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Comment by wyldfire | Hacker News Reader

Less common but still underrated as a risk is asteroid strike or near miss. Jupiter, Saturn, our Moon all keep us safer than other Earth-like planets. But it seems like a risk that should concern us. besides maybe subterranean shelter, IMO the best hedge is interplanetary colonization. Ridiculously expensive but not when measured against the cost of losing all of humanity.

Maybe galactic risks like these are part of the Great Filter.

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There very much is a back up plan, with bunkers, food, water and energy reserves, for such a catastrophic event. But not for you an I.

I think asteroids are overrated. A globally catastrophic asteroid will not hit earth within our life times with a high degree of certainty. Yet, we currently already have means to deflect it. When it finally happens, it will be deflected. That risk is minuscule. The chances of a random small asteroid destroying a city has a higher chance, but I wouldn’t really worry about that more than the building I’m living right now collapsing.

We don't have the technology to deflect a planet-killer.

If we detect it with enough time ahead (which is very likely we will), it’s not that hard to deflect. We have already landed on asteroids and we certainly have the technology to give it a light, but constant, push in the course of an year or so. We don’t have the technology in case it’s found only days before hitting the earth. But that is very unlikely.

The odds of a planet-killer hitting our planet in the lifetime of you, your children, your children's children, and the subsequent thousand generations is so low it may as well be zero.

Worrying about it is like stressing out over what you are going to name your super-yacht, when you're broke, unemployed, homeless, and are dying of lung cancer. It's the most absurdly pointless form of bikeshedding. There are plenty of immediate and catastrophic disasters that we should be preparing for, first.

Even so its still fun to think about!

A planet killer won't hit the Earth without 3-5 near misses first. Plenty of opportunity to paint it.

Paint it? I think I vaguely know what you're getting at, but please explain.

You only need a few thousand km of orbital change to move a direct impact to well clear of Earth. You can use the momentum of photons to get this orbital change. The effect of paint is a very small delta-V, but it works out that it's enough if done a few decades before the expected impact. This works out since it is extremely unlikely that a large asteroid would impact Earth without first having a series of near passes.

https://spaceguardcentre.com/asteroids-deflected-with-paint/

If something is going to hit Earth and wipe us out, we won’t have a few decades notice.

Yes we will. Planet killer sized asteroids are the easiest to detect and track and are also the least common. City killers are harder: there is a chance we won't one of those coming and there are many more of them out there.

Asteroids that hit Earth are in near-Earth orbits. For a first intercept event the odds of an asteroid just happening to hit an Earth sized target as opposed to, say, a 1000x Earth sized "near miss" target is incredibly unlikely. We get a few near-miss events of a given asteroid before an impact happens.

https://phys.org/news/2021-06-space-orbit-oort-cloud.html

The asteroid that wiped out the dinosaurs was between 3 and 10 miles diameter. 2014 UN271 is 100km-370km.

Would we have discovered 2014 UN271 earlier had its orbit brought it closer to Earth? It’s making its closest approach in 2031 (ok, 11AU away!), only 17 years after discovery.

And sure, incredibly unlikely something that size would hit Earth, but just an example of something that is very large, and discovered shortly before it makes its closest approach.

Objects with highly eccentric orbits are both very uncommon and very unlikely to intercept Earth. The period in which they have near misses is on the order of thousands of years at least. Even if we could see them from their high altitude we don't have enough information to predict where they'll be on the next intercept.

My understanding is that this kind of threat is usually lumped in with comets, whereas threat asteroids are lumped in with low eccentricity big rocks.

Thanks, I see the difference now.

If something is big enough to wipe us out, you WILL notice it a few decades ahead. The bigger asteroids are, the easier they are to detect.

The tech is not out of reach, it's just not getting funding. See B612 Foundation.

I read as detect, but if you detect it far enough in advance, it gives a lot of time to do something about it. Even a small nudge make a huge change in position decades later.

I'm quite sure we do, assuming that US military would be willing to share their experiences from the B61 Mod 11 project.

An "asteroid", under the strict astrophysical definition, isn't a big threat. Predictable orbits, and, as you say, all the big inner system objects are mapped, and smaller objects aren't global threats.

Comets, though. We keep spotting alarmingly large comets only when they get close enough to the sun to show up in low-sensitivity sky surveys, long after they've crossed the Earth's orbit. https://en.wikipedia.org/wiki/C/2020_F8_(SWAN)

A ultra-high eccentricity Oort object, covered with low-albedo goop from spending millions of years in the periphery of the solar system is going to be damn hard to detect. Worse yet, they'll be traveling fast. Kinetic energy is the square of velocity. Won't ever see em coming, and they'll hit hard. Worst of all worlds.

Does albedo refer only to visible spectrum? Could we detect these using other frequencies?

Thermal infrared would do it. (Lower the albedo, the more blackbody radiation is emitted) The JWST would be great for comet spotting, but unfortunately it's going to be used for stellar observation instead of sky surveys. Putting another ten of them in orbit would be great, but so far the JWST has taken 25 years and more than ten billion dollars to build. Infrared also won't spot a comet until it's far enough into the solar system to warm up.

Radar is tempting, but the tyranny of the inverse square law limits its use. (Worse-- the pulse has to make it out to the object, and then be reflected back. Inverse fourth power!) All those great radar images of asteroids made by Arecibo were done during their closest approach to Earth. This[1] claims its useful range was "~2 lunar distances". This was with a 305 meter dish and 1 MW of transmit power! Also, Arecibo collapsed, and nobody seems to be in a hurry to build a replacement. (The 500 meter dish in China doesn't have a transmitter)

Assuming you have infinite power, (With a moon-based dish, maybe, otherwise your gigawatts of power just ionize the air in front of the feed horn) you have other practical problems. Bigger the dish, better the gain, narrower the beam width. Arecibo had a 2 arcminute beam. 60 arcminutes to the degree: 21,600 arcminutes to a full circle. Assuming you're surveying out to Jupiter's orbit, 43 minutes for the pulse to get out there, 43 minutes back: 645 days to survey a strip 2 arcminutes wide and 360 degrees long. Which... wouldn't have caught C/2020 F8 (SWAN), which came in 110 degrees off the ecliptic. Surveying the entire night sky would take 6,074.9 years. Just out to Jupiter! You're not even looking into the cloud of outer objects, just spotting infalling ones.

1: https://www.hou.usra.edu/meetings/orbitaldebris2019/orbital2...

> Ridiculously expensive but not when measured against the cost of losing all of humanity.

there's no concept of money and cost left if humanity goes extinct ... so it can't be "expensive".

There is an argument to set the value of humanity to some finite, not ultra-enormous value: https://en.wikipedia.org/wiki/Pascal%27s_mugging

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