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tony873004
2016-Jan-24, 07:55 PM
The orbits of Earth-crossing asteroids change over time.
An asteroid can only strike Earth if its ascending or descending node crosses the ecliptic at the distance of Earth's orbit. Otherwise it will pass above or below the Earth.

Any asteroid whose orbit crosses Earth's has only a small window of time to impact Earth before its nodes rotate it out of harm's way.

Here's an animation from a simulation showing the orbits of several PHAs (potentially hazardous asteroids) and how they change over thousands of years.
Visible are also the orbits of the planets. Earth is the one that is alternating between blue and gray. Blue is Earth, gray is the Moon.

The asteroids orbits are brighter when above the ecliptic and dimmer when below. So the transition marks the ascending and descending nodes.
Since these are currently PHAs, their nodes should close to Earth's orbit. But as the animation progresses, they will rotate away. New currently-non PHAs will also change and become PHAs.

https://www.youtube.com/watch?v=3-0R27x8Irc
https://twitter.com/tony873004/status/691330879533154304

slang
2016-Jan-29, 09:48 PM
Speaking of Earth-crossing, did you ever create one to simulate one of the Theia scenarios?

tony873004
2016-Jan-29, 11:18 PM
I've heard that after the Theia impact that the stuff that got ejected into Earth orbit only took a few months to coalesce and form the Moon.

So I put 60 moonlets, each with a mass 1/100 that of the Moon in low Earth orbit to see how quickly they would coalesce. Here is the result.
This is not a youtube. It's a simulation that runs in your browser.
http://orbitsimulator.com/gravitySimulatorCloud/simulations/1433886939671_Moon%20Builder.html

efanton
2016-Jan-29, 11:25 PM
Speaking of Earth-crossing, did you ever create one to simulate one of the Theia scenarios?


Theres a web page regarding to the Theia collision with number of animation links in the top right
this web page propose an updated version of the planet collision theory citing that even though the previously accepted version accounts for the creation of the moon from the debris created by two planets impacting, it does not account for the differences in chemistry between the moon and Earth


A New Model for the Origin of the Moon


Therefore, the original giant impact model has a major problem: it can match the mass of the Moon and the rotation rates of the Earth and Moon, but not the chemistry of the Moon. Today, tides between the Earth and Moon slow Earth’s rotation and push the Moon’s orbit further away, but the total angular momentum (see glossary) is conserved. Going back in time, the early Earth had a day of only 5 hours when the Moon formed. With a post-impact spin period of about 5 hours, a giant impact could not loft enough Earth material into orbit to make the Moon match the chemistry of the Earth.

http://www.seti.org/node/1458

In the top right there are various links to animations of the impact

slang
2016-Jan-30, 12:06 AM
Whoops, forgot to add comment on the OP video: it's fun to see how much these orbits change over time.


I've heard that after the Theia impact that the stuff that got ejected into Earth orbit only took a few months to coalesce and form the Moon.

So I put 60 moonlets, each with a mass 1/100 that of the Moon in low Earth orbit to see how quickly they would coalesce. Here is the result.

Wow. That's pretty quick.


Theres a web page regarding to the Theia collision with number of animation links in the top right

Thanks, but I was specifically looking for one of tony873004's solar system animations, after seeing this animation (https://nl.wikipedia.org/wiki/Theia_(planeet)#/media/File:Big_Splash_Theia.gif) on the Dutch wikipedia page on Theia.

It does put a nice picture to the planet definition of "clearing its orbit", doesn't it? ;)

efanton
2016-Jan-30, 12:40 AM
I've heard that after the Theia impact that the stuff that got ejected into Earth orbit only took a few months to coalesce and form the Moon.

So I put 60 moonlets, each with a mass 1/100 that of the Moon in low Earth orbit to see how quickly they would coalesce. Here is the result.
This is not a youtube. It's a simulation that runs in your browser.
http://orbitsimulator.com/gravitySimulatorCloud/simulations/1433886939671_Moon%20Builder.html

really cool animation

I have had the gravity simulator for a while now. How did you get your finished simulation on the web. With the download version there only seems to be options top save to hard drive.

tony873004
2016-Jan-30, 12:52 AM
really cool animation

I have had the gravity simulator for a while now. How did you get your finished simulation on the web. With the download version there only seems to be options top save to hard drive.

The downloadable version only runs on Windows. I got a job at a school that uses nothing but Macs and iPads. So I re-wrote it in HTML5 and Javascript so it would run on any platform in a browser. Makes it MUCH easier to share simulations, even with Windows users because there's nothing to install.

efanton
2016-Jan-30, 02:04 AM
The downloadable version only runs on Windows. I got a job at a school that uses nothing but Macs and iPads. So I re-wrote it in HTML5 and Javascript so it would run on any platform in a browser. Makes it MUCH easier to share simulations, even with Windows users because there's nothing to install.

I'm impressed.

Being an IT guy and knowing the windows version of that program I can only imagine how much time and effort went into that.

tony873004
2016-Jan-30, 02:53 AM
Being an IT guy and knowing the windows version of that program I can only imagine how much time and effort went into that.
It's still a work in progress. I'm a teacher. It was one of my summer vacation projects last year.

The math parts were pretty straight forward. Lots of ctrl+h [sin] -> [Math.sin], etc, and 4000+ line-ending semi-colons.

The graphics took a while, but were actually fun to rewrite since HTML5 does so much more than Visual Basic 6.0.
There's lots more simulations for your browser here: http://orbitsimulator.com/gravitySimulatorCloud/simulations/
If you save one through the File menu and check the "share" button, it will add it to this list.