Plotting 2012 DA14 in Guide correctly

Bill Gray Jan 18, 2013

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 fixes.
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 after
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 geocentric
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 recently
got six observations over two nights, so we should now be able to get ephemerides
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 nights,
the observations were mis-timed by about five minutes. But I don't know
which night it was.)

-- Bill