1. A bhangmeter uses a silicon photodiode to record the temporal development of the irradiance of a transient event, as optical power per unit area. It is triggered to record when the incident irradiance and its rate of rise exceed pre-set levels. See G. H. Mauth, “Alert 747,” RS 1243/80/12, Sandia National Laboratories, 1 May 1980, available at nsarchive.gwu.edu/NSAEBB/NSAEBB190.

2. France, China, Pakistan, and North Korea were not members of the treaty.

3. Several histories, of varying detail, have been published over the years; they include in particular: Jeffrey Richelson, Spying on the bomb: American nuclear intelligence from Nazi Germany to Iran and North Korea, (WW Norton & Company, 2007); Leonard Weiss, “Flash from the past: Why an apparent Israeli nuclear test in 1979 matters today,” Bulletin of the Atomic Scientists, 2015, thebulletin.org/flash-past-why-apparent-israeli-nuclear-test-1979-matter-today8734 ; L. Weiss, “The Vela Event of 1979 (Or the Israeli Nuclear Test of 1979),” The Historical Dimensions of South Africa's Nuclear Weapons Program, 2012; L. Weiss, “The 1979 South Atlantic Flash: The Case for an Israeli Nuclear Test,” in H. Sokolski (ed.), “Nuclear Nonproliferation: Moving Beyond Pretense: Preliminary Findings of NPEC's Project on Nuclear Nonproliferation Policy,” Washington, DC, 2012, available at www.npolicy.org; L. Weiss, “Israel's 1979 Nuclear Test and the US Cover‐Up,” Middle East Policy, 18(2011): 83–95; David Albright and Corey Gay, “A Flash from the Past,” Bulletin of the Atomic Scientists, 53(1997): 15–17.

4. G. H. Mauth, Alert 747, op. cit.

5. Henry G. Horak, “Vela Event Alert 747,” LA-8365-MS, Los Alamos Scientific Laboratory, 1980, available at nsarchive.gwu.edu/NSAEBB/NSAEBB190.

6. C. J. Rice, “Search for Correlative Data,” The Aerospace Corporation, Space Systems Laboratory, TOR-0082(2640)-1, 1982, available at nsarchive.gwu.edu/NSAEBB/NSAEBB190.

7. J. P. Ruina et al., “Ad Hoc Panel Report on the September 22 Event,” Executive Office of the President, Office of Science and Technology Policy, 17 July 1980, available at fas.org/rlg/800717-vela.pdf; “Ad Hoc Panel Report on the September 22 Event,” 23 May 1980, available at nsarchive.gwu.edu/NSAEBB/NSAEBB190.

8. George N. Oetzel, and Steven C. Johnson, Vela Meteoroid Evaluation, T/8503/T/PMP, SRI Project 6914, SRI 0–4055, Special Technical Report 2, SRI International, 29 January 1980, available at nsarchive.gwu.edu/NSAEBB/NSAEBB190; Mauth, Alert 747, op. cit.; Horak, Vela Event Alert 747, op. cit.; Dale S. Sappenfield, David H. Sowle, and Trella H. McCartor, “Possible Origins of Event 747 Optical Data,” MRC-80-373, MRC-R-579, Mission Research Corporation, August 1980, available at nsarchive.gwu.edu/NSAEBB/NSAEBB190.

9. Guy E. Barasch, “Light flash produced by an atmospheric nuclear explosion,” LASL-79-84, Los Alamos Scientific Laboratory, 1979, available at nsarchive.gwu.edu/NSAEBB/NSAEBB190.

10. Ibid.

11. Ruina et al., Ad Hoc Panel Report, op. cit.; Phillip J. Klass, “Clandestine nuclear test doubted,” Aviation Week & Space Technology, 209(1980): 67–72.

12. Ruina et al., Ad Hoc Panel Report, op. cit.; Klass, “Clandestine nuclear test doubted,” op. cit.; The Radiological Situation at the Atolls of Mururoa and Fangataufa, STI/PUB/1028, International Atomic Energy Agency, 1998; Barasch, Light flash produced by an atmospheric nuclear explosion, op. cit.

13. Mauth, Alert 747, op. cit.

14. Background comes from reflection of sunlight off the Earth itself, and possibly from adjacent satellite structures. This is usually suppressed electronically so that it does not trigger the bhangmeter, but once triggered, e.g., by an atmospheric nuclear detonation or even lightning, this suppression no longer occurs. Therefore, as the satellite rotates the background may modulate, increasing (“tail up”) or decreasing (“tail down”) during the recording of the transient optical signal (Mauth 1980, Alert 747, op. cit.).

15. Barasch, Light flash produced by an atmospheric nuclear explosion, op. cit.

16. Ibid.

17. Sappenfield, Sowle, McCartor, Possible Origins of Event 747 Optical Data, op. cit.; Horak, Vela Event Alert 747, op. cit.; E. M. Jones, R. W. Whitaker, H. G. Horak, and J. W. Kodis, “Low-Yield Nuclear Explosion Calculations: The 9/22/79 VELA Signal,” LA-9062, Los Alamos National Laboratory, 1982.

18. Symbalisty et al., RADFLO Physics and Algorithms, op. cit.

19. It is worth noting here what the Ruina report did not say, namely that Alert 747 was not the signature of a nuclear explosion. Shortly after the release of the report one of the panel members was quoted as saying that when the panel first convened they thought the chance that it was a nuclear explosion was 4:1 while at the end of their deliberations it was 4:1 against (Eliot Marshall, “Debate continues on the bomb that wasn't,” Science, 209(1980): 572–573. Such odds are still quite reasonable.

20. This last nuclear detonation recorded over its full time history by both Vela 6911 bhangmeters was probably the French 4 kt Titania explosion on 30 June 1972 (and/or the 0.5 kt Umbriel explosion on 25 June 1972; there were no atmospheric tests by China during June 1972).

21. Based on the historical record, this must have been the 14 December 1978 Chinese test.

22. Mauth, Alert 747, op. cit.

23. The differences in these numbers are probably due to different criteria used to define zoo event. Eliot Marshall, “Scientists fail to solve Vela mystery,” Science, 207(1980): 504–506; Oetzel and Johnson, Vela Meteoroid Evaluation, op. cit.; Alert 747, op. cit.; Ruina et al., Ad Hoc Panel Report, op. cit.

24. J. S. Browning and J. L. Montoya, “Hypervelocity impact tests of optical sensors,” ”Proceedings of the conference of the American Physical Society topical group on shock compression of condensed matter, 370, (AIP Publishing, 1996): 1113 -1116; J. S. Browning and J. L. Montoya, “Hypervelocity impact testing of spacecraft optical sensors,” SAND95-11910, Sandia National Laboratory, Department of Energy, 1995, www.osti.gov/scitech/servlets/purl/76219; David F. Medina, Patrick J. Serna, and Firooz A. Allahdadi, “Reconstruction of a hypervelocity impact event in space,” “SPIE's 1996 International Symposium on Optical Science, Engineering, and Instrumentation,” International Society for Optics and Photonics, 1996, 137–147.

25. J. S. Browning and J. L. Montoya. “Hypervelocity impact tests of optical sensors,” “Proceedings of the conference of the American Physical Society topical group on shock compression of condensed matter,” 370, AIP Publishing, 1996, 1113–1116.

26. Mauth, Alert 747, op. cit.

27. See also OJ80, where Plots (a) and (b) reproduced in Figure 3 are discussed.

28. Despite unavailability of the full suite of zoo-on light curves, this is already suggestive that they and Alert 747 arise from different physical processes. A similar point has been made previously, but from the position of a personal sighting in early 1981 of all the zoo-on time histories, and where it was noted that “there was not a single ‘zoo animal’ that came close to the classic shape in duration and amplitude.” Leonard Weiss, “Flash from the past,” op.cit. Also, a feature of the bhangmeter data for the two zoo-ons from Ru80 was that they appeared to have fine structure in their time histories, seen in Figures 3c and 3d. The latter includes a “saw-tooth” like pattern on the more sensitive detector, between about 1 and 100 ms and with an amplitude which is a significant fraction of the main signal amplitude. Interestingly no such structure is evident in either the Alert 747 recordings, or the few other airburst bhangmeter readings that can be found in unclassified (or declassified) literature. They are instead relatively smooth. Such potential fine structure in bhangmeter pulses was one of the issues considered to impose constraints on the properties of a body which could reproduce the observation. Sappenfield, Sowle, McCartor, Possible Origins of Event 747 Optical Data, op. cit.

29. Ibid.

30. Sappenfield, Sowle, McCartor, Possible Origins of Event 747 Optical Data, op. cit., Figure 11.

31. Ibid.

32. Oetzel and Johnson, Vela Meteoroid Evaluation, op. cit.

33. Siegfried Auer, “Instrumentation”, in Eberhard Grün, Bo Gustafson, Stan Dermott, and Hugo Fechtig (eds.), Interplanetary Dust, (Springer-Verlag, Berlin, 2001), 385–444.

34. Sappenfield, Sowle, McCartor, Possible Origins of Event 747 Optical Data, op. cit.

35. Ibid.

36. Ibid.

37. Ibid.

38. Ibid.

39. Mauth, Alert 747, op. cit.; Oetzel and Johnson, Vela Meteoroid Evaluation, op. cit.

40. “Satellite Instruments,” SAND 89-0637, Sandia Technology, 13, March 1989, 4–6, prod.sandia.gov/techlib/access-control.cgi/1989/890637.pdf

41. Mauth, Alert 747, op. cit.

42. The exact orientation of the bhangmeters has not been made public; it is known, however, that they are separated by about 30 cm.

43. ”Characterization of Ejecta from HVI on Spacecraft Outer Surfaces,” IADC-11-05, Inter-Agency Space Debris Coordination Committee, April 2013.

44. R. K. Soberman, S. L. Neste, and K. Lichtenfeld, “Optical measurement of interplanetary particulates from Pioneer 10,” Journal of Geophysical Research, 79(1974): 3685–3694; D. H. Humes, “Results of Pioneer 10 and 11 meteoroid experiments: Interplanetary and near‐Saturn,” Journal of Geophysical Research: Space Physics, 85(1980): 5841–5852; D. H. Humes, J. M. Alvarez, R. L. O'Neal, and W. H. Kinard, “The interplanetary and near‐Jupiter meteoroid environments,” Journal of Geophysical Research, 79(1974): 3677–3684.

45. Humes, “Results of Pioneer 10 and 11 meteoroid experiments,” op. cit.

46. R. Jehn, “An analytical model to predict the particle flux on spacecraft in the solar system,” Planetary and Space Science, 48(2000): 1429–1435; O. Staubach, E. Grün, and R. Jehn, “The meteoroid environment near Earth,” Advances in Space Research, 19(1997): 301–308; Neil Divine, “Five populations of interplanetary meteoroids,” Journal of Geophysical Research: Planets, 98 (1993): 17029–17048.

47. Albright and Gay, “A Flash from the Past,” op. cit.

48. Browning and Montoya, “Hypervelocity impact tests of optical sensors,” op. cit.; Medina et al., “Reconstruction of a hypervelocity impact event in space,” op. cit.; Patrick J. Serna, “Data Report of Hypervelocity Micro-Particle Impact Light Flash Data and MOS Impact Detector Output,” PL-TR-95-1013, Phillips Laboratory, 1995.

49. Browning and Montoya, “Hypervelocity impact testing of spacecraft optical sensors,” op. cit.

50. Medina et al., “Reconstruction of a hypervelocity impact event in space,” op. cit.

51. Carolyn M. Ernst, ”Photometric, Thermal, and Spatial Evolution of the Impact Flash,” PhD thesis, Brown University, 2008.

52. Mauth, Alert 747, op. cit.

53. S. Close, P. Colestock, L. Cox, M. Kelley, and N. Lee, “Electromagnetic pulses generated by meteoroid impacts on spacecraft,” Journal of Geophysical Research: Space Physics, 115(2010): Charles Stein, “Hypervelocity Debris Initiated Spacecraft Discharging,” Spacecraft Charging Technology, 476(2001): 441; David A. Crawford and Peter H. Schultz, “Electromagnetic properties of impact-generated plasma, vapor and debris,” International Journal of Impact Engineering, 23 (1999): 169–180; Luigi Foschini, “Electromagnetic interference from plasmas generated in meteoroids impacts,” EPL (Europhysics Letters), 43(1998): 226.

54. Medina et al., “Reconstruction of a hypervelocity impact event in space,” op. cit. This could feasibly refer to the Vela satellites as they had another optical sensor onboard, a three-axis event locator system designated YBA which was designed to respond to the first maximum of a nuclear detonation. It did not trigger for Alert 747 due to the relatively low signal amplitude, hence the large uncertainty in the inferred event location which covered an area several thousand kilometers in diameter (Mauth, Alert 747, op. cit.).

55. Symbalisty et al., RADFLO Physics and Algorithms, op. cit.; Jones et al., Low-Yield Nuclear Explosion Calculations: The 9/22/79 VELA Signal, op. cit.; Horak, Vela Event Alert 747, op. cit.; Sappenfield et al., “Possible Origins of Event 747 Optical Data,” op. cit.

56. Barasch, “Light flash produced by an atmospheric nuclear explosion,” op. cit.

57. The picture is incomplete, however, since the time of minimum was not measured.

58. Symbalisty et al., RADFLO Physics and Algorithms, op. cit.

59. The calibration is based on Figure 11 in OJ80, converted to Watts, and multiplied by 4πR², where R is the Earth-Vela distance. Alert 747 time histories have been taken from a 26 November 1979 draft Sandia/Los Alamos document entitled “22 September 1979 Event” available at digitalarchive.wilsoncenter.org/document/119216.

60. Taken from Figure 8 in Symbalisty et al., RADFLO Physics and Algorithms, op. cit.

61. From Table 4 in Symbalisty et al., RADFLO Physics and Algorithms, op. cit., though without air chemistry.

62. Eugene M. D. Symbalisty, Some NUDET Effects due to Water Containment, LA-12775-MS, Los Alamos National Laboratory, 1994.

63. In some instances, classification restrictions may have prevented the Ruina panel from providing the complete information behind its analysis and findings.

64. These four approaches are i) time to minimum, ii) 3T, i.e. time after minimum at which the ‘well’ in the irradiance-time curve is a factor of 3 wide in time, iii) time to second maximum, and iv) integrated energy.