(357439) 2004 BL86

 is a bright sub-kilometer asteroid and binary system, classified as near-Earth object and potentially hazardous asteroid of the Apollo group, approximately in diameter. It was discovered on 30 January 2004 by astronomers of the Lincoln Near-Earth Asteroid Research at Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico. Its moon was discovered during the asteroid's close approach to the Earth in January 2015.
2015 Earth approach.
On 26 January 2015 at 16:20 UTC, passed , or 3.1 lunar distances, from Earth. The asteroid briefly peaked around apparent magnitude 9 and was near the celestial equator. The asteroid was visible in telescopes with objectives of or larger; high-end binoculars under a dark sky may also have worked. Near closest approach the asteroid was moving about 2.5 degrees per hour (2.5 arcseconds per second). The asteroid came to opposition (furthest elongation in the sky from the Sun) on 27 January 2015 at 04:37 UTC. Around 5:00 UTC, the asteroid was near M44 (the Beehive Cluster).
The 26 January 2015 approach of 3.1 lunar distances was the closest approach of for at least the next 200 years. For comparison, , about twice the size of , passed , or 1.3 lunar distances, from Earth on 31 October 2015.
Satellite.
A minor-planet moon, provisionally designated , was first detected by ground-based telescopes by Joe Pollock and Petr Pravec. Observations by the Goldstone Deep Space Communications Complex and Green Bank Telescope confirmed that it is a binary asteroid with a secondary roughly across. The secondary is estimated to orbit at least from the primary. About 16% of asteroids over in diameter are thought to be binaries.
Numbering and naming.
This minor planet was numbered on 27 March 2013 (). As of 2020, it has not been named.