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The 2019 Meteor Shower Activity Forecast for Low Earth Orbit 1 Overview

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The 2019 Meteor Shower Activity Forecast for Low Earth Orbit 1 Overview NASA METEOROID ENVIRONMENT OFFICE The 2019 meteor shower activity forecast for low Earth orbit Issued 15 October 2018 The purpose of this document is to provide a forecast of major meteor shower activity in low Earth orbit. Several meteor showers – the Draconids, Perseids, eta Aquariids, Orionids, and potentially the Androme- dids – are predicted to exhibit increased rates in 2019. However, no storms (meteor showers with visual rates exceeding 1000 [1, 2]) are predicted. 1 Overview Both the MSFC stream model [3] and the Egal et al. Draconid model [4] predict a second Draconid outburst in 2019. The 2019 Draconids are expected to have nearly the same level of activity as the 2018 Draconids, which reached a zenithal hourly rate (or ZHR) of about 100. Twin outbursts also occurred in 2011 and 2012 [5, 6]. The Perseids, eta Aquariids, and Orionids are expected to show mild enhancements over their baseline activity level in 2019. The Perseids are expected to be slightly more active in 2019 than in 2018, with a peak ZHR around 112. The eta Aquariids and Orionids, which belong to a single meteoroid stream generated by comet 1P/Halley, are thought to have a 12-year activity cycle and are currently increasing in activity from year to year. A review of eta Aquariid literature [7, 8, 9, 10] also indicates that the overall activity level is higher than previously believed, and this is reflected in higher forecasted rates (a ZHR of 75) and fluxes for this shower. We may see enhanced beta Taurid activity in 2019. The beta Taurids are part of the same stream as the Northern and Southern Taurids but produce daytime meteors. All members of the Taurid complex are thought to have periodic enhancements [11]; the beta Taurids may see ZHRs as high as 20 in 2019. The flux enhancement, however, will be modest. Finally, we may see minor Andromedid activity in 2019. The Andromedids are particularly difficult to predict; this is due in part to the break-up of its parent comet, 3D/Biela. Activity in 2019 is associated with some, but not all, possible solutions for its parent comet orbit [12]. The same orbit solutions were associated with activity in 2005 and 2012, when activity was minimal or absent. We tentatively predict a ZHR of 5 for the Andromedids in 2019, but note that unexpected outbursts of this shower have occurred in the past. This document is designed to supplement spacecraft risk assessments that incorporate an annual av- eraged meteor shower flux (as is the case with all NASA meteoroid models). Results are presented rela- tive to this baseline and are weighted to a constant kinetic energy. Two showers – the Draconids (DRA) and the Geminids (GEM) – attain flux levels exceeding that of the baseline meteoroid environment for 0.1-cm-equivalent meteoroids. This 1-mm diameter is the threshold for structural damage. These two showers, along with the Daytime Arietids (ARI) and Quadrantids (QUA), exceed the baseline flux for 0.3- cm-equivalent particles, which is near the limit for pressure vessel penetration. 1 Meteor shower fluxes drop dramatically with increasing particle size. For example, the Arietids con- 6 2 1 9 2 1 tribute a flux of about 2 10− m− hr− in the 0.04-cm-equivalent range, but only 4 10− m− hr− for ⇥ ⇥ the 0.3-cm-equivalent and larger size regime. Thus, a PNP (probability of no penetration) risk assessment should use the flux and flux enhancements corresponding to the smallest particle capable of penetrating a component because the flux at this size will be the dominant contributor to the risk. 2 Details The list of showers included in the 2019 forecast is very similar to 2018, and most shower parameters (aside from ZHR) are unchanged. We have revised the Draconid top-of-atmosphere speed from 20 to 23 km/s to better match observations, and we have changed our abbreviation of the Puppid/Velid complex to “PUP” to match the International Astronomical Union’s nomenclature. We have also changed one aspect of our forecasting algorithm. In the past, we have assumed that the Grun¨ et al. meteoroid flux [13] includes shower meteoroids. However, internal investigations indicate that shower meteors were either removed from [14] or comprised an insignificant fraction [15] of the data on which the Grun¨ et al. flux relies. We therefore now consider the Grun¨ et al. flux to describe only the sporadic meteor complex. Any small amount of shower activity is an enhancement over the sporadic flux, and thus our reported enhancement factors are now uniformly positive. Figure 1 gives the expected visual meteor rates (ZHR) for ground observers during calendar year 2019. The visual rate is dominated by the Quadrantids in early January, the Geminids in mid-December, and the Perseids in mid-August. The Draconids should produce notable activity in October. The eta Aquariids (ETA) have a high ZHR, but, as a southern shower, tend to receive less attention in the northern hemisphere. Finally, the Daytime Arietids (ARI) technically have a significant ZHR, but since they occur at dawn they are difficult to observe visually. Although meteor astronomers record and predict showers in terms of visual rates, ZHR does not directly correspond to meteoroid flux. The conversion from ZHR to flux must take into account the biases of the typical human observer, the speeds of the shower meteors, and the mass distributions of meteoroids belonging to these showers. The result is a flux profile that looks significantly different from the ZHR profile, and high flux does not necessarily correspond to a visually spectacular meteor shower. Showers typically contain proportionally more large particles than the sporadic background does; for this reason, showers are more significant at larger particle sizes. Figure 2 gives the flux profiles for four particle sizes; because damage is more closely related to kinetic energy than it is to particle size, we have computed the particle size for each shower that has the same kinetic energy as a 20 km/s particle with a diameter of 0.04, 0.1, 0.3, or 1.0 cm and a density of 1 g/cc. The sporadic meteoroid flux for each of these diameters is also shown for comparison (horizontal lines). Note that for small particle sizes, shower fluxes are less significant when compared to the sporadic flux. Figure 2 also indicates that, depending on the threat size considered, a handful of showers produce the highest fluxes. The basic characteristics of six major showers, including radiant position, are listed in Table 1 (the full list of included showers is provided at the end of this document in Table 2). For a spacecraft, the apparent directionality of a meteor shower (i.e., the aberrated radiant) will be shifted by the spacecraft’s geocentric velocity. 2 shower name radiant speed date of maximum 1 RA (◦) dec (◦) (km s− ) (UTC) Quadrantids 230 +49 41 2019-01-04 12:15 Daytime Arietids 44 +24 39 2019-06-11 04:04 Southern delta Aquariids 340 -16 42 2019-07-28 13:20 Perseids 48 +58 61 2019-08-13 08:15 Draconids 262 +55 23 2019-10-08 14:41 Geminids 112 +33 35 2019-12-14 19:26 Table 1: The six most active meteor showers of 2019 (in terms of flux). The radiant is the geocentric, unaberrated radiant and speed is taken at the top of the atmosphere. In order to facilitate risk assessments, including BUMPER PNP calculations, we provide flux enhance- ment factors for all of 2019 in 1-hour intervals (Figure 3). The larger flux enhancement factors in Figure 3 correspond to 0.1-cm-equivalent particles, which have lower absolute fluxes. The fluxes and enhancement factors presented in this memo may or may not apply to individual space- craft. For instance, we have not presented crater-limited fluxes; meteoroids incident on a surface at right angles penetrate deeper for many ballistic limit equations, and, for a surface directly facing the shower, this can further boost the significance of a shower relative to the background by another factor of approximately 2. Conversely, a surface tilted away from a shower radiant will encounter a less significant flux enhance- ment, and it is possible for the Earth to shield the spacecraft from all or part of a shower at a particular point in time. This forecast is designed for spacecraft in LEO; it does not cover spacecraft orbiting the Moon or near the Sun-Earth Lagrange points. 3 Contact information The Meteoroid Environment Office will update this forecast as necessary. Those with questions or special needs in the near future are encouraged to contact: Althea Moorhead NASA Meteoroid Environment Office EV44, Marshall Space Flight Center (256) 544-7352 [email protected] Bill Cooke Lead, NASA Meteoroid Environment Office EV44, Marshall Space Flight Center (256) 544-9136 [email protected] Contributors: Danielle Moser (Jacobs ESSCA, NASA MEO) 3 References [1] Y.-h. Ma, P.-x. Xu, G.-y. Li, and G. Bao. Astronautical definition of meteor storm. Chinese Astronomy and Astrophysics, 30:61–67, 2006. [2] G. W. Kronk. Meteor Showers. Springer New York, 2014. [3] D. E. Moser and W. J. Cooke. Updates to the MSFC Meteoroid Stream Model. Earth Moon and Planets, 102:285–291, 2008. [4] A. Egal, P. Wiegert, P. G. Brown, D. E. Moser, A. V. Moorhead, and W. J. Cooke. The Draconid Me- teoroid Stream 2018: Prospects for Satellite Impact Detection. The Astrophysical Journal Letters, 866:L8, 2018. [5] Q. Ye, P. G. Brown, M. D. Campbell-Brown, and R. J. Weryk. Radar observations of the 2011 October Draconid outburst.
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