Impacts large enough to sterilize a planet simply don't happen any more in our current solar system. Yes there were many impacts like that in the early solar system, which created the Hellas basin on Mars, Aitken crater on the Moon and the Caloris Basin on Mercury. However, those all date back to the time of the "late heavy bombardment". Perhaps this was caused by migration of giant planets, in the early solar system before they had cleared out all the dust and gas, deflecting comets and asteroids into the inner solar system. There are other hypotheses as well. Something did cause large numbers of huge objects to hit planets in the inner solar system. However, this is something that happened well over 3 billion years ago, getting on for 4 billion years ago, and we have had none of those for well over three billion years.

Jupiter protects us from the largest 100 kilometer scale comets from the outer solar system. It breaks up or deflects the largest comets, so that they hit the Sun, Jupiter, break up into smaller pieces through tidal interactions with Jupiter, evaporate in the heat of the Sun, or are deflected from the solar system. One way or another, they are destroyed or deflected away before they can get into an Earth crossing orbit. Jupiter doesn't do such a good job of protecting us from the 10 km sized asteroids, though it takes many of those "for the team" as well.

As for the asteroid belt, between Jupiter and Mars, its larger asteroids are in orbits stable over hundreds of millions of years timescales. Yes, true, our solar system is not completely stable even now. There is a small probability of Vesta eventually hitting Ceres, and a chance of a future resonance of Jupiter with Mercury disturbing it out of its orbit, but neither of those can happen any time soon.

This is confirmed by the cratering record. All the impacts that large on Mercury, Mars, the Moon are well over 3 billion years old. Earth has some impact craters significantly larger than for Chicxulub, the largest, the Warburton Basin in Australia is the result of two impactors both probably larger than 10 km in diameter probably from a larger asteroid that split just before impact. But that also would not make humans extinct. For other examples see this list of large unconfirmed craters (wikipedia)

Something like that would put so much soot into the atmosphere that it would turn day to night for years. Photosynthesis would be impossible. Only hardy spores and seeds of photosynthetic life would survive. At least in nature. But small mammals, and birds survived, also turtles. A few humans would survive also. Even large numbers with ingenuity, doing things like growing mushrooms on all the decaying wood, using insects as a food source directly or indirectly, fish, and any stored supplies. They could also use methane gas to grow methanogens as a food source directly or indirectly.

If given time to prepare we could also stockpile food, using food otherwise grown for biofuel or to feed to animals. So something like that does seem survivable, at least not an extinction event.

What about using LED lighting and growing crops as for BIOS-3?

Of course solar power won't work, but everything else will. The current world energy use per capita is 1,920 kilograms of oil equivalent (koe) in 2014 according to the World Bank. To get kilowatt hours, you multiply the koe by 11.63. To get average power in kilowatts, divide by the number of hours in the year, or 8766 (365.2422*24), to get 1920*11.63/8766 = 2.5 kilowatts. So bearing in mind that the BIOS-3 system needs only 1.5 kilowatts per person for lighting with efficient LED for crops (100 watts per square meter for 12 hours a day for 30 square meters per person) then it would be feasible to generate the power we need for crops - if we sacrifice other things to compensate.

However that's for all forms of power. We need it as electrical power. Some countries could find that much power from their electricity generation alone. Iceland has electricity consumption of 53,832 per capita per year (figure in 2014). That's a constant 6 kilowatts per person average. Only a quarter of that would be needed for LED lighting. Norway could also manage it with 2.6 kilowatts per person. Canada and a few other countries just squeak in with hardly anything to spare. The US generates 12,987 kilowatt hours per capita of electricity. That's just under our 1.5 kilowatts at 1.48 kilowatts average power usage per person. The world average is only 357 watts per person of electricity generated.

You could get enough power for LED lighting to feed a population of hundreds of billions that way for sure, and even perhaps a billion or more,if we were able to get all the infrastructure in place in advance and devote most of the electrical power to agriculture. But for the whole population, it would need a big increase in electricity power generation. You could do things such as to use the fuel for cars instead to power generators, and travel less.

Basically, I think it is fair to say that if you had space habitat type colonies on Earth they don't ned to use more than their "fair share" of Earth power generation to produce all their food using LED lighting. But they would have to be very efficient in how they use it and how they do everything else and it would be a huge effort to upgrade the whole Earth to a system like this.

However we don't need to worry that something like that could happen right now. Our asteroid surveys are already complete for NEO's of 10 kilometers upwards, and we have plotted their orbits. We now know that there are no asteroids as large as the Chicxulub one headed our way at least for the next several centuries.

What about comets? Well , less than 1 in 150 NEO's are NEC's (see the numbers of NEO's and NEC's in this table) so comets are very rare at present. That makes chance of one of those probably less than 1 in 150 million per century (given that there is about a one in a million chance of a 10 km diameter asteroid impact per century).

So, we can forget about ten kilometer diameter comets and asteroids, at least for a few centuries. If we find one headed our way, with a lead time of a couple of centuries, we have plenty of time to do something about it.

Now if we go back to a bit over three billion years ago, that's a bit different. We had impacts back then of objects of 30 miles diameter or more. There is evidence that they boiled our oceans. The ocean levels dropped probably by of the order of 100 meters, temperatures 500 C for weeks, sea boiling for a year. After something like that, the land would be completely sterilized of life. We couldn't survive that, except perhaps in a colony on the ocean floor. A subsurface self sufficient sea colony could survive something like that, perhaps more easily than a space colony. But luckily we don't have to prepare for this. More on this in the next section.

We haven't found all the one kilometer asteroids yet, but that risk is pretty much retired too with only 5% left to find, less than 50 probably.

By far the most likely impact is of the order of tens of meters up to perhaps 100 meters or so. However we shouldn't exaggerate the likelihood even of those. A 100 meter object would be far larger than the largest meteorite strike in all of recorded human history. We certainly have plenty of reason to track asteroids and protect ourselves from them. It is the one natural disaster we can predict exactly to the minute (and so, evacuate the impact zone and take precautions for those who live further away) - and given enough time,we can prevent it too, by deflecting the asteroid. The largest ones are devastating, but they are low probability, far less likely than volcanoes, earthquakes and tsunamis. And they can't make us extinct.

So, small impacts are possible, yes. Larger impactors 10 km across are possible, though there is none headed our way for a few centuries. However those would not make us extinct. The very large ones 100 km across probably could make us extinct, melting continents and boiling oceans, but we don't get those any more, and haven't had their like for billions of years. We haven't had their like since the end of the late heavy bombardment, and Jupiter protects us from them in the present day solar system.

So, there is no way we are going to go extinct from an asteroid impact.

What about a comet as large as Hale Bopp (60 km in diameter)?

We do occasionally get large long period comets come our way. And this one is indeed a whopper. At 60 km in diameter, it would be likely kill all land life and also destroy surface life in the oceans too, boiling away perhaps a hundred meters depth of the sea. .

Luckily there is no chance of this in the near to medium future. In the long term, it has a chance of about 2.54 per billion of impact with each encounter, in an orbit that currently takes over 2,500 years to complete. Right now the orbit crosses the ecliptic some distance away from Earth’s orbit, further away from the sun.

It passed us by in the 1990s and we won’t see it again for another 2,500 years. So, our civilization doesn’t need to worry about it for a fair while. Its impact if it were ever to hit Earth would be particularly devastating because of its inclined orbit, with an impact velocity of 52.5 km / sec.

We do have historical impacts to compare it with, from over three billion years ago, as described in the Smithsonian magazine:

“Researchers led by Don Lowe of Stanford University describe the effects of two asteroids measuring 30 to 60 miles across that hit about 3.29 and 3.23 billion years ago. (For context, the asteroid that killed the dinosaurs was probably a measly six miles across.) The dual impacts sent temperatures in the atmosphere up to 932 degrees Fahrenheit for weeks and boiled the oceans for a year, long enough that seawater evaporated and they dropped perhaps 328 feet. The researchers reported their findings in the journal Geology.”

Asteroid Impacts Once Made the Earth's Oceans Boil for A Whole Year

In metric units, that’s air temperatures of 500 C and evaporating 100 meters of ocean.

The paper is here: Geologic record of partial ocean evaporation triggered by giant asteroid impacts, 3.29-3.23 billion years ago

“recent studies in greenstone belts indicate that asteroids 20 km to 70+ km in diameter were still striking the Earth as late as 3.2 Ga at rates significantly greater than the values estimated from lunar studies. We here present geologic evidence that two of these terrestrial impacts, at 3.29 Ga and 3.23 Ga, caused heating of Earth’s atmosphere, ocean-surface boiling, and evaporation of tens of meters to perhaps 100 m of seawater”

Geologic record of partial ocean evaporation triggered by giant asteroid impacts, 3.29-3.23 billion years ago

Something like that is j ust not survivable, even a 30 to 60 miles (50 to 100 kilometers) impact. There would be no higher life left on land with the 500 C atmosphere. The land would be sterilized to some depth but rock is a good insulator. A few hundred meters below the surface, you wouldn't notice it.

With the oceans boiling there'd be hardly any photosynthetic life left either. Just microbes - and deep sea fish and giant squids and the like would survive. Not whales though, they can’t hold their breath for weeks for the atmosphere to cool down.

If this happened naturally on some other planet like Earth, without intelligent species to do a "backup", then perhaps the land would be recolonized by the descendants of squids , and crabs, which have the ability to survive on land for a short while already.

I suppose theoretically humans could survive this without leaving Earth,t if they could set up a deep sea colony, or one buried deep below the surface of the land in caves, insulated from the surface. The ocean colony seems somewhat easier perhaps. But it might well be easier to survive such an impact in space, rather than to be trapped beneath a 500 C atmosphere for weeks and a boiling sea for a year.

This would be the type of impact where our backup on the Moon would be really useful, and would let us recolonize the land quickly, not having to wait for the squids, crabs, and whatever might remain of photysnthetic life to do the job.

However the chance of Hale Bopp hitting Earth is tiny as we haven’t had an impact this large for over three billion years, and nor has the Moon, Mercury, or Mars. Also even close flybys by comets are rare. The closest is Lexell's Comet which passed by at only 6 lunar distances on July 1st 1770.

I have tried to find an article on the frequency of Hale Bopp or Swift Tuttle sized impacts but can’t find anything. Do say if you know of anything. Meanwhile, as a “back of the envelope” type first estimate, perhaps they happen at intervals of at least three billion years since that’s how long since the last one? That’s probably an over estimate of how often they happen, as back then Earth was still getting hit by more large objects than it is today, in the very tail end of the so called “Late Heavy Bombardment”.

That would make the risk per century of a land sterilizing impact like this less than one in thirty million, and probably far less.

So, how does that fit with the existence of Hale Bopp? Well it’s in an orbit that will let Jupiter deflect it from time to time. Over that time, it can hit Jupiter or the Sun, or, since it’s made of ice, then it can gradually evaporate away during close approaches as will probably happen to comet Halley. Also, it’s likely to be disrupted by Jupiter into multiple smaller comets during close flybys, as happened to comet Shoemaker Levy before it finally hit Jupiter. It could also just break up into two or more comets as it evaporates away some of the ice holding it together.

At any rate, Hale Bopp won't hit us for thousands of years.

The object that’s often mentioned as most dangerous for Earth in the very long term is Comet Swift Tuttle. We now know that it can’t get closer than 80,000 miles (130,000 km) for the next several centuries. But over thousands of years it can get closer. There’s a there is a one in a million chance that it will hit us in 4,479 AD. It’s not very likely and we have thousands of years to develop the technology to do something about it if needed. See my answer to What will happen if Comet Swift-Tuttle strikes the Earth in 2126?

Comet Swift Tuttle does close approaches to Earth - most recently in 1737, and 1862. The approaches below the shaded area are visible from Earth. The next such are in 2126, 2261 and 2392. However it can’t hit Earth on its current orbit as we now know, after working it out more exactly.

There’s a chance that it can hit Earth 4479 years from now, but it is only a one in a million chance. It would be devastating, not as bad as Hale Bopp but far worse than the dinosaur era asteroid impact. But we have more than 2000 years to develop the technology to do something about it in the remote case that we need to.

Even large asteroids and comets can be deflected given a timeline of decades or more to do something about it as then, only a tiny nudge to change its velocity will be plenty to change an impact into a miss decades later. A deflection of only two centimeters per second, the speed of a garden snail, can change a direct bullseye hit into a miss a decade earlier. If it does a fly past of Earth through a keyhole, before it hits, and you can deflect it a decade before the flyby, then you are talking about a nudge of microns per second. On those timescales, if we continue with our present day technology, never mind what we might develop by way of new methods by then, we could have many ways to deal with Hale Bopp. With over 2400 years before the encounter we have plenty of time to do something about it.

For more about impact hazards see my


I also look at all this in more detail in my

Also available on kindle as:

Giant Asteroid Is Headed Your Way? : How We Can Detect and Deflect Them (Amazon)

(Though as of writing this, it needs an update with new material on long period comets like Hale Bopp).