How far away could Earth detect a projectile travelling at relativistic speeds?

Suppose our alien neighbors woke up on the wrong side of the bed and decided humans would be less annoying if their home planet was hit with a giant artillery shell travelling at half the speed of light. At what range would modern day human astronomers be able to detect it?

The specifics of the situation I'm thinking of are a slug the size/shape of the Chrysler building made out of depleted uranium and titanium traveling at .6c on an "orbit" tangent to Earth's but about 30-45 degrees inclined from Earth's orbital plane, but I'm interested in similar scenarios just as well.

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How far away could Earth detect a projectile travelling at relativistic speeds?

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