Thread: Rock Around the Earth

Bill Haley and the Comets play on words seemed fitting.

Was trying to find anything regarding a rock or asteroid that may orbit the Earth like the Moon. There have been claims but tough to prove these quasi-satellites actually are out there.

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

Anyways I stumbled upon something called a Trojan, a celestial body that orbits the Sun along the same orbital path as Earth. I have wondered what LaGrange points were in the past until reading this article. Apparently other planets in our Solar System share their orbital path with Trojans also.

https://earthsky.org/space/astronome...oid-for-earth/

Only reason I was looking had to do with my curiosity about man-made vs natural space debris. If our scrap metal can orbit Earth then why can’t space rocks? We pass through the Perseids every year so why doesn’t some of that debris end up circling the Earth?

Originally Posted by zinjanthropos

If our scrap metal can orbit Earth then why can’t space rocks? We pass through the Perseids every year so why doesn’t some of that debris end up circling the Earth?

The scrap metal was placed into orbit using precise speeds and flight angle for orbital insertion.

For any large body like Earth, capturing an asteroid to form a natural moon is not at all easy.

Any asteroid must have just the right speed and trajectory (relating to its mass) when it approaches earth to become a natural moon. Otherwise, it either collides with Earth and is destroyed, or it will simply fly by without capture. To find out some more about this, check out wiki's link below:

https://en.wikipedia.org/wiki/Asteroid_capture

Our scrap metal was deliberately placed on trajectory which formed a closed orbit around the Earth. Solar system debris already has some velocity difference from the Earth due to it's different orbit around the Sun. For example, the Perseids are caused by debris left behind by the Comet Swift-Tuttle. This debris travels in roughly the same trajectory as the comet, which crosses Earth's orbit at a steep angle. This means it already starts with a high relative velocity to the Earth. Any such piece of debris falling towards the Earth, unless its trajectory causes it to collide with the Earth, will whip around it in an open hyperbolic trajectory. The only exception would be if if something slowed it down enough to put it in a closed orbit. But such interactions are extremely rare, and generally requires the object to be moving not much above orbital velocity to start with.

To give you an example, A piece of debris starting at twice the distance of the Moon from the Earth, and with a relative velocity to the Earth of just 1.03 km/sec is already moving too fast for the Earth to capture it into orbit without some secondary interaction.

Great responses guys. My thought was that at some time in the last 4 billion years there would have been enough opportunity for a rock to circle the globe as an independent moon but I suppose the odds are astronomically high against it.

I would imagine then that a ringed planet such as Saturn has what’s left of an unstable moon that crumbled? Do scientists know what happened there? There is no way that many rocks could be captured seeing how the odds are so stacked against even one making orbit. If instead of rock there is only dirty ice chunks in orbit then I can see being too close to the Sun as a ring deterrent. If comets, asteroids, etc leave icy fragments in their wake as they pass close to the Sun then could a few have made it into our orbit at least temporarily?

Originally Posted by zinjanthropos

My thought was that at some time in the last 4 billion years there would have been enough opportunity for a rock to circle the globe as an independent moon but I suppose the odds are astronomically high against it.

If comets, asteroids, etc leave icy fragments in their wake as they pass close to the Sun then could a few have made it into our orbit at least temporarily?

Found interesting wiki article on the possible moons of Earth.

This may have more info than you ever cared to learn about!

https://en.wikipedia.org/wiki/Claimed_moons_of_Earth

Just noticed you already referenced this link in your first post.

Understanding this better, my reading suggests that even if something small is captured, it will not stay in orbit due to gravitational interactions with the moon, the sun, and the minimoon's orbital characteristics. Also, over long periods of time, the many passing asteroids near it would also interrupt a small body's orbit around the earth, sending it off away from us.

If you read that link again, there are some smaller asteroids which have entered Earth's orbit, and stayed there for a while, but then drifted off ("temporary satellite capture") . It seems that most moons throughout the solar system formed by accretion.

Mars potentially has captured moons. Quoting from the link below :

"Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids.


https://en.wikipedia.org/wiki/Moons_of_Mars