Athos, Porthos, Aramis, & D'Artagnon:
Four Planning Scenarios for Planetary
Protection
David K. Lynch
Glenn E. Peterson
The Aerospace Corporation
AIAA-2004-1417
2004 Planetary Defense Conference: Protecting Earth from Asteroids
Feb 23-26, 2004
Athos, Porthos, Aramis, & D'Artagnon
Acknowledgements
The authors are indebted to Clark Chapman,
Rusty Schweickart, David Morrison, Ivan
Bekey, Don Yeomans and Louis Friedman for
critical comments (and we do mean critical) on
an early version of this paper.
Athos, Porthos, Aramis, & D'Artagnon
Topics
• Motivation and Purpose
• The Four Scenarios
• Comments on Mitigation Issues
– Determining Mass
– Determining Structural Properties
Athos, Porthos, Aramis, & D'Artagnon
Motivation and Purpose
•
•
•
•
•
Previous scenarios used different objects with different masses,
densities, orbits, impact speeds, times until impact, etc.
We constructed scenarios to cover a range of possibilities with as much
information as could be expected with in a few months of discovery.
The goal was to provide mission planners with defined threat (DEFT)
scenarios that were sufficiently specific to design realistic mitigation
missions.
We hoped that people attending this meeting would plan their papers
around one of the four missions. 1/4 of papers here address one of the
four scenarios.
We feel that in order to properly prepare for a threatening object,
building upon standardized scenarios is the only way to enable nextgeneration thinking and technology development.
Athos, Porthos, Aramis, & D'Artagnon
Design of the Four Scenarios
• The design process consisted of:
– Assume an impact date and impact location on Earth.
– Choose an initial impact velocity (magnitude & direction) and back
out the heliocentric orbit
– Alter the impact velocity until a reasonably realistic orbit is achieved
– Assign physical parameters (density, size, spin) based on typical
examples of real objects
– BUT make two of the asteroids on the smaller end of the scale in
order to make sure we have “doable” scenarios
– Assign uncertainties based upon reasonable tracking and current
levels of physical parameter knowledge
Athos, Porthos, Aramis, & D'Artagnon
The Four Scenarios
• Athos is a low inclination, prograde rubble pile discovered 11 years
before impact. 7 years before impact it is found to have a satellite
(deWinter), also thought to be a rubble pile.
• Porthos is a long period comet discovered less than 3 years before
impact. It’s a test of a “crash” program: What can we do, RIGHT NOW?
• Aramis is a C-type asteroid discovered 27 years before impact. The
asteroid’s properties in this scenario are presented as a series of
progressively more accurate information, as studies reveal mire detail and
precision.
• D’Artagnon is a solid body Aten. It too represents “crash” program,
with only5 years between discovery and impact, not unreasonable for an
Aten.
Athos, Porthos, Aramis, & D'Artagnon
The Four Scenarios
NEO Name
Mass
(g)
radius
(km)
Density
(g cm-3)
Properties
Discovery
Impact
Orbit
Athos
deWinter
1.1E13
6.3E11
0.2x0.1x0.07
~0.07 radius
3.5
3.5
S rubble
S rubble
2005
2009
2016
Low inc pro
Porthos
1.1E15
2x1x1
1.0
LP comet
2013
2015
Hi inc ret
D’Artagnon
2.7E12
0.13x0.12x0.11 3.0
S solid
2004
2009
Aten
Aramis
1.2E15
1.8x1.2x0.8
1.3
C rubble
2006
2033
*********************** MOST VALUES CHANGE WITH TIME AS MORE IS LEARNED **************************
Also present in Table 1&2 of the paper are impact, location and vector, orbital elements, spin properties, uncertainties on all
quantities and other information needed to plan a mitigation mission.
___________________________________________________________________
S
Asteroid type, metallic nickel-iron mixed with iron- and magnesium-silicates
C
Asteroid type, carbonaceous
LP
Long Period
Aten
Venus-crossing orbit.
Athos, Porthos, Aramis, & D'Artagnon
Determining Mass
• Mass M is estimated by
•
– Assuming an optical (~0.5 µm) albedo A
– Computing size (radius r), assuming reflectivity and sphericity
based on heliocentric and geocentric distances.
• r = [(I/Io)R2GD2/A]1/2 or r ~ A-1/2
– Adopting a density r.
M = (4/3)πr3r = (4/3)πr (I R2GD2/ Io A)3/2
• M ~ r /A3/2 and dM ~ A-3/2 dr - (3/2)rA-5/2 dA
• Io, R, D are easily determined to high accuracy (G less so).
• What are the uncertainties in A and r?
I = observed brightness, Io = solar constant, R = heliocentric distance, D = geocentric
distance, G geometric factor , A = albedo, r = density
Athos, Porthos, Aramis, & D'Artagnon
Uncertainties in A
• Albedo can only be directly determined by imaging it, i.e.
resolving it - a space craft fly-by is usually necessary.
• 15 asteroids have been measured well enough to determine A.
• For the vast majority of the others, the values derived from
occultations and thermal models can be used (Walker 2002) but
are much less reliable.
0.01 < A < 0.4
• If the object has not been well-observed in the thermal infrared,
a “best-guess” value is adopted.
• If good IR spectra are available, there can still be problems if
there is significant spectral structure present.
Athos, Porthos, Aramis, & D'Artagnon
Uncertainties in r
•
r can only be directly determined by resolving the
asteroid and measuring its mass M by its gravitational
perturbation on another body. r = M/(4/3)πr3
• 10 asteroids have had their densities directly determined.
•
•
•
•
•
•
•
•
•
•
•
Asteroid
1 Ceres
2 Pallas
4 Vesta
16 Psyche
20 Massalia
45 Eugenia
121 Hermione
243 Ida
253 Mathilde
433 Eros
Density (gm cm-3)
2.05 ± 0.05
4.2 ± 0.3
4.3 ± 0.3
1.8 ± 0.6
2.7 ± 1.1
1.2 (+0.6,-0.3)
1.8 ± 0.4
2.7 ± 0.4*
1.3 ± 0.2
2.67 ± 0.03
Volume Reference
Merline et al. 1996
Drummond & Cocke 1988
Thomas et al. 1997
Viateau 1999
Bange 1998
Merline, et al. 1999
Viateau 1999
Petit et al. 1997
Veverka et al. 1997
Yeomans, et al. 2000
Iron meteorites
have densities of
around 8 g cm-3.
Athos, Porthos, Aramis, & D'Artagnon
Summary of A and r Uncertainties
• Damage to Earth goes as kinetic energy MV2/2
• V will be accurately known. M is the greatest unknown. M
is derived from A and r.
• Significant uncertainties exist in r and A, and “average”
values cannot be used.
• Spacecraft flybys capable of imaging and gravitational
perturbation monitoring are required for any threatening
object.
• Such spacecraft add complications, cost and time to any
mitigation missions(s).
Athos, Porthos, Aramis, & D'Artagnon
Asteroids Are Not Spherical
Gaspara
Eros
Ida and dactyl
Toutatis
Mathilda
Antiope
From a “pushing” standpoint, none of them are round. Also, an orbiter’s
orbit may not be stable.
Athos, Porthos, Aramis, & D'Artagnon
Determining Structural Properties
•
There is growing evidence than many asteroids are rubble piles.
– fast rotating asteroids (Harris 1996; Pravec & Harris 2000)
– extremely low mean density derived for observed bodies (Yeomans et al. 1997)
– tidal disruption by Jupiter of comet Shoemaker-Levy 9 (Asphaug & Benz 1994)
•
•
We know that hard, solid asteroids with significant mechanical
strength exist because we see them as meteorites, usually chondrites.
To assess mechanical properties, we have to “thump” the asteroid and
see how it “rings”.
–
•
•
Seismologists are good at this.
There is also significant uncertainty as to the surface properties (hard
rock or layer of dust?)
We need orbiters and landers with significant remote sensing
capabilities and long lifetimes.
Athos, Porthos, Aramis, & D'Artagnon
Summary
•
We have presented four Earth-threatening scenarios and specified them
in greater technical detail than has been done before.
– The goal is to to give mission planners enough info to develop a
mitigation effort with a high degree of specificity, i.e to “dry-run” a real
mission.
•
•
To mount such a mission, a good deal of in situ remote sensing will be
necessary to characterize the NEO and define a successful mission.
An orbiter and lander will almost certainly be needed prior to actual
deflection, unless the lead-time is too short.