
NASA finds that the earth has a new “mini moon” - uptown
http://qz.com/712457/nasa-finds-that-the-earth-has-a-new-mini-moon/
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tantalor
> the asteroid technically moves relative to the sun, but it seems to be
> playing around the earth’s orbit, too

So does the Moon: [http://www.wired.com/2012/12/does-the-moon-orbit-the-sun-
or-...](http://www.wired.com/2012/12/does-the-moon-orbit-the-sun-or-the-
earth/)

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dalke
That's if you use what Asimov calls "tug of war" as your definition:
[https://en.wikipedia.org/wiki/Tug_of_war_(astronomy)](https://en.wikipedia.org/wiki/Tug_of_war_\(astronomy\))
.

I believe astronomers use the Hill sphere to define that bit of technicality.
Quoting
[https://en.wikipedia.org/wiki/Hill_sphere](https://en.wikipedia.org/wiki/Hill_sphere)
:

> In the Earth example, the Earth (5.97E24 kg) orbits the Sun (1.99E30 kg) at
> a distance of 149.6 million km. The Hill sphere for Earth thus extends out
> to about 1.5 million km (0.01 AU). The Moon's orbit, at a distance of 0.384
> million km from Earth, is comfortably within the gravitational sphere of
> influence ... All stable satellites of the Earth (those within the Earth's
> Hill sphere) must have an orbital period shorter than seven months.

This asteroid is outside of Earth's Hill sphere so is in orbit around the Sun.

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tantalor
Thanks!

Article mentions the asteroid's distsance is ~38x the moon's; that's ~15M km,
or 10x the Hill Sphere.

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dalke
The other way is to work backwards. The orbital period is 1 year, which is
longer than seven months, so it can't be in the Hill Sphere.

