4

The Meteoroid Environment and Its Effects on Spacecraft

BACKGROUND

Meteoroids are small, solid particles, formally defined by the International Astronomical Union as being considerably larger than an atom or molecule and smaller than an asteroid.1 Meteoroids are generated mainly from collisions between asteroids and the decay of comets, although a small percentage may originate from stellar activity outside the solar system. The precise size limits applying to the term “meteoroid” have been debated extensively in the literature,2 but for the purposes of this report an operational definition covers the size range from 1 micron to 10 cm in diameter. The lower bound represents solid particles, which, once created through collisions, are often expected to be on unbound orbits due to radiation pressure from the Sun; the upper (arbitrary) limit represents meteoroids, which are so infrequent as to be negligible as primary impactors on spacecraft. This is also the size at which the meteoroid flux is two orders of magnitude lower than the space debris flux in portions of low Earth orbit (LEO). Flux refers to the number of meteoroids of a certain mass or larger crossing a fixed surface in space per unit time.3

In contrast to the space debris environment, the meteoroid environment in near-Earth space is not affected by launch activity and subsequent operational practices. Beyond LEO, the flux of meteoroids up to the 5- to 10-cm-size range is the long-term averaged dominant impactor population for spacecraft, whereas in LEO meteoroids and orbital debris between 10 microns and 1 mm are comparable in flux. For sizes larger than 1 mm in LEO, orbital debris is the dominant impactor population.

The meteoroid population can be divided into two broad components: sporadic meteoroids and stream meteoroids. Stream meteoroids have a common, recent (less than 100,000 years) origin and thus all follow nearly identical heliocentric orbits. When this stream intersects Earth, each particle (or meteoroid) colliding with Earth’s atmosphere produces heat, light, and ionization. These phenomena associated with a meteoroid impacting a planetary atmosphere are collectively termed a meteor (i.e., a plasma). As meteors travel along parallel paths in the

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1 P.M. Millman, Meteor news: A report on meteor terminology, Journal of the Royal Astronomical Society of Canada 55:265-267, 1961, available at http://adsabs.harvard.edu/full/1961JRASC..55..265M, accessed July 13, 2011.

2 See also M. Beech and D. Steel, On the definition of the term ‘meteoroid,’ Quarterly Journal of the Royal Astronomical Society 36:281-284, 1995, available at http://articles.adsabs.harvard.edu//full/ 1995QJRAS..36..281B/0000281.000.html; A.E. Rubin and J.N. Grossman, Meteorite and meteoroid: New comprehensive definitions, Meteoritics and Planetary Science 122:114-122, 2010, available at http://cosmochemists.igpp.ucla.edu/definitions.pdf.

3 H. Klinkrad, Space Debris: Models and Risk Analysis, Springer Praxis, Chichester, U.K., 2006.



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4 The Meteoroid Environment and Its Effects on Spacecraft BACKGROUND Meteoroids are small, solid particles, formally defined by the International Astronomical Union as being considerably larger than an atom or molecule and smaller than an asteroid. 1 Meteoroids are generated mainly from collisions between asteroids and the decay of comets, although a small percentage may originate from stel - lar activity outside the solar system. The precise size limits applying to the term “meteoroid” have been debated extensively in the literature,2 but for the purposes of this report an operational definition covers the size range from 1 micron to 10 cm in diameter. The lower bound represents solid particles, which, once created through col - lisions, are often expected to be on unbound orbits due to radiation pressure from the Sun; the upper (arbitrary) limit represents meteoroids, which are so infrequent as to be negligible as primary impactors on spacecraft. This is also the size at which the meteoroid flux is two orders of magnitude lower than the space debris flux in portions of low Earth orbit (LEO). Flux refers to the number of meteoroids of a certain mass or larger crossing a fixed surface in space per unit time.3 In contrast to the space debris environment, the meteoroid environment in near-Earth space is not affected by launch activity and subsequent operational practices. Beyond LEO, the flux of meteoroids up to the 5- to 10-cm- size range is the long-term averaged dominant impactor population for spacecraft, whereas in LEO meteoroids and orbital debris between 10 microns and 1 mm are comparable in flux. For sizes larger than 1 mm in LEO, orbital debris is the dominant impactor population. The meteoroid population can be divided into two broad components: sporadic meteoroids and stream mete - oroids. Stream meteoroids have a common, recent (less than 100,000 years) origin and thus all follow nearly identical heliocentric orbits. When this stream intersects Earth, each particle (or meteoroid) colliding with Earth’s atmosphere produces heat, light, and ionization. These phenomena associated with a meteoroid impacting a plan - etary atmosphere are collectively termed a meteor (i.e., a plasma). As meteors travel along parallel paths in the 1 P.M. Millman, Meteor news: A report on meteor terminology, Journal of the Royal Astronomical Society of Canada 55:265-267, 1961, available at http://adsabs.harvard.edu/full/1961JRASC..55..265M, accessed July 13, 2011. 2 See also M. Beech and D. Steel, On the definition of the term ‘meteoroid,’ Quarterly Journal of the Royal Astronomical Society 36:281-284, 1995, available at http://articles.adsabs.harvard.edu//full/ 1995QJRAS..36..281B/0000281.000.html; A.E. Rubin and J.N. Grossman, Meteorite and meteoroid: New comprehensive definitions, Meteoritics and Planetary Science 122:114-122, 2010, available at http://cosmochemists.igpp. ucla.edu/definitions.pdf. 3 H. Klinkrad, Space Debris: Models and Risk Analysis, Springer Praxis, Chichester, U.K., 2006. 29

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30 LIMITING FUTURE COLLISION RISK TO SPACECRAFT TABLE 4.1 Major Nighttime Meteor Showers Visible from Earth Θ (×10–6) Shower Max Date ZHR RA DEC Speed (km/s) Parent Quadrantids Jan. 3 120 8.4 15 20 49 43 2003 EH1 Lyrids Apr. 22 20 4.6 18 10 34 48 C/1861 G1 (Thatcher) η-Aquariids May 6 60 6.4 22 30 –2 66 1P/Halley S. δ-Aquariids Jul. 29 20 6.2 22 44 –16 43 Perseids Aug. 13 90 6.0 3 08 58 60 109P/Swift-Tuttle Draconids Oct. 8 var var 17 28 54 23 9P/Giacobini-Zinner Orionids Oct. 21 20 2.2 6 20 16 67 1P/Halley S. Taurids Nov. 5 10 1.0 3 34 14 31 2P/Encke N. Taurids Nov. 12 15 1.4 4 00 22 30 2P/Encke Leonids Nov. 17 15 1.9 10 12 22 71 55P/Tempel-Tuttle Geminids Dec. 14 100 2.3 7 28 33 36 3200 Phaethon Ursids Dec. 22 10 2.2 14 36 75 35 8P/Tuttle NOTE: ZHR, zenithal hourly rate, the approximate number of shower-related meteors an observer would see under ideal conditions; Max Date, the date at which the highest flux of meteors is normally expected each year; Θ (×10–6), the flux of meteors brighter than astronomical absolute magnitude +6.5 per km–2s–1 at the time of the maximum; RA, the right ascension of the radiant in equatorial coordinates at the time of the shower maximum; DEC, the declination of the radiant in equatorial coordinates at the time of the shower maximum; Speed (km/s), the speed of a meteor at the top of the atmosphere; Parent, the parent body (comet or asteroid) from which the meteoroid stream is believed to originate. SOURCE: Adapted, courtesy of the Royal Astronomical Society of Canada, from the Observers Handbook (2011). atmosphere (having identical orbits about the Sun), they appear to emanate from a particular point in the sky (termed a “shower radiant”); the meteor shower’s name is derived from the constellation where this radiant is found. The comet or asteroid “parent” for many streams is known. Table 4.1 lists a selection of the strongest nighttime meteor showers visible at Earth, along with their parent bodies. Because meteor showers consist of many meteoroids trav - eling on similar orbits in a stream, which intersect Earth at a fixed point in its orbit during a short interval of time (typically on the order of days), the showers occur at about the same time each year. Several showers show strong variations in activity related to the details of how dust is produced and subsequently evolves in the stream. Such showers can produce strong flux enhancements for short periods in certain years. Examples include the Leonids in 1999 and the October Draconids in 1933 and 1946. Box 4.1 discusses meteor storms and spacecraft safety. Meteoroids not found in streams are termed “sporadic meteors.” While they do not have a clear common origin, the sporadic background meteoroid population as detected at Earth shows strong directionality, reflecting the general orbital properties of the meteors’ parent body population. At Earth, in particular, several major sporadic meteor sources are noticeable with radiant diameters on the order of 20 degrees, as shown in Figure 4.1. 4 Meteor showers from stream meteoroids are well known, whereas the sporadic meteoroid population is less well understood. Unlike knowledge of orbital debris, knowledge of meteoroids results in part from ground-based observations of the interaction of meteoroids with the atmosphere (used as a detector for this purpose) to form a meteor (the plasma) at an altitude of between approximately 70 and 140 km. Broadly speaking, ground-based optical and radar instruments can detect both the plasma formed around a meteoroid particle traveling at its velocity—called the “head” or “radar head echo”—and the quasi-stationary plasma that can extend for kilometers behind the meteoroid, called the “trail” or “wake.” Direct measurement of the meteoroid is not possible using ground-based observational techniques, and significant modeling effort is required to translate measured meteor parameters to meteoroid 4 J. Jones and P. Brown, Sporadic meteor radiant distributions—Orbital survey results, Royal Astronomical Society, Monthly Notices 265:524- 532, 1993, available at http://articles.adsabs.harvard.edu//full/1993MNRAS.265..524J/0000524.000.html, accessed July 13, 2011.

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31 THE METEOROID ENVIRONMENT AND ITS EFFECTS ON SPACECRAFT BOX 4.1 Meteor Storms and Spacecraft Safety The NASA meteoroid program established during the 1960s provided estimates of the background meteoroid impact environment, which was not found to be a show-stopping hazard to the Apollo program and subsequent crewed missions in low Earth orbit. That effort determined that the dominant impact threat from meteoroids was from the random background of particles, rather than the visually spectacular but less numerous (at the small dust sizes of concern to spacecraft) meteor showers. However, one possible exception to this rule of thumb is a rare phenomenon termed a “meteor storm.” Such meteor storms happen on average only once every few decades. The only major meteor storm of the early space age happened on November 17, 1966, when the Leonid meteor shower rained over western North America, providing spectators with a once-in-a-lifetime sight of up to tens of thousands of visible meteors in less than an hour. Although the number of meteoroids potentially hitting spacecraft spikes during a meteor storm, an additional potential danger exists in the speed of shower meteors, which tend to be many times faster than the average sporadic meteor. The Leonids, traveling at 71 km/s, are near the top of this scale, with other potential storm-producing showers like the Perseids (60 km/s) and Lyrids (43 km/s) also packing a substantial punch. The added velocity is a danger to spacecraft, not only for the added mechanical impact damage produced, but also because, at such high speeds, large amounts of plasma can be produced, which can damage sensitive spacecraft electronics. No similar storms were seen or expected after the 1966 Leonids until 1993. In 1992, the possibility of a storm from the well-known Perseid meteor shower became a sudden and real possibility with the surprise discovery of the Perseid parent comet, 109P/Swift-Tuttle, as it passed through the inner solar system. The prospect of a substantial storm the following year led to the reorientation of the Hubble Space Telescope near the time of the predicted storm peak and to a delay in the launch of space shuttle mission STS-51. The 1993 shower resulted in a strong surge in meteor numbers but fell short of a storm. Nevertheless, the Olympus telecommunication satellite suffered an impact, likely from a small Perseid meteoroid at the height of the shower, which ultimately led to the termination of that mission. After the experience of the 1993 Perseid shower and the Olympus impact, the space community became sensitive to the impact damage possibly associated with meteor storms. The Perseids proved a warm-up for the much more spectacular returns of the Leonids, which produced a strong shower in 1998 and true meteor storms in 1999, 2001, and 2002. Several major research efforts recorded the Leonid storms using video cameras and radars, some providing Leonid meteor numbers in real-time to space operators—the first real-time meteoroid “weather” reports. Many satellites were turned to present a mini- mal target area to the oncoming stream, and several satellite operators took additional precautions, such as turning off high-power subsystems at the time of the predicted peak and ensuring that extra ground support was available in case of an emergency. While major satellite damage did not occur during any of the Leonid storms, in part perhaps because so many satellites took precautions during the height of the storm periods, some smaller anomalies were reported by operators, which have been linked to the sudden increase in numbers of small, fast Leonids. In the short term, the 2011 October Draconids are predicted by some forecasters to produce a possible strong shower (or maybe even a storm) on October 8, 2011, which is likely to be the last meteor storm for at least a decade. properties (mass, bulk density).5 In addition to ground-based observations of meteors, direct in situ measurements are also made by means of space-borne dust detectors, analysis of the surfaces of returned spacecraft, and labora - tory measurements of a select suite of meteoroids in the form of airborne collected interplanetary dust particles (IDPs). In situ measurements must also be modeled and calibrated for proper interpretation of the impact signal. 5 For example, see S. Close, M. Oppenheim, S. Hunt, and A. Coster, A technique for calculating meteor plasma density and meteoroid mass from radar head echo scattering, Icarus 168:43-52, 2004, available at http://soe.stanford.edu/pubs/Icarus_scattering_sigridclose_5373.pdf; J. Borovička, Physical and chemical properties of meteoroids as deduced from observations, pp. 249-271 in Proceedings of the International Astronomical Union, Vol. 1, Cambridge University Press, Cambridge, U.K., 2005.

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32 LIMITING FUTURE COLLISION RISK TO SPACECRAFT 4.1 right from Word.eps 4.1 left from Word.eps FIGURE 4.1 (Left) The diagram shows Earth in orbit about the Sun as seen from the north pole of the ecliptic together with the ram direction (apex of Earth’s way) and the Sun (helion) and anti-Sun (antihelion) directions. (Right) The same coordinate sys- tem (termed the “Sun-centered ecliptic”) as seen from the vantage point of Earth looking outward, showing the apex of Earth’s way in the center of the plot, the Sun direction (at 0,0), and the anti-Sun direction (180,0). The circles represent the major sporadic meteor sources—i.e., areas on the sky where major concentrations of meteor radiants are present throughout the year. The meteoroid hazard to spacecraft takes the form of hypervelocity impacts of solid meteoroid particles onto spacecraft surfaces. The character of these impacts differs in several important respects, as compared to impacts from space debris.6 The average collisional velocity with meteoroids is higher than with orbital debris (in extreme cases, an order of magnitude higher), and the bulk density of meteoroids is typically quite different from that of space debris. Studies of the expected impact damage from meteoroids have tended to focus on mechanical effects and penetration, which is the main damage modality for space debris. 7 However, there is growing evidence that higher-velocity meteoroid collisions may produce plasma, generation of which in the vicinity of an impact may be more damaging in some cases than the purely mechanical effects. 8 This electrical damage can occur in the form of electrostatic discharges or electromagnetic pulses resulting from the direct ionization of the meteoroid and part of the spacecraft.9 Many ground-based experiments have shown that plasma is generated by hypervelocity impacts.10 With respect to electrical effects, meteoroid velocity is the dominant factor, with the amount of charge generated typically scal - ing as mass times velocity to the fourth power.11 At least two spacecraft have suffered electrical anomalies that might potentially have been caused by meteoroid impacts during Perseid meteor shower enhancements, including the Olympus satellite in 1993 (Box 4.2) and the Landsat-5 satellite in 2009. 12,13 Such anomalies are difficult to understand in connection with meteor showers, however, since current research suggests that the particle popula - M. Landgraf, R. Jehn, W. Flury, and V. Dikarev, Hazards by meteoroid impacts onto operational spacecraft, Advances in Space Research 6 33:1507-1510, 2003. 7 For example, see E.L. Christiansen, Handbook for Designing MMOD Protection, NASA TM-2009-214785, NASA Johnson Space Center, Houston, Tex., 2009. 8 D.R. Caswell, N. McBride, and A. Taylor, Olympus end of life anomaly—A Perseid meteoroid impact event?, International Journal of Impact Engineering 17:139-150, 1995. 9 G. Drolshagen, Impact effects from small size meteoroids and space debris, Advances in Space Research 41:1123-1131, 2008. 10 J. McDonnell, Microparticle studies by space instrumentation, pp. 337-426 in Cosmic Dust (J. McDonnell, ed.), John Wiley and Sons, New York, N.Y., 1978. 11 J. McDonnell, N. McBride, S. Green, P.R. Ratcliff, D.J. Gardener, and A.D. Griffiths, Near Earth environment, pp. 161-231 in Interplanetary Dust (E. Grun, B.A.S. Gustafson, S.F. Dermott, H. and Gechtig, H. eds.), Springer, New York, N.Y., 2001. 12 Caswell et al., International Journal of Impact Engineering, 1995. 13 W.J. Cooke, The 2009 Perseid Meteoroid Environment and Landsat 5 , NASA MEO Internal Report, NASA Marshall Space Flight Center in Huntsville, Ala., 2009.

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33 THE METEOROID ENVIRONMENT AND ITS EFFECTS ON SPACECRAFT BOX 4.2 Olympus Spacecraft Failure The Olympus spacecraft was a European Space Agency (ESA) experimental communications satel- lite that experienced multiple anomalies on August 11, 1993, near the peak of the Perseid meteor shower that year. Olympus lost Earth-pointing attitude and began spinning out of control after having been in orbit for 5 years. A previous temporary loss of the satellite in 1991 had required use of a large amount of fuel for recovery, and when ESA tried to recover the satellite in August 1993, the satellite used up most of its remaining fuel trying to reorient itself, thus making it impossible for the agency to reestablish service and forcing ESA to terminate operations and remove Olympus from geostationary orbit.1 Scientists and engineers cannot overlook the possible connection between the timing of this failure and the peak of the Perseid meteor shower, even though the failure occurred 3.5 hours prior to the peak of the Perseid storm. A team composed of specialists from ESA and industry (led by British Aerospace, the prime contractor for Olympus) conducted an investigation into the causes of the anomalies and the ultimate loss of Olympus.2 The team could not conclusively identify causes of the anomalies that occurred in some of Olympus’s systems. An internal electrical anomaly or a meteoroid impact generating a plasma, which then entered the spacecraft, were identified as possible failure modes. Based on the limited data available, the investigation report concluded that “although it was impossible to prove that the demise of Olympus was caused by the impact of a Perseid meteoroid, it does seem probable.” 3 1 European Space Agency, “Olympus: End of Mission,” Press Release No. 40-1993, available at http://www.esa. int/esaCP/Pr_40_1993_p_EN.html, 1993. 2 R. Douglas Caswell, Olympus end of life anomaly—A Perseid meteoroid impact event? International Journal of Impact Engineering 17:139-150, 1995. 3 Ibid., p. 149. tion in showers includes far fewer small meteoroids than does the sporadic background. 14 One possibility is that these impacts are related to natural debris produced by the near-Earth breakup of larger, fragile meteoroids in the stream, a mechanism first suggested as an explanation for the apparent impact clustering detected by the HEOS 2 dust experiment.15 Furthermore, such small meteoroids can be a particular threat to certain types of space-based sensors (such as focal-plane charge-coupled device sensors in x-ray telescopes), for which at least two impacts have been documented.16 Impact effects from small meteoroids remain a topic for future investigation, but the need to better characterize meteoroids with high velocities, even if the mass cut-off is below that needed for mechanical damage, is apparent. DETECTION AND MONITORING The primary methods for estimating the overall meteoroid flux, velocity, and physical properties of meteoroids include ground-based radar and optical measurements of meteors, in situ impact detections of meteoroids (includ - ing detections on returned surfaces), zodiacal brightness measurements, and IDP studies. In situ measurements include satellite dust detectors on interplanetary spacecraft and on Earth-orbiting satel - lites. There are several different types of detectors, such as (1) polyvinylidene fluoride (PVDF), which produces a current signal (for meteoroids of mass greater than 10–12 grams); (2) penetration detectors (for meteoroids of mass J. McDonnell et al., Interplanetary Dust, 2001. 14 H. Fechtig, E. Grün, and G. Morfill, Micrometeoroids within ten Earth radii, Planetary and Space Science 27:511-531, 1979, available 15 at http://www.sciencedirect.com/science/article/pii/0032063379901284 . 16 J.D. Carpenter, A. Wells, A.F. Abbey, and R.M. Ambrosi, Meteoroid and space debris impacts in grazing-incidence telescopes, Astronomy and Astrophysics 483(3):941-947, 2008.

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34 LIMITING FUTURE COLLISION RISK TO SPACECRAFT greater than 10–9 grams); (3) microphones that record the momentum transferred to the detector (for meteoroids of mass greater than 10–12 grams); (4) ionization detectors that measure the charge produced upon impact (for meteor- oids of mass greater than 10–15 grams); and (5) return surfaces such as lunar microcraters and spacecraft surfaces. Note that for many of these techniques the speed of individual particles is not known but is usually assumed to be ~20 km/s based on findings about mean speed from earlier radar and optical meteor measurements. More recently published estimates of meteoroid speed using high-power large-aperture radars (HPLA), however, are at variance with such low average velocities (of ~20 km/s).17,18 Recently proposed in situ dust detection systems offer the pros- pect of precise measurements of velocity and better measurements of mass on very small individual meteoroids. 19 Many of these methods preclude decoupling mass from velocity, and others involve a large error associated with the derived measurement. Due to limited collecting area, in situ measurements detect only the smallest meteoroids, since the number of meteoroids follows a power-law distribution. Finally, a very limited amount of meteoroid in situ impact data exists from outside Earth’s orbit, resulting in large levels of uncertainty in the mete - oroid distribution (particularly at larger sizes) beyond Earth. The inherent difficulty in interpreting in situ data, particularly when speed is not known, can result in great uncertainty in the measurements. 20 Optical measurements include video and photographic observations of meteors. The primary information extractable from optical measurements pertaining to the general meteoroid environment includes flux and veloc - ity distributions. With significant modeling (and assumptions), both mass and density can be inferred. The main limitation in utilizing optical measurements is characterizing luminous efficiency (the fraction of total initial energy converted into radiation in the instrument passband) in order to relate instrument magnitude to meteoroid mass. Additional instrument biases inherent in the system also have to be removed. 21 Ground-based radars transmit electromagnetic waves that scatter from the meteor plasma and return to the radar to be interpreted in the form of signal-to-noise ratio (SNR) or radar cross section (RCS). To measure the head plasma for smaller meteoroids, high-power instruments are generally needed because the scattered signal is weak. To measure a trail plasma, either the meteoroid must be traveling in a direction perpendicular to the line-of-sight of the radar (a “specular” trail), or the radar beam must be quasi-perpendicular to the background magnetic field (a “non-specular” trail). Trail data also exhibit a “height ceiling” effect: an altitude cut-off exists, above which a specular radar cannot detect trails as the meteor trail radius approaches the wavelength of the radar, leading to destructive scattering of the reflected wave. Since small, high-velocity meteoroids tend to ablate and form meteors at higher altitudes, there is a subset of high-velocity meteoroids that are notably difficult to detect with “specular-scattering” radars. A similar phenomenon may exist with respect to head echo data, although this is currently an area of active research. Additional uncertainties result when attempts are made to correlate observed meteor parameters with inferred meteoroid properties, such as quantifying the ionization efficiency. All of these unknowns result in a large number of biases associated with radar meteor observations. Measurements of the meteoroid flux both in situ and from atmospheric observations have been reported for many decades. Figure 4.2 summarizes recent meteoroid flux measurements in the size range of interest for mechanical hazards and includes attempts to remove inherent observational biases. Note that different sources use different assumptions to derive flux; in particular, different average velocities and differing means of converting from observed quantities to mass. Much uncertainty remains in all of these conversions. 22 Among the factors that remain only partly characterized for radar instruments, for example, is understanding the role of magnetic field 17 D. Janches, C.J. Heinselman, J.L. Chau, and A. Chandran, Modeling the global micrometeor input function in the upper atmosphere observed by high power and large aperture radars, Journal of Geophysical Research 111:A07317, 2006, available at http://www.cora.nwra. com/~diego/2006JA011628.pdf, accessed July 14, 2011. 18 D. Janches and J.L. Chau, Observed diurnal and seasonal behavior of the micrometeor flux using the Arecibo and Jicamarca radars, Journal of Atmospheric and Solar-Terrestrial Physics 67:1196-1210, 2005, available at http://www.cora.nwra.com/~diego/Janches-Chau.pdf, accessed July 14, 2011. 19 Z. Sternovsky et al., “Novel instrument for Dust Astronomy: Dust Telescope,” Aerospace Conference, 2011 IEEE, pp. 1-8, March, 5-12, 2011, doi: 10.1109/AERO.2011.5747300. 20 McDonnell, Cosmic Dust, 1978. 21 Z. Ceplecha, J. Borovička, W.G. Elford, D.O. ReVelle, R.L. Hawkes, V. Porubčan, and M. Šimek, Meteor phenomena and bodies, Space Science Reviews 84:327-471, 1998, available at http://www.springerlink.com/content/r2602605vm031517/, accessed July 14, 2011. 22 For a review, see Ceplecha et al., Space Science Reviews, 1998.

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35 THE METEOROID ENVIRONMENT AND ITS EFFECTS ON SPACECRAFT FIGURE 4.2 Flux of meteoroids at the top of Earth’s atmosphere as reported by various authors. Grün et al. (1985) is a compilation of in situ and meteor measurements made prior to the mid-1980s. Love and Brownlee (1993) reflects flux values derived from the returned surface of the Long Duration Exposure Facility. Thomas and Netherway (1989) describes meteoroid fluxes measured by a low-frequency over-the-horizon radar, while Ceplecha (2001) is a compilation of optical meteor measure - ments using recently reported values for the luminous efficiency. Halliday et al. (1996) is a determination of fireball fluxes as measured by the Meteorite Observation and Recovery Project. SOURCE: Data from E. Grün, H. Zook, H. Fechtig, and R.H. Giese, Collisional balance of the meteoritic complex, Icarus 62:244-272, 1985; S.G. Love and D.E. Brownlee, A direct mea- surement of the terrestrial mass accretion rate of cosmic dust, Science 262:550-553, 1993; R.M. Thomas and D.J. Netherway, Observations of meteors using an over-the-horizon radar, Proceedings of the Astronomical Society of Australia 8:88-93, 1989; Z. Ceplecha, The meteoroidal influx to the Earth, Astrophysics and Space Science Library 261:35-50, 2001; I. Halliday, A.A. Griffin, and A.T. Blackwell, Detailed data for 259 fireballs from the Canadian camera network and inferences concerning the influx of large meteoroids, Meteoritics and Planetary Science 31:185-217, 1996. effects, polarization properties, radar frequency, initial trail radius, and ionospheric effects on biases, while for optical instruments a major uncertainty involves meteor spectral energy distribution. The conversion of meteor brightness or radar cross section to equivalent mass, in particular, has a wide range of proposed forms as a func - tion of velocity and mass, used variously in the literature. Another cornerstone of all meteoroid environment models is the meteoroid velocity distribution at Earth. Most models use either the Harvard Radio Meteor Project radar-derived velocity distribution 23 or the distribu- A.D. Taylor, The Harvard radio meteor project meteor velocity distribution reappraised, Icarus 116:154-158, 1995. 23

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36 LIMITING FUTURE COLLISION RISK TO SPACECRAFT tion derived from super-Schmidt optical measurements from the Harvard meteor program (including the Harvard Photographic Meteor Program and the Harvard Radio Meteor Program). 24,25 No statistically significant in situ velocity measurements have yet been made for meteoroids.26 More recent measurements using radial scattering from head echoes as detected by high-power, large-aperture radar extending to very small masses suggest a very different (and much higher) speed distribution than is derived from the earlier estimates. 27 As shown in Figure 4.3, the mass-limited velocity distribution shows wide variation depending on the study; this may reflect a change in the velocity distribution with mass, an effect predicted by numerous authors,28 or result from instrumentation biases or deficiencies in the models that convert the measured signal of the plasma to meteoroid mass. If strong changes in the velocity distribution with mass are present in the environment, large errors in assessments of damage may occur, including both mechanical and electrical impact effects. A final major parameter required to interpret meteoroid impact effects is the bulk density of a meteoroid. This is a very difficult quantity to measure; all such measurements are necessarily indirect. Direct measurements of bulk density from recovered IDPs are strongly biased toward IDPs with low entry velocities due to mass-velocity selection effects, which make survivability less likely for high-velocity IDPs. 29 Values for the inferred bulk density of meteoroids from optical measurements coupled to ablation models show systematic differences due, in large part, to differing choices in (largely unconstrained) model parameters and the need to include effects such as mete - oroid fragmentation. Similar attempts to derive densities from radar data also exhibit such tendencies. All studies, however, suggest that the true distribution in densities covers a wide range (more than an order of magnitude). 30 Finding: The models used to relate measurements of plasma to fundamental parameters of a meteoroid contain large uncertainties and errors. These models include, but are not limited to, electromagnetic scattering models, luminous emission models, and meteoroid fragmentation models. Finding: Because the scientific community infers the properties of a meteoroid indirectly from its effects on the atmosphere (a meteor) or the effects of its impact on a spacecraft, it is imperative to understand observational biases inherent in each instrument that affect the detection of these secondary effects. MODELING AND SIMULATION The meteoroid hazard to a spacecraft can be estimated in a probabilistic manner by knowing (1) the flux of meteoroids above some specified size or mass, (2) the velocity distribution of the impacting population, (3) the bulk density of the impacting meteoroids, and (4) the directionality of the meteoroid flux relative to a spacecraft surface. In addition, in order to understand the hazard as it relates to electrical damage, it is important to know (1) the spacecraft material properties, including surface potential, and (2) the proximity of electrical components and shielding relative to impact location. These considerations are commonly bundled in a single environment model and, when combined with a model for impact damage (i.e., a BUMPER-like model), provide a specific estimate for spacecraft mechanical damage given a known spacecraft orbit. The dominant parameters for assessing mechanical damage are meteoroid mass and velocity, whereas the primary parameter for assessing electrical damage is velocity. It is therefore crucial that small, fast meteoroids be considered, even if the mass threshold lies below that needed to produce mechanical damage. J.E. Erickson, Velocity distribution of sporadic photographic meteors, Journal of Geophysical Research 73:3721-3726, 1968. 24 D.J. Kessler, Average relative velocity of sporadic meteoroids in interplanetary space, AIAA Journal. 7:2337-2338, 1969. 25 26 McDonnell et al., Interplanetary Dust, 2001. 27 Janches et al., Journal of Geophysical Research, 2006. 28 See also T.R. Kaiser, Interplanetary dust cloud, pp. 323-342 in International Astronomical Union Symposium #33: Physics and Dynamics of Meteors (L. Kresak and P.M. Millman, eds.), Springer-Verlag, New York, 1968; P.A. Wiegert, J. Vaubaillon, and M. Campbell-Brown, A dynamical model of the sporadic meteoroid complex, Icarus 201:295-310, 2009. 29 S.G. Love and D.E. Brownlee, A direct measurement of the terrestrial mass accretion rate of cosmic dust, Science 262:550-553, 1993. 30 J. Borovička, Physical and chemical properties of meteoroids as deduced from observations, pp. 249-271 in Proceedings of the International Astronomical Union, Vol. 1, Cambridge University Press, Cambridge, U.K., 2005.

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37 THE METEOROID ENVIRONMENT AND ITS EFFECTS ON SPACECRAFT FIGURE 4.3 Meteoroid velocity distributions as reported in the literature for meteors from gram-size (>4 × 10 –1 g) to ap- proximate nanogram masses (>1 × 10–11 g) as reported by radar. Data sources for the Harvard Super-Schmidt Photographic data (>4 × 10–1 g; Erickson, 1968; Kessler, 1969), the Advanced Meteor Orbit Radar (>3 × 10 –7 g; Galligan and Baggaley, 2004), the Canadian Meteor Orbit Radar (>4 × 10–4 g; Brown et al., 2005), the Harvard Radio Meteor Project (>1 × 10 –5 g; Taylor and Elford, 1998), the ARPA Long-Range Tracking and Instrumentation Radar (>1 × 10–5 g; Hunt et al., 2004), the Jicamarca Radio Observatory (>1 × 10–6 g; Janches and Chau, 2005), and the Arecibo Radar (>1 × 10–11 g; Janches et al., 2006). SOURCE: J.E. Erickson, Velocity distribution of sporadic photographic meteors, Journal of Geophysical Research 73:3721-3726, 1968; D.J. Kessler, Average relative velocity of sporadic meteoroids in interplanetary space, AIAA Journal 7:2337-2338, 1969; D.P. Galligan and W.J. Baggaley, The orbital distribution of radar-detected meteoroids of the Solar system dust cloud, Monthly Notices of the Royal Astronomical Society 353(2):422-446, 2004; P. Brown, J. Jones, R.J. Weryk, and M. Campbell-Brown, The velocity distribution of meteoroids at the Earth as measured by the Canadian Meteor Orbit Radar (CMOR), Earth, Moon and Planets 95(1-4):617-626, 2005; A.D. Taylor and W.G. Elford, Meteoroid orbital element distributions at 1 AU deduced from the Harvard Radio Meteor Project observations, Earth Planets Space 50:569-575, 1998; S.M. Hunt, M. Oppenheim, S. Close, P.G. Brown, F. McKeen, and M. Minardi, Determination of the meteoroid velocity distribution at the Earth using high-gain radar, Icarus 168:34-42, 2004; D. Janches and J.L. Chau, Observed diurnal and seasonal behavior of the micrometeor flux using the Arecibo and Jicamarca radars, Journal of Atmospheric and Solar-Terrestrial Physics 67:1196-1210, 2005; and D. Janches, C.J. Heinselman, J.L. Chau, and A. Chandran, Modeling the global micrometeor input function in the upper atmosphere observed by high power and large aperture radars, Journal of Geophysical Research 111:A07317, 2006. There are two contemporary NASA meteoroid models in use today: NASA SSP 30425, developed in the late 1980s and early 1990s primarily to address the issue of meteoroid damage to the space station, 31 and the NASA Meteoroid Environment Model.32 Both models use as their flux reference the Grün et al. Interplanetary Flux Model (Grün IFM).33 31 B.J. Anderson and R.E. Smith, Natural Orbital Environment Guidelines for Use in Aerospace Vehicle Development, NASA TM-4527, NASA, June 1994, available at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940031668_1994031668.pdf, accessed August 16, 2011. 32 H. McNamara, R. Suggs, J. Jones, W. Cooke, and S. Smith, Meteoroid Engineering Model (MEM): A meteoroid model for the inner solar system, Earth, Moon, and Planets 95:123-139, 2004, available at http://www.nasa.gov/pdf/195790main_McNamara_04-MEM.pdf, accessed August 16, 2011. 33 E. Grün, H. Zook, H. Fechtig, and R.H. Giese, Collisional balance of the meteoritic complex, Icarus 62:244-272, 1985.

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38 LIMITING FUTURE COLLISION RISK TO SPACECRAFT The Grün IFM34 is an empirical compilation of many sources of data and is used as the standard reference for meteoroid flux in near-Earth space. It is based on in situ dust detector fluxes from missions prior to the mid- 1980s, lunar microcratering, measurements of zodiacal brightness, and early measurements of meteor flux. The IFM ignores the directionality in the sporadic meteoroid flux and uses a mean velocity (rather than a velocity distribution) in computing flux values. It also ignores temporal variations in the background sporadic meteoroid flux, which have been measured as having an amplitude near a factor of 2 throughout the year. 35 For masses of less than 100 µm the IFM is well validated by numerous (and varied) sources of data. Recent (after the mid-1980s) meteoroid flux measurements have been found to be in general agreement with the Grün curve within uncertainty bounds applying at these smaller masses. However, for meteoroids with larger masses, order-of-magnitude dis - parities exist (see Figure 4.2). At sizes greater than 0.1 mm, fluxes can be derived only from meteor data, and the large uncertainty in mass-brightness-velocity conversions between earlier data and more recent measurements has become apparent. The SSP 30425 meteoroid model uses as its basis the Grün IFM but makes assumptions about the veloc - ity distribution that are incompatible with the velocity assumptions in the IFM. This inconsistency results in a roughly factor-of-2 underestimation of the flux relative to the IFM (which is the input to SSP 30425). This result is independent of the uncertainty in the flux caused by uncertainty in derived meteoroid mass from measurements, which produces an additional error factor of approximately 3 at submicrogram masses. 36 SSP 30425 also assumes isotropy in impact directions, which is at variance with observations. 37 The meteoroid bulk density distribution used in SSP 30425 incorporates estimates guided by early radar and photographic measurements of bulk density and chosen to vary as a function of mass from 2 g/cm3 for mass less than 10–6 g, to 1 g/cm3 for mass between 10–6 and 10–2 g, and 0.5 g/cm3 for mass greater than 10–2 g,38 although the actual variation in bulk density has been shown to correlate strongly with orbit type rather than mass alone. 39 A physics-based model, the Meteoroid Environment Model (MEM) starts with an assumed parent source population (comets and asteroids) and propagates released meteoroids forward in time in a Monte Carlo manner until they encounter Earth. This distribution of Earth-impacting particles is calibrated in its directionality and velocity distribution by radar measurements and adopts the IFM for flux. 40 It has several variants (see Table 3.1 in Chapter 3) but includes Earth-shielding and gravitational focusing where appropriate, and it adopts a single meteoroid density of 1 g/cm3 to provide compatibility in final flux values with the IFM, given the difference in mean velocity between MEM and the IFM. Validation for MEM has come only from data from near-Earth space, while the Grün IFM (implicitly used in MEM) provides meteoroid flux at 1 AU. Although some in situ measure - ments of very small meteoroids, together with measurements of zodiacal light, provide limited data on meteoroid populations at other heliocentric distances, much greater uncertainties exist in the measurements of meteoroid flux at distances other than 1 AU, with contradictory results particularly evident in measurements in the outer solar system.41 Confident extension of MEM to other heliocentric distances and larger particle sizes likely will require additional measurements beyond those that exist at present. Recently published work offers the prospect of a higher-fidelity evolutionary model of sporadic meteoroids, 42 although such newer work does not entirely agree on source populations and their relative strengths. Development of more physically realistic meteoroid models that fit the available constraints from all data sources remains to be done. Grün et al., Icarus, 1985. 34 M. Campbell-Brown and J. Jones, Annual variation of sporadic radar meteor rates, Monthly Notices of the Royal Astronomical Society 35 367:709-716, 2006, available at http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2005.09974.x/pdf, accessed August 16, 2011. 36 Anderson and Smith, Natural Orbital Environment Guidelines for Use in Aerospace Vehicle Development, 1994. 37 Jones and Brown, Royal Astronomical Society, Monthly Notices, 1993. 38 Anderson and Smith, Natural Orbital Environment Guidelines for Use in Aerospace Vehicle Development, 1994. 39 Ceplecha et al., Space Science Reviews, 1998. 40 McNamera et al., Earth, Moon, and Planets, 2004; Jones and Brown, Royal Astronomical Society, Monthly Notices , 2003. 41 C. Leinert and E. Grün, Interplanetary dust, pp. 207-282 in Physics of the Inner Heliosphere I1 (R. Schewenn and E. Marsch, eds.), Springer-Verlag, Berlin, Germany, 1990. 42 See Wiegert et al., Icarus, 2009; D. Nesvorny, P. Jenniskens, H.F. Levison, W.F. Bottke, D. Vokrouhlický, and M. Gounelle, Cometary origin of the zodiacal cloud and carbonaceous micrometeorites: Implications for hot debris disks, Astrophysical Journal 713:816-836, 2010.

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39 THE METEOROID ENVIRONMENT AND ITS EFFECTS ON SPACECRAFT Finding: The Meteoroid Environment Model incorporates in its predictions the latest available data on the meteoroid environment, including the directionality and full velocity distribution of the meteoroids. It is currently the NASA model that is most consistent with the known meteoroid environment, although some major uncertainties still remain. Recommendation: The NASA meteoroid and orbital debris programs should establish a baseline effort to evaluate major uncertainties in the Meteoroid Environment Model regarding the meteoroid environ- ment in the following areas: (1) meteoroid velocity distributions as a function of mass; (2) flux of meteor- oids of larger sizes (>100 microns); (3) effects of plasma during impacts, including impacts of very small but high-velocity particles; and (4) variations in meteoroid bulk density with impact velocity. Finding: The earlier SSP 30425 meteoroid model does not reproduce existing observational meteoroid data with a fidelity equal to that of the Meteoroid Environment Model. Numerous disparate sources of data have been fused to produce the current meteoroid flux model used by NASA, sometimes incorporat- ing differing underlying assumptions. Finding: The Meteoroid Environment Model currently does not extend to prediction of the meteoroid environment in the outer solar system, and the measurements it incorporates are poorly constrained in the cis-martian region. Recommendation: An effort should be made to re-examine earlier data used in the Grün Interplanetary Flux Model and to reconcile the data with more recent measurements in the literature on meteoroid flux, and a technical evaluation should be undertaken to synthesize and document such data as it is incorpo- rated into the Meteoroid Environment Model (MEM). Updates of the MEM and technical development should follow a technical pathway as rigorous as that being taken for updates of the Orbital Debris Environment Model. Recommendation: NASA should adopt the Meteoroid Environment Model for agency-wide official use and extend its capabilities to the outer solar system.