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The Primary Meteoroid Flux at the Moon and at the Lunar South Pole

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The Primary Meteoroid Flux at the Moon and at the Lunar South Pole The primary meteoroid flux at the Moon and at the lunar south pole Althea Moorhead NASA Meteoroid Environment Office, MSFC We model shower and sporadic meteoroids separately. Shower meteors occur at a certain time of year and share similar orbits. Sporadic meteors occur throughout the year, have varied orbits, and pose more risk[ref]. 100 sporadic ) 1 Geminid − 10−2 yr 2 −4 − 10 10−6 flux (m 10−8 Photographs by David Kingham 10−6 10−4 10−2 100 limiting mass (g) 2 / 19 MEM describes the sporadic component of the meteoroid environment. Meteoroid models primarily consist of populations of meteoroid orbits. The environment varies across interplanetary distances, not km, which allows us to extrapolate from Earth-based observations. The gravity and size of the Earth and Moon affect the local environment, and the spacecraft's motion factors in to the apparent velocity of the meteoroids it encounters. vˆ vˆ m − sc MEM then describes the resulting vˆ m environment relative to a given spacecraft vˆ sc trajectory [ref, ref, ref, ref, ref]. 3 / 19 MEM provides impactor size, angle, speed, and density. ) 1 102 − 0 yr 10 −2 2 10 − 10−4 10−6 10−8 flux (m 10−6 10−3 100 limiting mass (g) ) ) 1 1 −1 10 − − 0.15 yr yr −2 2 2 10 0.1 − − 10−3 0.05 −4 0 10 flux (m flux (m 0 20 40 60 0 2 4 6 8 speed (km s−1) density (g cm−3) 4 / 19 MEM has limitations that we'd like to eventually remove. Limited to inner solar system: 0.2 { 2 au Limited to within 5◦ of the ecliptic ∼ Limited to 10−6 { 10 g 103 10−6 1 relative flux 0:5 10−15 0 10−11 10−5 101 −0:5 −1 limiting mass (g) −2 0 2 5 / 19 MEM 3's improvements include a correct handling of planetary gravity Planets (and moons) bend and block the paths of meteoroids MEMR2 8 MEM 3 7 flux 6 5 104 105 altitude (km) This effect was erroneously large in MEMR2, and has been corrected in MEM 3. The flux is now lower at low altitudes and higher at high altitudes. 6 / 19 MEM 3's orbital populations match their observed strength at Earth. helion 0.20 tor. MEMR2 0.15 apexl MEM 3 60% total observed flux 0.10 40% 0.05 0.00 20% 0.20 0.15 0% flux 0.10 helion toroidal lapex 0.05 0.00 This re-weighting of the orbital populations also 10 20 30 40 50 60 70 changes the speed distribution. speed (km s−1) 7 / 19 MEM 3 introduces a new, bimodal density distribution. water rock iron data 20 MEM 3 MEMR2 data MEM 3 10 0 0.5 1 3 5 10 density (g cm−3) This distribution is based on detailed modeling of meteors in the atmosphere [ref, ref]. 8 / 19 MEM 3 predicts a higher risk for most spacecraft. MEMR2 MEM 3 flux fraction MEMR2 MEM 3 relative freq. 104 105 20 40 60 80 0.5 1 3 5 10 altitude (km) speed (km s−1) density (g cm−3) MEM 3 has higher flux (at some altitudes), faster speed distribution, and new high densities. All of these factors will increase the risk [ref]. How do we know that's justified? 9 / 19 MEM 3 matches the available in situ data. no extrapolation MEM 3 + CP MEM 3+CP MEMR2 + CP MEMR2 + CP MEM 3 + WA MEM 3 + WA MEMR2 + WA MEMR2 + WA extrap. to small masses observed meteoroids mets. + debris 0 0:5 1 1:5 2 0 50 100 −2 −1 Pegasus penetration rate (m yr ) number of craters on LDEF MEM 3 lies between the two best sets of in situ data we have in the threat regime: 1 µg - 1 g [ref] 10 / 19 Cratering rate at the Moon (2-mm-deep in aluminum) 0.02 equator 45◦ S 0.015 south pole 0.01 craters per year 0.005 0 east west north south zenith nadir total 11 / 19 Meteor showers are a small but varying fraction of the environment. Shower meteoroids constitute 1-5% of the risk, but this risk varies with time. We handle meteor showers in separate \forecasts" that describe their activity over the course of a year 150 150 QUA GEM (λ0, ZHR0) PER 100 100 ARI ETA ZHR 50 / 10Bp (λ −λ0) / 10−Bm(λ −λ0) 50 0 0 256 258 260 262 264 266 268 Jan Apr Jul Oct ◦ λ ( ) hi 12 / 19 ZHRs are converted to flux for a given altitude and limiting parameter. 13 / 19 We made a series of improvements starting in 2016. First, we updated many meteor shower activity profiles using data from CMOR ... 2020 Radiant dispersions 2019 Gravitational focusing 2018 Other altitudes 2016 Code & data updated 2014 Python version Second, we updated the algorithms. For instance, we improved our calculation of the 1998 First forecast average shower contribution to the meteoroid flux [ref] 14 / 19 We expanded the code to handle other altitudes in 2018. We generalized the code in 2018 to handle additional altitudes, the Moon, and Lagrange 2020 Radiant dispersions points [ref] 2019 Gravitational focusing L1 Gaia 2018 Other altitudes 4;000 2016 Code & data updated 2;000 ZHR 2014 Python version 0 195:2 195:4 195:2 195:4 ◦ ◦ λ ( ) λ ( ) 1998 First forecast This was done in part to enable a 2018 Draconid forecast near L1 and L2. 15 / 19 We can now generate forecasts for specific trajectories. In 2019, we incorporated local gravitational 2020 Radiant dispersions focusing and planetary shielding. In 2020, we 2019 Gravitational focusing improved this to take stream dispersion into 2018 Other altitudes account [ref]. 2016 Code & data updated 100 0 2014 Python version 10 0 1 enhancement 1998 First forecast 0 8R⊕ 16 / 19 Particularly tricky showers may require more extensive modeling. We and colleagues at the University of Western Ontario conducted detailed simulations of the Draconids in advance of our 2018 and 2019 forecasts [ref]. 5000 MSFC sims. Egal et al. 4000 fit 3000 N 2000 1000 equivalent ZHR 0 -0.01 -0.005 0 0.005 0.01 195:1 195:2 195:3 195:4 ◦ r r⊕ (au) solar longitude ( ) − Based on these simulations, we issued an advisory for the Sun-Earth L1 point. 17 / 19 We have begun to issue forecasts for the lunar surface NASA METEOROID ENVIRONMENT OFFICE The 2020 meteor shower activity forecast for the lunar surface Issued 8 November 2019 The purpose of this document is to provide a forecast of major meteor shower activity on the lunar surface. While the predictions in this document are for the surface, spacecraft orbiting the Moon at low altitudes will encounter meteoroids at similar rates. Typical activity levels are expected for nearly all show- ers in 2020; only the Geminids, which are gradually increasing in strength over time, are expected to be stronger than in previous years. No meteor storms or outbursts are predicted for 2020. 1 Overview No meteor shower outbursts are predicted for 2020. The fluxes and enhancement factors in this docu- ment correspond to the “typical” level of activity of the showers in our list in most cases. We have made 18 / 19 two notable changes to our shower parameters this year: we have increased the predicted activity level of the Geminids, and we have updated the size distribution of the Daytime Arietids. Further discussion of these showers is available in the meteor shower activity forecast for low Earth orbit [1]. This document is designed to supplement risk assessments that incorporate an annual averaged meteor shower flux (as is the case with all NASA meteoroid models). Results are presented relative to this baseline and are weighted to a constant kinetic energy. Note that meteor shower fluxes drop dramatically with increasing particle energy. Thus, a PNP (probability of no penetration) risk assessment should use the flux and flux enhancement factors corresponding to the smallest particle capable of penetrating a component because the flux at this size will be the dominant contributor to the risk. The fluxes given in this forecast are those at each shower’s subradiant point. An individual spacecraft or single, fixed location on the lunar surface – such as the lunar south pole – will experience significantly reduced fluxes from showers whose radiants lie far from local zenith. Some showers will be blocked en- tirely. Please contact the Meteoroid Environment Office (MEO) with latitude and longitude information if a location-specific forecast is needed, or with trajectory information if a spacecraft-specific forecast is needed. 2 Details The expected visual meteor rates (ZHR) for Earth-based observers during calendar year 2020 are avail- able in Fig. 1 of Ref. [1]. The visual rate is dominated by the Quadrantids in early January, the Perseids in mid-August, and the Geminids in mid-December. However, while meteor astronomers record and predict showers in terms of visual rates, ZHR does not directly correspond to meteoroid flux and thus is not mean- ingful at the lunar surface. We therefore convert ZHR to flux, taking 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, where a high flux does not necessarily correspond to a visually spectacular meteor shower. 1 Summary MEM is our sporadic environment model: I It covers the bulk of the meteoroid environment and is useful during the design phase of a mission.
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