[meteorite-list] Space Drifters

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:48:13 2004
Message-ID: <200110191753.KAA06755_at_zagami.jpl.nasa.gov>

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).
Received on Fri 19 Oct 2001 01:53:58 PM PDT