Hi Grant, all,

On 01/27/2013 12:30 PM, grantcblair wrote:
>>From past experience, I would wait until nearer the desired observing date
> to update the orbital elements. I missed a close approach by about 20 arcmin
>last year by having slightly older elements loaded into Guide.

Shouldn't be a problem in this case. With the January data, the ephemerides
are known to within an arcminute or two at closest approach.

The problem to which you are referring may be that if you have elements for
an epoch far away from the flyby, you won't get an accurate result. But these
elements are for an epoch _at_ the flyby. Also, they are geocentric, so
the perturbations ignored are the solar and lunar ones, which don't matter
very much over the short period of the flyby.

You may also have run into trouble with a prediction made before the object
was recovered. That could have been an issue with 2012 DA14, too; we had a
fair bit of data from last year, but not enough to tell you exactly where it
would be this February (except "it's going to be close to us").

But we do have observations made on 9 and 11 January, so the orbit is really
pretty well defined (as mentioned above, to within an arcminute or so.) And
ephemerides made with unperturbed geocentric elements (the sort I've provided)
will only add about an arcminute or so of error.

> ..it simply hadn't occurred to me they would change quite so much during a close pass.

They don't, normally. But objects don't normally come this close to us.
Here are heliocentric elements before and after the encounter (DON'T USE THESE
IN ANY PLANETARIUM PROGRAM, INCLUDING GUIDE; they're just to show how much
the orbit will change due to the earth's perturbations) :

Orbital elements: 2012 DA14
Perihelion 2012 Nov 30.198617 +/- 3.45e-5 TT = 4:46:00 (JD 2456261.698617)
Epoch 2013 Feb 14.0 TT = JDT 2456337.5 Earth MOID: 0.0003
M 74.4252 +/- 0.000039 Find_Orb
n 0.9818449 +/- 1.17e-7 Peri. 271.2560 +/- 0.00006
a 1.0025532 +/- 8e-8 Node 147.1835 +/- 0.000035
e 0.109258 +/- 6.35e-7 Incl. 10.4426 +/- 0.00006
P 1.00/366.65d H 24.4 G 0.15 U 2.0
q 0.8930159 +/- 6e-7 Q 1.1120905 +/- 6.9e-7
187 of 197 observations 2012 Feb. 23-2013 Jan. 11; mean residual 0".320.

Orbital elements: 2012 DA14
Perihelion 2012 Sep 26.608258 +/- 0.0276 TT = 14:35:53 (JD 2456197.108258)
Epoch 2013 Feb 18.0 TT = JDT 2456341.5 Earth MOID: 0.0003
M 164.0031 +/- 0.020 Find_Orb
n 1.1358206 +/- 8.17e-5 Peri. 195.5702 +/- 0.015
a 0.9097656 +/- 4.36e-5 Node 147.0647 +/- 0.000039
e 0.090129 +/- 6.22e-5 Incl. 11.7101 +/- 0.00045
P 0.87/316.95d H 24.4 G 0.15 U 6.4
q 0.8277690 +/- 9.63e-5 Q 0.9917623 +/- 9.72e-6
187 of 197 observations 2012 Feb. 23-2013 Jan. 11; mean residual 0".320.

Note that the orbital period has gone from 366 days down to 317 days.
The inhabitants of 2012 DA14 will have to do some calendar reform, as well
as get used to some global warming: the perihelion distance will drop from
.89 AU to .83 AU. (Fortunately, it's a pretty small globe. They could
probably just put up a sunshade.)

-- Bill