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(meteorobs) Interstellar article
Lets try this again... now that we're up and running again!...
Guys -
Lew had suggested that I summarize an article in our April 1997 RASC Journal,
entitled "Do Some Meteorites Come from Interstellar Space?", by Robert L.
Hawkes and Sean C. Woodworth.
As the abstract is not too long, I have posted it verbatim, followed by a
summary of the article itself.
- Cathy
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Abstract:
"We review observational evidence which suggests that a small percentage (<2%)
of incoming meteors are in hyperbolic orbits, and that the percentage increases
with decreasing mass. Because of the magnitude of random stellar motions in the
solar neighbourhood, interstellar meteors are expected to arrive at Earth with
geocentric velocities ranging from about 21 km/s to 78 km/s. Classical
meteorite theory precludes meteorite survival above a geocentric velocity of
about 28 km/s, and the atmosphere ablates most of their mass even for velocities
in the low twenties. Hence, while interstellar meteorites are possible, there
are strong biases against them. A case has been made that the Pultusk meteorite
of 1868 had a well-determined hyperbolic orbit. However, that is based on
visual observations and leads to other conclusions regarding the trajectory
which seem unreasonable, so we discount the claim. Micrometeorite, or more
precisely ablation product, survival is also a strong function of entry
velocity, with very little residual mass surviving for velocities above 40 km/s.
It is unlikely that interstellar material would be represented in interplanetary
dust particle (IDP) collections, and even IDP's of cometary origin are strongly
discriminated against. Radiation pressure from solar-type stars is effective
for ejecting meteoroids of radius 0.4 um and smaller. Conversely, particles of
that size can be slowed upon entry, and could survive. It is unclear if such
small particles are part of existing IDP collections, but they should be
strongly favoured."
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A summary of the rest of the article follows.....
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Why is it important to identify if even a few micro/meteorites are from
interstellar space? It would provide evidence regarding the formation and
characteristics of planetary systems - chemical and physical.
Studies have been based on various detection methods - rocking mirror
techniques, Super-Schmidt photographic studies, Pioneer 8 and 9 space dust
collectors, and Ulysses dust detection at high ecliptic latitudes.
Photographic studies have found near or total absence of interstellar meteors.
Spacecraft, on the other hand, have found a much higher percentage of dust in
apparently hyperbolic orbits. This dust arrival also seems to dominate at an
ecliptic latitude of 10d +/- 10d and an ecliptic longitude of 280d +/- 30d,
which corresponds to a direction only about 20 degrees from the direction of the
galactic center. The spacecraft experiments provide statistical evidence for
interstellar meteoroids, based on the angle of arrival of the dust impact - but
not orbit determination for a single event.
The smallest masses found photographically are about 10 to the -5 kg.. The
Ulysses spacecraft detects masses as large as only 10 to the -12 kg. The New
Zealand AMOR radar can detect a minimum mass of about 10 to the -9 kg., ie.
corresponding to about visual mag. 13. In 1993, the AMOR-detected meteors
showed about 0.9 % of their sample to be hyperbolic. Hawkes & Woodworth, on
reviewing the first multi-station television meteor study, found 5 meteors from
a sample of 77 to be apparently hyperbolic, but once stats were applied, only 1
likely to be in hyperbolic orbit. Different surveys have found different
percentages of hyperbolic orbits, but all very small.
There is also the possibility that differences in numbers of hyperbolic meteors
detected differs based on the months when they were detected. Apparently one
study (Ueda & Fujiwara, 1995) took a sample, in which 1/3 of the meteors
observed were at Leonid time. In fact, one of the supposed hyperbolic meteors
is listed as a Leonid meteor. Problems with slight errors in derived orbits are
discussed!
The direction of arrival for interstellar meteoroids is discussed, assuming that
the majority result from ejection of material from stars spread throughout the
galaxy, and that multiple effects are rare. This means that there should be a
strong concentration originating from our galactic center in Sagittarius (17h45m
-29) Hence, from mid-northern latitudes, we should detect more hyperbolic
meteors between April and July, when the expected radiant is above the horizon
for more night hours. We should also expect to see more when the apex of the
sun's motion in the galaxy relative to nearby stars is high in the night sky,
ie. spring and early summer. Another probable direction for hyperbolic
meteoroids is from the direction of flow of neutral hydrogen gas near the sun -
in some studies, corresponding to a direction of arrival located in Ophiuchus
(16h55m +7). This gas flow would be most effective for small meteoroids, and
ties in with the Ulysses observations.
The probable range of geocentric entrance velocities is discussed. The
parabolic heliocentric velocity at 1 AU is 42.2 km/s (varies slightly depending
on aphelion & perihelion distances, from 41.9 to 42.6). The geocentric velocity
of the meteor is the vector sum of its heliocentric velocity and the velocity of
the earth, about 29.9 km/s at 1 AU (varies from 29.4 to 30.4). So, a
heliocentric hyperbolic orbit means any geocentric velocity over 73 km/s, ie.
42.6 plus 30.4. Taking into account both relative motion of stars at the sun's
location in the Milky Way, and also random velocity components, larger in older
stars, it is calculated that we would expect interstellar meteors to have
heliocentric velocities on the order of 46.7 km/s. The orbits detected by the
Harvard Super-Schmidt don't indicate any velocities over that limit, and almost
none above 45 km/s. The conclusion drawn is that interstellar meteors are
probably not present on photographic surveys. Taking all factors into
consideration (some equations here), expected geocentric velocities of
interstellar meteors would range from 21 km/s to 78 km/s.
Next, the changes in mass of the meteoroid are discussed, as affected by the
encounter with the earth's atmosphere. Basically, it depends on the geocentric
entry velocity. Most meteors stop being luminous at about 8 km/s. All ablation
stops at a velocity of about 2 km/s. For a stony type meteorite, for a 30 km/s
entry velocity, less than 1 millionth of the original mass would remain - and
that's not taking any fragmentation into account. It is stated that it is
possible for meteorites to result from interstellar meteoroids, but that only a
fraction would have a velocity slow enough to produce meteorites, and that the
possibility of large objects would be just about zero.
The Pultusk meteorite is briefly mentioned. It fell on January 30, 1868, in
Poland, and generated about 100,000 small individual fragments. An analysis
based on visual observations yielded, according to some sources, a hyperbolic
orbit, with a heliocentric velocity of 57 km/s, and an interstellar velocity of
37 km/s for the parent body. Hawkes & Woodworth state that they believe the
observational data is not accurate enough to reach those conclusions.
Micrometeorites are discussed. It mentions that they are meteoroid masses small
enough to decelerate in the atmosphere before intensive evaporation. If they
are melted and resolidified, losing part of their mass, they are called ablation
products. Together they are called IDP's, Interplanetary Dust Particles, and
are collected by rockets, and sea sediment and ice sampling. Different sizes
are also detected depending on the sampling method. Regardless, the entry
velocity must be less than about 35 km/s in order to really survive getting
through the atmosphere. It is stated that it is highly unlikely that there are
interstellar IDP's, and that the data is biased against cometary origins as
well. A study by Kortenkamp & Dermott in 1996 argues that the 2 asteroid
families, Themis and Koronis, account for over half of all IDP's.
Mechanisms for producing interstellar meteoroids are discussed. These include
ejection from planetary systems by collision and near-collision processes and by
radiation pressure from stars, ejection material from novae and supernovae, and
ejection of material from comets.
In conclusion, Hawkes & Woodworth state that observations indicate that a small
percentage of incoming meteors have hyperbolic orbits, with that percentage
increasing with decreasing mass. Television and radar surveys may show 1% of
their total detected meteors to have hyperbolic orbits. Photographic meteors -
brighter ones - show less to no interstellar origin. Micrometeorites and
ablation product have, statistically, the best chance for interstellar origins,
and further studies of IDP collections and data should be undertaken.
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