You are an expert at summarizing long articles. Proceed to summarize the following text: 
neptune s triton is the only large planetary satellite to orbit retrograde relative to the planet s rotation . @xcite and later @xcite suggested that triton is a captured satellite , whose originally eccentric orbit was circularized due to tidal dissipation within triton . @xcite postulate that triton was captured from heliocentric orbit by a collision with a pre - existing satellite , and its initial high - eccentricity orbit then evolved due to tidal dissipation alone . they showed that the tidal evolution timescale is significantly shorter than the age of the solar system ( a few times @xmath2 years ) , even when the variations in triton s eccentricity with @xmath3-yr period are accounted for . however , @xcite noted that the criteria for non - disruptive capture are much stricter than @xcite calculated . if the original regular satellites of neptune and uranus were similar , a collision with the largest moons ( preferred due to their cross - sections ) would disrupt triton , with re - accretion on an orbit inclined to neptune s equator being impossible . @xcite suggested instead that triton was captured and its orbit was evolved by aerodynamic drag in neptune s primordial protosatellite nebula , and that after its orbit ewas circularized triton s gravity would be sufficient to clear a disk gap and thus halt further evolution . gas drag has been suspected as a capture mechanism for small and distant irregular satellites , but capture of triton would require unprecedented gas densities , requiring very close approaches to neptune .    @xcite propose a three - body capture scenario for triton . they suggest pre - capture triton may have been a member of a binary , whose disruption during a neptune encounter led to triton s capture and its companion s escape . their work addresses only the capture itself , leaving the physics of post - capture evolution unchanged . any capture mechanism , be it collision , gas drag or 3-body interaction , is likely to introduce triton on a large , highly eccentric orbit . distant satellite orbits are perturbed primarily by the sun , inducing precession of angular variables and oscillations in eccentricity and inclination , with minor semimajor axis variations . the two important periodic perturbations are those associated with @xmath3 of the planet s orbital period ( `` evection '' ) and @xmath3 of the precession period of the argument of pericenter @xmath4 ( `` kozai behavior '' ) . for early triton , evection was first noted by @xcite ; conservation of orbital angular momentum during tidal evoution implies initial pericenter of 7 neptune radii ( @xmath5 ; triton s present orbit has @xmath6 ) , and evection - induced oscillations in @xmath7 produced minimum pericenters of @xmath8 . however , @xcite ignored kozai oscillations in @xmath7 , which must have been present if triton s inclination @xmath9 at closest approach was same as now ( @xmath10 , measured with respect to neptune s equator ) . kozai osillations require that when @xmath11 or @xmath12 , both @xmath7 and @xmath13 are at a maximum ( @xmath7 and @xmath9 oscillate in phase for retrograde satellites ) . since almost all tidal dissipation occurs during this high-@xmath7 phase of the kozai cycle ( when the pericenter distance is smallest ) , this @xmath14 inclination will be conserved as the maximum one for the kozai cycle , while the minimum one ( coinciding with @xmath15 and @xmath16 ) will be affected by dissipation . using more complete treatment of tides , @xcite show that tides raised on triton could not have led to a significant inclination change as long as it was in cassini state 1 . trapping in the much less likely cassini state 2 would have caused a rapid increase in its inclination ( i.e. , closer to @xmath16 ) , rather than a decrease @xcite , so triton s inclination relative to the local laplace plane was always @xmath17 . this assumes orbital evolution slower than the nodal precession , preserving inclination relative to the local laplace plane , which would initially be close to neptune s orbital plane but would subsequently move closer to neptune s equator . this condition is true for the tidal model @xcite but not the gas - drag model @xcite . [ peric ] presents two illustrative short orbital evolutions . using initial conditions of @xmath18  km@xmath19 , and varying inclinations , we re - create two possible post - capture orbits for triton . both evection ( @xmath20 yrs ) and kozai oscillations ( @xmath21 yrs ) are clearly visible in the evolution of the inclined orbit , while the @xmath22 case , shows only evection - related oscillations ( whose amplitudes are in agreement with results from @xcite fig . 2 for @xmath23 ) . however , if tidal dissipation alone evolved triton s orbit , only the inclined case can lead to present inclination of @xmath24 . this conclusion points to a paradox . @xcite modelled the tidal orbital evolution with the standard relation : @xmath25 where @xmath26 and @xmath27 are respectively the semimajor axis , time , pericenter distance , mean motion , tidal love number and tidal disspation factor , and @xmath28  km is triton s radius . we numerically averaged eq . [ goldreich ] over several full kozai periods , based on the output of the integration shown in fig . [ peric ] . using @xmath29 and @xmath30 for triton @xcite , the resulting timescale for @xmath31 reduction in @xmath32 is 3.5 gyr for the inclined orbit and 0.11 gyr for the near - coplanar orbit ( cf . goldreich et al . the requirement of an inclined orbit means that tides alone are not capable of circularizing triton s orbit . neptune s satellite system predating triton s capture was likely similar to that of uranus , as the planets are virtual twins in many important characteristics . uranus possesses five sizeable satellites at @xmath33 , with total mass @xmath3440% of triton s . it is likely that triton s capture led to destruction or ejection of any existing regular satellites outside @xmath8 .    to explore the fate of these moons , we integrated five massless particles under the influence of triton on the @xmath13 orbit of fig . [ peric ] . the innermost satellite was put at @xmath35 , and the others on @xmath36 ; all the orbits were circular and in the plane of neptune s equator ( using current values of neptune s obliquity and @xmath37 ) . we found that the orbital crossing of the pairs 4 - 5 and 3 - 4 only a few centuries into the simulation . to calculate the collisonal timescale , we assigned our three outermost particles the radii of umbriel , titania and oberon , and we took the satellites to be uniformly distibuted over all radii @xmath38 , and latitudes @xmath39 . under these ( admittedly rough ) approximations , we estimated the collision probability for every output interval in our simulation and kept track of the cumulative collision probability for each pair . in both cases , the cumulative probability reaches unity at about 1000 years . while our approximations likely underestimate collision timescales , a more sophisticated method would not raise the result more than a factor of a few . due to its long orbital period ( about 7 yrs ) , most of it spent far from neptune , eccentric triton would be unlikely to collide with any of the satellites on such short timescales . the collision would likely destroy both moons and create a debris ring around neptune . this debris would rapidly disrupt the remaining regular satellites , grinding them down to small pieces incapable of destroying triton . the resulting massive disk out to about @xmath40 would interact strongly with triton , whose perturbations would prevent re - accretion of the disk particles from taking place ( cf . banfield & murray , 1992 ) . the mutual interaction of the retrograde triton and prograde disk , either through tidal torques or direct collisions , would cause decay of triton s angular momentum ; here we focus on the latter .    to find the approximate evolution timescale of triton s orbit due to the debris disk , we modeled the disk as having uniform surface density and extending out to @xmath40 , and then integrated triton s orbit giving it a instantaneous `` kick '' every time it goes through neptune s equatorial plane within @xmath40 . like @xcite , we find that triton s lifetime depends largely on its inclination : inclined orbits survive much longer than coplanar ones . our timescales for the decay of orbits with @xmath13 at @xmath11 are on the order of a few times @xmath0  yrs , while the orbit with @xmath41 at @xmath11 would have an @xmath7-folding time of a few times @xmath42  yrs . while our disk model and resolution of the passages are crude , the robust conclusion is that the debris - drag timescale for orbital evolution is much shorter than the tidal one . although our disk is much less massive than that of @xcite ( who use a minimum mass nebula ) triton can still evolve quickly due to deeper penetration into the disk . finally , since the disk is less massive than triton , there is a natural end to this stage of the evolution , so there is no need for specific mechanisms ensuring triton s survival .    if triton s evolution had been this fast ( @xmath43  yrs ) , then the assumption of conservation of inclination relative to local laplace plane is incorrect , as the nodal precession timescale ( at the boundary between an inner region dominated by neptune s oblatneness and an outer region dominated by the sun ) is longer than @xmath0  yrs ( the transition distance depends on @xmath37 as the fifth root , making the influence of the additional material from the debris disk modest ) . therefore , the pre - transition inclination of triton could have been anything from @xmath44 to @xmath45 if the evolution was fast . because passages through the disk would at that point ( @xmath46 ) happen for a wide range of values for @xmath4 ( and not only @xmath47 when @xmath7 is high ) , the constraints on the initial inclination are very weak . nereid and the other five now - known irregular satellites of neptune @xcite can help us constrain triton s post - capture orbit . during its early orbital history , triton s orbit would intersect those of the irregulars , making gravitational scattering possible . there is reason to think that neptune s irregulars had their orbits modified after capture : almost all irregular satellites of the other three giant planets have pericenters @xmath48 around 100 and 200 planetary radii , in case of direct and retrograde satellites , respectively . four outer neptunians do not approach the planet closer than about @xmath49 , while s/2002  n1 and nereid come within about @xmath50 ; the latter two objects will be addressed in the next section . can large-@xmath48 irregulars result from triton s passage ? [ lt ] shows six simulations using a symplectic integrator that includes triton evolving under the influence of the disk while interacting with a swarm of test particles representing `` original '' irregulars . the 2044 test particles were originally put at @xmath51 , @xmath52 , @xmath11 and with fixed @xmath53 ( for direct ) or @xmath54 ( retrograde orbits ) . triton s initial conditions were varied ( see fig . [ lt ] caption ) . the curves in fig . [ lt ] give the fraction of surviving bodies over the 1  myr integrations , normalized to the number of survivors when triton is not present ( some particles are inherently unstable ) .    fig . [ lt ] shows that a significant number of irregulars survives triton s passage only if the duration of the interaction is below @xmath55  yrs . in such cases , triton s apocenter drops below the inner edge of the irregular population , and the depletion stops . in these models such rapid evolution occurs only for @xmath56 , for then triton s pericenter is almost continuously within the disk . an orbit with this maximum inclination will occur for a large fraction of @xmath57 post - capture orbits . also , requirement for @xmath58  yrs practically excludes tidal evolution _ independently _ from arguments in section 2 . [ aqplot ] compares the semimajor axes and pericenters of nereid and the five other irregulars with the surviving test particles in the simulation represented by the topmost curve in fig . the scattered particles bridge the gap between initial conditions corresponding to other planetary irregulars and neptune s . however , the mechanism leaves many satellites at @xmath59 . interestingly , nereid s apocenter distance seems to separate the bulk of the surviving particles and the observed moons . to test the importance of scattering by nereid on the irregulars , we ran two simulations similar to the ones already described , except that now non - evolving nereid was the scatterer and the simulations lasted @xmath60 and @xmath61 years , respectively . there was a noticeable effect on the stability of particles , and some particles in the center of the `` stable zone '' were observed to escape . nereid s escape speed ( @xmath62 at apocenter ) apparently makes it an efficient scatterer , perhaps allowing the moons on orbits permanently out of nereid s reach to dominate the surviving population . nereid orbits between 55 and 385@xmath5 , and has been previously suggested to be a regular satellite scattered to its current orbit by triton . we report preliminary findings on the secular orbital evolution of nereid - like objects just after the end of close encounters with triton . by `` nereid - like '' we mean that the precession is dominated by triton , rather than the sun . the radius of the zone within which the planetary oblateness dominates solar perturbations is @xmath63^{1/5}$ ] , where @xmath64 and @xmath65 are the masses of neptune and the sun , and @xmath66 is the planet s semimajor axis . we estimate the importance of triton by putting @xmath67 , where subscript @xmath68 refers to triton . if we take @xmath69 , we find @xmath70  km . therefore , `` nereid - type '' objects are those orbiting within about 7% of neptune s hill sphere ( for nereid , @xmath71  km@xmath72 ) . the most important secular interaction between two non - intersecting orbits is through the quadrupole term in the interaction hamiltonian ( i.e. one containing @xmath73 ; subscripts 1 and 2 refer to inner and outer body , respectively ) . the disturbing function for the outer body is @xcite : @xmath74.\ ] ] where @xmath75 is the outer body s semiminor axis , and @xmath76 and @xmath77 are those of triton . if we assume triton s orbit precesses much more slowly ( due to neptune s @xmath37 ) than the outer body s ( due to triton and the sun ) , we can replace @xmath77 with @xmath78 , the longitude of the ascending node of the outer body , measured from triton s pericenter , in triton s orbital plane ( the angles are geometrically identical ) . the expression above has no dependence on @xmath79 , so @xmath80 . using lagrange s eqations , expressions for the evolution of @xmath81 and @xmath78 obey : @xmath82,\ ] ] @xmath83 where @xmath84 . putting @xmath85 ( which puts triton close to intersection with nereid , assuming current pericenters ) , we numerically integrated eqs . [ domdt ] and [ didt ] for initial conditions of @xmath86 and @xmath87 ( fig . [ ioplot ] ) . suprisingly , objects with @xmath88 can have nodes librating around @xmath89 ( or @xmath12 , as the results for @xmath90 are symmetric ) , and their inclination ( relative to triton ) can change from direct to retrograde . we tested this model using a numerical integration , in which triton had its current @xmath9 and @xmath48 but @xmath85 . test particles were placed outside its apocenter , on @xmath91 orbits in neptune s equator plane . several particles show librating @xmath78 and repeatedly change their sense of revolution ( fig . [ ioplot ] ) . inclinations in fig . [ ioplot ] are relative to triton s orbit , which has @xmath13 relative to neptune s equator , which itself has an obliquity of @xmath92 relative to the planet s orbit . objects in libration island can end up with a wide range of inclinations relative to the sun . in particular , bodies starting within the equatorial plane of neptune can be shuffled through the libration island to both direct and retrograde orbits . one recently discovered irregular , s/2002  n1 , has @xmath93  km , @xmath94  km , and @xmath95 @xcite . @xcite report that n1 s colors are similar to that of nereid and suggest that they are members of a collisional family . this is implausible if n1 was produced at the present epoch , but is consistent with both objects being pieces of circumplanetary debris ejected by triton and mixed by this process . we propose that , after its initial capture by neptune ( via any process ) , triton strongly pertubed all of the pre - existing satellites , both regular and irregular . regular satellites rapidly collide with each other , creating a debris disk to be subsequently swept up swept up by triton . the debris drag evolved triton s orbit rapidly enough ( @xmath96  yrs ) to preserve some of the irregular satellites , scattering some to high @xmath48 . bodies in the inner part of the hill sphere later suffered triton s secular perturbations , which in some cases `` flipped '' their inclination between direct and retrograde . the closest irregular satellites were subsequently depleted through encounters with nereid , producing the distribution observed today . agnor , c. b. , & hamilton d. p. 2004 , , 36 , 1169 banfield , d , & murray , n. 1992 , , 99 , 390 benner , l. a. m. , & mckinnon , w. b. 1994 , , 114 , 1 chyba , c. f. , jankowski , d. g. , & p. d. nicholson 1989 , , 219 , l23 goldreich , p. , murray , n. , longaretti , p. y. , & banfield , d. 1989 , science , 245 , 500 grav , t. , holman , m. j. , & fraser , w. c. 2004 , , 613 , l77 holman , m. j. et al . 2004 , , 430 , 865 innanen , k. a. , zheng , j. q. , mikkola , s. , & valtonen , m. j. 1997 , , 113 , 1915 jankowski , d. g. , chyba , c. f. , & nicholson , p. d. 1989 , , 80 , 211 mccord , t. b. 1966 , , 71 , 585 mckinnon , w. b. 1984 , , 311 , 355 mckinnon , w. b. , & leith , a. c. 1995 , , 118 , 392 mckinnon , w. b. , lunine , j. i. , & banfield , d. 1995 , neptune and triton , d. p. cruikshank , tucson : u. of arizona press , 807
we present simulations of triton s post - capture orbit that confirm the importance of kozai - type oscillations in its orbital elements . in the context of the tidal orbital evolution model , these variations require average pericenter distances much higher than previously published , and 
 the timescale for the tidal orbital evolution of triton becomes longer than the age of the solar system . 
 recently - discovered irregular satellites present a new constraint on triton s orbital history . 
 our numerical integrations of test particles indicate a timescale for triton s orbital evolution to be less than @xmath0 yrs for a reasonable number of distant satellites to survive triton s passage . 
 this timescale is inconsistent with the exclusively tidal evolution ( time scale of @xmath1 yrs ) , but consistent with the interestion with the debris from satellite - satellite collisions . 
 any major regular satellites will quickly collide among themselves after being perturbed by triton , and the resulting debris disk would eventually be swept up by triton ; given that the total mass of the uranian satellite system is 40% of that of triton , large scale evolution is possible . 
 this scenario could have followed either collisional or the recently - discussed three - body - interaction - based capture .