Farewell, until the next time, 2004 BL86: a personal view

This image shows the track of near-Earth asteroid 2004 BL86 against the stars of southern Cancer over the course of ten minutes during which time it moved almost 0.4° or 24-arcminutes. This is a stack of five two-minute exposures at ISO1600 with a 65mm f/6.5 astrographic refractor from 00:13—00:23 UT on 27th January. Image credit: Ade Ashford
This image shows the track of near-Earth asteroid 2004 BL86 against the stars of southern Cancer over the course of ten minutes during which time it moved almost 0.4° or 24-arcminutes. This is a stack of five two-minute exposures at ISO1600 with a 65mm f/6.5 astrographic refractor from 00:13—00:23 UT on 27th January. Image credit: Ade Ashford
Intermittent high cloud cleared from mid-Norfolk around 11 pm GMT last night enabling me to get a first good look at this intriguing near-Earth asteroid with an 11-inch Schmidt-Cassegrain telescope. By this time, 2004 BL86 was at altitude of 40° in the south-southeast and predicted to be around magnitude +9.4, so I knew it wouldn’t be too hard to find.

However, I wasn’t prepared for just how quickly the asteroid would fly through the eyepiece field of view! Around 11:30 pm it was heading north at a rate of 2.4° per hour, or the apparent diameter of the Moon every thirteen minutes — this was easy to see in an 8mm eyepiece at 350x magnification, particularly when the asteroid passed between two field stars or close by another. Little did I know at the time that NASA’s Deep Space Network antenna at Goldstone, California was making radar observations that would reveal a diminutive moon for 2004 BL86 (needless to say, I didn’t spot the moon with my 11-inch ‘scope).

Keen to record an image of the NEO, I attached an astromodified Canon 1100D at the focus of my TS 65mm f/6.5 quadruplet refractor, a setup that delivers a 3° x 2° field of view. All was mounted on an elderly (but thoroughly dependable) unguided Vixen GP mount that I knew would give good results with exposures up to two minutes duration. The attached photograph is therefore a stack of five such images, so any apparent wiggles in the asteroid’s track are due to periodic errors in the mount’s drive system, or vibrations due to gusts of wind.

As I flitted between camera and telescope eyepiece, I couldn’t help but wonder how many other as yet undiscovered mountain-sized rocks will pay Earth a close visit in coming years. This close brush with 2004 BL is the nearest it will get to our planet over the next 200 years, and the closest any known asteroid this size will get until asteroid 1999 AN10 flies past our planet in 2027.