On Sun, 25 Dec 2005 20:27:19 GMT, mack wrote:

I often wondered the same thing myself. Apparently initial orbital
calculations for comets assume they're parabolic at least removing
one degree of freedom from the calculations. Presumably initial
calculations for asteroids are that they are in circular orbits.

See my reply to Eric. There is a method, developed by Olbers, that
assumes a parabolic orbit. However, more general methods are usually
used, and these produce orbital elements without any initial assumption
about the type of orbit. That is, they simply produce the elements,
which determine whether the orbit is parabolic, hyperbolic, or
elliptical.


I understand that initial calculations aren't very accurate, but then
the next thing you here is that they've found the object in photos
from 100yrs back. Presumably you need total precision to get back
like that.

The initial calculations can be very accurate- it depends on how large a
section of the orbit the measurements are collected. One problem with
interpolating back in time is that the effect of perturbations can
become significant. Identifying an object in an old photo is usually one
part calculation and one part luck. Objects are seldom exactly where
they are calculated to be. Normal orbital elements assume a two-body
system. That is usually a good approximation in the short term, but not
over long periods.


Hubble has just recovered a satellite of Uranus (Perfida ?) originally
seen in Voyager II photos. How they can tell whether it's taken n
orbits or (n+1) orbits in the intervening time I don't know.

I'm not familiar with this specific case, but normally, the initial
error in calculating the orbit is small enough that it would take a very
long time before the positional uncertainty would become so large that
you didn't even know which orbit you were in.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com