How close will it be in 2021?
Or is that asking “the wrong question”? (Cue spooky music...)
You might not wanna know......................
They cannot really project its orbit that far in advance due to the Yarkovsky effect..........................
The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies.
Will it get close enough that I won’t need to bother with a haircut?
1.36 AU, or 1.36 x 92.5 million miles = 125 million miles
http://mel.ess.ucla.edu/radarwiki/index.php/FutureTargetsJDG20100601
Hold on. I’m not sure if that 125 million miles in 2021 is correct. According to that chart I linked to, it’s at that same exact distance every time it makes a “close approach”, including this recent one (Dec 2015) where the posted article says it’ll come to within 6.7 million miles. So that number I provided obviously can’t be correct.