Title pretty much says it. How close could a extinction-size Asteroid (or Comet) get to Earth, but without causing any/or very minimal harm to Earth/living things on it. Meaning, it would come close, but entirely miss the Earth and everyone or almost everyone would continue about their day. If it were on a slightly different path and hit the Earth, game over for us.
And also, what would that look like if I were to look out and see it passing by.
Planet-killers can be as small as 10 miles across. That’s about one ten-millionth of the mass of the moon, which means it could be 1/3172 as far away and exert the same tidal forces on the earth as the moon. That’s about 75 miles, i.e. inside the atmosphere, lower than low earth orbit, so that’s a separate limitation.
So ISTM it could squeak by at the height of the lowest artificial satellites we have up there (~200 miles or so), safely outside the earth’s atmosphere, and there really wouldn’t be any discernable effects - not even a noteworthy tide, especially since it’d be screaming by so rapidly. Depending on its albedo, and where/when it passes by, you might see something bright, or you might not. No streak of atmospheric plasma, since it’d be outside the atmosphere - just reflected sunlight.
Tidal forces are pretty much irrelevant to whether it’ll cause any harm. The big question is how far into the atmosphere it can go without exploding/disintegrating. The answer will depend on the constitution of the body and perhaps other factors, but probably between 50 and 100 km. Of course, it could hit a satellite on its way in or out and cause damage that way.
With how thin the atmosphere is compared to the radius of the Earth, there’s almost no difference between “skimming the atmosphere” and hitting the surface.
I would guess even lower than that.
Thanks for the responses. So just nothing. I’m not sure what I was expecting. I suppose I didn’t appreciate how small it could/would be.
For the sake of clarity, would it be similar to me almost being hit by a bullet? Deadly on impact or shortly thereafter if it hits me, but absolutely harmless if it misses by even (say) a centimeter and totally unnoticed (assuming I didn’t hear it or appreciate what the sound really was).
Yes, but those Earth grazers have been small. An Extinction Level Event Asteroid is going to be on the order of 10 km. With the larger diameter, it’ll encounter lots more atmosphere, so I’d expect it to disintegrate/explode at a higher altitude. A data point: the Chelyabinsk meteor (~20 m diameter) exploded at an altitude of 30 km.
Right. As long as it isn’t close enough to cause lots of heating in the atmosphere, it is nothing. A miss by 100 miles is as good as a miss by 100 million.
And the atmosphere that hits it will be much less significant. The square/cube law applies. (The general rule of thumb is that a meteoroid is slowed to a halt when it passes through a column of air equal to its mass.) also, when you get that size and distance, you really have to think about if it is the center of the asteroid that is a certain distance or the closest edge. But as I mentioned, mine was just a guess, not claiming that is an authorotative estimate.
A tide that’s twice as high as normal is not irrelevant. If the object passed between the moon and the earth at a distance of 75 miles, you might expect a very high tide that causes damage. It fits the OP’s metric of “very minimal harm” and identifies a meaningful potential answer to their question.
An airborne explosion would likely deliver a lot of the object’s kinetic energy as heat, and would indeed be trouble. But even without disintegration (e.g. a solid iron bolide), an object passing through the atmosphere at hypersonic speed would generate a shock wave that could cause damage. Hard to guestimate how bad it could be. The space shuttle created an appreciable but non-damaging sonic boom by doing just Mach 1.5 at an altitude of 20 km, and it (the shuttle) was pretty small. a 16-km earth-grazing meteor will pass by at higher altitude, but it’s also far larger and travels far faster, on the order of Mach 60. I suspect the shock wave it produces would be much more of a problem than the space shuttle’s ear-pleasing double-thud.
If supersonic, a bullet produces a small but real sonic boom. Likely not a problem for most of your body, but if it passed in very close proximity to your unprotected ear, I’d be concerned about the potential for hearing loss. Here’s a video of a guy shooting an AR-15 toward a target. He’s 350 yards away and the camera is positioned very close to the target. If you watch starting at 3:20, the first two shots are deliberate misses - no impact sound - so you can hear the actual sonic boom as the bullets go by.
The reason tides would be irrelevant would be because the asteroid will be going too fast to raise them. The water (and ground) has too much inertia to respond to a rock quickly zipping through the neighborhood. The Moon raises tides because it’s a relative slow mover, but even those actually move ahead (carried by the Earth’s rotation) of the sub-lunar point on Earth by a fair amount. The inertia of the water keeps it from ever “catching up” to the sub-lunar point.
And the effect would be very localised.
The effect of the Moon on tides is felt all over the Earth at approximately the same strength; the tides caused by a small asteroid would fade away quickly with distance. The strongest pull would be directly under the object, but 200 km away the effect would be almost negligible. Since the object would be travelling at very roughly 50 km per second, the object would be 200 km away in less than four seconds.
You’re right of course, and I’m inadvertently arguing both sides of the matter (my first post even said the tidal effects for such a rapid transient would be negligible).