A question that has bugged me for a long time is how planets capture asteroids, etc into an orbit. My understanding of orbital dynamics is that a body approaching a planet would need to be "braked" in order to be captured into orbit. In the same manner that our space probes use their rockets to slow them down enough or they would shoot past.

So for moons like Triton, Deimos, and Phobos (as well as the small, distant moons of Jupiter/Saturn) how were they captured? What provided the braking?

Gravity. In specific, gravity from a third (and even fourth or fifth) bodies. Around Mars, I'd guess that Jupiter generated a gravitational resonance that braked Diemos and Phobos enough to be captured. Tidal influences can then work to circularize the orbits.

I find it fascinating that, according to one current theory, Uranus and Neptune may have formed *between* Jupiter and Saturn. I can just image the gravitational resonances that must have disturbed the entire Solar System when those two giants were flung out into the outer reaches...

-the other Doug

Things don't always need to be braked to be captured - as I understand it if the velocity relative to the target is less than the escape velocity at that point then it will be captured. The reason spacecraft need to be braked is because of the fast transfer orbits that need to be chosen to get them to their targets in a reasonable length of time\fuel efficient manner.

A lot would depend on relative velocity. If an object were in a similar orbit moving at a similar speed, not a lot of breaking would be needed. Atmospheric drag also can play in, but such objects probably would decay quickly. It is also possible, in the case of Triton, that it was a combination of a low relative velocity encounter with a collision with some primordial Neptunian moon.

Those moons are the leftovers from a Kardashev Type 2 Civilization astroconstruction project that had to be abandoned when the Galactic Empire switched ruling political parties.

Apparently they didn't count all the votes on Floridon 12 and Ohyox 3 until it was too late to reverse the decision.

LJK:

According to *my* issue of the Guide (42nd Edition, Megadodo Publishing House, Ursa Major Beta) it was actually because of a mix-up between Galactic Imperial Quarts and the exchange rate of the Trigellian Pu. The Management Consultants brought in afterwards made sure that poor old Slartibartfast got the blame for the whole affair, and then the mice naturally promoted him to deal with some project called Dirt, or Mud, or something like that. Compost, was it?

Obviously, no such mistake could happen here!

Bob Shaw

I wonder if it's friendly?

-the other Doug

We use gravity assist for probes like Voyager, Cassini etc to transfer kinetic energy from the planets orbital motion to that of the craft. When we want to put something like Cassini into orbit we can do the reverse as well i.e. transfer some of the kinetic energy from the craft to the planet to slow it down by approaching from a different angle and then add some rocket full to complete the task. The maximum energy removal happen when a object approaches from a direction that puts it into a retrograde orbit ( as I understand it) and that is why many captured outer moon are like this.

The whole process is difficult to explain easily. Velocity is a vector and therefore defined by magnitude and direction. The planets gravity does not change the speed of the probe but by changing its direction it can change its velocity relative to the sun and therefore can add or remove orbital energy.

If the relative velocity of the planet and the potential moon is low enough, third-body gravitational effects can jockey the moon into orbit around the planet -- that is, the fact that both objects are orbiting the Sun at about the same distance out can by itself allow the moon to approach the planet slowly enough to be captured into orbit by it when it reaches the planet's vicinity. But for this to happen, their orbits and thus their velocities must be very close to begin with -- which was always one problem with the "capture" hypothesis for the origin of Earth's own Moon.

Instead, most captured moons are captured by one of two mechanisms: either they brushed through the cloud of gas that was still around a forming planet and were slowed down by friction, or they collided with a hunk of debris from the large cloud of solid debris that was still orbiting (or being pulled into) the planet while it was forming and got braked into orbit that way. There is a debate as to which of these mechanisms was more important in the early Solar System, and whether we'll ever be able to estimate whether a particular captured moon was captured by one or the other. (This is also one reason for the continuing dispute over the origin of Phobos and Deimos: it's hard to conceive of either mechanism being strong enough around the forming Mars for their capture to be likely.)

Bruce:

The recent classic instance of a more-or-less (Solar) co-orbital object being captured by a planet must be the Apollo 12 S-IVB, but without some actual change in velocity such captures will be quite 'loose' - like the outermost semi-moons of the giant planets, which may not know quite whether they are asteroids, Trojans or moons. Phobos and Deimos, however, seem to me to defy all logic, though the very fact there's two of them surely suggests some common factors...

Do you have any good links regarding capture mechanisms?

Bob Shaw

Though Phobos and Deimos look like planetoids and have therefore been considered captured ones since at least Mariner 9, what about the possibility that they are the remains of a larger moon that was formed with Mars long ago? Perhaps another moon smashed into it and Phobos and Deimos are what's left of the cosmic collision.

The following is _very_ vaguely remembered information from a term project I wrote up about seven years ago -- my apologies if any of it is obsolete or just plain wrong:

In addition to three-body effects, atmospheric friction and collisions, there's also internal tidal dissipation to consider. If a fairly large "proto-satellite" makes a close enough pass by a larger planet, the side of the proto-satellite that is closest to the planet will be significantly closer to the planet's centre of mass than the side farthest away. The "near side" will then want to zip past at a different velocity than the "far side". This is what leads to deformation of the proto-satellite, and it generates *a lot* of frictional heat (assuming the proto-satellite doesn't rip apart entirely). That heat energy has to come from somewhere and it ends up coming from the kinetic energy of the proto-satellite, relative to the planet. If enough is lost, the beast can be slowed down to the point where capture can occur.

Part of this vague recollection of mine is that this model was at one time proposed for Triton's capture around Neptune. Right after capture, the initial planet-centred orbit inevitably ends up being _very_ elliptical, so this model works best when you're far, far away from the Sun's greedy grasping fingers. Once the moon is securely captured, further tidal dissipation can round-out the orbit over time. This last point is of course crucial in Triton's case since its orbit is nearly circular. I believe that interaction with Neptune's atmosphere was also considered in this model, but, as I said, it's been a while.

As regards Luna: There's a book called "Origin of the Moon" from about 1986 in which the capture hypothesis of the origin of the Earth's Moon is described in some detail. However this book has been superseded by one or two more recent volumes. All of them are conference proceedings.

Gravity is certainly the answer here...
The large planet Jupiter (gas giant in some way a failed star) acts as a vacuum cleaner and sucks in matter that comes to close (remember 1994... Shoemaeker-Levy comet).
Pioneer 10 had a dust particle counter and when it closed in towards Jupiter, the particle count went X100

Phobos and Deimos still make no sense, though!

Bob Shaw

I've always thought the main question regarding Phobos and Deimos is: What is their origin? The two main models are (1) the two moons are captured asteroids or (2) they co-accreted with Mars. Not surprisingly, there is evidence to support both. While both models have attractive components, however, they also have some rather glaring holes.

For a more rigorous treatment of the subject, I would refer the reader to Joe Burns's chapter in the classic reference work Mars [H.H. Kieffer et al., Eds. (Univ. of Arizona Press, Tucson, AZ, 1992)], which, while a little of out date being published in 1992, is still de rigueur reading on anything related to Mars.

At first glance, the "captured asteroids" model seems to be the more attractive of the two. The two moons, for all intents and purposes, do "look" like asteroids. And the close proximity of the asteroidal main belt offers a convenient source. That said, though, even first order observations supporting this view are somewhat puzzling. For example, the spectra of the leading hemisphere of Phobos (i.e., the Stickney-dominated region) best fit the curves for T-class asteroids, while Phobos' trailing hemisphere (and, incidentally, Deimos' leading hemisphere) match spectra from D-class asteroids.

Even assuming these spectral observations are truly indicative of captured asteroids, as Burns points out there are problems in the capture mechanism. With aerocapture, presumably by the primordial Martian nebula or proto-Mars atmosphere, the problem is not so much with its mechanics, which, though problematical, can be made to work, but rather with its timing. Moreover, capture scenarios should, ideally, show a good fit to the observables.

For example, tidal evolution theory vis-à-vis Phobos's secular acceleration needs to account for the timing of the Sun's putative T-Tauri stage and associated stage solar wind, which narrows the window for aerocapture and prevention of rapid orbital decay. In short, if the T-Tauri stage came first, then the captures most probably would not have happened (i.e., no extended atmosphere). If the T-Tauri stage came afterwards, then the moons should have decayed a long, long time ago. This is a true puzzle.

Looking for a way out, Burns modelled the particular case of a planetesimal that was captured by the proto-Mars nebula and subsequently evolved down to areosynchronous orbit. At this position, orbital decay would virtually cease due to the low relative velocities between the planetesimal and the Martian nebula. Subsequently, the planetesimal was shattered by another, resulting in two or more fragments that resulted in Phobos ending up below areosynchronous orbit and Deimos above. The former would undergo secular acceleration (i.e., orbital decay), which has been documented and is well known. The latter, Deimos, would undergo relatively little orbital evolution, which is consistent with the observables. Indeed, given the nature of orbital dynamics, it is possible to integrate Phobos' orbital history backwards in time to infer that the moon, even under an accretionary origin model, originated at ~5.7 Martian radii (Rm). This, of course, assumes that its orbit has always been roughly circular and conveniently ignores chaotic evolution, resonances, etc.

Of course, one will note that the above model relies on a series of rather unique events to account for what we see today. Mainly, such a model contains rather precise timing, and I'm not sure it does not avoid the dreaded "Tooth Fairy" hurdles (i.e., one is allowed to invoke "miraculous" events only once per model). That said, it still does not mean it did not happen.

It's obvious that highly detailed in situ and/or sample return studies are needed to progress further, else the "modellers" will continue to dominate the literature. To approach a resolution, especially on the co-accretionary model, one needs a dedicated mission(s). Hopefully, a sample return concept such as Gulliver: Deimos Sample Return Mission or something similar to the Aladdin mission concept (for details click here and here), which was proposed a couple of times for the Discovery Program, gets approved. The Russians have also made noises with their PHOBOS-GRUNT mission concept but, as I mentioned elsewhere, I'll believe in this mission when I see it.

Alex:

Absolutely!

Bob Shaw

Source of the information, click here

About a theory of Mars' moons: Phobos and Deimos. They were captured by Mars soon after the desintegration of a ancient planet: Astra when it was approaching to hit on Mars.

The Presence of Two Tiny Satellites

Mars has two small satellites named Deimos and Phobos. Both are pock-marked or pitted as if they have encountered many smaller asteroidal collisions. Deimos, the outer one, is the smaller of the two. Its dimensions in miles are about 6 x 7 1/2 x 10. Phobos, the inner one, is about 12 x 14 x 17.

Not only Mars has captured two asteroid-like satellites. Jupiter has directly captured another eight.[12] And in addition, Jupiter's immense gravity has so influenced the asteroids as to have gathered two clusters of them into parallel orbits. These are named the "Trojan" asteroids after mythology relating to Mars coming from Homer and the Trojan War, around 864 B.C.

The Trojans are about a dozen asteroids which have assumed two strange orbits in our solar system. One group is at 2/3 and the other is at 3/4 of the orbital period of Jupiter. For each group the positions of the Trojans in their orbits always make an equilateral triangle with the Sun and Jupiter being the other two points of the triangle.

How could little Mars capture two tiny asteroids when Jupiter, with its massive gravity, has managed to collect only six or eight? Jupiter's mass is about 3000 times greater than Mars' paltry mass and gravitational attraction. According to Richards: [13]

Phobos and Deimos are the same size as many of the asteroids. On the face of it, such a hypothesis [evolutionary capture by chance of the two trabants] sounds quite possible, but upon closer examination it does not stand up so well. A planet only one-tenth as massive as Earth could not easily effect a capture. Suppose that eight small satellites of Jupiter are captured asteroids.

Then Mars, with a mass only 1/2950 that of Jupiter, has done extraordinarily well to have been able to latch onto two such bodies. The asteroids revolve in orbits that have no particular relationship to the orbit of Mars.

Suppose one of the satellites is a captured asteroid, captured in such a way that it revolves in a circular orbit in the plane of the planet's equator.... It does seem incredible that Mars could have effected two such very special captures. Further speculation along this line is useless.

Richards has correctly pointed out that tiny Mars could hardly grab a small asteroid moving about 6000 miles per hour on the fly. Any asteroid buzzing Mars would be well past it in the timespan of an hour or two and beyond any hope of capture.

Richards, however, has not considered the possibility that Mars in ancient times once had a different orbit, wherein it caused the fragmentation of Astra. Richards has suggested that Deimos and Phobos were exploded backwards from Astra's trajectory. This had a tendency to cancel or neutralize the velocities of these two trabants or to brake them temporarily. Within our model the capture of these two bodies is not only possible but also the most probable explanation, effecting a capture along with some minor asteroidal debris.

Deimos and Phobos' pitlets, quite numerous on such small trabants, give testimony to the possibility of other, smaller debris also gathering in rings around Mars.[14] And perhaps, if Figure 1 validly portrays Mars' ancient orbit, later interactions with the Earth cleared away all of that debris except Deimos and Phobos.

Retrograde Direction of Mars' Moons

Figure 6 illustrates the capture of Deimos and Phobos. In order for these two bodies to be captured retrogradely, the geometry requires that Astra approach Mars from the sunward side. This suggests Astra was receding toward its aphelion whereas Mars was just past aphelion and beginning its long 720-day journey toward perihelion.

All planets in the solar system revolve directly, which is counter-clockwise as viewed from Polaris. Almost all of the satellites also revolve in direct motion. Only four of the small asteroid-like moons of Jupiter, the two moons of Mars, and one moon of Neptune revolve in retrograde motion.[15]

Perihelions of the Largest Asteroids

If this fragmentation occurred around 205,000,000 or 210,000,000 miles from the Sun, then how did the asteroids come to arrive at regions considerably more distant? For instance, among the ten largest asteroids, the average perihelion distance is 240,000,000 miles.

There are two factors to consider in answering this question. First, Astra was apparently moving in its orbit away from the Sun when it was at the fatal scene. This motion was imparted to each of the fragments plus a variety of vectors added from the force of explosion. The exploding force would accelerate some fragments, retard others, and disperse yet others in sideward directions.

But also, if fragments nearly hit Mars—yet just missed, Mars' gravity would have acted upon them precisely as the massive gravities of Jupiter and Saturn acted on some recent spacecraft, which was to act as a booster. Thus three factors (at least) need to be examined in the trajectories of the current asteroids. These are:

1. The motion of Astra.
2. The direction, and level, of energy imparted by the explosion.
3. The booster effect of the asteroids using Mars as a turnpoint for a new orbit .
4. Finally, the subsequent influences of Jupiter and Saturn, which necessarily come into effect.

These seem to be the motions or forces which have brought the variety of asteroids into their current orbits.

Conclusion

Some thirteen levels of evidence have been presented in support of this concept of Mars and the asteroids. Perhaps some day a young celestial mechanic will program the asteroid orbits on a computer and measure and adjust for the perturbing effects of Jupiter and Saturn. In such a scenario many asteroids could be traced back to one location some 205 to 210 million miles from the Sun. And they could be trailed back to one certain moment of time.

We suspect this fragmentation was a recent event in the solar system, and will be considered "ancient" only in terms of thousands not millions or billions of years .

Further, we suspect that such a location will be posited in the general region of 270 degrees from the Earth's autumnal node. This is the region directly overhead at midnight in mid- or late June.

An analysis of the physical geography of both hemispheres of Mars has been made. Both hemispheres have shown evidence for such massive fragmentation.

Rodolfo

Wow. The D-class asteroids get around. Or rather the material with the distinctive signature does. From the outer asteroid belt, to the Jupiter Trojans and outer satellite horde of Jupiter, and the Cassini Regio on Iapetus (and I am willing to bet the dark crater floor material of Hyperion) D- type spectrums are very well represented.

I had wondered how close to the sun the 'material' would be stable. Mars distance is quite a jump from the outer belt. Would the D-type spectra be more of a tracer for a 'ubiquitious' material that can form across a wide range of solar distances rather than for a common source (like an ancient odd asteroid that was disrupted) of it?

never mind