# What is a Quasi-satellite

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#### Danzi

##### Guest
Ok, so today in calss, out teacher said 'Earth has 2 moons,' now when she said this i was qucik to challenge her, and se told me about how they discovered a second much smaller moon and it was called, Cruithne.

I quickly got in from school and used hjandy old google search. When i found about this object, it said it was a Quasi-satellite, and NOT A MOON. Now, the not a moon part i was expecting, but i what is a Quasi-satellite?

Thats my question for all you astronomers, what is a Quasi-satellite? Why was this Cruithne once called a second moon?

Thank you

p.s - I am going to tell the teacher this, but i doubt she will believe me, as she didn't even believe me when i informed her that pluto is no longer a planet.

oh, and please can you try and answer as simple as possible, as i am only in my first year of GCSE's, and as much as i love astronomy and physics, some of the words confuse me a lot.

M

#### MeteorWayne

##### Guest
Hmmm, well it's hard to explain simply, but I'll give it a try.

Lets' start with the moon; both the moon and the earth orbit each other around the center of mass of the total earth-moon system. If you look at it from above, you will see both making circles around a common point. If you look at the solar system from above, you would see the earth-moon system orbiting the sun together

An objects such as Cruinthne is really in orbit independantly around the sun, but because of it's proximity and gravitational interactions with the earth never gets very far away. It doesn't orbit the earth (circle around it) but rather travels around the sun in the same direction, always on one side (ahead of the earth).
It is in what is called a resonant orbit.

In a way, pluto is like that. The dwarf planet has a resonant orbit with Neptune; for every 3 times Neptune goes around the sun, pluto makes exactly 2 orbits, so every 3 Neptune and 2 pluto orbits they reach the same relationship with each other.

I don't know if that makes sense to you, if it's not clear enough, let me now and I'll try and explain it better.

Wayne

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#### Danzi

##### Guest
ahh, thank you Wayne, that does make sense and i leave this thread learnign somthing new about space

Thank you again

M

#### MeteorWayne

##### Guest
Rather than a "quasi-moon", a better term would be the title of the article in Nature:
"An asteroidal companion to Earth"

MW

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#### Saiph

##### Guest
If you have problems visualizing Cruithne's orbit wikipedia has a good page with animations on it: http://en.wikipedia.org/wiki/3753_Cruithne

The reason it's a 'quasi-satellite' is that it has, from earth's perspective, a horse-shoe shaped orbit, not a more familiar circular one.

here's the two simple reasons cruithne is not really a moon: a) it's size, at only 5 km across I wouldn't call that a moon, a satellite yes, but not a moon. The difference? It's akin to a square is always a rectangle, but a rectangle is not always a square. A moon is a satellite, but not all satellites are moons.

b) Here's the big reason, Cruithne is almost always outside of earth's "hill sphere" the volume of space where earth's gravity holds dominance.

I.e. calling it a moon is not technically wrong, but is a pretty strong exaggeration. Still an interesting object though.

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#### bdewoody

##### Guest
Saiph":2z4q9n2p said:
If you have problems visualizing Cruithne's orbit wikipedia has a good page with animations on it: http://en.wikipedia.org/wiki/3753_Cruithne

The reason it's a 'quasi-satellite' is that it has, from earth's perspective, a horse-shoe shaped orbit, not a more familiar circular one.

here's the two simple reasons cruithne is not really a moon: a) it's size, at only 5 km across I wouldn't call that a moon, a satellite yes, but not a moon. The difference? It's akin to a square is always a rectangle, but a rectangle is not always a square. A moon is a satellite, but not all satellites are moons.

b) Here's the big reason, Cruithne is almost always outside of earth's "hill sphere" the volume of space where earth's gravity holds dominance.

I.e. calling it a moon is not technically wrong, but is a pretty strong exaggeration. Still an interesting object though.

So by your definition are Mars' satellites moons?

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#### silylene

##### Guest
bdewoody":3nnx850s said:
So by your definition are Mars' satellites moons?

Phobos and Diemos are moons.

Your question reminded me of some trivia I had long buried, which made me have to look it up. The Soviet's old spacecraft, Phobos 2, was for some period of time in an orbit that made it a quasi-satellite of Phobos, one of Mars's moons.

By the way earth has 4 quasi-satellites: 3753 Cruithne, 2002 AA29, 2003 YN107, and 2004 GU9. Venus has one.

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