What would happen if a block of matter with a diameter of 1 meter, but with the density of a neutron star, impacted Earth with the average speed of a meteor?

Well, a few things would happen, and the speed of the object is not terribly important.

Neutron stars have a density of about 10^18 kg/m^3, so our roughly 1 cubic metre object would have a mass of 10^18kg.  (The earth is about 6E24kg, so about six million times heavier).

Meteorites travel at all sorts of speeds, but let's go with 50,000 m/s.

So the kinetic energy involved is roughly 10^27 J, by E=0.5mv^2.  That is about 5,000 times more energy than the Chicxulub event, and is equivalent to ten billion of the largest atomic bomb ever exploded.

That's the end of the world, then.

But it gets worse.  (If it could get worse - or maybe better, because it is a bit more interesting).  Why?  Because neutron stars are strange things.

If this metre-sized lump of neutron star just appeared at the edge of our atmosphere, the kinetic energy is the least of our concerns.  Matter that dense is essentially just a bunch of neutrons rubbing shoulders, without the usual separation full of electrons, nothingness and quantum oddity.  The gravitational forces inside a neutron star causes this density, but also prevents the neutrons from doing what comes naturally to neutrons - i.e. decaying.

Free from the gravitational forces, the neutrons immediately expand to a gigantic sphere, and will decay.  The difference in mass between the neutrons and the resulting decay products (proton, electron, neutrino) will be released as energy.  That ends up being around 1/1000th of the mass.

We can work out how much energy that is using E=mc^2, so 1/1000th of our 10^18kg gives:

E=(10^15 x 3x10^8 x 3x10^8) = 10^32J

Oh dear, that is 100,000 times the kinetic energy of this world-killing object.  We now have enough energy to melt much of our planet, remove the atmosphere and the seas, and if the moon is in the wrong place, blow it up like a watermelon hit with a hammer.

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