The primary meteoroid flux at the Moon and at the lunar south pole
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
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
flux (m−2 yr−1 )
Geminid
10−2
10−4
10−6
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.
v̂m − v̂sc
MEM then describes the resulting
v̂m
environment relative to a given spacecraft
v̂sc
trajectory [ref, ref, ref, ref, ref].
3 / 19
MEM provides impactor size, angle, speed, and density.
flux (m−2 yr−1 )
102
100
10−2
10−4
10−6
10−8
10−6 10−3 100
limiting mass (g)
flux (m−2 yr−1 )
flux (m−2 yr−1 )
10−1
0.15
10−2 0.1
10−3 0.05
10−4 0
0 20 40 60 0 2 4 6 8
speed (km s−1 ) density (g cm−3 )
4 / 19MEM 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
relative flux
10−6
1
0.5
10−15
0
10−11 10−5 101
−0.5
−1 limiting mass (g)
−2 0 2
5 / 19MEM 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 / 19MEM 3’s orbital populations match their observed strength at Earth.
helion
0.20
tor.
MEMR2
0.15 apexl
flux
60% MEM 3
total
observed 0.10
40% 0.05
0.00
20% 0.20
0.15
flux
0%
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 / 19MEM 3 introduces a new, bimodal density distribution.
water rock iron
MEMR2
data
20 MEM 3
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 / 19MEM 3 predicts a higher risk for most spacecraft.
MEMR2
relative freq.
MEM 3
fraction
flux
MEMR2
MEM 3
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 / 19MEM 3 matches the available in situ data.
no extrapolation
MEM 3 + CP MEM 3 + CP
MEMR2 + CP MEMR2 + CP
mets. + debris
observed
meteoroids
MEM 3 + WA MEM 3 + WA
MEMR2 + WA MEMR2 + WA
extrap. to small masses
0 0.5 1 1.5 2 0 50 100
−2
Pegasus penetration rate (m yr−1 ) 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 / 19Cratering rate at the Moon (2-mm-deep in aluminum)
0.02
equator
45◦ S
0.015 south pole
craters per year
0.01
0.005
0
east west north south zenith nadir total
11 / 19Meteor 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
ZHR
ARI
ETA
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 / 19ZHRs are converted to flux for a given altitude and limiting parameter.
13 / 19We 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 / 19We 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
ZHR
2,000
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 / 19We 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
enhancement
0
2014 Python version
10
0
1
1998 First forecast 0 8R⊕
16 / 19Particularly 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.
equivalent ZHR
Egal et al. 4000 fit
3000
N
2000
1000
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 / 19We have begun to issue forecasts for the lunar surface
NASA M ETEOROID E NVIRONMENT O FFICE
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 / 19Summary
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.
I MEM 3 was released mid-2019 and offers many improvements over previous
versions. It predicts higher risk but matches meteor and impact data.
I It’s the most appropriate tool for simulating the primary meteoroid flux onto the
Moon.
Separately, we issue meteor shower forecasts:
I The code we use has gradually increased in complexity and fidelity and is beginning
to resemble MEM.
I Inputs are based on both typical shower activity and model predictions.
I We have begun to issue forecasts specific to the lunar surface.
19 / 19You can also read
