I think it’s largely because the majority of those that could hit the Earth have akready done so; the local neighbourhood has been cleared, we have the Moon as a shield, and any others that could come in from the outside of the Solar System tend to have to deal with Jupiter first which has such a huge gravity well.
Most of space is basically empty. There are still some chunks out there which could hit Earth, but it’s unlikely to be on the scale of the Late Heavy Bombardment. There are plenty of asteroids in the Solar System but most have very stable orbits, and Earth is a very, very small target.
How long a period of time was the Late Heavy Bombardment, based on the wiki text? I mean, imagine that the earth had been hit by thirty thousand large asteroids in that period: what would the average time between impacts be?
4.1 to 3.8 billion years ago is a period of 300 million years. Thirty thousand asteroids in that period works out to an average of one asteroid strike every ten thousand years.
The Late Heavy Bombardment was a discrete event, and was probably caused by a rapid change in the position of the planets in the Solar System. Trouble is, the exact nature of that change is still open to debate.
From that wiki page
The Nice model (developed in Nice, France) suggests that Uranus and Neptune swapped places, disturbing a massive asteroid belt that once existed out there; this caused a bombardment that lasted about three hundred million years (to answer For You).
The LHB eventually stopped, it seems; all the dangerous objects were presumably cleared out by interactions with Jupiter and the other planets (including Earth and the Moon).
Note there are other theories, including an extra planet or two that have been expelled or flung into the Sun; however ideas of an exploding planet are almost certainly wrong.
Which, if I am very much mistaken, is longer than we have written historical records for. Not that I am saying thirty thousand is a meaningful number (I picked it out of a hat to make the math simple), but it sounds like a lot to me. That decimal point sure does a lot too obscure the magnitude of numbers.
True, but we have pretty good records of climate, plant populations (pollen lasts a long time), sea-levels, etc. for many tens of thousands of years. If there had been an extinction-level asteroid hit in the last few hundred thousand years, we’d probably find evidence of it.
The Wiki article is a little confusing on this point. It estimates about 22,000 strikes with an impact crater of > 20 km, which comes out to a rate similar to what you have (actually at around once every 14,000 years). But then it says:
“Serious environmental damage would occur about every 100 years, although life is not known to have existed on Earth at this time.”
I don’t know how big an impact crater would need to be created by an asteroid that would do “serious environmental damage”. Would it be a lot smaller than 20 km? If so that would explain the variance between once every 13,000 years and once every 100 years.
If you’re implying that Earth could still be experiencing LHB-levels of impacts without us knowing, it should be pointed out that impact craters tend to stick around, even on geologic timescales, and that we don’t rely on written records of such events to know they occurred. We know an awful lot about our world, much of which happened before 10,000 years ago.
I’m not even sure how you’d define environmental damage without any life forms. It’s not like a semi-molten crust of rock cares what temperature it is, how much ash there is, etc.
If you look at the Tunguska event, it’s probably an example of the difference. Most people believe there was no crater left at all, but it knocked over trees on nearly 1000 square miles. Had it hit a city, we could have seen a million deaths. On a world full of life, that surely qualifies as serious environmental damage.
As a reference, the crater from the asteroid that caused the extinction of dinosaurs is about 180 km in diameter and caused about 30% of marine life at the time (probably more over land) to become extinct. Also, from this list, there are several impact craters in the 20 km range that occurred since then (most recently about 14 million years ago), with no mass extinctions (not that one today wouldn’t be noticed and would probably have adverse climate effects for a few decades).
ETA: Since the area of a crater increases as diameter squared, a 180 km crater requires at least 81 times the energy to form, and probably a lot more.
The Chesapeake Bay impact crater is thought to have been caused by a bolide (solid rock) that was 100-150m across, and it is believed to have disrupted the geology, hydrology, and local climate of the area for decades following the impact. Energy scales as a cube of diameter, so a 1 km diameter bolide would have one thousand times of the energy. This may not correspond linearly to ecological damage, but it is fair to say that a >1 km bolide would do widespread damage, and larger still would do global damage.
As for why we aren’t still getting pounded, the answer is that the current inner solar system has been largely cleaned out of large objects that have highly eccentric paths that cross the orbit of the Earth. The Moon is less of a “shield” in this regard, and more of a batter; that is, objects that tend to periodically intercept Earth orbit are perturbed and tend to be flung into high or lower orbits. However, this doesn’t protect us from long period elliptical objects or objects in the Oort cloud that are perturbed toward the inner system. While the probability that such an object will intercept the Earth in any human lifespan is pretty low, the probability compounds over longer timescales, and the severity of incidence may be severe or catastrophic, dwarfing the worst natural terrestrial or man-made disasters.
Wikipedia says that evidence of liquid water exists as far back as 3.8 Ga, the end of the LHB. Hence, earth had a significant outgassed atmosphere well before that time, it was not a semi-molten lump of rock. At the same time, it was probably not very hospitable to even primordial ooze until at least a few million years after that, but it did have some sort of stable environment. I imagine “damage” would mean with respect to the environment’s ability to give rise to life.