Hacker News Reader

Comment by Syzygies | Hacker News Reader

It missed Earth by 73,000 kilometers; Earth has radius 6,371 kilometers.

(79371 * 79371) / (6371 * 6371) = 155.2

This was a game of darts in a galactic pub. The dart hit the dart board, but not the bullseye (Earth). One in every 155 times we play this game, it ends badly.

View on HN

How often do we play this same game, though? If a 1Km size asteroid seems to hit the Earth about once every 500,000 years, it would seem like they would pass this close to the Earth only once every 32,000 years or so.

So either the 500Ky figure is wrong, or we've been extremely lucky to witness a once-in-30Ky event, or we're missing something.

This was only 100m. Those probably hit semi-regularly and would have usually gone unnoticed or at least unrecorded before modern times.

Incredibly in Australia there are impact events recorded in indigenous oral tradition dating back thousands of years.

http://www.aboriginalastronomy.com.au/content/topics/craters...

Even if it hit the earth the odds of it landing anywhere near a city are slim. 3/4 of the time it hits the ocean, Siberia, the Canadian north, or the Antarctic.

Serendipitously cooling the surface of the earth down by exactly 2.2 C.

Bring it, punk!

Perhaps the solution to climate change is dropping a rock the size of a stadium on Canada every few years.

Mine some ice from an asteroid and drop it into the ocean, with larger blocks every few years as pollution increases :-)

Much of the ice would vaporise on entry. Water vapor is a greenhouse gas.

Yes. But more importantly, that was a Futurama reference: https://www.youtube.com/watch?v=6cjx4gJFME0

It can either both prevent sunlight from entering or leaving depending on the altitude.

Although an asteroid of that size, depending on its makeup, can have other damaging effects. For example, by area, the pacific ocean covers a lot of the planet. An asteroid hitting the pacific can cause a tsunami that could seriously damage any coastal city in range.

Even a 390 ft nickel-iron asteroid wouldn’t have enough energy to create a meaningful tsunami. We’re talking energy along the lines of Castle Bravo, not the asteroid that killed the dinos.

An asteroid of this size would need to land within 1000 ft of shore to have any meaningful wave height, and would only significantly impact maybe a mile of shoreline. You need to move an impressively large volume of water to create a Fukashima-type event.

One thing to note: the Tunguska meteor was ~50-200m in diameter, and was equivalent to a 3-30 mega-tonne explosion.

You massively underestimate the destruction caused by a 1km asteroid. Remember some scientists estimate the asteroid that killed dinosaurs was as small as 11km. In other words a 1km one may be 1/10th the diameter needed to cause an extinction-level event.

A 1km asteroid will cause massive casualties no matter where it impacts. If it hits the ocean it will cause a tsunami. If it hits a remote land it will eject so much debris in the atmosphere that they will block sunlight around the globe and reduce temperatures by multiple degrees, thus reducing food crop yields and causing worldwide food shortages and famine.

This was a 100m asteroid (city killer), not 1km.

And a factor of 10 difference in linear size (radius, diameter, etc.) implies a factor of 1000 difference in mass.

How does the math change when you take into account the fact that Earth exerts a gravitational pull on the asteroid?

Interesting question, not sure. As the speed of the astroid is goes up, the relevance of gravity goes down. As the speed goes down, the damage it can do also goes down, and also reduces the probability of an encounter.

And does that make it more likely to miss? Ie it gets slung around by gravity instead of hitting?

The degree of gravitational focusing depends on the speed of the incoming object, but I seem to recall that for typical interplanetary velocities it's of order 1, ie not huge.

It may not change this encounter much, but it could change the orbit just enough for some future earth-orbit crossing - as could the gravitational effect of any of the other planets, their moons, and all the other asteroids change the orbits of anything out there. Solar systems can be messy places, over geologic time frames. Sometime, sooner or later, we will have a significant impact.

Thinking out loud time! My favorite!

To matter for a single pass, the object must be small and/or traveling slowly. Otherwise, itll just keep on cruising.

Iteratively is a different story. Many objects repeatedly cross earths path. How that orbit changes over time i suppose is a function of the other gravity wells the objects pass when they arent near earth. I expect there are so many confounding gravities that only a certain belt\torus exists in the universe where, inside that torus, space stuff is capable of running into us.

Side note: this article is clickbaity because it implies no one knew asteroids almost crash into us without us knowing, but thats not true. People know. There are hard reset buttons floating around in space and we might just press one without warning.

https://en.m.wikipedia.org/wiki/Potentially_hazardous_object

Why would the size of the object matter in this instance?

Mass, not really size. The smaller the object relative to earth, the more its course is changed by earths gravity.

You may want to refer to Galileo's leaning tower of pisa experiment...

It probably has no effect: objects that are not in stable orbits around the earth are going to be in hyperbolic orbits relative to the Earth's gravitational field. A dangerous asteroid would need a periapsis lower than the Earth's radius by a certain amount to hit the Earth, so the risk of collision is still roughly proportional to Area. If gravity is a factor I would expect it to be a high order term. Atmospheric effects are probably more significant.

Assuming no other conditions change, and that the object crosses Earth's orbit as the planet is passing by, and so on.

I'd be willing to bet, however, that this object's orbit permutes a bit with every pass, especially dipping that deeply into the gravity wells of other, much more massive, objects.

It was described as "astronomically small" that something like this would hit us so I imagine there's factors not being considered here

How often do we play this game?

How often would it hit something we care about on the dartboard?

> One in every 155 times we play this game, it ends badly.

That assumes only one degree of freedom. If you consider the area of the dart-board vs the area of the bullseye, the odds are much lower.

No, the GP specifically talked about area.

On galactic scales that's more like glancing off the ring of the bulls eye.

On galactic scales it’s a meaningless analogy.

Your math is off because the bullseye gravitationally attracts the dart

Only significantly so if the dart is going very slow

Reply on news.ycombinator.com