Stephen Tonkin Jan 18, 2013
On 18/01/13 17:38, Bill Gray wrote:
> Hello all,
> I've gotten a few comments from people about this object, which is to
> come within about 34000 km of us on 15 February. At such a close distance,
> the earth's gravity obviously becomes a big factor. If you just get
> orbital elements from MPC, the resulting ephemeris will be very far off.
> (Note that this is _not_ MPC's fault. If Guide had numerical integration
> built into it, the ephemerides would be Just Fine. But it doesn't.)
> The way to get it to work in Guides 7, 8, and 9 is to switch from
> heliocentric elements to geocentric ones. In the latter case, it's now
> the sun's gravity that is ignored. But that's not so significant for
> the month or so around the flyby.
> First thing you'll need to do, if you haven't already, is to go to
> and download the current software. (It's for Guide 9, but as you will see,
> there is a way to use it to update Guide 8.) This includes a long list of
> useful improvements and fixes.
> Unfortunately, getting 2012 DA14 to be on position is not one of those
> As a _very_ near-earth object, 2012 DA14 requires some special treatment.
> You'll need to also download this small text file to your Guide folder,
> overwriting a copy that is currently there :
> Run Guide, and 2012 DA14 will then appear within a few arcminutes of the
> current prediction, for dates from the present to dates a month or so
> its encounter with us. The orbit used is the one from
> You'll see that this page provides geocentric ephemerides, starting at
> 12 February.
> The positions you get from Guide (if you set that date and time and a
> location) should match pretty closely, with some deviation due to the
> fact that
> in this case, only the earth's gravitational pull is considered.
> Please note that this is not, as yet, the final word on the orbit for
> 2012 DA14
> during the flyby. We'll need more observations for that. Until last
> week, the
> orbit was almost hilariously inaccurate, because the object hadn't
> been observed
> since last May. However, the folks at (304) Las Campanas Observatory
> got six observations over two nights, so we should now be able to get
> to within about a quarter of a degree near perigee (better than that
> when the object
> is further out).
> The orbit will improve when we get some more observations. The object
> is still quite
> faint, and it may be a little while before we get any more data.
> (There's some
> additional uncertainty because I'm pretty sure that on one of those
> the observations were mis-timed by about five minutes. But I don't know
> which night it was.)
> -- Bill
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