ALERT! Close Approach of Near Earth Asteroid 2012 TC4, 11-12 October
Tuesday, October 9, 2012 at 11:30PM | by Ian Musgrave
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left - Path of Asteroid 2012 TC4 as seen from Mayhill NM on the evening of October 11 at the start of astronomical twilight. Position ticks are every 15 minutes, click to embiggen.
The 22 meter wide asteroid 2012 TC4 (see also here) will pass within 0.25 Lunar Distances (0.0006 AU) of Earth on the evening of October 11-12, being closest at October 12, 6:35 UT.
Imaging this asteroid will be a significant challenge, it will be moving around 10" a second near closest approach. The asteroid wll be around magnitude 13.8, so while not bright it is not dim enough to be a problem with 60-120 second exposures.
As well, shortly after astronomical twilight, the asteroid rapidy drops below the iTelescopes limit of travel, so there is approximately only an hour for imaging.
You won't see it at closest approach, that occurs when te asteroid has set from the point of view of the New Mexico and Spanish scopes. But it will be moving at a fair clips even before this, so still a challenging capture.
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left - High power view showing the fields of view of T14 (large rectangle) and T5 (small rectangle). The tick marks are 5 minutes apart. The track is the geocentric position, which is somewhat removed from the Topocentric position, the circle shows the predicted view from Mayhill from the MPEC (click to embiggen, but use a proper topocentric ephemeris or the 2012 TC4 comet/NEA dialog, rather than this chart, for indicative purposes only).
Choosing an imaging instrument is a tradeoff; T14 is nice wide-field instrument that performs wel, but you can't drive it faster than sidereal rate, and the blur that is 2012 may be too faint to show up. T20 is also wide-field, although narrower than T14.
T4 and T5 can be driven (just) fast enough to track the asteroid in tracking mode, but positional uncertainties (see below) may mean the tracking might be a bit off (and trailing bright stars may override the asteroids position). Yes, I know there are other instruments, but this gives an idea of the considerations you have to juggle.
As the asteroid is quite close to Earth, there will be a significant parallax error between geocentric ephemeris and the position as seen from Mayhill (see chart above for an example), most astromomy programs will give misleading positions when an asteroid approaches this close to Earth.
Use a proper topometric ephemeris instead (eg using the MPEC ephemeris generator, if you enter the observatory code for Mayhill - H06, into the box in the ephemeris generator, and make sure the Epoch is set to October 11, 2012 and it will create a topocentric ephemeris for Mayhill). The position of 2012 TC4 is being continually refined, so use the latest elements.
If you take this approach, remember that it takes time for the iTelescope scopes to get to tracking position. T14 can take up to 5 minutes (depending on there being reliable stars in the field for tracking), so offset you inital position by around 4 minutes or so (this will be a significant distance) so the asteroid will be in field. I used this technique to image 2011 MN on T12, which was moving at about 10" per second.
The planning guides to viewing a previous close approach asteroid, YU55, here gives step by step instructions for this approach.
If you use the one line MPEC elements and the comet/NEA option for the iTelescope systems (for the iTelescope 05/04 instruments, you can choose the track option so the instrument will track the asteroid) this will be less hassle, but at the risk that the positioning may be off for such a close object.
While this is challenging, iTelescope users have captured rapidly moving faint asteroids before, 2010 TD4 and 2011 MD.
