If aliens existed and wanted to kill us, they would use a relativistic kill vehicle, not send an object the size of ATLAS. An RKV is an object accelerated to very high speeds, so high that when it hits its target it unleashes enormous amounts of energy, destroying everything.

What makes RKVs great from an offensive perspective is that they are relative to their destructive power, very small. The kinetic energy formula is E = (1/2)mv^2. Notice that velocity is squared. If you double velocity energy quadruples. Increase it tenfold energy goes up 100x.

For some perspective, a piece of rock or metal with a mass of 5,000 kg (like, the size of an African elephant) accelerated to .2c unleashes about 10^19 Joules. Enough energy to destroy an entire metro area, and that's just one. You can send hundreds at once.

That velocity squared is a double edged sword. In order for an object at rest to have "x" kinetic energy, you need to put "x" amount of energy into it. (See work-energy theorem). More destructive power equals more energy usage, which also increases quadratically.

So, the optimal speed for a relativistic kill vehicle usually ranges between .1 to .2c due to these constraints. Given we have theoretically devised a mean to achieve these speeds using solar sails, aliens, with their superior technology would certainly be able to do so.

Unlike an ICMB, it has no exhaust plume or heat signature, so we'd need to spot it visually. A 5000kg chunk of metal at .2c would be so so small we couldn't detect it with our current equipment. Relativistic speeds makes it harder, by the time you see it, it's 20% closer to you.

It would be impossible to shoot down. You'd need to be able to intercept something moving 20% of the speed of light. Nothing we have can do that. From the distance of the Moon that's six whole seconds away. And you'd need just as much energy to do it.

To make matters worse, if you *did* shoot it down, its fragments are still travelling at relativistic speeds. Now you get a pellet of objects that could have as much power as a single nuke, striking random places. But, forget that, in practice, we would die instantly. No warning.

Should you be worried about a RKV instantly killing you? Well, probably not. Due to the anthropic principle, the fact that we are still alive suggests it's not coming. If there were an alien civ the time to kill us would have been in the past, not now. We wouldn't exist.

[Editor's Note]: For relativistic speeds a different formula is used and energy requirements tougher. It's more complicated to explain. Basic physics is roughly the same.