Could the earth have another satellite?

I was wonder if it would be possible, in theory, that a meteor or some other rock of good size come at Earth and instead of crashing into it, or burning up in the atmosphere, become captured by Earth’s gravity and become another satellite of Earth.

I guess another satellite the size of the moon would be out of the question, but a lot of planets have “moons” of all different sized. Also would there be an upper limit to how big the meter (or whatever) would have to be captured?

I think it’s unlikely that another natural satellite could start orbitting the Earth. IIRC, the current theory is that all the natural satellites going about planets have been doing so since the planets and moons formed as the solar system coalesced.

Any new moon-sized object coming towards Earth would be going too fast to be captured by Earth’s gravity. We might significantly change its course (and it ours).

Yes, it is possible. There are a lot of reasons that it is rare, but rare over a billion years or so means infrequent. Ten billion years and it becomes unremarkable.

Most of the moons in the solar system are not captured external objects. Well, probably not. If a moon is in the plane perpendicular to the axis of rotation of the planet, it is fairly likely that it formed with the planet. Most are.

The capture of a moon requires that the new moon enter the orbit of Earth at a velocity low enough that its interaction with the earth and the moon result in an orbit that isn’t hyperbolic (escape). At the same time it must be moving fast enough not to decay from atmospheric interaction, or be moved into an unstable orbit by the moon. It ends up being a pretty tough billiard shot. Among the billions of objects in the relative nearby orbits, some number of them have orbited the earth for a while. But, unless they are in certain specific relationships with the Earth and Moon, their orbits become unstable, and the object either leaves Earth orbit, or hits one or the other.


3753 Cruithne

Asteroid 2002 AA29


There’ve been a couple more objects that, like J002E3 , came into earth’s orbit through the L1 Lagrange point. Sooner or later, one of them will be a rock, rather than a piece of old hardware.

That’s not true of the giant planets. Saturn’s moon Phoebe is thought to be a captured object, as are 92 other moons orbiting giant planets.

It’s easier for giant planets to capture satellites than it is for terrestrial planets. To capture a satellite, a planet needs to dissipate some of the satellite’s excess orbital energy. Giants can do this through movements in their atmospheres- since terrestrial planets don’t have nearly as much in the way of atmosphere as giants, it’s harder for them.

Not necessarily true; several moons of the gas giants are thought to have been captured. However, with a large moon the likelyhood of capture (which involves, as Triskadecamus describes, a very delicate set of parameters) is very low. There is essentially no way the Earth could naturally capture something the size of Luna, for instance. (Nor could an object the size of the Moon form “naturally” around the Earth, leading to the widely-accepted theory that the Moon was formed from an impact of a large body on the primordial Earth.)

We could easily capture a slow moving Near Earth Asteroid, however, though it would take a long time for such an orbit to stabilize into a regular elliptical orbit. This would have to occur due to drag of the upper atmosphere on close approach to the Earth, or some kind of momentum-killing collision with another object, or something like; otherwise it’ll remain in a chaotic orbit and eventually be cast back out.

None of these, strictly speaking, are satellites of the Earth, but rather objects in Solar orbit that are irregularly coupled to the Earth.


J002E3 qualified for a while. Here’s an animation of its orbit around the Earth.

I say we mine materials from Luna and build another Earth orbiting satellite at a Lagrange point. We could call it “The Death Star”.

Since there is no bottom-line size limitiation to a “satelite” I would not be shocked if there is some softball sized chunk already in orbit. Certinaly a Phobos or Deismos sized object could be fitted in, but having it be in a stbale orbit what wth Luna and all coul dbe a problem.

Such as 6R10DB9 which is pretty definitely a natural object and not a piece of space junk. It just recently left Earth’s orbit, but will probably return. See also the Sky & Telescope article on it.

So I didn’t think it was possible to “leave” an orbit unless as a result of an exerted force of some kind. (By which I mean a collision, or use of a rocket, etc.)

What’s an orbit then?


Any path that an object follows as a result of gravitational forces involving one or more other objects is an orbit. Some orbits are very regular, such as Earth and moon orbiting around the sun. The combined system is dominated by the gravitation of the Sun, and the minor changes in the orbit caused by other objects are very slow to change the period or shape of the orbit of the system.

In the case of the Earth, and Moon, the Earth dominates, but by a much smaller factor than the Sun. A third object would enter the system, and could end up in an unstable orbit that repeatedly caused the new, and smaller object to move outside of the the influence of the Earth, and it would enter an orbit around the Sun. The new orbit might well include a path that passed within the near proximity of the Earth or Moon again after a number of years. At that time, it might be “recaptured” into another, and probably very dissimilar orbit around the Earth/Moon. This new orbit would almost certainly be unstable as well.

There are a very small number of orbits around two objects (or actually, the center of gravity of the two objects) that are stable. There are a nearly infinite number of orbits that are less stable. Many of these are hyperbolic, rather than elliptical. Hyperbolic orbits result in “escape” from the system. In the case of Jupiter, there is a significant chance that it will accelerate an orbiting small object so much that it will leave the solar system. That new orbit might first send the object very close to the Sun, and perhaps cause it to interact with the many other bodies in the Solar system. It is believed that Jupiter’s mass was very important in the formation of the Solar system, sweeping up, and ejecting many smaller objects, over billions of years.


Then, strictly speaking, doesn’t this mean it’s impossible to leave orbit? For example, doesn’t the Earth exert a (very small but still) non-zero gravitational force on everything, no matter how far away? Within the solar system, isn’t this effect even measurable?


by Bill Higgins and Barry Gehm c. 1978, used with malice:

Of course, it isn’t zero-gee, it’s freefall: You are still intimately affected by the Earth, the Sun, and the Moon, otherwise you would go sailing in a straight line under your own momentum. I don’t know if there’s anywhere in the Universe you can go to find zero-gee.

This illustrates an important point: While a two-body system (such as the Earth and the Moon) can capture outside objects into orbit, it’s an unstable orbit. Any object which can be captured in such a way can also be ejected in the same way. If you finesse it just right, it might stay in that unstable orbit for a very long time, but it’s still unstable. The only way to capture something into a stable orbit is to also capture yet another object meanwhile, and then eject that other object at just coincidentally the right time.

Zero-gee and freefall are exactly synonymous. An object in freefall is sailing in a straight line under its own momentum. It’s just that the presence of those other bodies causes that straight line to be warped enough that its spatial component is closed. That said, I still prefer the term “freefall” (when scansion allows it), since it generally leads to less confusion.

Sorry, I missed this, earlier.

Yeah. You are currently orbiting the center of mass of the entire universe, and the resultant orbit might be closed, or hyperbolic. That’s a big deal point of contention in Astrophysics.

Now, lets talk about the measurable part.

You really can’t measure the effect of planetary sized objects upon stuff further away than say, one astronomical unit, without multiple years of observations, or extremely accurate equipment, or both. Once you measure them, you get authoritative answers that predict near earth objects that will fly by either in 2018, or perhaps 2040, within a million kilometers of the Earth. Or, maybe not.

It’s called the “n body problem” and it is very recent indeed that we have been able to tackle even fairly small values of n when doing the calculation on fairly large computers.

The practical effect is that we don’t pay much attention to the gravitational effects of Uranus, except when we are measuring things out beyond Jupiter, and even then, only when they are in the same general direction.

The convention is to say you orbit something when your motion is not a hyperbola with respect to the center of gravity between you, and the object. The closed ellipse is an orbit, a hyperbola is a fly by. Both are examples of gravitational interactions. From a layman’s point of vew, if it ain’t comin’ back, it ain’t orbiting.


Is there such thing as a center of mass for the universe? I’m fairly sure there isn’t.

Possibly you are talking about wether the universe itself is open or closed, but that would strain the meaning of the word orbit to apply it to that. And last I checked, the cosmologists had concluded that the universe has exactly enough mass to make it flat (neither hyperbolic nor closed).

Really? Wow. I am a fair bit short of sure of pretty much anything that subtle.

So, cosmologists have determined the nature of the orbit around something you are sure doesn’t exist. I am in awe.


Now, see, this is what happens when you try to discuss physics in a clumsy, imprecise language like English (or French, or Korean, or Swahili, or Esparanto). The math all works out just fine; it’s just in trying to translate that math into English that the problems arise.