Re: {MPML} Orbit determination exercise + SkyMorph image artifact ?

Alessandro Jan 29, 2011

Thanks Bill and thanks to the many people who have answered my question.

I understand that having only 3 observations there are many orbits which are equally plausible because they are all coherent with the observations (given their inherent uncertainty).

I must tell you that my exercise is not completely invented because these 3 observations have been found by the program Astrometrica (feature called moving detection). I am aware that such a feature is not "magic" , i.e. there is a high likelihood of false detections !

This is the reason why I still prefer to refer to this object as to an hypothetycal object, it would be nonsense to attach a physical reality to image artifacts.

Playing a little with Astrometrica setting, I produced a slightly different hypothetycal MPCReport:

COD 857
COM Long. 110 36 07.2 W, Lat. 31 39 56.5 N, Alt. 1545m
CON Alessandro Odasso [alessandro_odasso@...]
OBS Alessandro Odasso
MEA Alessandro Odasso
TEL 0.61-m f/10 reflector + CCD
ACK MPCReport file updated 2011.01.30 02:39:19
AC2 alessandro_odasso@...
NET UCAC-3
MYOBJ C2011 01 11.43745 11 05 20.01 +38 02 33.7 17.00R 857
MYOBJ C2011 01 11.47912 11 04 30.30 +38 05 12.3 16.58R 857
MYOBJ C2011 01 11.52055 11 03 40.79 +38 07 47.7 17.93R 857
----- end -----

When I submitted this report to FIND_ORB and after pressing the autosolve button, I got what you would say is an even more weird orbit:

Orbital elements:
MYOBJ
Perihelion 2010 Nov 19.405151 TT = 9:43:25 (JD 2455519.905151)
Epoch 2011 Jan 12.0 TT = JDT 2455573.5 Earth MOID: 0.4806
M 55.47152 (2000.0) P Q
n 1.03501604 Peri. 12.74324 0.49594204 -0.02608990
a 0.96791672 Node 299.30648 -0.85814882 -0.16752957
e 0.9038868 Incl. 84.47557 -0.13274818 0.98552176
P 0.95/347.81d H 18.3 G 0.15 q 0.09302952 Q 1.84280392
From 3 observations 2011 Jan. 11 (2.0 hr); RMS error 0.000 arcseconds

Then I produced the Pseudo-MPEC ephemerides and I gave a look at the link that connects you to SkyMorph.

The following set of images are listed:

NEAT Observations for: EPOCH=2455573.500|EC=0.903887|QR=0.093030|TP=2455519.905151|OM=299.306484|W=12.743247|IN=84.475574|H=18.3

I am surprised to see that in the images labeled:
2002-12-14 10:23:52
2002-12-14 10:39:07
2002-12-14 10:53:56


... and in the previous images, the triplet image shows what seems a moving dot (blue, green and red)! But this moving dot is shown twice: two blue, two green ... two red !

Are these image artifacts ?


Cheers,
Alessandro






--- In find_orb@yahoogroups.com, Bill J Gray <pluto@...> wrote:
>
> Hi Alessandro,
>
> (Side note: this discussion appears to be continuing on both the
> Find_Orb and MPML lists. Fortunately, both are open to viewing by
> non-members.)
>
> With the short arc you have here (two hours long), you can get
> an astonishing variety of orbits. That's basically standard
> procedure for very short arcs, except in some unusual situations
> (mostly involving objects that are really close to us). We can
> say, though, with confidence, that the object isn't more than
> .447 AU away. If it were, the orbit would have to be hyperbolic,
> and nobody has seen that happen (yet). That upper distance limit
> is just about the only constraint on the orbit right now.
>
> Within .447 AU, there is a huge range of orbits. Find_Orb
> looks through a broad sample of them and picks one that looks most
> "reasonable", given what we know about the distribution of orbits
> among observed objects. Usually, that leads to a main-belt
> solution. (This is the approach that says, "When you hear
> hoofbeats, expect horses, not zebras".)
>
> In this case, Find_Orb's default orbit has i=27, q=.13, Q=1.53.
> That's a pretty weird orbit. Find_Orb should have looked further
> (i.e., tried out some more possible orbits). With some searching,
> I found one with i=4, q=.57, Q=1.89, though the residuals are
> a bit high (rms error = .705 arcseconds).
>
> One _could_ just look for the orbit with the lowest residuals.
> In this case, with only three observations, the lowest residuals
> would be exactly zero. As has been pointed out in this thread,
> that leads to a somewhat exotic orbit with i=55, q=.09.
>
> There are a couple of problems with doing this. For one thing,
> with a very short arc like this, it's common for the lowest-residual
> orbit to be something _really_ exotic, like a Warp 9 orbit out past
> Alpha Centauri. Sometimes, it diverges completely. Or, it
> converges on something pretty weird.
>
> The usual way to tackle this sort of very short arc problem
> would be with statistical ranging. SR says: We compute a slew
> of possible orbits at various ranges and radial velocities, and
> keep those with non-hyperbolic orbits. We also add in some "noise"
> that reflects our guess as to how exact our observations are. All
> the resulting "virtual asteroids" are at least _possible_ places
> the object could be. (Though if both main-belt solutions and
> exotic solutions appear, it'll probably be a main-belter: again,
> expect horses, not zebras.)
>
> This is the approach used for the NEOCP uncertainty charts: each
> of triangle in the chart represents a possible orbit that fits the
> observations tolerably well. Dave Tholen's KNOBS software, I gather,
> does the same thing, and I think OrbFit and CODES have this
> capability. As does Find_Orb... except %$!*^ it, I tried it on this
> object and got three orbits, and then it failed. I'll have to check
> that out. (I only added SR recently; it's a "work in progress".)
>
> -- Bill
>