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When it passed
closest to the Earth - just 1.5 times the distance to the Moon - it was
too close to the Sun to be visible. Asteroids approaching from this
blind spot cannot be seen by astronomers.
That sounds like a long way - its 375 000 miles.
However, think about that in terms of time. The Earth moves at a speed of
about 70 000 miles per hour, so it missed by a little over 5 hours. Floats
around for 3 or 4 billion years and misses by 5 hours. Thats like driving
a half hour to a meeting and being late by ~10 nanoseconds.
[Of course, this is assuming the very unlikely event that it passed through
the orbital plane at closest approach. It probably passed through the
orbital plane elsewhere, but its still a useful analogy when someone says -
"but that's so far away"]