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

Bill Gray Dec 14, 2015

Hi Andrew,

Take a look in 'environ.dat'. You should (if you're using a somewhat
recent version of Find_Orb) have a line that says

TIME_RANGE_OBS=1000,2020

This should probably say something more along the lines of "no future
observations", rather than cutting them off at 2020. (Or, perhaps,
have Find_Orb do what it does for many other errors, and have it
give you a warning message.) Anyway, change that to 2089 or whatever
your maximum date is, and you should be all set.

Now, that said... I'm still not clear on how adding in the Horizons
positions will help. Your best bet would be to get all current data
from the NEODyS system (MPC doesn't have radar data, quite important
in these cases). I got the data from

http://hamilton.dm.unipi.it/~astdys2/mpcobs/numbered/2/2340.rwo

The Yarkovsky effect will cause some trouble for a small object such
as this over a long enough time span. So take another look in 'environ.dat'
for the 2009BD line. This has a bit of explanation above it, showing
how it enables a "standard" outward-forcing radiation force, plus an
"along orbit" one. Set 2009BD=1, and start up Find_Orb and load the
2340.rwo file. "Auto-Solve" will get you a decent orbit to start
with, but with high residuals and no non-gravs.

Go into Settings, and turn on the 'comet non-grav' model, and
click OK. Then use "Filter Observations to throw out the outliers.
I threw out those beyond two sigmas. After I did this a few times,
I got a "No changes made!" message.

At this point, I set the epoch to 2069 Oct 21 and did a 'full step',
and got

Orbital elements: (2340)
Perihelion 2069 Aug 11.291296 +/- 0.000174 TT; A1=6.31e-12, A2=-2.64e-14
Epoch 2069 Oct 21.0 TT = JDT 2477040.5 Earth MOID: 0.0058 Ve: 0.0904
M 90.04958 +/- 0.00026 Me: 0.0261 Find_Orb
n 1.27352894 +/- 5.65e-7 Peri. 40.55329 +/- 0.00007
a 0.84294033 +/- 2.49e-7 Node 210.63965 +/- 0.000017
e 0.4510473 +/- 1.14e-7 Incl. 5.93055 +/- 0.000015
P 0.77/282.67d H 20.4 G 0.15 U 3.1
q 0.46273433 +/- 2.04e-7 Q 1.22314633 +/- 3.22e-7
383 of 426 observations 1976 Oct. 25-2014 Dec. 6; mean residual 0".44

You'll note that by doing this, you get orbital elements that have
formal uncertainties. (Actually, you would get sigmas if you used
Horizons "observations", too, but without a knowledge of how good
the Horizons ephems were, the sigmas would be meaningless.)

-- Bill

On 12/13/2015 04:43 PM, andrewlowe@... [find_orb] wrote:
>
>
> Hi Bill,
>
> I've created an astrometry file of future positions of (2340) Hathor, using ephemerides
> from JPL's Horizons program, to determine accurate current-epoch elements with Find_Orb
> (unfortunately, Horizons has an unconventional epoch, while the MPC's accuracy is
> insufficient for my purposes). In particular, I'd like to add Horizons ephemerides
> after the close approaches to the earth in 2069 and 2086, in order to nail down the
> elements precisely.
>
>
> My problem is that Find_Orb is not accepting the future astrometry. I suspect that it
> recognizes any measurements from the future as not possible and therefore not
> allowed(for example, if I change the date from 2021-01-01 to 2011-01-01, the line of
> astrometry will appear in Find_Orb), but given my particular goal, is there any way this
> feature could be disabled?
>
>
> Thanks!
>
>
> Andrew Lowe
>
> http://members.shaw.ca/andrewlowe/home.htm
>
>
>
>
>
>