)
500 frames / sec
60o impact angle
Gravity control
Strength dominated


Cone detaches
Volatiles – drive
ejecta, fill in cone
Gravity dominated

Expected scenario
Simulations: J. Richardson
Ejecta Plume Simulations
Modelling Mass / Density


Viewing time 900 s
Use velocity to est M
Simulations: J. Richardson
Ground-Based Support

Characterize nucleus

Size & Albedo


Rotation period & pole







RN = 2.6 +/- 0.2, pv = 0.07
Periods 22.104, 42.091 hr
(a,d) = 283+/-3, 18+/-3,
(a,d) = 62+/-3, 73+/-3
a:b = 3.3+/-0.2
a = 5.4, b=c=1.6+/-0.2
Phase Function
Baseline for activity
Dust Environment
10 microns
R band

Dust models  velocity distn, size distn, Qdust




Evaluate motion of dust after leaving comet
Add up the scattered light from grains
Fit to observations of surface brightness of coma versus time
Want observations spread so observing geometry changes a lot


Dust
Small dust (fast) – many images/short time (mostly anti-solar)
Large dust – equally spaced – long periods (monthly) (along orbit)
Critical periods
 Mar-Apr 04


Onset
Feb-Jul 05

STSP
Feb 15 2005
Jan 1 2004
Mar 1 2004
Apr 15 2005
May 15 2005
May 1 2004
Jun 15 2005
Mauna Kea: Keck 10m,
UH2.2m














M. Belton
N. Samarasinha
B. Mueller
P. Massey
R. Millis
CTIO: 4m, 1.5m


KPNO: 4m,
Wiyn3.5m, 2.1m

M. Mateo
N. Suntzeff
K. Krisciunas
T. Farnham

Bohyunsan 1.8m
(Korea)



Lowell 72”
42”

Y-C. Choi
D. Prialnik
Wise 1.1m
(Israel)

Participating Observatories
McDonald: 2.7m
82”

H. Boehnhardt
O. Hainaut
K. Meech



G. P. Tozzi
J. Licandro
ESO: VLT8.0m,
NTT3.6m,
Dan1.5m



K. Meech, M. F. A’Hearn
M. Belton, C. Lisse
Y. Fernandez, J. Pittichova
H. Hsieh, G. Bauer
S. Sheppard, P. Henry

TNG 3.6m


Y-C. Choi
D. Prialnik
M. Buie
Comet Paradigms

“Comets are the most
pristine things in the
Solar System”

“Comets tell us about
the formation of the
Solar System
Stardust Mission

Timeline






Launch 2/7/99 – Delta II
Dust 1: Feb-May 2000
Dust 2: Aug-Dec 2002
Enter coma: Dec 31, ’03
Earth Return 1/15/06
Science Goals



Comet imaging – 81P/Wild 2
ISM Dust collection
Comet dust collection
Earth collection



Arrival 1/15/06
Final descent via parchute
Curation and study – Johnson Space Center
Dust Collection

Captured in aerogel



99.8% air
40x more insulation
than fiberglass
No heating at 6.1 km/s