Deep Impact First Look Inside a Comet

Deep Impact First Look Inside a Comet
Michael F. A’Hearn

Deep Impact - First Look Inside a Comet
Outline Scientific Objectives, Mission Overview, Context Cratering Physics The Target and Environment Measurements and Observations EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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 Deep Impact - First Look Inside a Comet

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, In 2001: A Space Odyssey. Chapter 18 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Science Team M. F. A’Hearn, PI Management, Emission Spectra, Coma relation to nucleus, PDS Archiving M. J. S. Belton, Deputy PI Imaging, Rotation 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 Deep Impact - First Look Inside a Comet

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 Deep Impact - First Look Inside a Comet

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 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Spacecraft Overview Instruments MRI, ITS, HRI EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Inter-Planetary Trajectory
Mars at Encounter EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Interplanetary Trajectory
EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

* data rates without Reed-Solomon encoding
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 Flyby S/C Deflection Maneuver E-23.5 hr 500 km Science and Autonav Imaging to Impact sec Flyby S/C Science And Impactor Data at 175 kbps* Shield Mode Attitude through Inner Coma Flyby Science Realtime Data at 175 kbps* TCA + TBD sec Flyby S/C Science Data Playback at 175 kbps* to 70-meter DSS Look-back Imaging * data rates without Reed-Solomon encoding EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Flyby Geometry Varies With Launch Date
The spacecraft’s position with respect to the Moon for A) Opening, B) Middle, and C) Closing Launch Dates. A. CALIBRATION SEQUENCES WILL BE VERY LAUNCH DATE DEPENDENT! B. C. EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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 proto-planetary disk Comets break apart under small stresses 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 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Everything We Know Directly
H. U. Keller EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

What We Won’t Know Shape Details and Topography Phase Function Density Mass Dust environment Rotational axis Cratering Physics EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Nuclear Models EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Interior Model? EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Context – Other Small-Body Missions
Mission Launch Encounter Encounter Encounter End Mission Goal NEAR /02/ /06/ /12/ /02/ /02/12 surface ( 253)Mathilde (433) Eros (rend) DS /10/ /07/ /09/ surface (9969) Braille 19P/Borrelly Stardust /02/ /01/ /01/15 sample 81P/Wild return Muses C /12/xx /09/xx /06/xx sample 1998 SF return CONTOUR 02/07/ /11/ /06/ (08/08/18) diversity 2P/Encke P/S-W (6P/d'Arrest) Rosetta /01/ /07/ /07/ /07/ /xx/xx ~1m deep ( 4979)Otawara (140)Siwa P/Wirtanen tomog. Deep Impact 04/01/ /07/ ~25m deep 9P/Tempel 1 Dates are yy/mm/dd EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Cratering Physics

Possible Scenarios Crater formation on an intact nucleus Gravity controlled crater Compression controlled crater Aerogel-like capture of the impactor Split 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 D. K. Yeomans CSR page 1-12 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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 Deep Impact - First Look Inside a Comet

Cratering Flow Pattern

Crater Scaling Floor Depth Below Surface Bulk Density = 0.3 g/cc Sand Bulk Density = 0.8 g/cc Pumice Excavation Depth Below Surface 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 Deep Impact - First Look Inside a Comet

Formation Time Scaling
T ~ m1/6 T ~ rc-2/3 T ~ Rc-2/3 Bulk Density = 0.3 g/cc 800-sec observing window provides large margin for extreme cometary properties, even down to bulk density 0.1 g/cc Bulk Density = 0.8 g/cc Most important thing is to know impactor properties EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Impactor Designed to Optimize Cratering
Debris Shields note: radiator not shown is debris shield too Launch vehicle adaptor not shown Radiator shown in translucent blue 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

Speed of Early Ejecta Solid Copper Porous Copper Ejecta Velocities Comparison PLATE porous ~ 0.5 to 1.5 km/s CAP solid ~ up to 5 km/s , high initial temperatures CAN hollow ~ km/s Porosity of plate reduces ejecta velocity! Easier to track ejecta! J. D. O’Keefe EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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 Deep Impact - First Look Inside a Comet

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 Deep Impact - First Look Inside a Comet

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 Deep Impact - First Look Inside a Comet

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 Deep Impact - First Look Inside a Comet

Slow Rotation of 9P/Tempel 1

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 Deep Impact - First Look Inside a Comet

IRAS Survey Image Tempel 1 Dust Trail in Orbit Plane Best image of dust trail from comet Tempel Zodiacal Dust Density of old dust in orbit plane is low compared to dust currently released near nucleus! EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Revised IRAS Results Limit Dust
R. Walker 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 421 R=1.56 D=1.26 Trail visible but very faint HCON Jul 13, T+5 days HCON Aug 24, T+46 days EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

IRAS and Keck Results Recalibration of IRAS data constrains size distribution of large dust 5 days after perihelion. 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.) EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Design models allow flight system design to be mature now!
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 Deep Impact - First Look Inside a Comet

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 _________________ 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 Deep Impact - First Look Inside a Comet

Phase Functions Design Case Selected from Many

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

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

Conservative design model has good margin for uncertainties
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. Conservative design model has good margin for uncertainties EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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

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

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 MRI Instrument EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

ITS Optics and Electronics Fit Into Allocated Impactor Volumes
ITS Instrument Thermal Radiator ITS Thermal Strap ITS Electronics EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Instrument Functional Block Diagrams
HRI Dichroic Beamsplitter Filter Wheel Shutter CCD Electronics LVDS to S/C (Vis) CCD Telescope LVDS to S/C (IR) IR FPA IR Electronics Radiative Cooler Electronics Controller IR Spectrometer 1553 Bus SIM Bench HRI Electronics 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

Visible Imagers Parameter HRI MRI ITS FOV [mrad] 2.05 10.2 IFOV [mrad] 2 10 Dl [mm] PSF FWHM <1.3 <0.6 Full Frame Rate [s-1] 1/1.7 Radiometric Sensitivity Stars to m~11.3 in 0.1 s Boresight Alignment <1 mrad N/A CCDs 1024x1024 active area Bilateral frame transfer (2 1024x512 shielded areas) EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

IR Spectrometer Parameter Capability Units Slit FOV 2.6 Mrad IFOV 10 mrad Dl mm PSF FWHM <1 Resolving Power, l/dl Radiometric Sensitivity CO 300 kR/dl Full Frame Rate 1/1.75 s-1 for 1.75s exposure Detector Rockwell HgCdTe with “Hawaii” Mux 1024x512 with 2x2 readout binning EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

CO Lines Drive HRI IR Sensitivity
Background SNR=1 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

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

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 S-band link from impactor to flyby Best resolution if no dust hits - 20 cm Best resolution if dust hits are major problem 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

Flyby Measurements Before impact Monitor rotation of nucleus (brightness) & coma activity for weeks Map coma with narrow-band filters Map nucleus & innermost coma in filters and with spectrometer At time of impact 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 Images of ejecta cone Spectra of down-range ejecta Track ejecta with images After crater complete 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

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 gas-driven jet pushing through ejecta Determine layering of regolith from crater walls 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 EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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

Earth-Based Elevations

Impact Time CTIO Paranal Goldstone DSN Redundancy La Palma Madrid Current baseline 2100 2300 0100 Canberra 3/4 July 2005 (UT) IMPACT! Baseline at CSR orals Mauna Kea Palomar Goldstone IMPACT! 0500 0600 0700 0800 4 July 2005 (UT) EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

Earth-Based Observing
Feature CSR Orals Baseline PDR Baseline Prime Region Chile: CTIO, La Silla, Campanas, Paranal Hawai’i:- Mauna Kea, Haleakala Weather - photometric, usable CTIO - .44, .75 La Silla - .42, est .8 Paranal - .72, est >.9 MKO - .58, .83 Haleakala - ?, ? Backup Region Canaries La Palma - ?, .96 S. California Palomar - ?, est >.9 HST window ±45 min ±25 min EMCSC 22 June 2001 Deep Impact - First Look Inside a Comet

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

Deep Impact First Look Inside a Comet

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