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. "

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Lilith and Other Discoveries of Earth's 2nd Moon

Many of you interested in anstronomy, or just the planets of the Solar System in general, might find this information [arizona.edu] 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 [arizona.edu].

how does this affect my Zodiac?

as a devoted follower of the Zodiac, and how it affects me (i am totally aware of how my sign interacts with the planets to prevent me from obtaining any sort of good job or even a menial one) i am concerned... will this moon exert any pull over me as i try to appease the gods and the signs? will this alter my chances of attaining zen, although, being a devotee of astrology, i don't follow zen? should i worry that i was born under this moon and will never live long enough to see it return? this is too much for someone with no sense of self-determination!!!!!

Time

If memory serves me correctly, Crithne is an intricate part of the Sci-Fi novel, Time by Stephen Baxter. It mentions the horseshoe orbit and some other interesting things known about it. I didn't realise when I was reading it that the thing was for real. Very cool. You should check the book out if you're a Baxter fan. It's very good IMHO.

"Sir, I'd stake my reputation on it."
"Kryten, you haven't got a reputation."

More information on the asteroid

Here is a link with some more [yorku.ca] info on the asteroid. Some cool animation as well showing the orbit. Gino

...by the pricking of my thumbs,

Re:Old news

Exactly. Before anyone goes around talking about things they know nothing about (oops, too late), look at this [yorku.ca].

Just for informational purposes, the asteroid was discovered in 1986, and the paper on its orbit was published in 1997.

At the end of time

For it was fortold, that when the two moons shall be seen together that the oceans would rise, and the sun would never come up again. So it is written.

Alternatively if we never see the two moons again might be for the same reason we never see Peter Parker and Spiderman or Bruce and Batman at the same time. When we are only "seeing" the dark side of the moon maybe it's getting into a slightly smaller dress and posing as the other one.

Wow, this is cool!

Hey!

You get plenty of kids asking for the earth for their birthdays, but on mine I get a moon! Not bad for a start, is it?

:-)

Greg

What's a moon, anyway?

Not all of us are astrogeeks here. When does an orbiting body become a moon rather than as asteroid?

~~~~~~~~~
auntfloyd

other moons also seen and reported last night

At approximately 12:50 last night, some sorority girl somwhere screamed in disgust as a frat butt loomed threateningly from a car window. Scientists say the new moon is stationary and appears to have a random orbit. If confronted by the moon, stay calm and move slowly as it may attack. Don't try to feed it as we do not need domesticated moons in the area. Thank you.

Scientists Report New Hope For Cheese

[Queen Mary and Westfield College, London]

Scientists ignited renewed hopes for Extra-Terrestrial Cheese today, as they reported discovery of a second "moon" orbiting the earth. The asteriod, named Cruithne, was first discovered in 1986, but has only recently been subjected to detailed analysis of its highly eccentric orbit and dairyon emissions.

The asteroid will remain in it's orbit for at least 5,000 years. "That's plenty of time for proper ageing," said one astronomer. "And with a diameter of 5 kilometers, this asteroid could supply the earth with Space Cheese for centuries."

Thanks to whoever posted the article "Hidden Agenda [slashdot.org]".

your horoscope

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

Re:unstable orbit

Cruithne truely is in a stable (ok it might decay in 5000 years according to the story) orbit but it isn't a moon. It is actually orbiting around one of the two stable points caused by the interaction of the gravity of the Earth and the Sun. If the Sun disappeared the Moon would continue in it's orbit of Earth, but Cruithne would not.

The stable points in question are called the familar L4 and L5 (L is for Lagrange the French mathematician who originally discovered them). If I recall correctly, L4 trails the Earth in it's orbit by 60 degrees while L5 precedes the Earth by the same angle.

I suspect that the reason Cruithne wasn't found before is because it probably has an exceptionally elongated orbit and is rather small. It is about the same distance away from Earth as the Sun is!

Incidentally, for those that love to speculate on these things, I don't think it would take much effort to push Cruithne into a more stable orbit. If Cruithne happens to be solid rock (rather than a pile of gravel weakly bound by gravity), then it would make an ideal base for a space station or shipyard.

A bit confusing to a layperson...

I took a look at a few of the links below that people have posted about the meteor/satellite. According to the article, it is a "Trojan Satellite," which according to this [arizona.edu] link, which seems to imply that Trojan Satellites share the same orbit as the moon.

However, This [yorku.ca] link says this asteroid actually shares an orbit with the Earth around the sun.

Now, I know that this object has a rather unusual "orbit" of the Earth, but what exactly qualifies it as a bonafied Satellite rather than just a near Earth meteor?

"You ever have that feeling where you're not sure if you're dreaming or awake?"

Where is Cruithne Now?

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 [arm.ac.uk] 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.

Re:What defines a big rock/asteroid vs a moon?

There's also good reason (IMO better reason) to suspect that Charon was originally part of Pluto, or that they were formed at the same time as the result of a collision.

• Pluto's low mass would make it difficult for it to capture an independent Charon.
• Charon has a very high mass relative to its parent body
• In fact, Charon and Pluto revolve about their relative center of mass about halfway between the center of Pluto and Pluto's surface

An interesting note (that would be I would be hard-pressed to claim as evidence supporting or countering my claim that Charon was not captured by Pluto) is that during Pluto's summer, its atmosphere encompasses Charon. It's so thin, though, that it doesn't affect Charon's orbit. During Pluto's autumn, the atmosphere "snows-out", and it won't have any atmosphere during winter. This is one of the reasons why launching a Pluto Fast Flyby was such a hot item a few years back -- they wanted to be able to study the atmosphere before it snowed-out. Now, it's not likely a probe will be able to do the flyby in time, but we can always hope & vote.
Christopher A. Bohn

Observational Information

Just in case anyone wanted to go look for this puppy.

It's a numbered asteroid (3753) so the orbit is well-determined. Right now it's within 60 degrees of the Sun so that's a little challenging (but Venus is always within 47, Mercury 17ish, so that gives some perspective).

According to the Minor Planet Center [harvard.edu] it's presently at magnitude 16.2 in Scutum (approx R.A. 18h 34m, Dec. -14 11', but of course that's changing fairly quickly), with a solar distance of 1.205 AU and a distance from Earth of 0.56 AU. It has a diameter of about 17.5 km.

Since it's in Scutum, that means it's also in the Milky Way so the chances of there being few 16th magnitude stars nearby is well, astronomical! :-)

You can get up to date positions, etc. from the website listed above. Please be gentle - it's not a terribly fast server, and a lot of dedicated amateurs/professionals rely on it being available!

a moon is...

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.)

Re:unstable orbit

The orbit is not unstable; according to the Cruithne FAQ [yorku.ca], it will be stable for at least 10,000 years. The FAQ does not say if anyone has projected Cruithne's orbit back in time, let alone what the results were. Given the chaotic nature of 3-body interactions, we probably cannot determine the origin of Cruithne from its orbit alone. We will have to determine its composition to get an idea of where it came from, and send a probe past it to get some idea of its bombardment history (and thus where in the solar system it's spent most of its past). From this we might... might... be able to make some good guesses as to how long it's been Earth's orbital partner.
--

"Trojan" satellites

A quick history & science lesson. The first "trojan" objects discovered where clusters of asteroids in Jupiter's orbit, but 60 degrees ahead and behind of it. The name "trojan" refers to mythology (since Troy was considered a myth at the time), but I don't recall the details of why that name was picked.

This cluster wasn't hard to explain - the "three body problem" can't be analytically solved for the general case, but it can be easily solved for cases where M1 >= M2 >> M3. This solution shows five points there the gravitational attraction of the two large bodies balance. IIRC all of these points are "stable," but objects can orbit those points for billions of years before friction with the solar wind, gravitational attraction from other objects, etc., cause the object to return to a normal orbit.

The five Lagrange points are named L1-L5. As I recall, if M1 >> M2 then

L1 = on M1-M2 line, opposite of M2 (e.g., "counter-earth")

L2 = on M1-M2 line, between M1 and M2, (e.g., the solar observer satellite)

L3 = on M1-M2 line, beyond M2

L4 = 60 degrees ahead of M2 on M2's orbit

L5 = 60 degrees behind M2 on M2's orbit

Since it's been twenty years since I thought about this, I might have L1-L3 permuted and L4-L5 reversed.

Suitability for a space station: nil

how exactly is it ideal for a space station?

Near as I can tell, it's probably not suitable at all.

1. The orbit is inclined 20 degrees to Earth's. Change-of-plane maneuvers are very costly in terms of propellant.
2. Being so close to the Sun, there is a smaller likelihood of Cruithne having deposits of the essentials for life-support: water, ammonia or methane ices. These would make it more attractive to set up shop there, because resupply costs would be drastically reduced.

As it is, we're probably better off looking for something like an extinct comet nucleus, regardless of its orbit, if we want to set up a space station far from Earth.
--

"Trojan" satellites, addendum

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

Observatory? To observe what?

But it could possibly make a sweet deep space observatory.

What could you see from there that you couldn't see from here?

• You still have the Zodiacal light fogging your pictures of nebulae; to get away from that, you need to go further from the Sun.
• You can't see much more of the Sun from a 20-degree inclined orbit. For that you really want a polar orbit. If you want to see solar flares and prominences from a different angle, you'd want to be at the Earth-Sun L4 or L5 point instead of on Cruithne, which varies its angular separation from the Earth-Sun line throughout the year.
• You can't see much more of the sky from Cruithne than you can from the Moon's L4 or L5 point, and it's a lot faster to get data back from the shorter distance.
• Last and most significant: what kind of observatory needs to be gotten away from Earth and put on a rock somewhere out in the middle of nowhere, so that you can still only see the half of the sky it doesn't block? Why not a free-flying probe?

An observatory which is best suited to Cruithne than somewhere else would have to have some very specialized requirements. I can't think of anything that has requirements remotely like that. Note: I am not an astronomer.
--

Space Race Revisited?

Does this mean that Russia can beat JFK's challenge of the early '60s to send men to the moon first?

You still have a chance, Russia! RUN! RUN!