Within the space of less than an hour on September 5, the Mount Lemmon Survey discovered two objects which will both pass by the Earth on September 8 at a distance closer than the Moon! This unprecedented coincidence provides an exciting observing challenge for amateurs although those observing from the UK will not have the best views.

The intrinsically smaller object, 2010 RF12, will be the more favourable observing target in that it passes closest at about 0.21 lunar-distance, i.e. about 80,000 km. Tonight from the UK (Sep 6/7) this object will be 17th magnitude but by tomorrow (Sep 7/8) it will be brightening rapidly from 16th to 15th magnitude and be accelerating from an apparent speed of about 30 "/min to 50 "/min. It passes closest around 2100 UT on the 8th but by then it will be difficult from any location on the Earth.

The larger object, 2010 RX30, only approaches to within about 0.66 lunar-distances of the Earth but will be more favourably placed for UK observers and should be able to be followed to within about 6 hours of closest approach which takes place around 1000 UT on the 8th. It will be visible all night on Sep 7/8 being 16th magnitude at first but then brightening to 15th mag. The problem however is its apparent speed in that it will be racing across the sky at between 2-5 ARCSEC/SEC. Observers with access to telescopes located in other parts of the world especially in the southern hemisphere could witness 2010 RF12 reach 13th magnitude around 1600-1700 UT on the 8th. As seen visually in a large telescope (30-cm aperture or more), its motion across the sky would be very apparent in real time. Details of the position of the objects can be found here

Instructions for using this webpage are as follows:

First, enter "2010 RF12″ or '2010 RX30″ in the main box as required. Then enter the date/time in 'Ephemeris start date' box, e.g. "2010 09 07 1200″ for 12:00 UT on September 7th.

Then choose the 'Ephemeris units': I suggest a time interval of "5″ 'minutes' would be appropriate.

In the 'Observatory Code' box select an IAU observatory code of the observatory location of the telescope you will be using (or one that is relatively nearby it). N.B. DO NOT use a lower case letter in the observatory code or else it defaults to the (unwanted) geocentric position.

Finally click on 'Get ephemerides/HTML page' and you will see a list of data from which you can select the positions (RA and Dec) corresponding to the times for your observing session.

Richard Miles
Director, Asteroids and Remote Planets Section