[ExI] Asteroid on track for possible (updated probability of 1:28) Mars hit

[Amara Graps]

Spike:
>The size of particles that make for really good meteors are the
>pea-gravel-ish size and larger, ja?  Gravity would be the only force
>which would do much to those orbits on the time scale of a few hundred
>years,

The size is on the cusp when the forces get really interesting...!
One can't assume gravity is the dominant force.

Look here:
"The Meteoroid Environment: Shower and Sporadic Meteors"
http://www.mpi-hd.mpg.de/dustgroup/~graps/dips2005/120_CampellBrown_FINAL.pdf
from this book:
http://www.mpi-hd.mpg.de/dustgroup/~graps/dips2005/dipschapters.html


<begin quote>
Also of interest is the size distribution of the meteoroids: this is
influenced both by the formation of the stream and its subsequent
evolution. The cumulative mass distribution is the index of the power
law of the masses of the shower; it can be found from the slope of a
plot of log cumulative number against log of mass (or equivalently echo
amplitude, for radar echoes). An example (from the 2005 Draconid
outburst) is shown in Figure 4. Shower meteoroids tend to have small
mass distribution indexes, since larger particles are less perturbed
from their original orbits and the dominant mass input from cometary
decay is in larger (mm- to cm-sized) meteoroids. Some streams also show
mass sorting as a function of solar longitude near their maximum
activity, with larger and smaller particles having undergone different
histories and having different density distributions in the stream [39].

The links between shower meteoroids and their parent bodies is one of
the most interesting aspects of the study of meteor showers. The timing
and duration of meteor showers, coupled with the dispersion in radiant
and speed for a single shower, give insights into the ejection processes
and the evolution of meteoroid streams after ejection. The light curves
(plots of luminous intensity against time or height) of ablating meteors
can give valuable insights into the physical structure and chemical
composition of meteoroids, and therefore of their parent bodies. This
allows researchers to sample material from a large number of objects
whose orbits cross the orbit of the Earth, and also to sample larger
material than dust collectors in space can harvest. Studies of light
curves [19],[3] have shown that the ablation of meteoroids is
inconsistent with solid, meteorite-like objects: cometary meteors appear
to be dustballs, or loose agglomerations of small solid grains. Such
structures produce relatively symmetrical light curves, rather than
curves with slow increases in brightness followed by rapid drops, as
expected from modelling solid stone spheres. All cometary showers
studies, as well as most sporadic meteors (eg [26]), show this sort of
symmetric light curve.
<end quote>

For other forces than gravity, see

1) the Yarkovsky Effect, which begins having an
influence at sizes of ~10 cm and larger:

http://en.wikipedia.org/wiki/Yarkovsky_effect

One recent application:
http://asymptotia.com/2007/09/09/origins-of-a-species-killer/

and

2) radiation pressure force, which strongly influences particles a
little bit smaller (say < 1 mm).

---

There is also another effect: "photophoresis". But I don't think the
modelers are considering that effect yet (new idea).

http://www.journals.uchicago.edu/doi/abs/10.1086/432087

Amara

-- 

Amara Graps, PhD      www.amara.com
Research Scientist, Southwest Research Institute (SwRI), Boulder, Colorado