Brown, Peter

Campbell-Brown, Margaret

Cooke, Bill

Moorhead, Althea

NASA Technical Reports Server

Optical and radar measurements of
the meteor speed distribution
Althea Moorhead, Bill Cooke
NASA Meteoroid Environment Office
Peter Brown, Margaret Campbell-Brown
University of Western Ontario
Meteoroids 2016
June 7, 2016
https://ntrs.nasa.gov/search.jsp?R=20160008878 2019-08-31T02:06:17+00:00Z
Meteoroid speed distribution(s)
10 20 30 40 50 60 70
10−2
10−1
100
101
velocity (km/s)
re
la
ti
ve
fl
u
x
Brown (2004)
Fentzke & Janches (2008)1
Nesvorny´ et al. (2010)1
1 As represented in Janches et al. (2014)
Goals
Have: Start with the meteor speed distribution
to constant limiting radar amplitude
Improve: Re-weight the radar speed distribution
to constant limiting KE
I Use improved bias estimations
I Use modern forms of β
New: Characterize associated uncertainties
New: Re-weight the radar speed distribution
to constant limiting magnitudes and
compare with optical measurements
Correcting to a limiting mass
q ∝ mavb, flux ∝ m−α → N>mref = Nv−bα/a (Taylor, 1995)
20 40 60 80
10−3
10−2
10−1
100
v (km/s)
re
l.
n
o
Ionization efficiency
I Jones (1997) predicts
q 6∝ vb
I Experiments confirm this for
iron (Sternovsky, 2015)
I Radar detections show a
“cliff” near 9.5 km/s

XXXXXq ∝ mavb
q = −β(v)µv dmdt
10 15 20 30 70
10−4
10−2
100
v (km/s)
β
Sternovsky (2015)
Jones (1997)
8 10 12
0
200
400
v (km/s)
N
Mass ablation rate
dm
dt
∝ m2/3 dm
dt
∝ m
dm
dt
= −ΛA
2ξ
(
m
mfrag
)x ( m
ρm
)2/3
ρav
3
m
Kinetic energy distribution
I Impact experiments are KE-limited
p = 5.24× d19/18BH−1/4
(
ρp
ρt
)1/2(vp cosβ
cs
)2/3
= 0.739× KE19/54 BH−1/4 ρ
4/27
p
ρ
1/2
t
v
−1/27
p
(
cosβ
cs
)−2/3
I Meteor observations are closer to being KE-limited than
mass-limited
q ∼ m v3.5
∼ KE v1.5
Kinetic energy distribution
+
0 0.5 1
0
0.1
0.2
0.3
crater depth/diameter
fr
ac
ti
on
100 102 104
0.00
0.05
0.10
0.15
density (kg/m3)
fr
ac
ti
on
Kinetic energy distribution
−5 0 5 10 15 20
−25
−20
−15
−10
−5
0
log10 KE (J)
lo
g 1
0
fl
u
x
(m
−2
d
ay
−1
)
Radar bias corrections
I Pulse repetition effect
I Finite velocity effect
I Initial trail radius effect
I Empirical relation
(with uncertainties!)
from Jones &
Campbell-Brown
(2005)
I Beam pattern/radiant
visibility
−50 0 50 100
0
200
400
dec (◦)
co
lle
ct
in
g
ar
ea
Corrected speed distribution
10 20 30 40 50 60 70 80
10−5
10−3
10−1
101
103
I 8 variations on β
I 2 KE distributions
I random mass exponent
I random ITR correction coefficients
I log-random min attenuation
v (km/s)
re
l.
n
o
Gravitational focusing constraints
Micrometeoroid flux near Earth–Sun L1 303
in turn yields an estimate of the degree to which incoming dust
is focussed by the Earth’s gravity.
2. Measurements
The space-exposed surface investigated here was a highly
polished aluminum plate. Various literature sources and Gene-
sis mission documents refer to it as the “saddle collector,” the
“polished aluminum collector,” the “saddle side-canister ther-
mal shield collector,” and the “kidney.” For brevity we adopt
the last term. The kidney was machined from a single piece of
6061-T6 aluminum stock 0.2 in (5.08 mm) thick. After milling,
most of the surface was 0.025 in (0.635 mm) thick, supported
by four stiffening ribs 0.1 in (2.54 mm) thick. The kidney was
roughly triangular. It measured about 15× 20 cm, with an area
of 235 cm2. As shown in Fig. 1, the kidney was located in the
sample return canister and faced in the same direction as the
solar wind collector arrays. The collector arrays pointed toward
the Sun, offset by 4.5± 2.0 degrees toward the direction of the
Earth’s orbital motion for optimal alignment with the average
direction of the solar wind. The spacecraft rotated about the col-
lector array pointing axis at a rate of 1.6± 10% rpm (Burnett
et al., 2003). The kidney was exposed to space for the same du-
ration as the bulk solar wind collectors: 852.83 days (Reisenfeld
et al., 2005) or 2.33 years. Part of the sky as seen from the cen-
ter of the kidney was blocked by surrounding structures. These
obstructions decreased the number of incoming particles that
could hit the kidney. Impacts on the same obstructions may have
produced high-speed secondary particles that cratered the kid-
ney.
The kidney was bent, scratched, and contaminated during
landing. Fig. 2 shows its condition after recovery. Under the
microscope, many features that hinder impact crater studies are
evident. Parts of the surface are marred with scratches large
enough to degrade or destroy small craters. Other areas are
covered with white deposits, apparently left by the evapora-
tion of mineral-laden water droplets that fell on the kidney
after the muddy hard-landing. Some evaporite deposits are thick
enough to cover craters that might lie beneath them, and many
Fig. 1. The polished aluminum kidney installed on the Genesis spacecraft be-
fore launch.
are round in shape, making them potentially mistakable for
craters. Small transparent crystals on the kidney surface may
also mimic the bright rim and dark interior of an impact crater
under the oblique lighting conditions needed for this study.
Most of the∼760 microscope fields examined in our scans con-
tained enough optical clutter to make identification of smaller
craters less than certain. All of these effects complicate identi-
fication of impact craters, opening the door for “false positives”
(features that appear to be craters but really are not) and “false
negatives” (craters that have been obscured beyond recogni-
tion). The potential for misidentification is less at large sizes.
With the equipment we used, craters bigger than about 40 µm
are easy to spot and impossible to counterfeit.
Before the kidney was made available for this study, a 45 cm2
portion of it was scanned for an inventory of surface features
(Allton et al., 2005), then sawed off and allocated for use in an
unrelated investigation. We make use of archived images from
the separated part of the kidney to extend our crater counts to
that area.
For this study, we affixed the kidney’s permanent mount-
ing plate to a motor-driven x–y translating stage on an optical
bench. The stage allowed us to move any part of the kidney into
the field of view of a Leica MZ16A stereomicroscope with mo-
torized focus and zoom controls. We used Image Pro software
to photograph and measure features, and Aardvark software to
control the coordinate frame and stage positioning. The x and
y extent of the kidney was tiled into a rectangular array of 34
by 40 visual fields under 20× magnification. Because of the
irregular shape of the kidney, many of the fields were off the
sample. Fields that included part of the sample were searched
by eye for craters. Computer-displayed CCD camera images
from the microscope had insufficient resolution for searching,
but were used to photograph and measure features under 115×
magnification. The independent scan of the separated 45 cm2
portion of the sample employed the same equipment and proto-
col.
For each field, we estimated what fraction of the surface was
clean and undamaged enough for craters to be found. Of our
Fig. 2. The kidney after recovery.
0 5 10 15 20 25
0
5
10
15
x (cm)
y
(c
m
)
I Cratering rate on Genesis near L1 was within 40% of
near-Earth rate (Love & Allton, 2006)
Weighted average speed distribution
10 20 30 40 50 60 70 80
10−4
10−3
10−2
10−1
100
101
102 vavg = 19.5 km/s
vrms = 21.5 km/s
v (km/s)
re
l.
n
o
Luminous efficiency
0 10 20 30 40 50 60 70
10−5
10−4
10−3
10−2
10−1
v (km/s)
τ
Opik (1955)
Verniani (1964)
Hill et al. (2005), corrected
Ceplecha & McCrosky (1976)
Weryk & Brown (2013)
constant 0.7%
Radiant coverage
90 0 270 180
−90
−45
0
45
90
λ− λ (◦)
β
(◦
)
CAMO influx camera speed distribution
Limiting magnitude of +6.5
10 20 30 40 50 60 70 80
10−2
10−1
100
v (km/s)
re
l.
n
o
CMOR weighted to +6.5
CAMO influx cameras
NASA all-sky network speed distribution
Limiting magnitude ∼ −1
10 20 30 40 50 60 70 80
10−2
10−1
100
v (km/s)
re
l.
n
o
CMOR weighted to -1
NASA all-sky network
Luminous efficiency
log10(β/τ) = cv log10 v − cL log10 L− cc
Conclusions
Improved: Radar speed distribution to constant limiting KE
I Improved treatment of β yields more slow
meteors
New: Characterized associated uncertainties
I Large uncertainty remains for slowest bins
New: Good agreement with video data for some βs
Future: Better characterization of τ , especially at low
speed
Future: Refine speed distribution with additional in-situ
constraints


Optical and Radar Measurements of the Meteor Speed Distribution

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