Earth's Second Moon

sbryant writes "According to the Daily Telegraph (free login required): the earth has a second moon called 'Cruithne', which was discovered by a British team. The moon was previously thought to be an asteroid, but the experts now tell us that Cruithne, which is 3 miles across (4.8km), is in fact a Trojan asteroid, and has an eccentric horseshoe orbit around Earth which takes 770 years to complete. "

Comments

Lilith and Other Discoveries of Earth's 2nd Moon

[Anonymous Coward]

Many of you interested in anstronomy, or just the planets of the Solar System in general, might find this information very interesting. It's an account of other people who claimed to have discovered that Earth had a second moon.

I'm not trying to discredit the British team's discovery in any way, but it's still a very interesting read.

The main page of the site (called Nine Planets) is here.

your horoscope

[The Queen]

Born under the wobbling 'moon' Cruithne, you resonate with the sign of the Waffle. Your life is one of inconsistency, you bounce from job to job, relationship to relationship, never quite making that final connection. (On the other hand, you avoid conflict at all cost.) People see you as something of an enigma, following a different path than your peers. Since the conditions at the time of your birth will not come around again for another 700 years or so, you will never find contentment in this life. Hope that you are born again under a planet with a shorter orbit.

The Divine Creatrix in a Mortal Shell that stays Crunchy in Milk

Where is Cruithne Now?

[szyzyg]

Cruithne is a named Near Earth Asteroid which has been known about for some years. Asteroids only get names and numbers after their orbit is well known - which means a good few solar orbits.

I've gone and higlighted it on my NEO map so you can all see where it is right now. (look near Venus). I presume this is the same object they're talking about.

It's not actually a moon of the earth, at least I wouldn't consider it a moon. Trojan objects aren't bound to their objects in the same way that moons are. Certainly the Earth's influence acts to stabilise the orbit, but if that's teh only criteria for an object being a moon then perhaps we should consider Pluto a Moon of Neptune since pluto is help in the 3:2 resonance with Neptune.

Oddly enough - the 1:1 resonance of trojan objects with respect to the Earth make it almost dynamically impossible for the object to ever become a true satellite of the Earth.

a moon is...

[ChristTrekker]

I remember reading _Asimov on Astronomy_ many times as a kid. Here's what I remember from one of his articles:

• A true satellite's "primary" gravitational influence is it's primary, naturally enough. (What's that? Well Terra is considered Luna's primary. Sol is Terra's primary. It's the body you revolve around.) Asimov did the calculations for most of the then-known satellites in the solar system. The outermost satellites of Jupiter and Saturn are not "true" satellites in this sense. Sol actually has more influence on them, but the primary has enough effect to keep them close by. They are probably captured bodies, and they may drift away in a couple centuries.
• Interestingly enough, Luna is not a true satellite of ours. Our moon isn't really a moon! We're more like a double planet.
• Double planets are coorbital. They each have their own orbit around the sun. The orbits are nearly identical, but they are convex. If the secondary body's orbit w.r.t. Sol is ever concave, then it is a satellite of the planet, i.e., the planet has more influence than the sun does. Terra and Luna do influence the orbit of each other gravitationally away from an ellipse, but not enough to cause a "dimple" in the shape.

Anyway, as I read it, this asteroid is not a true moon in the sense of being a satellite of our planet. It still revolves around the sun. However, it is in a gravitational relationship with Terra, as our Luna is. In that sense I suppose it is "a moon".

I think in that same book, maybe even that article, Asimov discussed the Trojan satellites. At that point I don't think anyone had discovered any for earth, but dust clouds were detected in the L4 and L5 positions.

Astronomy is only a neglected hobby of mine, so I may not be totally correct. (I almost got a minor in it, but that's been years ago.)

"Trojan" satellites, addendum

[Robert Link]

The name "Trojan" was picked because the first several asteroids found trapped in the Jupiter-Sun Lagrange points were named after heroes from Homer's Iliad. For that reason I would argue that "Trojan" should apply only to objects in the Jupiter-Sun Lagrange points, although the analogy to other Lagrange point objects is obviously pretty strong.

Note also that there is a difference between "equilibrium" and "stability". Equilibrium just means that there exists a static solution to the equations of motion (albeit static in a rotating frame of reference in this case). Stable means that the static solution, if perturbed, will return to the equilibrium instead of drifting off into some dynamic orbit. The L1-L3 points are stable to perturbations in one direction (the tangential, if I recall), but they are unstable to perturbations in the other direction. Consequently, one doesn't expect to find long-lived orbits at these locations. The L4 and L5 equilibria are stable against all perturbations, so one might expect to find objects in long lived orbits at these locations, like the Trojan asteroids, for instance.

The original poster's confusion arose because the earth has two sets of Lagrange points associated with it. One comes from the earth-moon system (in which the earth is M1 and the moon is M2), and the other comes from the earth-sun system (M1==sun, M2==earth). An object at L4 or L5 in the earth-moon system would appear to orbit the earth in synch with the moon, while an object at an earth-sun lagrange point would appear to orbit the sun in synch with the earth. So, it's a little far-fetched to call an object at the earth-sun Lagrange points a "second moon", although such an object is in some sense more deeply associated with the earth than an object in some random solar orbit.

Finally, one should remember that the Lagrange points are solutions of the restricted 3-body problem which presumes that there are only three bodies in the system. Obviously, that isn't the case in our solar system. For instance, I've seen people argue that the earth-moon L4 and L5 points aren't really stable because of the sun's influence. Similarly, the lagrange points between the sun and most of the outer planets are thought to be disrupted by Jupiter's gravity. Consequently, I wouldn't be surprised if Venus' gravity had a tendency to destabilize the earth-sun Lagrange points, resulting in objects trapped there eventually escaping into regular solar orbits.

-r