The night of 26 Jan into the morning of 27 Jan presents an opportunity to spot asteroid 2004 BL86, a 0.5km diameter lump of rock destined to pass just 1,191,000 km from Earth. Actually, that’s not really anything to worry about as it’s just over 3x the distance of the Moon!
So, assuming the UK’s skies clear on the night of 26/27 January, what’s the best way to go about spotting the asteroid?
First let’s look at its brightness. As the sky darkens on 26 January, the asteroid will appear as a small dot of magnitude +9.7 – that’s doable with an average pair of binoculars given a clear dark sky but if your visual limit is around mag. +5.0 you’re going to struggle.
As the night, or rather morning, draws on, so the asteroid will brighten. It reaches its peak brightness of mag. +9.0 at 04:30 UT. This is when the asteroid is passing up through Cancer the Crab, heading towards the eastern side of the magnificent binocular object, M44 or the Beehive Cluster. In fact, this object is very useful because it gives you a means to calibrate your binoculars in terms of how deep they can go. Using a pair of binoculars, if you can see stars which are the same brightness or dimmer than the asteroid then you know you’re in with a shot of spotting it. The graphic below shows the Beehive Cluster and identifies the main pattern within it – the bit I always think looks like a beehive, albeit, an upside down one! It should be relatively easy to pick this out with binoculars. Once you’ve done this, jump to the first arrowed pattern and then, once you’re used to the view, see how faint you can go. If you’re not familiar with magnitudes, the larger the number, the fainter the object.
Assuming you’re able to see stars equal to or dimmer than the asteroid, the next job is to locate it. This is trickier as it’ll appear exactly like a star. However, it’s presence will be given away by the fact that it’ll move noticeably over the space of a few minutes. The downside is that there are a lot of stars around the mag. +9.0 to +10.0 range in that part of Cancer. The chart below gives the location for a given time. The asteroid will be at mag. +9.3 at 00:00 UT on Jan 27 and +9.2 at 01:00, +9.2 at 02:00, +9.1 at 03:00, +9.0 at 04:00, +9.0 at 05:00 and +9.2 at 06:00. At 06:00 the Beehive Cluster will have an altitude of 19 degrees in the west.
The position of the asteroid will vary very slightly with your location due to parallax but the chart below should allow you to find it. This chart doesn’t sugar coat the situation and shows it how it is. The stars picked out go down to mag. +10.0 which means that these are will be slightly fainter than the asteroid.
When trying to locate something like this, identify patterns that stand out to you which lie near to where the asteroid is supposed to be. Use these patterns as your navigational beacons, and use them to home in on the position of 2004 BL86.
Good luck and clear skies!
Update – Here’s a 60s exposure of the field containing the asteroid. 2004 BL86 is shown as a streak in this relatively short exposure.