Next week, there will be a chance to observe the close approach of the potentially hazardous asteroid (PHA) 2005 YU55. This 400m size object will pass by the Earth at about 325,000km (about 0.85 of the distance from the Earth to the Moon) on November 8th. However it will be low in the south west at sunset, so observations from the UK will be tricky. Over the following two nights, though, it will be more accessible.

Chart for 2005 YU55 (in Pisces) on 2011 November 9/10 - Martin Mobberley

BAA Asteroids Section Director, Dr. Richard Miles explains more…

Seen from the UK on the evening of Tuesday, November 8/9, a 400-metre size asteroid 2005 YU55 will pass within 0.85 lunar-distances of us travelling at a speed of 13.7 km/s relative to the Earth.  This encounter will be the closest known for an asteroid of this size between the years, 1976 and 2028. So in this respect, it is a once in a 52-year opportunity to witness this particular skirmish.

It should first become visible from the UK and Europe on Tuesday evening, low in the west mainly in Aquila, passing 22 degrees south-west of Comet Garradd (C/2009 P1).  It should prove fascinating to follow as it approaches.  Seen from southern England (Dorset), it may be first detectable using a CCD camera and large telescope at about 18:00UT some 23 degrees altitude in the south-west direction at about magnitude 15 moving at 5 “/sec. It will brighten by about a factor of 10 over the next 4 hours so that by 22:00UT it will be 12th magnitude and moving at around 8 “/sec, i.e. crossing the sky at a rate of 1 Moon-diameter every 3.6 minutes.  Even exposures of a few seconds will show the asteroid as a trail and only telescope mounts set up to track moving objects will be able to register it as a point source. It will attain 11th magnitude at closest approach (Nov 8 at 23:28UT) at an altitude of just 6 degrees (as seen from Dorset) and thereafter will set below the western horizon.  With a very high apparent speed (reaching almost 9 “/sec), the object will be visible through large telescopes (25cm or greater) looking like a moving point of light crossing the field of view in a minute or two. Quite an observing challenge!

2005 YU55 will be much better placed for observers and easier to see on the evening of November 9/10 when at about 18:00UT it will be 12th magnitude and moving at <1 “/sec in the east close (12 degrees away) to the nearly full Moon.  Martin Mobberley has kindly generated a finder chart showing the general position of the asteroid for the nights of Nov 9/10, 10/11 and 11/12.  The chart can be found at:
http://martinmobberley.co.uk/images/2005YU55chart_mpm.jpg

If you are planning to observe then you will need to generate an ephemeris for a geographical location within a few hundred kilometres of your observing site.  You may find the Minor Planet Center website convenient to use for this purpose, located at:
http://minorplanetcenter.net/iau/MPEph/MPEph.html
Be sure to enter an observatory code in the relevant box: “J95″ would be a good one to use by anyone in southern England. You will have to pick a short ‘ephemeris interval’ say 5 minutes so that you can point your telescope at a convenient spot which the asteroid will reach some minutes after the telescope has been trained on a suitable R..A. and Dec. Enter “2005 YU55″ in the large box and an ephemeris start date using the following format, “2011 11 09 1800″.

The object was last observed in 2010 April when the Arecibo radio telescope was used to generate a radar image of the near-spherical object, and which was shown to be very dark and a slow rotator turning just once every 18 hours or so.  See for example:
http://www.jpl.nasa.gov/asteroidwatch/newsfeatures.cfm?release=2010-144

Although a potentially hazardous object, we do know that this is the closest approach the object will make to the Earth during the next 100 years.

All observations welcome. Good luck with the weather,

Richard Miles
Director, Asteroids and Remote Planets Section
BAA
arps@britastro.org