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danielravennest t1_itfzxq7 wrote

Reply to comment by Devil-sAdvocate in As many as a fifth of all asteroids in the asteroid belt may have been formed out on the fringes of the solar system. Findings suggest that both Ryugu and the CI chondrites originate from the same region of space, and could even have shared the same parent body. by marketrent

Exactly. Japan sent a probe to the asteroid Ryugu and brought back samples. The samples indicate its material formed in the outer Solar System. but now it's orbit crosses Earth's (that's why the picked it to visit).

More generally, there are 30,000 known asteroids whose orbits come closer than 1.3 times the Earth's distance to the Sun. That compares to over a million other asteroids. These "near Earth asteroids)" have an orbit half-life of 10 million years, which is very short compared to the age of the Solar System.

So these asteroids need to be constantly resupplied, otherwise there would be none left. The giant planets, which is mostly Jupiter since it is heavier than everything else combined, are the main cause of orbit changes.

The near Earth asteroids are the ones most likely to hit Earth, which is why NASA has a search program for them, and why they just tested changing an orbit by hitting it. What we have going for us is space is really empty. Earth only takes up one part in 500 million of our orbital region. So the chance of any given random rock hitting us is low. On the other hand, there are many rocks. Dust and sand-sized ones hit us every day. That's what meteors in the night sky are.

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thejoeymonster t1_itnaai0 wrote

Earth's orbit is 500million times it's diameter?

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danielravennest t1_itpxg9q wrote

No. Our orbit is 23,455 times the size of the Earth. But there is a second dimension, above and below our orbit.

This plot shows the size of asteroid orbits relative to ours on the horizontal scale, and the tilt in degrees relative to the Earth's orbit. You can see the tilts go up to 30-40 degrees.

Any tilt above zero means when they cross our orbit's distance, they can be crossing above or below where the Earth will pass. They can only hit us if they cross our distance exactly on our path, and the Earth also happens to be at that point in our orbit at the same time.

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