Technical Report No. 32-731 The Perturbations of a Hyperbolic Orbit by an Oblate Planet Carl G. Sauer, Jr. OTS PRICE I XEROX MICROFILM $ JET PROPULSION LABORATORY C-AtlFORNiA 1NSTlTUTE OF TECHNOLOGY PASADENA.CALIFORNIA January 15, 1963 NATIONAL AERONAUTICSAND SPACE ADMINISTRATION CONTRACTNo. NAS 7-100 Tecbnical Report No. 32-731 The Perturbations of a Hyperbolic Orbit by an Oblate Planet Carl G. Sauer, Jr. C. Gates, Chief Systems Analysis Section JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA,CALIFORNIA January 15, 1963 Copyright @ 1963 Jet Propulsion laboratory California Institute of Technology The material in this Report appeared in the ARS Journal, Vol. 32, No. 5, pp. 714-717. JPL TECHNICAL REPORT NO. 32-131 CONTENTS 1. Introduction ..................... 1 II. The Perturbative Potential ................2 111. Variation of the Orbital Elements ............. 2 IV. Example of an Earth-Escape Mission ............4 References ....................... 8 1. Example of an Earth-escape mission ..............5 2 . Asymptotic value of the orbital elements ............5 3 . Asymptotic perturbations of the hyperbolic excess velocity vector ....6 4 . Calculated and observed perturbations at 240 min .........6 5 . Position and velocity at T = 240 min ..............7 FIGURES 1. Perturbation of the semi-transverse axis ............5 2 . Perturbation of the eccentricity ................5 3 . Perturbation of the time of peri-focal passage .......... 6 4 . Perturbation of the inclination ................6 5 . Perturbation of the argument of perigee .............6 6 . Perturbation of the longitudeof the ascending node ........7 7. True anomaly vs time (e = 1.25) ...............7 JPL TECHNICAL REPORT NO. 32-131 The perturbations of the hyperbolic orbital elements of a vehicle in the gravitational field of an oblate planet are derived as functions of the initial osculating elements. Assumptions are made that atmos- pheric drag is absent and that the gravitational potential of the planet may be represented adequately by the principal term and the second harmonic. An example of an Earth-escape mission is presented in which a comparison is made between calculated orbital perturbations and results from a numerical integration of the equations of motion. 1. INTRODUCTION Although the variations of the orbital elements of Earth perturbations of an interplanetary probe by an oblate satellites have been derived numerous times, nevertheless planet at encounter. a treatment of the variation of the orbital elements of a vehicle for an escape mission is lacking except for the 'Since 1960, when this report was originally prepared, there have case of perturbed hyperbolic motion in the equatorial been two additional papers published concerning the perturbing plane (see Ref. 1 ) There are several instances in which effects of an oblate Planet upon a vehicle on a hYFrbolic tra- jectory (Ref. 2 and 3). Of particular interest is Ref. 3 by G. Hori the Of this paper are Of interest, the first being in which the Von-&ipel was used to derive the pefiw&d the perturbations Of the escape from Earth Of motion. The present paper uses the method of Poisson-LaGrange, a lunar or interplanetary probe, and the second being the as does Ref. 2. 1 JPL TECHNICAL REPORT NO. 32-131 II. PERTURBATIVE POTENTIAL If an axially symmetric density distribution of the planet where is assumed, the gravitational potential function U may be 1 P, (sin+) = -(3sin, 4 - 1 expanded in terms of surface zonal harmonics of the form 2 (3) (Ref. 4 and 5). The distance from the gravicenter T and gravicenter lati- tude + expressed as functions of the osculating elements of the vehicle are where T = p/(l + ecosv) (4) J, = the coefficients of the zonal hormonics P, ( sin 4 ) and k2 = GM = gravitational constant of attracting body r = radial distance from gravicenter in equatorial radii sin4 = sinisin (W + V) (5) 0 = gravicenter latitude where In an analysis of the perturbations of the bound orbit p = h2/k2= ( -u) (e' - 1) = semilatus rectum of an Earth satellite, it is necessary to consideral several of a = semitransverse axis the harmonic terms to describe accurately the motion of h = angular momentum the vehicle over an extended period of time (Ref. 6 and e = eccentricity i = inclination to the equatorial plane 7). However, only the second harmonic J2 contributes significantly to the perturbations of the elements of a non- w = argument of perigee bound or hyperbolic orbit, since the time spent in the v = trueanomaly vicinity of the planet is short and the effects of the higher The disturbing function R, when expressed in terms of order harmonics are negligible. the osculating elements of the orbit, becomes The perturbing potential R due to the second harmonic R = ],(k2/2pS)(1 + BCOS v)3 [l - -sin23. i J2 is given by 2 + 3 ;z sin2 i cos (2, + 2~)](6) 111. VARIATION OF ORBITAL ELEMENTS The solution of the differential equations representing E = - k2/2a = undisturbed energy the variations of the orbital elements with respect to time is derived by a method analogous to that employed by h, = h cos i = component of angular momentum along Kozai (Ref. 8) in investigations of elliptic motion. The polar axis orbital elements E, h, h,, T, O, and sz are employed, since they both form a canonical set and are also well behaved T = time of perifocal passage for eccentricities near unity. The elements not previously defined are sz = longitude of the ascending node 2 ~ ~~ JPL TECHNICAL REPORT NO. 32-131 The rate of change of the orbital elements with time n( t - T), where the mean motion n is a function of the may be expressed in terms of the differential coefficients element E through the harmonic law n = ( 2E):’’2k-2.The of R as variable A.1 corresponds to the mean anomaly for hyper- bolic motion and is a function of both the true anomaly and the eccentricity, and hence the disturbing function involves the element h both explicitly and also implicitly through the true anomaly. dddt = -?R/ah (9) Since an axially symmetric density distribution is as- sumed, the disturbing function R is independent of 0,and Eq. ( 12) becomes dhz/dt = 0 (14) and h, = h,(O) = const (15) Since the orbital elements of the vehicle change only the component of angular momentum along the polar slightly due to the effects of the oblateness, the pertur- axis being unaffected by the oblateness. bations of the first order may be derived by considering the elements appearing in R on the right side of Eq. (7) The perturbations of the remaining five orbital ele- through (12) constant. If the elements are assumed con- ments are given in Eq. ( 16) through (20) where M and stant, the true anomaly may be regarded as a known appearing in Eq. (20) are the mean anomaly and mean function of time, and the independent variable may be motion, respectively. These expressions for the perturba- transformed from time to the true anomaly by tions are to be evaluated between the limits v0 and V. SE = R1’ JVO h sin2 i Sh = J, - 3e cos + ”) + 3 cos (2w + 2”) + e cos (20 + 3”) (17) 4p? I:, cos i 60 = -I., -i6(” + e sin “) - [3e sin (2, + v) + 3 sin (2” + 2”) + e sin (2~+ 3v)] - 4pX 1 >:, e 6” + cos is0 = J, 3e3 sin (2w - (12 - 21e’) sin (2w - 36e sin (2, 2v) - 48e v) + + v) + (28 + lle?) sin (2, + 3“) - 18e sin (2, + 4”) - 3e2 sin (20~+ 5~) The perturbations of the semitransverse axis a, eccen- d/dt = h/r2 d/dv (13) tricity e, and orbital inclination i are found from The disturbing function involves the element E and ‘T implicity through the true anomaly from the relation M = 3 JPL TECHNICAL REPORT NO. 32-131 ah and 4h being the right ascension and declination of the hyperbolic excess velocity vector and Vh being the angle between the hyperbolic excess velocity vector and peri- 8i = cot i (6h/h) (23) apsis, The expressions for the variations of the elements E, The angle Vh is related to the eccentricity of the hyper- h, h,, and 0 are well behaved for eccentricities near unity; bola by however, the expression for the variation of the time of perifocal passage breaks down when e = 1. Fortunately, cos Vh = - l/e (28) in most cases of interest, the change in the time of peri- focal passage is of not great importance. The perturbation of the magnitude of the hyperbolic excess velocity vector is given by Of additional importance in the analysis of interplan- etary escape trajectories is the velocity vector of the 8vh = 8E/Vh (29) vehicle at a great distance from Earth. The magnitude of this hyperbolic excess velocity vector is given by the whereas the perturbations of the right ascension and dec- following function of the energy: lination are given by The unit vector S in the direction of the hyperbolic excess velocity vector and also in the direction of the outgoing asymptote of the hyperbola has direction cosines of the form cos i sin (w + VI,) sin i cos (0 + Vh) 84h = (80 f 8Vh) cos $‘h si + cos 4h S, = cos C#3h sin ah = sin 0 cos (W + vIl) + The perturbation of the angle between S and periapsis cos 0 sin (W + vII)cos i (26) is given by S, = sin 41,= sin (W + Vh) sin i (27) IV. EXAMPLE OF AN EARTH-ESCAPE MISSION As an example of the application of the deriviation of 6 as functions of the true anomaly. The time along the the perturbations, an Earth-escape mission with the initial trajectory may be determined from Fig. 7 as a function of parameters shown in Table 1 was used. The perturbations the true anomaly. The maximum true anomaly is approxi- of the elements a, e, i, T, W, and R were calculated from mately 2.5 rad; in the following discussion the time of Eq.