Go ahead a look up what kind of telescope you need for this, your choice for wavelength. Poster was saying, mass drivers were useless in space war because you could dodge. Painting it black was a joke because it does not matter at all.
A 10 kg sphere of DU is conveniently almost exactly 5 cm in diameter. Let me save you some trouble, the Hubble has a resolution of about 0.1 arcseconds, which means a football stadium on the moon (about 384,000 km) is needed to resolve as a single pixel on the Hubble CCD's (radar has worse resolution, higher frequency gives better resolution). So for convenience, lets assume our slug is exactly 20000 times smaller in diameter, which means it would have to be 20000 times closer (19.2 km) to be imaged by the Hubble -- giving you a grand total of 0.001 seconds to dodge the incoming round. A additional problem, you have to be pointing your telescope directly at the slug in order to see it. High mag. scopes have a limited field of field. Also, due to orbital mechanics and a long time of flight you could easily lob thousands of slugs on different trajectories all designed to arrive at the time time.
K/E weapons are truly difficult to defend against. Now, a 10 kg slug at 20 kps exceeds the capabilities for any existing railgun I know of, it won't for long though, maybe a few decades. It will still be a heavy piece of equipment for some time to come. But given sufficient motivation they will eventually end up is space unless we find something better first.