Re: [find_orb] "Future" Astrometry Ignored in Find_Orb

Bill Gray Dec 15, 2015

Hi Andrew,

Hmmm... do keep in mind that you can just solve from the observations,
rather than an Horizons ephemeris, and change the epoch. Normally, that
would have assorted advantages, such as getting uncertainties. In your
case, where you aren't using the uncertainties anyway, the Horizons
ephems ought to work; you won't get an advantage from using them, though.

You do have one small disadvantage in using the Horizons ephems: they
are based on observations up to 2014 October 24 (I assume they recomputed
after getting radar data). The actual observations go to 2014 December 6.
(Not much of an effect, but I hate ignoring data.)

On 12/14/2015 04:46 PM, 'Andrew Lowe' andrewlowe@... [find_orb] wrote:
> One final thing; a purely gravitational solution is lousy. Horizon's
> ephemerides include an A2 term to fit the data better (if you check
> out the residuals in the MPC's solution, they throw out entire
> apparitions because they don't include the A2 term).

They tossed all the 2007 data and much of the 2011-2012 apparition.
NEODyS also uses a "normal" solution without non-gravs, but they _did_
get most of the 2007 observations to fit, albeit with ugly residuals
and systematic errors.

> Horizon's value is -3.03e-14. As a check, when you turn on the comet
> non-grav term within Find_Orb, it computes A2=-2.84e-14. Not too bad
> at all, is it Bill?

Actually, I did see that as a problem. But the sigmas from both
Horizons and Find_Orb are about 0.15e-14, so the mismatch is about
what one would expect. (There's another source of mismatch, which
is that Find_Orb is solving for both A1 and A2. Apparently, JPL is
assuming that this object is pushed in a tangential manner, ignoring
the radial force. I know they've done that sort of thing with other
objects... it really doesn't seem like a good idea to me, since the
Yarkovsky force does have both components. But I'm not especially
knowledgeable about this; I'll have to ask them why they do that.)

> While most PHAs have well-behaved solutions, there are at least
> two objects, Hathor and Hermes included, where multiple close
> approaches to the earth can really affect the accuracy if the
> initial orbit is deficient.

The comment at the bottom of the MPEC linking "current" Hermes
data to the original 1937 observations is interesting:

http://www.minorplanetcenter.net/mpec/K03/K03U04.html

Steve Chesley and Paul Chodas did some very impressive work to
figure out the link. (Don't bother trying to do it in Find_Orb!
Of course, if you have subsequent observations, the link becomes
quite easy to do.)

-- Bill