MODELING THE NEOs ORBITAL DISTRIBUTION AND NEO DISCOVERY STRATEGIES
A. Morbidelli (OCA)
R. Jedicke (Spacewatch)
W.F. Bottke (SWRI)
P. Michel (OCA)
P. Tanga (OCA, Obs. Turin)
ESA Contract No. 14018/2000/F/TB
ASTEROIDS CAN ESCAPE FROM THE MAIN BELT AND BECOME NEOs
By numerically integrating the dynamics of a large number of particles we can quantify the statistics of the orbital evolutions
WE HAVE DEVELOPED A NEO DISTRIBUTION MODEL BY:
1. USING NUMERICAL INTEGRATIONS
2. CALIBRATING THE FREE PARAMETERS USING THE OBSERVATIONAL DATA
Our approach consists of 5 steps.
Step 1: Find “Primary” NEO Source Regions
Nu6
Each source produces NEOs with a distinctive orbital distribution
MCMC
OB 3:1
JFC
PRINCIPLE:
The distribution of the residence times is equal to the steady-state orbital distribution of the NEOs coming from the considered source.
Step 1 (continued): Determine the orbital distribution of NEOs coming from each Source
Nu6
Step 2: Combine NEO Sources
IMC 3:1 Outer MB JFC
n u 6 IM C 3:1 O u ter M B JF Cs
C om b in e N E O S ou rcesR (a ,e,i)
Step 3: Create Model NEO Distribution
C om b in e N E O S ou rcesR (a ,e,i)
A b s. M ag. D is trib u tionN (H)
D eb iased N E O O rb its M odel (a ,e,i,H ) = R (a ,e,i) N (H )
We cannot compare our NEO model with data until we account for observational biases!
Step 4: Create Biased NEO Distribution
O b servation al B iasesB (a ,e,i,H)
D eb iased N E O O rb its M odel (a ,e,i,H)
" O b served " N EO D istrib u tionn (a ,e,i,H ) = B (a ,e,i,H ) M odel (a ,e ,i,H)
Combine NEO model with the probability than an object with given (a,e,i,H) with be discovered by Spacewatch.
Step 5: Compare Biased Model with NEO Data
O b servation al B iasesB (a ,e,i,H )
n u 6 IM C 3:1 O u ter M B JF Cs
C om b in e N E O S ou rcesR (a ,e,i)
A b s. M ag. D is trib u tionN (H)
D eb iased N E O O rb its M odel (a ,e,i,H )
" O b served " N EO D istrib u tionn (a ,e,i,H )
C om p are w ith S p acew atch N E O D atan (a ,e ,i,H ) = "K now n N E O s"?
Continue U
ntil “Best-F
it” Found
(4)
(3)
(1)
(2)
(5)
Comparison Between NEOs and Best-Fit Model
Source contributions
6 0.37 ± 0.08
IMC 0.25 ± 0.03
3:1 0.23 ± 0.08
Outer MB 0.08 ± 0.01
JFC 0.06 ± 0.04
Model fit to 138 Spacewatch NEOs
with H < 22
Debiased Orbital and Size Distribution of NEOs
There are ~ 970 NEOs with H < 18 and a < 7.4 AU.
~50% of them have been found so far.
60% come from the inner main belt (a < 2.5 AU).
Amor: 32%; Apollo: 62%; Aten:6%; IEO: 2%
A SPECTRAL DISTRIBUTION MODEL
From the spectral distribution of the bodies in/close the 5 main NEO sources we compute the spectral distribution of NEOs as a function of their orbital distribution.
We estimate that:
1) the C/S ratio for an H-limited sample of the NEO population is 0.25 +/- 0.02
2) The C/S ratio for a size-limited sample of the NEO population is 0.87+/- 0.05
Using our NEO albedo distribution model we predict 834 bodies with D>1km, against 963 with H<18
To obtain a mass distribution we assume, in agreement with recent determinations by flyby missions or satellite detections, that:
1) C-type NEOs have density 1.3 g/cm
2) S-type NEOs have density 2.7 g/cm
With this, we have all ingredients to estimate the frequency of NEO collisions with the Earth as a function of impact energy:
ENERGY
(MT)
AV. INT.
(Y) H D
(M)
COMPLET.
(%)
1,000 63,000 20.5 277 18
10,000 241,000 19.0 597
37
100,000 925,000 17.3 1287 49
3
3
WE PREDICT 4X LESS IMPACTS THAN PREVIOUSLY ESTIMATED
Difference is likely due to an estimated smaller number of NEOs, different orbital distribution, improved bulk densities etc.
The « measured » formation rate of 4 km craters on the Moon is: 3.3+/-1.7x10 -14 km2/y; our NEO model predicts : 2.73x10-14km2/y
NEO DISCOVERY STRATEGIES
How to achieve the Spageguard goal (90% of H<18 NEOs within 2008) and beyond (90% of H<20.5 NEOs)?
• Characterization of existing major surveys (LINEAR)
• How to achieve the Spaceguard goal with a LINEAR-esque
strategy
•Space-based strategies
We have constructed a pseudoLINEAR simulator, that simulates the average sky coverage of LINEAR and its average limiting magnitude V=18.5
QUALITY TEST I:
In 2 years LINEAR increased the detected population of the NEOs with H<18 from 273 to 449.
Our pseudoLINEAR simulator takes 2.14 years
QUALITY TEST II:
The orbital-magnitude distribution of the first 469 NEOs with H<18 discovered by our pseudoLINEAR simulator mimics very well that of the 469 objects discovered so far by LINEAR and other surveys
PROSPECTS FOR ACHIEVING THE SPACEGUARD GOAL WITH A GROUND BASED SURVEY
LINEAR
LSST?
Current completeness
Current time
SPACE BASED SURVEYS
A space-based survey that duplicates the strategy of ground-based surveys will never be competitive in term of cost.
A space-based survey must take advantage of the location of the instrument in space by either:
•Observe at small solar elongation or,
•Search for NEOs from a point closer to the Sun than the Earth
NEO sky density viewed from 1 AU
Discovery efficiency of satellites with V=18.5 on NEOs with H<18
(Ideal situation with daily full sky coverage, except 45deg close to the Sun)
WARNING: the fact that a space-based survey detects NEOs that are not visible from the ground, implies that one cannot count on ground-based recoveries for follow-up and orbital determination
Ground-based ecliptic coordinates of NEOs at the time of their discovery form a space observatory inside Venus’ orbit
A space-based survey must be able to do its own follow-up
CONCLUSIONS
• We have a model of the (a,e,i,H) NEO distribution
• We estimate 963 NEOs with H<18 and 855 with D>1km
• Our model predicts 4x less collisions than Shoemaker’s
• We predict one 1,000MT collision every 63 Kyear
• These collisions are caused in average by H ~ 20.5
• The Spaceguard goal should be extended to H=20.5 NEOs
CONCLUSIONS
• We have a model of the (a,e,i,H) NEO distribution
• We estimate 963 NEOs with H<18 and 855 with D>1km
• Our model predicts 4x less collisions than Shoemaker’s
• We predict one 1,000MT collision every 63 Kyear
• These collisions are caused in average by H ~ 20.5
• The Spaceguard goal should be extended to H=20.5 NEOs
CONCLUSIONS
• We have a model of the (a,e,i,H) NEO distribution
• We estimate 963 NEOs with H<18 and 855 with D>1km
• Our model predicts 4x less collisions than Shoemaker’s
• We predict one 1,000MT collision every 63 Kyear
• These collisions are caused in average by H ~ 20.5
• The Spaceguard goal should be extended to H=20.5 NEOs
CONCLUSIONS
• We have a model of the (a,e,i,H) NEO distribution
• We estimate 963 NEOs with H<18 and 855 with D>1km
• Our model predicts 4x less collisions than Shoemaker’s
• We predict one 1,000MT collision every 63 Kyear
• These collisions are caused in average by H ~ 20.5
• The Spaceguard goal should be extended to H=20.5 NEOs
CONCLUSIONS
• We have a model of the (a,e,i,H) NEO distribution
• We estimate 963 NEOs with H<18 and 855 with D>1km
• Our model predicts 4x less collisions than Shoemaker’s
• We predict one 1,000MT collision every 63 Kyear
• These collisions are caused in average by H ~ 20.5
• The Spaceguard goal should be extended to H=20.5 NEOs
CONCLUSIONS
• We have a model of the (a,e,i,H) NEO distribution
• We estimate 963 NEOs with H<18 and 855 with D>1km
• Our model predicts 4x less collisions than Shoemaker’s
• We predict one 1,000MT collision every 63 Kyear
• These collisions are caused in average by H ~ 20.5
• The Spaceguard goal should be extended to H=20.5 NEOs
CONCLUSIONS (2)
• To achieve a satisfactory completeness on the H<20.5 NEO population, ground based surveys should be pushed to V=24
• Spaced-based surveys can be effective only if– They observe at small solar elongation– They observe from a point placed a smaller
heliocentric distance than the Earth
• Space-based surveys must do their own follow-up work
CONCLUSIONS (2)
• To achieve a satisfactory completeness on the H<20.5 NEO population, ground based surveys should be pushed to V=24
• Spaced-based surveys can be effective only if– They observe at small solar elongation– They observe from a point placed a smaller
heliocentric distance than the Earth
• Space-based surveys must do their own follow-up work
CONCLUSIONS (2)
• To achieve a satisfactory completeness on the H<20.5 NEO population, ground based surveys should be pushed to V=24
• Spaced-based surveys can be effective only if– They observe at small solar elongation– They observe from a point placed a smaller
heliocentric distance than the Earth
• Space-based surveys must do their own follow-up work
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