(meteorobs) Fw: [meteorite-list] Space Drifters

  • Subject: (meteorobs) Fw: [meteorite-list] Space Drifters
  • From: Ed Majden
  • Date: Fri, 19 Oct 2001 12:06:18 -0700
----- Original Message -----
From: Ron Baalke <baalke@zagami.jpl.nasa.gov>
To: Meteorite Mailing List <meteorite-list@meteoritecentral.com>
Sent: Friday, October 19, 2001 10:53 AM
Subject: [meteorite-list] Space Drifters

> http://www.guardian.co.uk/Print/0,3858,4279294,00.html
> Space drifters
> Duncan Steel
> The Guardian (United Kingdom)
> October 18, 2001
> A peculiar pseudo-force has changed the way astronomers think meteorites
> reach Earth, explains Duncan Steel.
> The Guardian
> Meteorites are mostly chips off bigger blocks. Out in the main belt,
> between Mars and Jupiter, there are billions of asteroids. Inevitably
> there are collisions between them, and some of the debris eventually
> reaches us.
> A simple picture, but there is a puzzle in the details. Most meteorites
> appear to be too young, in terms of the time spent on independent orbits
> after escaping their parent asteroids. Subject to the assumption that
> the gravitational tugs of the planets are the only forces at play,
> astro-mathematicians are able to trace how the paths of interplanetary
> objects wander. Such calculations lead to an estimate that meteorites
> need about a hundred million years to reach us, much longer than they
> actually take.
> This transit time is known from a meteorite's space exposure age. This
> duration is quite different from the period it may have lain on the
> ground before discovery (between seconds and millennia), or its age from
> formation as measured using radioactive dating.
> Space exposure ages are determined using cosmic rays. Within a much
> larger asteroid, an eventual meteorite is shielded by an overlying layer
> of rock. After an inter-asteroid collision, the freed meteoroid is
> suddenly exposed to the high-energy elementary particles that permeate
> space.
> When these cosmic rays hit the meteoroid, they penetrate a centimetre or
> so. Characteristic tracks are left in the rock, which may be studied
> under a microscope. By counting the numbers of tracks it is possible to
> determine how long it took for the meteorite to travel from its parent
> asteroid to the Earth's surface. Typical values are a few million years.
> This implies that meteoroid orbits must evolve much faster than purely
> gravity-based computations would indicate. Something else must be going
> on. What could it be?
> Consider the famous experiment of two cannonballs of different size
> being dropped from the Leaning Tower of Pisa. Both reach the ground at
> the same time, despite their differing masses: only gravity matters
> here.
> This is not the case if a feather is substituted, because its large
> cross-section compared to its mass means that air resistance is
> substantial. In a vacuum the feather falls at the same rate as the iron
> balls. Now think again about meteoroids in space. Are there any
> influences that are size-dependent, causing them to evolve dynamically
> at a rate faster than pure gravity would allow? There is no air, but is
> there some other sort of resisting medium affecting their orbits,
> helping them migrate inwards on a crash course with Earth?
> The solar wind, the stream of charged particles moving outwards from the
> sun, imposes a small force. A greater pressure derives from the photons
> of sunlight. These two factors are important for tiny interplanetary
> dust grains, but a meteoroid the size of a basketball is essentially
> unaffected.
> The sunlight absorbed by meteoroids can have other effects. They are
> heated by this flux, and that energy is then re-emitted as infrared
> radiation. The emission is not isotropic, though: it is not the same in
> all directions. This leads to two types of pseudo-force affecting
> orbiting objects.
> The first was discovered in 1903 by a British physicist, John Poynting,
> who spent his career at the University of Birmingham. Howard Robertson
> of the California Institute of Technology further explored this concept
> in 1937: in astronomical jargon it is known as the Poynting-Robertson
> effect.
> Poynting reasoned that because the meteoroid is moving in its orbit with
> a speed of more than 10 miles per second, there are differing Doppler
> shifts on the infrared photons emitted in opposing directions.
> Forward-emitted radiation is shifted to a shorter wavelength, while
> radiation emanating in the reverse direction is pulled out to a longer
> wavelength. As a result, more momentum is emitted forwards than
> backwards, and there is a retarding force on the meteoroid causing it to
> spiral slowly in toward the sun. Although this is important for objects
> less than a few centimetres in size, it is not significant for larger
> bodies.
> The second pseudo-force has only recently been recognised to be of
> consequence. The idea is not new: it was simply forgotten over the
> several decades since two Russian astronomers, named Yarkovsky and
> Radzievskii, explored how a warm object's spin may affect its path.
> The easiest way to comprehend the so-called Yarkovsky force is to think
> about the Earth rotating on its axis. Split its surface into four time
> zones. On the dayside are the morning and afternoon zones, on the
> nightside there are pre- and post-midnight segments. Because it takes
> some hours for the temperature to rise during the morning, on average
> the afternoon zone is hottest, and so a greater share of the radiation
> emitted into space emanates from there. Diametrically opposed to that
> segment is the post-midnight zone, which is the coolest, and so the
> least radiation escapes from that region.
> This non-isotropic emission of radiation provides a slight shove in the
> direction away from the afternoon segment, accelerating the planet
> slightly. In the case of a huge body like the Earth, there is no
> measurable effect. But for a small spinning meteoroid, the influence is
> substantial.
> It appears that the Yarkovsky effect causes a hurry-up of the orbital
> evolution, and so can explain the brief space exposure ages of
> meteorites.
> There is another important implication of this work. In making
> predictions of the tracks of asteroids and so possible impacts on the
> Earth, we generally assume that only gravity affects their motion. In
> such a complicated situation, even a tiny additional perturbation like
> the Yarkovsky effect may make the difference between a near-miss and a
> bull's-eye, the target being this little sphere in space we call our
> home.
> Duncan Steel works at the University of Salford. His most recent book
> is Target Earth (Time Life).
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