1. Elizabeth A. Silber, Douglas O. ReVelle, Wayne N. Edwards, Peter G. Brown The University of Western Ontario Presented at the Bermuda Infrasound Technology Workshop, November 3-7, 2008

2.  Introduction & Overview  Previous infrasonic influx estimates (AFTAC)  Method  Results  Summary and conclusions

3.  Flux of meter-sized and larger meteoroids still inadequately understood  Why do we care about influx rate?  Meter sized and larger objects can penetrate the atmosphere and produce craters  Critical to dating of young planetary surfaces  Helps quantify the risk of small asteroid impact with Earth  Constrains models of asteroid evolution  Confirms idea that most meter-to-hundred meter sized NEOs are asteroidal (not cometary)  Influx rate estimates come from a variety of sources:  Lunar cratering record (Werner et al., 2001)  Lunar Impact-flash record (Ortiz et al., 2007)  Infrasonic pressure sensor arrays (ReVelle, 1997)  US DoD Satellites (Brown et al., 2002)  Ground-based photographic fireball networks (Halliday et al., 1996)

5.  Global network of ground-based arrays looking for nuclear events  Operated from ~1960 to August 1974  Microbarometer pressure sensors placed 6-12 km apart at 16-25 locations worldwide  Low and high frequency bands, each with its own frequency response filter, 440 – 44 s and 25 s – 8.2 Hz, respectively  Various sources detected (volcanic eruptions, earthquakes, aurora, bolides, etc.)  Ten bolide events detected – most of them were also verified by other methods (seismic, etc.)  Events range from 0.2 kt up to 1,100 kt (S. African bolide)  The S. African bolide is very significant

6.  The most comprehensive estimate to date examined satellite data (Brown et al., 2002)  Various flux estimates differ in results and assumptions – another independent study would be very valuable S. African bolide

7.  Shoemaker & Lowery (1968), Gault (1970): AFTAC infrasound data  “Airwave” objects (Revelstoke, etc.) - First recognition that AFTAC large events were bolides  Abstracts ONLY!!!! No details of analysis procedures…  Wetherill and ReVelle (1978):  Entire AFTAC bolide infrasound database  Ten very large events over a ~14 year period- Global data set: 0.2 kt to 1.1 Mt;  All bolide data confirmed by at least one other technique. The entire technique was discussed in a Carnegie Institution of Washington (CIW) Annual Report (DTM- Department of Terrestrial Magnetism).  ReVelle (1980, 1995 -1997):  Careful reanalysis of the entire historic, infrasonic bolide data  Examined percent coverage during summer/winter, source energy for each event (average versus individual values), etc.  First “real” published estimate

8.  Given the following inputs:  Source energy (from the AFTAC empirical relationship, etc.)  Percent coverage of the Earth as a function of source energy and season, i.e., the relative detection probability of each bolide event.  Total time of operation of the infrasonic network  Cumulative bolide source energy prediction: ∑N = Cumulative (integral) number of bolides at any source energy, E S A E = Surface area of the Earth Percent coverage of the AFTAC global network = f(E S, season)A E, where f is known from a large database of explosive events for nuclear explosions of comparable source energy ∆t O = Time of operation of the infrasonic network N(E S ) = ∑ N{ 1 /[f(E S, season)A E ]}{ 1 /∆t O } N(E S ) = Cumulative number of bolides/(over the Earth per unit time) as a function of the deduced source energy

9.  Source E estimates typically rely on empirical relations derived from known datasets  AFTAC – originally derived for nuclear explosions (ReVelle, 1978)  log (E/2) = 3.34 log (P) – 2.58E/2 ≤ 100 kt  log (E/2) = 4.14 log (P) – 3.61E/2 ≥ 40 kt  Empirical Infrasound amplitude/range relation (Edwards et al., 2006)  E = 10 3(a-kv)/b R 3 A -3/b  We used all of these approaches to derive energies

10.  Complete hardcopy dataset carefully digitized  Applied correction to the original cylindrical pen recordings to transform the to linear quantities  Applied instrument response to the airwave signals  Re-measured all quantities (raw digitized and modified digitized)  Apply wind fields Before After

11.  Below: Corrections applied for the band-pass filter as given in Flores and Vega, JASA, 1975  Right: Infrasonic probability of detection as a function of yield and the season (AFTAC)

12. Before: After:

13. The S. African bolide waveform (JB)

14. Had to use ‘no- wind’ relations E averaged across stations for multi- station events AFTAC scaled => all events are scaled to the ‘benchmark’ event (confirmed by other reliable methods, 03-Jan-65) The S. African bolide remains a high energy event

15. AFTAC modified AFTAC (ReVelle, 1997)

16. AFTAC modified & scaled

17. AFTAC (ReVelle, 1997) Empirical, < 7 kt

18. AFTAC (ReVelle, 1997) Empirical, > 7 kt

19. Cumulative influx curve showing data from: - a global debiasing of all telescopic surveys [Harris, 2008], - individual detailed debiased flux values from the Spacewatch and NEAT programs [Rabinowitz et al., 2000]. - lunar cratering data [Werner et al., 2002] - satellite observations [Brown et al. 2002] - the power law fit and extrapolation - The NASA 2003 NEO SDT estimated flux [Stokes et al., 2003]. Our two new sets of data points from the digitized historic bolide data set using: - i) the AFTAC energy-period relation [ReVelle, 1997], and - ii) the empirical E > 7 kt relation [Edwards et al., 2006]

20.  Various empirical relations used to determine source energies  Some multi-station events show variations in measured energy estimates  These variations could be contributed to the shockwave originating from different locations along the trajectory trail  Wind field could not be reliably applied (standard deviations are higher than measurement itself)  The S. African event is still very energetic (0.7 – 2 Mt)  Previous and new AFTAC measurements are in agreement within a factor  Amplitude/range relation generally shows a significant divergence from previous and new AFTAC energy estimates