How to measure the flux
of large meteoroids?
Geert Barentsen
University of Hertfordshire
International Meteor Conference 2012
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I don’t know.
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But it’s a good question!
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The frequency of small bodies colliding with Earth
(Brown et al. 2002)
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The frequency of small bodies colliding with Earth
(Brown et al. 2002)
Fireball -5 mag (~10cm)
every few minutes somewhere on Earth
(Note: brightness also depends on density, velocity, etc.)
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The frequency of small bodies colliding with Earth
(Brown et al. 2002)
Fireball -5 mag (~10cm)
every few minutes somewhere on Earth
Fireball -9 mag (~1m)
~ once a week
(Note: brightness also depends on density, velocity, etc.)
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The frequency of small bodies colliding with Earth
(Brown et al. 2002)
Fireball -5 mag (~10cm)
every few minutes somewhere on Earth
Fireball -9 mag (~1m)
~ once a week
Fireball -13 mag (~10m)
~ once a year (e.g. 2008 TC3)
(Note: brightness also depends on density, velocity, etc.)
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The frequency of small bodies colliding with Earth
(Brown et al. 2002)
Fireball -5 mag (~10cm)
every few minutes somewhere on Earth
Fireball -9 mag (~1m)
~ once a week
Fireball -13 mag (~10m)
~ once a year (e.g. 2008 TC3)
Tunguska (~50m)
~ once every 1000 years?
(Note: brightness also depends on density, velocity, etc.)
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The frequency of small bodies colliding with Earth
(Brown et al. 2002)
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All-sky
cameras
The frequency of small bodies colliding with Earth
(Brown et al. 2002)
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All-sky
cameras
Telescopic
asteroid
surveys
The frequency of small bodies colliding with Earth
(Brown et al. 2002)
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All-sky
cameras
Telescopic
asteroid
surveys
Military satellites
& infrasound
The frequency of small bodies colliding with Earth
(Brown et al. 2002)
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Data on large meteoroids
1. Military satellites
• US Defense and Energy departments operate satellites to
detect nuclear explosions;
• detected ~300 bolide detonations between 1994 and 2002;
• sensitive down to ~1 meter objects (as far as reported?)
2. Ground-based infrasonic/acoustic data
• 19 events in Brown et al. (2002);
• biased towards deeply penetrating (asteroidal) bodies.
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Problem
• Data on meteoroids between 20cm and 10m
is either sparse or restricted.
• All-sky cameras tend to miss this size range because
• the objects are very infrequent;
• brightness estimates are tricky due to saturation.
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A pre-planned exception
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A pre-planned exception
Wake of 2008 TC3 detected by Meteosat 8
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Yet we know that meteoroid
streams may contain big fragments!
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Broke into dozens of sub-km
fragments in 1995 and 2001
(e.g. Vaubaillon & Reach 2010)
Comet 73P/Schwassman-Wachmann 3
Image: Spitzer Space Telescope
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Comet McNaught
Image: Robert McNaught
Orientation of “striae” suggests fragmentation
(e.g. Sekanina et al.)
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Comet Hartley 2
Fly-by of EPOXI spacecraft revealed meteoroids
sized 3 to 30 cm (A’Hearn et al. 2011)
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Comet Hartley 2
Fly-by of EPOXI spacecraft revealed meteoroids
sized 3 to 30 cm (A’Hearn et al. 2011)
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Comet Hartley 2
Fly-by of EPOXI spacecraft revealed meteoroids
sized 3 to 30 cm (A’Hearn et al. 2011)
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Tunguska event (1908)
Timing and direction of Tunguska object
appears consistent with Taurid complex
(e.g. Kresak 1978, Jopek 2008)
Two other major airbursts coincided with
Perseids on 13 Aug 1930 and Geminids
on 11 Dec 1935 (Napier & Asher 2009)
Taurids, Geminids and Arietids are
associated with km-sized asteroids
(e.g. Jenniskens et al. 2008)
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Do our current meteoroid streams
harbour large objects?
• Pro: decameter-sized bodies may have sublimation lifetimes
lasting dozens of perihelion passages (Beech & Nikolova 2001)
• Con: they may disintegrate quickly due to thermal and tidal
stresses, radiative spin-up, collisions (e.g. Davidsson 1999)
=> Need to measure their flux to determine
just how frequent (or rare) they are!
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Do our current meteoroid streams
harbour large objects?
• Pro: decameter-sized bodies may have sublimation lifetimes
lasting dozens of perihelion passages (Beech & Nikolova 2001)
• Con: they may disintegrate quickly due to thermal and tidal
stresses, radiative spin-up, collisions (e.g. Davidsson 1999)
=> Need to measure their flux to determine
just how frequent (or rare) they are!
=> Important because it puts constraints on the
fragmentation process and the frequency of Tunguska events.
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So, how to measure the
frequency of large meteoroids?
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I still don’t know.
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But here are two ideas...
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1. Point a telescope at a meteoroid stream
2. Exploit fireball sightings by humans
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• Barabanov et al. (1996) and Smirnov & Barabanov
(1997) reported the detection of five decameter-sized
objects during the Perseids, using a 1m-telescope.
• A repeat experiment by Beech et al. (2003) failed to
detect any such objects.
• Draconids 2011 offered an excellent opportunity to
repeat such experiment.
Pointing a telescope at a stream
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La Palma
15 000 km
50cm
Draconid
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La Palma
15 000 km
50cm
Draconid
Brightness = 17th magnitude
(assuming albedo 0.04, elongation 84deg)
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La Palma
15 000 km
50cm
Draconid
Brightness = 17th magnitude
(assuming albedo 0.04, elongation 84deg)
A 10-meter object even reaches
17th magnitude at 500 000 km!
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La Palma
ω = 0.1 degrees/second @ 15 000 km
Impedes detection :-(
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La Palma
If you point within 0.5 degrees
from the radiant...
ω < 2 arcseconds/second :-)
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Liverpool Telescope (La Palma)
2.0 meter robotic Cassegrain
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We used this camera to take
7500 x 0.8 second exposures
during the Draconid outburst
Andor DW435 ‘RISE’ camera
E2V frame-transfer CCD
No readout overhead!
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17 mag
12 mag
9 arcminutes
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Draconids
No convincing detections; hence upper limit on the flux:
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Need to cover a larger area in
space to obtain tighter constraints
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World’s largest sensor network
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Human fireball sightings
• Amount of atmosphere monitored by humans
remains much larger than that by all-sky cameras
• Brightness range
• CCD chips: 6 magnitudes
• Humans: >20 magnitudes
• Databases of fireball sightings remain useful, but ...
• No global database?
• Tricky selection effects (e.g. different reports forms)
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Hold on.
There is a global database.
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• Designed to share text messages
of 140 characters with the world.
• 500 million active users (!!);
340 million messages per day
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5.6 million messages since 2010 contained one of the words
“meteor(s), meteorite(s), meteoro(s), meteorito(s), fireball(s)”
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e.g. during the Perseids:
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But also at unexpected times, e.g. 15 April 2010
3:06 UT
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15 April 2010
3:06:41 @sarahrattenborg HOLY BALLS. METEOR.
3:07:06 @JazzieBabeee I just saw a meteor!
3:07:21 @zeroethic I swear to Bob I just seen a fireball ...
3:07:24 @OhJorden Just saw like, a plane explode ...
3:07:25 @BJWEISFLOG just saw a huge meteor ...
3:07:25 @AdamPeters WHO JUST SAW THAT HUGE METEOR ...
Followed by 600 similar messages within the hour.
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Data Mining Twitter
• Possible project: measure the fireball
frequency using natural language processing.
• Assume the number of messages is a
function of brightness?
• Message counts can be normalized using the frequency
of unrelated messages.
• Some geospatial information is attached to
each message.
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Conclusion
• Measuring the flux of large meteoroids is tricky
• Until we get access to satellite data,
we’ll have to be creative!