[BAA-ebulletin 00836] 2004 BL86 - An unusually bright close-approaching asteroid

[BAA electronic bulletins service]

On the night of January 26/27, asteroid 2004 BL86 will make a close pass of 
the Earth becoming, for a short time, the brightest natural near-Earth 
object (NEO) that we know of (other than the Moon) over the next 12 years. 
The object will be very favourably placed for observation, especially for 
observers based in the UK and Europe.

BACKGROUND:
This potentially hazardous asteroid (PHA) is quite large, some 0.4-1.0 km 
across, and so will appear as bright as 9th magnitude when it passes the 
Earth at the safe distance of 1.2 million kilometres. Although it orbits the 
Sun every 1.84 years ranging between 0.90 AU and 2.11 AU, the forthcoming 
event marks its closest geocentric approach for several centuries when it 
will pass almost 3 lunar-distances away on January 26 at 16:20 UT. This 
close approach is especially unusual (for a PHA) in that it will continue to 
brighten by more than a factor of 2 after closest approach whilst it moves 
towards opposition, reaching the remarkably low phase angle of 1.1 degrees, 
and attaining a V magnitude of 9.0 on Tuesday, January 27 between 
03:40-05:10 UT, during which time interval its apparent speed will have 
slowed to ~2"/sec.

Please note that its visibility as seen from Europe and North America is 
very favourable, and so you will not have to wait up until the early hours 
to spot it since (seen from the UK) the 9th magnitude object will rise and 
become readily visible soon after 19:00 UT. Its motion should be very 
obvious as seen through a small telescope (10-cm aperture or larger 
instrument) when it heads northwards leaving the constellation of Hydra and 
entering Cancer at around 23:10 UT on the 26th.

CHARTS:
Charts showing the object's track across the sky for the night of January 
26/27 (for UK-based observers) have been prepared by Steve Harvey, Director 
of our Computing Section.
For Jan 26.5-27.0, the chart is at:
http://britastro.org/computing/ch/357439_2004_BL86_2015Jan26Jan26.html
Likewise for Jan 27.0-27.5, Steve's chart is here:
http://britastro.org/computing/ch/357439_2004_BL86_2015Jan27Jan27.html

OBSERVING HINTS:
At its brightest, large binoculars should reveal the interloper as it slowly 
migrates across the field of view. Between 05:05-05:45 UT on the 27th, the 
asteroid glides past the western (following) edge of the Beehive star 
cluster (Messier 44) as seen from southern England. Parallax will 
significantly affect its apparent position as seen by observers elsewhere. 
So if you wish to obtain exact celestial coordinates for your location, 
these can be downloaded from the Minor Planet & Comet Ephemeris Service:
http://www.minorplanetcenter.net/iau/MPEph/MPEph.html
You will need to enter your latitude and longitude (or observatory code - in 
uppercase - at or near your location) using the online form together with 
the time (in UT) when you are planning to observe. Using large telescopes 
with small fields of view, you might consider pointing the scope at a point 
in the sky where the asteroid is due to pass say 5 or 10 minutes later. You 
will hen be able to monitor the object coming into view and traversing your 
field before disappearing at the opposite edge of the frame.

REPORTING:
Updates as to the nature of this object and links to charts, together with 
observations by members, will be posted on the Asteroids and Remote Planets 
Section homepage:
http://britastro.org/asteroids/
Watch out for brightness fluctuations as it spins on its axis: that's if 
2004 BL86 turns out to be rather elongated in shape, or indeed if it happens 
to be a binary system. It is also worth observing with different filters to 
see whether its colour changes as it spins, or even spectroscopically to 
classify its type. Astrometry made from observatories assigned an IAU code 
should be reported to the Minor Planet Center in the usual way. Images and 
photometry will be collated by myself as section director - arps [at] 
britastro.org

Good luck to all weather-wise.

Richard Miles
Director, Asteroids and Remote Planets Section
British Astronomical Association

2015 January 22