A boxcar filled with gold would weigh 4,000 tons, about the same as a modern coast-guard cutter, and exactly the same as Unexpected Finger, when she was first sighted approaching Earth.
Now, I am not saying Finger rammed the Earth. That would be a spoiler. But just, you know, out of curiosity, if she did ram the Earth, how bad would it be?
The destructive power of a bullet depends mainly on its kinetic energy, determined by its mass and velocity. Finger's mass was 4000 tons, and its velocity was 0.2c.
Wait, did I say 0.2c? That is pretty fast. Is Wasp a sci-fi fairy tale that assumes magical technology?
No. Finger relies on two technical innovations: A photon rocket (a simple if idealized concept routinely analyzed in physics classrooms) and an unspecified means to make heat, the simplest form of energy. In other respects, Finger is primitive. Electronic computers? We don't need no computers. Computers are for pussies. But we really could use a decent flashlight.
Back to Finger's kinetic energy, which was given by the formula e = ½mv2, where mass m = 4◊106kg, and velocity v = 0.2c = 6◊107m/s.
So Finger's kinetic energy was e = ½(4◊106kg) (6◊107m/s)2 = 7.2◊1021 kg m2/s2, which is 7.2◊1021 Joules.
The good news is, this was only two percent of the (estimated) 4◊1023 J of the dinosaur killer.
The bad news is, a megaton is only 4◊1015J. So Fingerís total kinetic energy was (7.2◊1021 J) / (4◊1015 J/Mt) = 1.8◊106Mt, almost two million megatons, or half a megaton per kilogram.
Three feet of lead won't really help.
But let's not dwell on that. For Finger to build up that much energy, she must have burned a lot of fuel.