1. 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

2. ASTEROIDS CAN ESCAPE FROM THE MAIN BELT AND BECOME NEOs

3. By numerically integrating the dynamics of a large number of particles we can quantify the statistics of the orbital evolutions

4. 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.

5. Step 1: Find “Primary” NEO Source Regions Nu6 Each source produces NEOs with a distinctive orbital distribution MC OB 3:1 JFC

6. 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

7. Step 2: Combine NEO Sources IMC 3:1Outer MBJFC

8. Step 3: Create Model NEO Distribution We cannot compare our NEO model with data until we account for observational biases!

9. Step 4: Create Biased NEO Distribution  Combine NEO model with the probability than an object with given (a,e,i,H) with be discovered by Spacewatch.

10. Step 5: Compare Biased Model with NEO Data Continue Until “Best-Fit” Found (4) (3) (1) (2) (5)

11. Comparison Between NEOs and Best-Fit Model Source contributions 66 0.37 ± 0.08 IMC0.25 ± 0.03 3:10.23 ± 0.08 Outer MB0.08 ± 0.01 JFC0.06 ± 0.04 Model fit to 138 Spacewatch NEOs with H < 22

12. 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%

13. 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

14. Using our NEO albedo distribution model we predict 834 bodies with D>1km, against 963 with H<18

15. 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

16. 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 km 2 /y; our NEO model predicts : 2.73x10 -14 km 2 /y

17. 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

18. 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

19. 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

20. PROSPECTS FOR ACHIEVING THE SPACEGUARD GOAL WITH A GROUND BASED SURVEY LINEAR LSST? Current completeness Current time

21. 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

22. NEO sky density viewed from 1 AU

23. 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)

24. 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

25. CONCLUSIONS We have a model of the (a,e,i,H) NEO distribution We estimate 963 NEOs with H 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

26. CONCLUSIONS We have a model of the (a,e,i,H) NEO distribution We estimate 963 NEOs with H 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

27. CONCLUSIONS We have a model of the (a,e,i,H) NEO distribution We estimate 963 NEOs with H 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

28. CONCLUSIONS We have a model of the (a,e,i,H) NEO distribution We estimate 963 NEOs with H 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

29. CONCLUSIONS We have a model of the (a,e,i,H) NEO distribution We estimate 963 NEOs with H 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

30. CONCLUSIONS We have a model of the (a,e,i,H) NEO distribution We estimate 963 NEOs with H 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

31. 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

32. 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

33. 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