Icarus141, 420–425 (1999)

Article ID icar.1999.6189, available online at http://www.idealibrary.com on

NOTE

On the Detectability of Satellites of Small Bodies Orbiting the Sunin the Inner Region of the Edgeworth–Kuiper Belt

I. Toth

Konkoly Observatory, P.O. Box 67, Budapest H-1525, HungaryE-mail: tothi@konkoly.hu

Received March 17, 1998; revised June 17, 1999

The detection and observation of satellites of the small bodies orbiting theSun in the outer Solar System is extremely important in order to determinethe orbit and masses of the components in the binary system as well as toknow their sizes to derive their mean bulk density. This note provides aformal estimation of the possible separation distances, orbital periods, andapparent brightnesses of possible satellites of the objects in the inner regionof the Edgeworth–Kuiper Belt. c© 1999 Academic Press

Key Words: Centaurs; Kuiper Belt objects; satellites, general; celestialmechanics, stability; asteroids, dynamics: binaries.

Binary asteroids: Orbits, masses, and bulk densities.Binary objects amongthe small bodies of the Solar System can add immensely to our knowledgan orbit can be determined for a binary asteroid system, the mass of the prim

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observations during the mid-1970s, and discussed the orbital characteristics ofpossible asteroid satellites.

The binary systems among the small bodies of the Solar System (asteroids,sses

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and secondary can be calculated. Once the masses and approximate dimeare known, densities can be determined and the total mass of the small bin the outer Solar System can be estimated. Based on knowledge of the dties, constraints on the composition or on internal structure can be establiThe frequency and other characteristics of duplicity also provide importantstraints upon our understanding of the origin and evolution of the outer asteor comet belt.

The knowledge of mass and bulk density of KBOs is interesting due torelationship between them and short-period comets. Recent investigationsclude that the reservoir of the short-period comets is basically the EdgewoKuiper Belt in which the larger bodies (100–1000 km in diameter) had format about 30 AU (e.g., Farinella and Davies 1996, Stern and Colwell 1997).delivery process of KBOs onto the regions of Jupiter and Earth including thenegligible impact frequencies of transferred KBOs were calculated by Levand Duncan (1997). Thus, the determination of the mass and mean bulk deof the KBOs is important and it could also yield information on the origin aevolution of the short-period comets.

With the high resolution and detector sensitvity of the modern large telescboth from the ground and by spaceborn optics like the Hubble Space Telesfor instance, it should be possible to directly observe binary minor field objin the outer Solar System. The principal goal of this note is to draw attentiothe aspect of the existence of orbiting companions to the small bodies orbthe Sun in the inner region of the Edgeworth–Kuiper Belt and to computesizes of stability domains of orbiting satellites and angular resolution limittheir detectability as well as the elapsed time intervals to possible synchron

Duplicity among small bodies in the Solar System, origin of binary asteroInterest in binary asteroids has been high for the past 20 years. The subjereviewed by van Flandernet al. (1979) and Weidenschillinget al. (1989), whodescribed the early histories of the suggestions of the existence, recounte

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comets) could be (1) primordial; (2) created in the impacts, as by the proceof rotational fission, formation from orbiting ejecta (yields close orbits toprimary), and fragmentation with mutual capture; or (3) captured from helioctric orbit. So, there are various processes which can form binary systems asmall bodies.

The collisional disruptions in the Kuiper Belt can create binaries in the caof both monolithic and rubble pile colliding bodies. The bifurcated contactnaries can split mainly by rotational fission, and other bodies can disintegraorbiting ejecta or fragmentation with mutual captures. The captures from hcentric orbit or soft collisions can also form binary systems since the collisis not a negligible process in the Kuiper Belt (Stern 1995, Farinella and Da1996, Stern and Colwell 1997). Both the primordial and recent processes iSolar System can form a rubble pile structured body with larger componen

At present the question is open about the structure of KBOs: whetherhave monolithic or rubble pile structure. This is connected to the formationoccurence frequency of satellites because these depend also on the structinternal configuration and internal material strength of bodies in the casecollisional processes of binary formation. A rubble pile body structure colead to form binary (or multiple) systems more easily among small bodieboth soft collisions and disintegration processes. The natural consequenccomet formation process (Donn and Hughes 1986, Weissman 1986, SternRickman and Huebner 1990) is a rubble pile structured body. A well stuexample of the KBOs is 1993 SC (Williamset al. 1995, Davieset al. 1997).Early photometric observations showed apparently that the major to minorratio is 1.6 : 1 (Williamset al.1995), allowing both an elongated monolithic oa rubble pile body. This was rejected by Davieset al. (1997), showing a slightvariation in the lightcurve with less than 0.2 magnitude in amplitude, soobject is rather a spherical shaped monolithic body (or was it observed at aa pole-on aspect geometry?). So, until now there is no evidence for rubblebody among KBOs. Of course, the planetesimals and comets can also be cvia other processes. Recent models of the planetesimal and comet formprocess were given by Weidenschilling (1994, 1997a,b).

In the transneptunian region the Pluto–Charon Binary (PCB) (ChristyHarrington 1978) is the only known primary+ companion system; howevePluto is considered as a major planet. The properties and origin of theare summarized in detail by Stern (1992) and Levison and Stern (1995).stability domain for satellite orbits around Pluto extends≈100 times furtherthan the radius of Charon’s orbit. This motivated a deep CCD search for disatellites of Pluto by Sternet al. (1991) and they reported that no detectiowere made at 90% confidence upper limits on the absence of potential satwithin 10 arcsec from Pluto to the edge of its stability domain. Pluto is knoto have one satellite.

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Stability of the binary asteroids in the outer Solar System.The question ishow the binary systems can be stable against gravitational perturbationslarge the orbital stability domain is for a satellite companion orbiting aroua primary body. The effects of the gravitational perturbations will be mpronounced on a binary asteroid than on a binary star or on a planetary sadue to the relatively small masses of the components of a binary asteroid.

The former studies on the effects of the perturbations of the Sun and Juon a Main Belt binary asteroid system made by Whipple and White (19have shown that the binary system to be stable against these perturbaTheir results indicate that the existence of binary asteroids is possible everelatively small components and relatively large separation distances betthe components. Finally, the discovery of the satellite 1993(243)1 Dactyasteroid 243 Ida by the GALILEO spacecraft showed that satellites coulcommon around asteroids (Chapmanet al.1995).

The precise calculations and correct evaluations can lead to the concluon the long-term stability and evolution of the small binary objects in the SSystem. Detailed studies were prepared on the stability and dynamical evoof the binary asteroid systems (Hamilton and Burns 1991, Hamilton and Kr1997, and numerous other references therein). However, the discussionlong-term behavior of these systems is beyond the scope of this note and wsimplified assumptions as follows.

A quantitative measure of stability based on Hill’s definition is evaluateddirect and retrograde satellite orbits. The size of the solar-tidal stability reis defined by Szebehely’s stability criterion for satellites in nearly circular or(Szebehely 1967, 1978). However, different criteria of stability limits for satemotion were compared by Szebehely and McKenzie (1978). The lowest rand therefore the most conservative estimate is obtained by the simple formSzebehely (1967). To estimate the size of the orbital stability region Szebeh(1967) criterion is employed here in our further calculations. It is a criti(maximum) value of the satellite’s orbital radius found for stability and it iremarkably simple function of the mass-parameterµ,

Rs = ap(1− ep)( µ

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whereap is the semimajor axis,ep is the eccentricity of the orbit of the primarbody orbiting the Sun, andµ=mp/(m¯ +mp) is the mass-parameter with masof the primary bodymp and solar massm¯. Rs is not the radius of the planetar(primary body) zero velocity Hill oval, but ratherRs= (1/3)RHill . For objectsorbiting betweenRs and RHill instability is possible; objects orbiting outsidRHill must be unstable.

Computational results and discussion.The results of calculations of orbistability radii, orbital period at the stability radii, separations at the periheconditions of Centaurs, and KBOs with known sizes at present are summarizTable I. The diameters of Centaurs are taken from the compilation by Weiss(1995), and for Centaur 1997 CU26 from Jewitt and Kalas (1998), the diamof KBOs are updated by Jewitt and Luu (1995, 1996, 1998), Jewitt (199Recently, a new value of 2060 Chiron’s size became available by AltenhoffStumpff (1995). The orbital elements are taken from lists of Centaurs and Kcompiled by Marsden (1998).

The body size and bulk density range is limited by the observational cstraints: KBOs are thought to be ice–rock remnants that have survivedthe epoch of formation of the gas giant planets; they may also supply theperiod comets (the bulk density of their nuclei is low: around 1 g cm−3), andthrough collisions, they may be a source of interplanetary dust. Presently,than 60 such objects are known, ranging from the 2200-km-diameter Plunumerous 100-km diameter objects at the limit of optical surveys (JewittLuu 1998).

For example, assume spherical shaped bodies and use a plausible bulk dvalue of 2 g cm−3. Taking the actual radii of the bodies as well as the correspoing values of the mass-parametersµ in the restricted problem of three bodies, thsizes of stability domains were calculated employing Eq. (1). Chosing a vof the satellite/primary mass ratio ofµ12= 0.2 the orbital periods of satellite

were computed employing Kepler’s third law for the selected mass-paramµ12 of the binary systems. These correspond to the Pluto+Charon parameter

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TABLE IOrbit Stability Radii and Maximum Separations for Possible

Satellites among Centaurs and KBOs

qp ap Rp Rs Psat SeparationObject (AU) ep (AU) (km) (km) (day) (arcsec)

Centaurs1997 CU26 13.072 0.168 15.713 151.0 1.10(5) 1.75(3) 12.61994 TA 11.744 0.303 16.853 8.5 5.58(3) 1.49(3) 0.47066 Nessus 11.821 0.518 24.545 22.0 1.45(4) 1.51(3) 1.95145 Pholus 8.664 0.571 20.217 60.0 2.91(4) 9.46(2) 6.92060 Chiron 8.452 0.380 13.633 84.0 3.96(4) 9.10(2) 5.1

KBOs1997 CW29 36.271 0.079 39.375 171.0 3.46(5) 8.09(3) 11.91997 CV29 35.487 0.183 43.433 117.0 2.32(5) 7.83(3) 7.61997 CU29 41.975 0.032 43.383 161.5 3.79(5) 1.01(4) 11.91997 CT29 42.321 0.030 43.610 133.0 3.14(5) 1.02(4) 9.61997 CS29 43.515 0.006 43.774 296.0 7.19(5) 1.06(4) 23.41997 CR29 41.560 0.073 44.823 126.5 2.93(5) 9.92(3) 9.21997 CQ29 41.083 0.074 44.379 150.5 3.45(5) 9.76(3) 15.41996 TS66 38.526 0.128 44.177 191.0 4.11(5) 8.86(3) 18.51996 TR66 33.160 0.223 42.687 109.5 2.03(5) 7.08(3) 6.41996 TQ66 34.613 0.129 39.723 121.0 2.34(5) 7.54(3) 8.21996 TP66 26.373 0.338 39.828 123.0 1.81(5) 5.02(3) 6.21996 TO66 38.612 0.114 43.599 379.5 8.19(5) 8.89(3) 30.01996 TL66 35.038 0.588 84.965 275.0 5.38(5) 7.67(3) 24.81996 KY1 35.712 0.096 39.517 63.0 1.26(5) 7.91(3) 4.31996 KX1 35.704 0.097 39.543 65.5 1.31(5) 7.90(3) 4.61996 KW1 46.602 0.000 46.602 140.5 3.66(5) 1.18(4) 14.31996 KV1 40.223 0.111 45.229 134.0 3.01(5) 9.45(3) 12.01995 KK1 31.981 0.190 39.475 83.0 1.48(5) 6.70(3) 5.01995 KJ1 43.468 0.000 43.468 180.5 4.38(5) 1.06(4) 14.61995 HM5 29.482 0.250 39.304 80.5 1.32(5) 5.93(3) 4.31995 GA7 34.751 0.119 39.455 101.0 1.96(5) 7.59(3) 6.71995 GJ 39.006 0.091 42.907 150.5 3.28(5) 9.02(3) 11.31995 FB21 42.426 0.000 42.426 84.5 2.00(5) 1.02(4) 7.41995 DC2 40.784 0.070 43.853 169.0 3.85(5) 9.65(3) 16.51995 DB2 40.069 0.135 46.334 133.0 2.98(5) 9.40(3) 12.21995 DA2 33.684 0.070 36.209 84.5 1.59(5) 7.24(3) 8.61994 VK8 41.795 0.026 42.903 194.5 4.54(5) 1.00(4) 16.61994 TG2 42.448 0.000 42.448 70.5 1.67(5) 1.02(4) 6.81994 TH 40.940 0.000 40.940 108.5 2.48(5) 9.70(3) 8.21994 TG 42.254 0.000 42.254 116.0 2.74(5) 1.02(4) 13.21994 TB 27.043 0.322 39.893 129.0 1.95(5) 5.21(3) 8.51994 JR1 34.756 0.117 39.362 119.0 2.31(5) 7.59(3) 8.31994 JQ1 41.763 0.049 43.893 191.0 4.45(5) 9.99(3) 17.71994 JV 35.251 0.000 35.251 127.0 2.50(5) 7.75(3) 9.91994 JS 33.005 0.218 42.207 131.5 2.42(5) 7.03(3) 7.31994 GV9 40.978 0.057 43.465 132.0 3.02(5) 9.72(3) 10.61994 EV3 40.682 0.047 42.684 133.5 3.03(5) 9.61(3) 14.11994 ES2 40.322 0.114 45.526 79.5 1.79(5) 9.49(3) 6.21993 SC 32.276 0.191 39.906 159.5 2.88(5) 6.80(3) 12.11993 SB 26.891 0.322 39.690 94.0 1.41(5) 5.17(3) 6.91993 RP 34.863 0.114 39.329 48.0 9.34(4) 7.62(3) 4.21993 RO 31.485 0.206 39.637 69.5 1.22(5) 6.54(3) 4.01993 FW 41.539 0.045 43.479 143.0 3.32(5) 9.91(3) 16.01992 QB1 40.891 0.078 44.338 141.5 3.23(5) 9.68(3) 11.0

Note. qp, ep, ap, perihelion distance, eccentricity, semimajor axis of the pmary; Rp, radius of the primary body;Rs, orbit stability radius of a satellite

eterRsat, orbital period of a satellite atRs; Separation, apparent separation distanceat perihelion.

422 I. TOTH

FIG. 1. Body sizes and extents of orbital stability domain for all Centaurs and KBOs with known sizes and for the PCB in the (ap, ep) plane. The symbol sizesare proportional to the real body sizes and stability radii.

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ranges (e.g., Sternet al. 1991, 1992). The bulk density of bodies are assumequal and are comparable with ice+ rock and Pluto values. Of course, the actuvalues of masses, mass ratios, and bulk densities were unknown for the Ceand KBOs until now, and these parameter values are to be determined mocurately from observations of possible satellites; however, the above valueused in the computations to characterize these binary systems.

The body sizes and extensions of the stability domains of the corresponobjects including the PCB system are plotted with size-proportional symbothe (ap, ep) plane (Fig. 1). This is based on the formula for the stability radRs, which suggests the perihelion distance dependence sinceqp=ap(1− ep), aswell as the mass-parameter, contains the adopted mass (shape, volume adensity) of the bodies.

The first inspection of the figure shows that the stability domain sizes areor two magnitudes smaller for Centaurs than for KBOs and PCB systemsto (i) the smaller perihelion distances and (ii) relatively smaller sizes (masof Centaur primary bodies. The stability radius dependence on the actualparameters are displayed in Fig. 2 with lines corresponding to the varioushelion distances. The Centaurs, KBOs, and PCB are also separated into digroups according to their perihelion distances and mass-parameters. Theing values for satellite distances are about 105 km for Centaur 1997 CU26 anonly 3.9× 104 km for 2060 Chiron, while for the smallest known object amoboth the Centaurs and KBOs, the Centaur 1994 TA, this stability radius is5.5× 103 km. The KBOs with largest stability domain radii for satellites arefollows: 1996 TO66 (8.1×105 km), 1997 CS29 (7.1×105 km), and 1996 TL66(5.3×105 km). The apparent maximum angular separation distances definthe stability radii visible at perihelion (geocentric distance is1=qp− 1 AU)are listed for each object (Table I). The Centaur 1994 TA has only subarcseseparation, but for other Centaurs and KBOs the separation ranges fromarcseconds to 30 arcsec. It has a consequence in the case of satellite seaKBOs: in many cases a relatively large portion of the sky should be survey

the edge of stability domain during the perihelion of the objects. (In comparisPluto has≈190 arcsec for the visible stability domain; Sternet al.1991).

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The orbital period of a satellite ranges from about 102 to 103 days for Cen-taurs and about 103 to 104 days for KBOs for the adopted mass-parameterµ12

of the binary systems (for an orbit with orbital semimajor axis equal to thebility radiusRs). The orbital period of a satellite depends on its orbital semimjor axisasat≤ Rs. This dependence is much stronger than the mass-param

FIG. 2. Mass-parameter dependence of the orbit stability radius for

onCentaurs and KBOs with known sizes and for the PCB. The lines correspond tothe given perihelion distance.

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dependence in the binary system (Kepler’s third law). The primary radiuchosen asRp= 250 km, corresponding to the sizes of larger KBOs. The shoorbital periods (∼10–∼102 days) correspond to the orbital semimajor axes ranfrom 5.0× 103 to∼104 km, but close to the orbit stability limit the periods amuch longer.

We turn now to the question of synchronization of a binary system amKBOs. The angular momentum transfer during the spindown to synchronrequires a time interval for the binary systems. The elapsed time duringsynchronization process at this phase of the tidal evolution is given, e.gWeidenschillinget al. (1989). The key parameters of the problem are mateand internal structure dependent: these are the modulus of rigidityµrigid andthe specific internal dissipation functionQ. Moreover, the final separation distance arises over a large power exponent; thus the effect of the initial orsemimajor axis is negligible (Weidenschillinget al.1989). The values ofµrigid

andQ were estimated for Comet 1P/Halley by Peale and Lissauer (1989) taµrigid= 4× 109 N m−2 (water ice) and 1≤ Q≤ 100. The cometary materiastrength estimations were summarized by Meech (1996), as well as dicuand given by Rickmanet al. (1996) based on laboratory experiments (KOSIPDs, meteorites etc.). A value ofµrigid Q= 5.0× 1011 N m−2 was suggestedfor comets by Jewitt (1998b) according to the estimation given by Harris (19In comparison, for the PCB system the time to reach orbital synchronici2× 103 to 106 years assuming an internal dissipation factor 5≤ Q ≤ 103 forPluto, in accord with all the terrestrial planets and icy planetary satellites (Set al. 1991). However, we have no direct evidences on the parameter vaof µrigid Q for KBOs; therefore an adequate value in magnitude as givenPeale and Lissauer (1989), Harris (1994), and Jewitt (1998b) is used hethe following computations. Of course, for the moment the validity of analobetween the material parameters of KBOs and comets as well as the PCBtem is questionable, but the characteristics of the icy region of the Solar Syand relation between KBOs and short-period comets, moreover between Kand the PCB, establish and support this analogy in terms of material paramabove. For example, take a KBO spherical primary body with radius of 250bulk density of 2 g cm−3, µrigid Q = 5.0× 1011 N m−2, satellite/primary massratio of 0.2, and a final orbital separation distance of 103 km (close binary). In

FIG. 3. Elapsed time to synchronicity vs separation of satellite fromprimaryasat/Rp for a set of differentµrigid Q (N m−2) curves. Other parameterof the binary system are fixed. The adopted range ofµrigid Q for comets andsome asteroids is∼1011 N m−2 (Peale and Lissauer 1989, Weidenschillin1989, Harris 1994, Jewitt 1998b). The time interval for coming to synchroni

for the Pluto–Charon binary system (PCB) is indicated by a vertical bar (Stet al.1991). The dashed line limits the age of the Solar System.

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this case the elapsed time to synchronization is about 1.8× 103 years. For thesame primary body, when the separation distance is 104 km it is 5.8×109 years,while for a separation distance of 105 km it is longer than the age of the SolaSystem, so the synchronicity is impossible for wide binaries with paramegiven above. For a binary system with the same parameters of primary andratio as defined above, the elapsed time to synchronicity vs domain of sepais shown in Fig. 3 for different values ofµrigid Q ranging from∼102 to∼1011 Nm−2 compared with the elapsed time range for the PCB system given by Set al. (1991). The recent knowledge about the values ofµrigid Q for comets andcometary analog materials yield elapsed time ranges from∼103 to∼108 yearsor more if the separation is larger and if the product of rigidity and dissipafactor is smaller.

In order to discuss the apparent brightness and detectability of a compathe geometric albedo and phase coefficient have to be taken into accocalculations. The geometric albedo in the optical range could be influencethe bounded atmosphere of a large object as well as by the surface composby the consequences of collisional resurfacing. The latter scenario was sugby the observations of some KBOs (Jewitt and Luu 1998, Luu and Jewitt 19The usual values of theRgeometric albedo ranges from 0.04 (dark comet nuclsurface covered by dirty ice or dark irradiated material) to 0.40 (Charonacceptable for most of KBOs with diameters of 100 km (without atmospheffects). The phase effect is practically negligible for a groundbased obseran instrument in an Earth-bound orbit because of the small phase angle ranthe KBOs; otherwise the usual value of the linear phase coefficient can beas 0.035 mag deg−1. The apparent brightness difference is 7 mag between a Kcompanion with 10 and 250 km in radii. The heliocentric distance dependof theRapparent magnitude of outer Solar System objects for different radibeen calculated for different values of geometric albedo from 0.04 to 0.40body surface with a geometric albedo ofpg= 0.40 is about 2.5 mag brightethan a body withpg= 0.04 for all heliocentric distances and sizes. The appaR magnitude of an object with a radius of about 70 km and with geomealbedo of 0.04 at heliocentric distance of 100 AU is about 28.0 magnitudopposition; however, in the case of a body with higher albedo it could be brig

Conclusions: Detectability conditions and suggestions for orbit determtion. The heliocentric distance region of interest is limited by recent stuby Jewittet al. (1996) and Jewittet al. (1998): the population estimates bason limited sampling of the ecliptic suggest that more than 70,000 bodiesdiameters larger than 100 km are to be found in the 30- to 50-AU distance rwith combined mass on the order of 0.1 Earth masses. Jewittet al.(1998) expectthat probably a discrete edge to the belt does not exist at about 45–50 AUthat more distant regions of the Kuiper Belt may be revealed by furtherimaging observations; i.e., the region from 30 to 50 AU is the inner regiothe Edgeworth–Kuiper Belt.

The new generation of telescopes (e.g., HST, NGST, VLT) can reachmagnitudes inR at 100 AU and they would be able to see objects as sma∼70 km in radius. The detection of companions of the KBOs with radius lathan∼10 km is expectable with extraatmospheric deep optical imaging surfor wide binaries, e.g., with the HST WFPC2 within 10 arcsec in radius toprimary, which amounts to 220 PC2 pixels, and for observations to be perfowith speckle interferometric methods for close double or bifurcated binarytems. The detection of companions to the larger KBOs (e.g., 1996 TO66,CS29, and 1996 TL66) could be hoped for groundbased deep imaging suwith large optical telescopes. The typical orbital period in a binary KBO sysotbiting well within the stability radius ranges from∼10 days to∼102, which isin the range of usual few-months seasonal visibility from Earth. Although bthe satellite’s orbit determination and the finding of the total mass for the syare complicated, there are possibilities of getting these studies under waythe classical methods of monitoring the relative motions of the componentsonly known example until now of smaller binary objects in the outer Solar Stem is the Pluto–Charon system reviewed by Stern (1992). The modern veof classical astrometric methods of studying a visual binary system as wspeckle interferometry were applied to these faint objects as in the case

ernPluto–Charon binary system. The new determination of Charon’s orbit by usingHubble Space Telescope observations is reported by Tholen and Buie (1997).

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The recent examples of mass ratio determination by astrometric methodgiven for the Pluto–Charon system (Younget al.1994, Foustet al.1997); how-ever, there are some discrepancies between the results of different groupsshows the difficulties involved in studying binary systems at large distancea first approximation a binary system orbiting in the outer Solar System caconsidered in the frame of the two-body problem and analogously the clasmethods of orbit determination of the visual binary stars can be applied (Aitken 1963). Speckle interferometry could be a useful tool for revealing cbinary systems among Centaurs and KBOs, but applying new techniques.the groundbased optical images are disturbed by atmospheric seeing andlight level the signal over the noise ratio is poor for such faint objects as KBthe groundbased speckle interferometry is not a very effective method fovealing faint KBO binaries due to atmospheric degradation of the observatApplication of the adaptive optics systems could improve the quality of bthe imaging and the speckle interferometric observations. However, it is wtrying to apply this tool using spaceborn astronomical instrumentation maextraatmospheric observations. (A speckle interferometric survey for astduplicity was performed at the KPNO 4-m telescope during 1982–1984 oasteroids no companions were detected; Robertset al. 1995.) The speckle in-terferometry is also a useful tool for determining the orbit of the componein a close binary system. The orbital elements of Charon were determinedspeckle interferometry by Beleticet al. (1989). Monitoring the lightcurves canlead to observing eclipsing binary-like features, but this requires an exceptaspect geometry; moreover the contact binaries can show similar lightcuThe albedo features (albedo variegation, spots) can also occur without anylite companion. The resulting thermal emissions of KBOs calculated by Thoet al. (1997) suggest that the thermal infrared flux of 10–25 and 100–400can be detected in the cases of large Centaurs (e.g., Chiron) only at 120 µm, respectively. Using the archived ISO (ISOCAM) observations larcompanions to the Centaurs could be detected only at about 10–16 AUthese presumably should have been discovered already by optical observaSo, the greatest chance of detecting well detached KBO binary systemsperform deep optical surveys with large ground and space telescopes wthe detectability and angular resolution limits of recent techniques, and cbinaries can be revealed employing speckle interferometry.

ACKNOWLEDGMENTS

The valuable comments and notes by anonymous referees are gratefuknowledged. This work was partly supported by OTKA Grants T14963T025049.

REFERENCES

Aitken, R. G. 1963.The Binary Stars.Dover Publications, New York.

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