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Comment by mydriasis | Hacker News Reader

> ... the collision also shook off 37 boulders, each measuring from 3 to 22 feet across. None of the boulders is on a course to hit Earth, but if rubble from a future asteroid deflection were to reach our planet, Jewitt said, they’d hit at the same speed the asteroid was traveling — fast enough to cause tremendous damage.

Holy hell! OK -- do they burn up on entry? What's the criteria for that?

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What's the criteria for that?

It's complicated. Depends on the cross-sectional area and the materials the boulders are made of. Ice, for example, burns up (melts) on reentry much more readily than iron!

Also depends on the angle of attack. Heading straight down (perpendicular to the ground beneath it) passes through much less atmosphere than a glancing blow, so has a lot less time to burn up.

Interesting, thank you!

I wonder how often perpendicular to the ground impacts occur? I seems that they could only occur if an asteroid approached in a head on collision with our orbit or perpendicular from a higher or lower orbit as it passes or is passed by the earth. That makes it statistically unlikely, doesn't it?

The statistical likelihood depends entirely on the timeframe. Within our lifetimes? Extremely unlikely for any large impact. Over the next billion years? A lot more likely!

Nope. I was referring the likelihood of perpendicular collisions out of the set of all possible collisions. No time dimension.

Not to mention asteroids made of iron would be much harder to knock boulders off of.

Yeah if there's some solid iron asteroid out there heading right for us we're totally screwed. That's like a core of a planetessimal! Very dense stuff!

It could also be some lighter rock (a matrix) with iron clasts, created from a collision between a huge, light asteroid and smaller iron planetessimal fragments. Then if our planetary defense system breaks off a bunch of boulders, some could contain the heavy iron fragments and some might not, so different things would happen on reentry for each boulder.

> if there's some solid iron asteroid out there heading right for us we're totally screwed

Isn’t this among the easier to deflect? You literally smash other asteroids into it without much fear of it breaking up.

No. That’s going to be extremely difficult to deflect due to the momentum. You’d have to catch it much earlier.

As for smashing “other asteroids” into it, that’s likely not feasible due to the vast distances between asteroids and the masses involved.

> going to be extremely difficult to deflect due to the momentum

Momentum is a function of mass and velocity. The momentum of two similarly-massive asteroids, one rock the other metal, is the same. Given existing technologies, deflecting a solid-ish body is easier than a loose pile of gravel.

The average distance between asteroids (and their nearest neighbours) in the asteroid belt is about 8 times the earth-moon distance. Most of these asteroids have roughly the same orbit, so they have almost no relative velocity. So to achieve a collision we have to dramatically change the velocity of one of them in order to hit the other in a reasonable time frame.

But this doesn’t take into account the masses. Asteroids vary in size from about 1 metre across to the dwarf planet Ceres, at 1000 km across. A giant asteroid is unlikely to have other giant asteroids nearby. If one giant asteroid is on a collision course with earth, what is the likelihood we’ll find a similar-enough sized one within a reasonable enough distance to be able to make them collide? Extremely remote.

The point is composition has no influence on momentum. Nobody doubts causing two bullets, already mid-flight, to collide by shooting other bullets at them is nontrivial. To the degree I have seen it discussed, it’s in using a dense asteroid to gravitationally perturb a loose one. Precisely because dense, metallic asteroids’ orbits are easier for us to manipulate than rubble piles’.

> Space rocks smaller than about 25 meters (about 82 feet) will most likely burn up as they enter the Earth's atmosphere and cause little or no damage.

https://www.nasa.gov/mission_pages/asteroids/overview/fastfa...

Could we wrap the planet in some kind of space safety blanket? Like say an thicker atmosphere full of CO2 or methane?

Ah, the Venus Defense.

Too much work. And prime real estate on Venus is dirt cheap, if you're smart enough to get in before the big rush.

What if I told you I could get you a cubic mile in the Venusian atmosphere for only $99??

> What if I told you I could get you a cubic mile in the Venusian atmosphere for only $99??

I'll say you're a scammer. I'm willing to sell it for half the price. Comes with a certificate demarcating the plot as well.

What we should do is try and crash some moons or something into Jupiter, increasing its mass and gravity so it will attract all asteroids.

We would need a lot of mass. If we could crash Saturn into it that would be a good proof of concept.

Don't worry we're already working on it.

It's kind of like making yourself a fireproof asbestos blanket.

So then why would these ones cause tremendous damage, I’m given how small they are?

Journalistic hyperbole.

Satellites?

Tremendous damage to the boulders.

The wouldn't, and a journalist is wrong on the internet.

Heat the atmosphere too much and we all cook

To/dr the small ones probably will, but it depends on the angle of approach, their composition, their speed, and even their shape and internal structure.

Yes, those sizes would very likely burn up completely before impact. 10-20m required to reach ground.

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