Chapter 25 Celestial Mechanics

Transcription

1 Chapter 5 Celestial Mechanics 5.1 Introduction: The Kepler Problem Planetary Orbits Reducing the Two-Body Problem into a One-Body Problem Energy and Angular Momentum, Constants of the Motion The Orbit Equation for the One-Body Problem Energy Diagram, Effective Potential Energy, and Orbits Circular Orbit E = E min Elliptic Orbit E min < E < Parabolic Orbit E = Hyperbolic Orbit E > Orbits of the Two Bodies Kepler s Laws Elliptic Orbit Law Equal Area Law Period Law Worked Examples... Example 5.1 Elliptic Orbit... Example 5. The Motion of the Star SO- around the Black Hole at the Galactic Center... Example 5.3 Central Force Proportional to Distance Cubed... 4 Example 5.4 Transfer Orbit... 6 Appendix 5A Derivation of the Orbit Equation A.1 Derivation of the Orbit Equation: Method A. Derivation of the Orbit Equation: Method... 3 Appendix 5B Properties of an Elliptical Orbit B.1 Coordinate System for the Elliptic Orbit B. The Semi-major Axis B..3 The Location x of the Center of the Ellipse B..4 The Semi-minor Axis B..5 Constants of the Motion for Elliptic Motion... 36

2 5B..6 Speeds at Nearest and Furthest Approaches Appendix 5C Analytic Geometric Properties of Ellipses

3 Chapter 5 Celestial Mechanics...and if you want the exact moment in time, it was conceived mentally on 8 th March in this year one thousand six hundred and eighteen, but submitted to calculation in an unlucky way, and therefore rejected as false, and finally returning on the 15 th of May and adopting a new line of attack, stormed the darkness of my mind. So strong was the support from the combination of my labour of seventeen years on the observations of Brahe and the present study, which conspired together, that at first I believed I was dreaming, and assuming my conclusion among my basic premises. But it is absolutely certain and exact that "the proportion between the periodic times of any two planets is precisely the sesquialterate proportion of their mean distances..." Introduction: The Kepler Problem Johannes Kepler first formulated the laws that describe planetary motion, I. Each planet moves in an ellipse with the sun at one focus. Johannes Kepler II. III. The radius vector from the sun to a planet sweeps out equal areas in equal time. The period of revolution T of a planet about the sun is related to the semi-major axis a of the ellipse by T = k a 3 where k is the same for all planets. The third law was published in 1619, and efforts to discover and solve the equation of motion of the planets generated two hundred years of mathematical and scientific discovery. In his honor, this problem has been named the Kepler Problem. When there are more than two bodies, the problem becomes impossible to solve exactly. The most important three-body problem in the 17 th and 18 th centuries involved finding the motion of the moon, due to gravitational interaction with both the sun and the earth. Newton realized that if the exact position of the moon were known, the longitude of any observer on the earth could be determined by measuring the moon s position with respect to the stars. In the eighteenth century, Leonhard Euler and other mathematicians spent many years trying to solve the three-body problem, and they raised a deeper question. Do the small contributions from the gravitational interactions of all the planets make the planetary system unstable over long periods of time? At the end of 18th century, Pierre 1 Kepler, Johannes, Harmonice mundi Book 5, Chapter 3, trans. Aiton, Duncan and Field, p. 411 As stated in An Introduction to Mechanics, Daniel Kleppner and Robert Kolenkow, McGraw-Hill, 1973, p

4 Simon Laplace and others found a series solution to this stability question, but it was unknown whether or not the series solution converged after a long period of time. Henri Poincaré proved that the series actually diverged. Poincaré went on to invent new mathematical methods that produced the modern fields of differential geometry and topology in order to answer the stability question using geometric arguments, rather than analytic methods. Poincaré and others did manage to show that the three-body problem was indeed stable, due to the existence of periodic solutions. Just as in the time of Newton and Leibniz and the invention of calculus, unsolved problems in celestial mechanics became the experimental laboratory for the discovery of new mathematics. 5. Planetary Orbits We now commence a study of the Kepler Problem. We shall determine the equation of motion for the motions of two bodies interacting via a gravitational force (two-body problem) using both force methods and conservation laws Reducing the Two-Body Problem into a One-Body Problem We shall begin by showing how the motion of two bodies interacting via a gravitational force (two-body problem) is mathematically equivalent to the motion of a single body acted on by an external central gravitational force, where the mass of the single body is the reduced mass µ, 1 µ = 1 m m µ = m 1 m m 1 + m. (5..1) Once we solve for the motion of the reduced body in this equivalent one-body problem, we can then return to the real two-body problem and solve for the actual motion of the two original bodies. The reduced mass was introduced in Chapter 13 Appendix A of these notes. That appendix used similar but slightly different notation from that used in this chapter. Consider the gravitational interaction between two bodies with masses m 1 and m as shown in Figure 5.1. Figure 5.1 Gravitational force between two bodies 5-3

5 Choose a coordinate system with a choice of origin such that body 1 has position r 1 and body has position r (Figure 5.). The relative position vector r pointing from body to body 1 is r = r 1 r. We denote the magnitude of r by r = r, where r is the distance between the bodies, and ˆr is the unit vector pointing from body to body 1, so that r = r ˆr. Figure 5. Coordinate system for the two-body problem The force on body 1 (due to the interaction of the two bodies) can be described by Newton s Universal Law of Gravitation F,1 = F,1 ˆr = G m 1 m r ˆr. (5..) Recall that Newton s Third Law requires that the force on body is equal in magnitude and opposite in direction to the force on body 1, F 1, = F,1. (5..3) Newton s Second Law can be applied individually to the two bodies: F,1 = m 1 d r1 dt, (5..4) F 1, = m d r dt. (5..5) Dividing through by the mass in each of Equations (5..4) and (5..5) yields F,1 m 1 = d r1 dt, (5..6) 5-4

6 F 1, m = d r dt. (5..7) Subtracting the expression in Equation (5..7) from that in Equation (5..6) yields F,1 F 1, m 1 m = d r1 dt d r dt = d r dt. (5..8) Using Newton s Third Law, Equation (5..3), Equation (5..8) becomes 1 F,1 + 1 m 1 m = d r dt. (5..9) Using the reduced mass µ, as defined in Equation (5..1), Equation (5..9) becomes where F,1 is given by Equation (5..). F,1 µ = d r dt F,1 = µ d r dt, (5..1) Our result has a special interpretation using Newton s Second Law. Let µ be the mass of a single body with position vector r = r ˆr with respect to an origin O, where ˆr is the unit vector pointing from the origin O to the single body. Then the equation of motion, Equation (5..1), implies that the single body of mass µ is under the influence of an attractive gravitational force pointing toward the origin. So, the original two-body gravitational problem has now been reduced to an equivalent one-body problem, involving a single body with mass µ under the influence of a central force F G = F,1 ˆr. Note that in this reformulation, there is no body located at the central point (the origin O ). The parameter r in the two-body problem is the relative distance between the original two bodies, while the same parameter r in the one-body problem is the distance between the single body and the central point. This reduction generalizes to all central forces. 5.3 Energy and Angular Momentum, Constants of the Motion The equivalent one-body problem has two constants of the motion, energy E and the angular momentum L about the origin O. Energy is a constant because in our original two-body problem, the gravitational interaction was an internal conservative force. Angular momentum is constant about the origin because the only force is directed 5-5

7 towards the origin, and hence the torque about the origin due to that force is zero (the vector from the origin to the single body is anti-parallel to the force vector and sinπ = ). Because angular momentum is constant, the orbit of the single body lies in a plane with the angular momentum vector pointing perpendicular to this plane. In the plane of the orbit, choose polar coordinates (r, θ) for the single body (see Figure 5.3), where r is the distance of the single body from the central point that is now taken as the origin O, and θ is the angle that the single body makes with respect to a chosen direction, and which increases positively in the counterclockwise direction. Figure 5.3 Coordinate system for the orbit of the single body There are two approaches to describing the motion of the single body. We can try to find both the distance from the origin, r(t) and the angle, θ(t), as functions of the parameter time, but in most cases explicit functions can t be found analytically. We can also find the distance from the origin, r(θ), as a function of the angle θ. This second approach offers a spatial description of the motion of the single body (see Appendix 5A) The Orbit Equation for the One-Body Problem Consider the single body with mass µ given by Equation (5..1), orbiting about a central point under the influence of a radially attractive force given by Equation (5..). Since the force is conservative, the potential energy (from the two-body problem) with choice of zero reference point U ( ) = is given by U (r) = G m 1 m r. (5.3.1) The total energy E is constant, and the sum of the kinetic energy and the potential energy is E = 1 µ v G m 1 m r. (5.3.) The kinetic energy term µv / is written in terms of the mass µ and the relative speed v of the two bodies. Choose polar coordinates such that 5-6

8 v = v r ˆr + v θ ˆθ, v = v = d r dt, (5.3.3) where v r = dr / dt and v θ = r(dθ / dt). Equation (5.3.) then becomes E = 1 µ dr dt + r dθ dt The angular momentum with respect to the origin O is given by G m m 1. (5.3.4) r with magnitude L O = r O µ v = rˆr µ(v r ˆr + v θ ˆθ) = µ rvθ ˆk = µ r dθ ˆk L ˆk (5.3.5) dt L = µ r v θ = µ r dθ dt. (5.3.6) We shall explicitly eliminate the θ expression in Equation (5.3.6), dependence from Equation (5.3.4) by using our dθ dt = L µ r. (5.3.7) The mechanical energy as expressed in Equation (5.3.4) then becomes E = 1 µ dr dt + 1 L µ r G m m 1. (5.3.8) r Equation (5.3.8) is a separable differential equation involving the variable r as a function of time t and can be solved for the first derivative dr / dt, dr dt = µ E 1 L µ r + G m m 1 r. (5.3.9) Equation (5.3.9) can in principle be integrated directly for r(t). In fact, doing the integrals is complicated and beyond the scope of this book. The function r(t) can then, in principle, be substituted into Equation (5.3.7) and can then be integrated to find θ(t). Instead of solving for the position of the single body as a function of time, we shall find a geometric description of the orbit by finding r(θ). We first divide Equation (5.3.7) by Equation (5.3.9) to obtain 1 5-7

9 dθ dr = dθ dt dr dt The variables r and θ are separable; = L µ E dθ = L µ E L L ( 1/ r ) µ r + G m m 1 r ( 1/ r ) µ r + G m m 1 r 1/. (5.3.1) 1/ dr. (5.3.11) Equation (5.3.11) can be integrated to find the radius as a function of the angle θ ; see Appendix 5A for the exact integral solution. The result is called the orbit equation for the reduced body and is given by where r = r = r 1 ε cosθ (5.3.1) L µg m 1 m (5.3.13) is a constant (known as the semilatus rectum) and E L ε = 1+ µ(g m 1 m ) 1 (5.3.14) is the eccentricity of the orbit. The two constants of the motion, angular momentum L and mechanical energy E, in terms of r and ε, are L = (µg m 1 m r ) 1/ (5.3.15) E = G m 1 m (ε 1) r. (5.3.16) The orbit equation as given in Equation (5.3.1) is a general conic section and is perhaps somewhat more familiar in Cartesian coordinates. Let x = r cosθ and y = r sinθ, with r = x + y. The orbit equation can be rewritten as r = r + ε r cosθ. (5.3.17) 5-8

10 Using the Cartesian substitutions for x and y, rewrite Equation (5.3.17) as Squaring both sides of Equation (5.3.18), (x + y ) 1/ = r + ε x. (5.3.18) x + y = r + ε x r + ε x. (5.3.19) After rearranging terms, Equation (5.3.19) is the general expression of a conic section with axis on the x -axis, x (1 ε ) ε x r + y = r. (5.3.) (We now see that the horizontal axis in Figure 5.3 can be taken to be the x -axis). For a given r >, corresponding to a given nonzero angular momentum as in Equation (5.3.1), there are four cases determined by the value of the eccentricity. Case 1: when ε =, E = E min < and r = r, Equation (5.3.) is the equation for a circle, x + y = r. (5.3.1) Case : when < ε < 1, E min < E <, Equation (5.3.) describes an ellipse, y + Ax B x = k. (5.3.) where A > and k is a positive constant. (Appendix 5C shows how this expression may be expressed in the more traditional form involving the coordinates of the center of the ellipse and the semi-major and semi-minor axes.) Case 3: when ε = 1, E =, Equation (5.3.) describes a parabola, x = y r r. (5.3.3) Case 4: when ε > 1, E >, Equation (5.3.) describes a hyperbola, where A > and k is a positive constant. y Ax B x = k, (5.3.4) 5-9

11 5.4 Energy Diagram, Effective Potential Energy, and Orbits The energy (Equation (5.3.8)) of the single body moving in two dimensions can be reinterpreted as the energy of a single body moving in one dimension, the radial direction r, in an effective potential energy given by two terms, U eff = The energy is still the same, but our interpretation has changed, E = K eff +U eff = 1 µ dr dt L µ r G m m 1. (5.4.1) r + L µ r G m m 1, (5.4.) r where the effective kinetic energy K eff associated with the one-dimensional motion is K eff = 1 µ dr dt. (5.4.3) The graph of U eff as a function of u = r / r, where r as given in Equation (5.3.13), is shown in Figure 5.4. The upper red curve is proportional to L / (µr ) 1/ r. The lower blue curve is proportional to Gm 1 m / r 1/ r. The sum U eff is represented by the middle green curve. The minimum value of U eff is at r = r, as will be shown analytically below. The vertical scale is in units of U eff (r ). Whenever the onedimensional kinetic energy is zero, K eff =, the energy is equal to the effective potential energy, E = U eff = L µ r G m m 1. (5.4.4) r Recall that the potential energy is defined to be the negative integral of the work done by the force. For our reduction to a one-body problem, using the effective potential, we will introduce an effective force such that U eff, B U eff, A = B A F eff dr B = F eff r dr (5.4.5) A 5-1

12 Figure 5.4 Graph of effective potential energy The fundamental theorem of calculus (for one variable) then states that the integral of the derivative of the effective potential energy function between two points is the effective potential energy difference between those two points, B du eff U eff, B U eff, A = dr (5.4.6) dr Comparing Equation (5.4.6) to Equation (5.4.5) shows that the radial component of the effective force is the negative of the derivative of the effective potential energy, A F r eff = du eff dr (5.4.7) The effective potential energy describes the potential energy for a reduced body moving in one dimension. (Note that the effective potential energy is only a function of the variable r and is independent of the variable θ ). There are two contributions to the effective potential energy, and the radial component of the force is then Thus there are two forces acting on the reduced body, with an effective centrifugal force given by F eff r = d dr U = d L eff dr µ r G m m 1 r (5.4.8) F r eff = F r, centifugal + F r, gravity, (5.4.9) 5-11

13 F r,centrifugal = d L dr µ r = L µ r (5.4.1) 3 and the centripetal gravitational force given by F r, gravity = G m 1 m r. (5.4.11) With this nomenclature, let s review the four cases presented in Section 5.3. Figure 5.5 Plot of U eff (r) vs. r with four energies corresponding to circular, elliptic, parabolic, and hyperbolic orbits Circular Orbit E = E min The lowest energy state, E min, corresponds to the minimum of the effective potential energy, E min = (U eff ) min. We can minimize the effective potential energy = du eff dr r=r = L µ r + G m m 1. (5.4.1) 3 r and solve Equation (5.4.1) for r, r = L G m 1 m, (5.4.13) reproducing Equation (5.3.13). For E = E min, r = r which corresponds to a circular orbit. 5-1

14 5.4. Elliptic Orbit E min < E < For E min < E <, there are two points r min and r max such that E = U eff (r min ) = U eff (r max ). At these points K eff =, therefore dr / dt = which corresponds to a point of closest or furthest approach (Figure 5.6). This condition corresponds to the minimum and maximum values of r for an elliptic orbit. (a) (b) Figure 5.6 (a) elliptic orbit, (b) closest and furthest approach The energy condition at these two points E = is a quadratic equation for the distance r, L µ r G m m 1, r = r r min = r max, (5.4.14) There are two roots r + G m 1 m E r r = G m m 1 E ± G m 1 m E L µe =. (5.4.15) µe + L 1/. (5.4.16) Equation (5.4.16) may be simplified somewhat as r = G m 1 m E L E 1± 1+ µ(g m 1 m ) 1/ (5.4.17) From Equation (5.3.14), the square root is the eccentricity ε, 5-13

15 and Equation (5.4.17) becomes A little algebra shows that EL ε = 1+ µ(g m 1 m ) r = G m 1 m E 1, (5.4.18) (1± ε). (5.4.19) r 1 ε = L / µg m 1 m L E 1 1+ µ(g m 1 m ) = L / µgm 1 m L E / µ(gm 1 m ) (5.4.) = G m 1 m E. Substituting the last expression in (5.4.) into Equation (5.4.19) gives an expression for the points of closest and furthest approach, r = r 1 ε (1± ε) = r 1 ε. (5.4.1) The minus sign corresponds to the distance of closest approach, r r min = r 1+ ε (5.4.) and the plus sign corresponds to the distance of furthest approach, Parabolic Orbit E = r r max = r 1 ε. (5.4.3) The effective potential energy, as given in Equation (5.4.1), approaches zero (U eff ) when the distance r approaches infinity ( r ). When E =, as r, the kinetic energy also approaches zero, K eff. This corresponds to a parabolic orbit (see Equation (5.3.3)). Recall that in order for a body to escape from a planet, the body must have an energy E = (we set U eff = at infinity). This escape velocity condition corresponds to a parabolic orbit. For a parabolic orbit, the body also has a distance of closest approach. This distance r par can be found from the condition 5-14

16 E = U eff (r par ) = L µ r par G m 1 m r par =. (5.4.4) Solving Equation (5.4.4) for r par yields r par = L µg m 1 m = 1 r ; (5.4.5) the fact that the minimum distance to the origin (the focus of a parabola) is half the semilatus rectum is a well-known property of a parabola (Figure 5.5) Hyperbolic Orbit E > When E >, in the limit as r the kinetic energy is positive, K eff >. This corresponds to a hyperbolic orbit (see Equation (5.3.4)). The condition for closest approach is similar to Equation (5.4.14) except that the energy is now positive. This implies that there is only one positive solution to the quadratic Equation (5.4.15), the distance of closest approach for the hyperbolic orbit r hyp = r 1+ ε. (5.4.6) The constant r is independent of the energy and from Equation (5.3.14) as the energy of the single body increases, the eccentricity increases, and hence from Equation (5.4.6), the distance of closest approach gets smaller (Figure 5.5). 5.5 Orbits of the Two Bodies The orbit of the single body can be circular, elliptical, parabolic or hyperbolic, depending on the values of the two constants of the motion, the angular momentum and the energy. Once we have the explicit solution (in this discussion, r(θ) ) for the single body, we can find the actual orbits of the two bodies. Choose a coordinate system as we did for the reduction of the two-body problem (Figure 5.7). 5-15

17 Figure 5.7 Center of mass coordinate system The center of mass of the system is given by R cm = m r m r. (5.4.7) m 1 + m Let r 1 be the vector from the center of mass to body 1 and r be the vector from the center of mass to body. Then, by the geometry in Figure 5.6, and hence r = r 1 r = r 1 r, (5.4.8) r1 = r 1 R cm = r 1 m r m r = m ( r 1 r ) = µ r. (5.4.9) m 1 + m m 1 + m m 1 A similar calculation shows that r = µ r. (5.4.3) m Thus each body undergoes a motion about the center of mass in the same manner that the single body moves about the central point given by Equation (5.3.1). The only difference is that the distance from either body to the center of mass is shortened by a factor µ / m i. When the orbit of the single body is an ellipse, then the orbits of the two bodies are also ellipses, as shown in Figure 5.8. When one mass is much smaller than the other, for example m 1 << m, then the reduced mass is approximately the smaller mass, µ = m 1 m m 1 + m m 1 m m = m 1. (5.4.31) 5-16

18 Figure 5.8 The elliptical motion of bodies interacting gravitationally The center of mass is located approximately at the position of the larger mass, body of mass m. Thus body 1 moves according to and body is approximately stationary, 5.6 Kepler s Laws Elliptic Orbit Law I. Each planet moves in an ellipse with the sun at one focus. r 1 = µ r r, (5.4.3) m 1 r = µ m r 1 r. (5.4.33) m m When the energy is negative, E <, and according to Equation (5.3.14), E L ε = 1+ µ(g m 1 m ) 1 (5.5.1) and the eccentricity must fall within the range ε < 1. These orbits are either circles or ellipses. Note the elliptic orbit law is only valid if we assume that there is only one central force acting. We are ignoring the gravitational interactions due to all the other bodies in the universe, a necessary approximation for our analytic solution. 5-17

19 5.6. Equal Area Law II. The radius vector from the sun to a planet sweeps out equal areas in equal time. Using analytic geometry in the limit of small Δθ, the sum of the areas of the triangles in Figure 5.9 is given by ΔA = 1 (r Δθ) (r Δθ)r + Δr (5.5.) Figure 5.9 Kepler s equal area law. The average rate of the change of area, ΔA, in time, Δt, is given by ΔA = 1 (r Δθ)r Δt In the limit as Δt, Δθ, this becomes + (r Δθ) Δr Δt. (5.5.3) da dt = 1 r dθ dt. (5.5.4) Recall that according to Equation (5.3.7) (reproduced below as Equation (5.5.5)), the angular momentum is related to the angular velocity dθ / dt by and Equation (5.5.4) is then dθ dt = L µ r (5.5.5) da dt = L µ. (5.5.6) Because L and µ are constants, the rate of change of area with respect to time is a constant. This is often familiarly referred to by the expression: equal areas are swept out in equal times (see Kepler s Laws at the beginning of this chapter). 5-18

20 5.6.3 Period Law III. The period of revolution T of a planet about the sun is related to the semi-major axis a of the ellipse by T = k a 3 where k is the same for all planets. When Kepler stated his period law for planetary orbits based on observation, he only noted the dependence on the larger mass of the sun. Because the mass of the sun is much greater than the mass of the planets, his observation is an excellent approximation. In order to demonstrate the third law we begin by rewriting Equation (5.5.6) in the form Equation (5.5.7) can be integrated as µ da dt = L. (5.5.7) µ da = Ldt, (5.5.8) orbit T where T is the period of the orbit. For an ellipse, Area = da = π ab, (5.5.9) where a is the semi-major axis and b is the semi-minor axis (Figure 5.1). orbit Figure 5.1 Semi-major and semi-minor axis for an ellipse Thus we have Squaring Equation (5.5.1) then yields T = µπ ab. (5.5.1) L 5-19

21 T = 4π µ a b L. (5.5.11) In Appendix 5B, Equation (5.B.) gives the angular momentum in terms of the semimajor axis and the eccentricity. Substitution for the angular momentum into Equation (5.5.11) yields 4π µ a b T = µgm 1 m a(1 ε ). (5.5.1) In Appendix 5B, Equation (5.B.17) gives the semi-minor axis which upon substitution into Equation (5.5.1) yields T = 4π µ a 3 µgm 1 m. (5.5.13) Using Equation (5..1) for reduced mass, the square of the period of the orbit is proportional to the semi-major axis cubed, 5.7 Worked Examples T = 4π a 3 G(m 1 + m ). (5.5.14) Example 5.1 Elliptic Orbit A satellite of mass m s is in an elliptical orbit around a planet of mass m p >> m s. The planet is located at one focus of the ellipse. The satellite is at the distance r a when it is furthest from the planet. The distance of closest approach is r p (Figure 5.11). What is (i) the speed v p of the satellite when it is closest to the planet and (ii) the speed v a of the satellite when it is furthest from the planet? Figure 5.11 Example

22 Solution: The angular momentum about the origin is constant and because r O,a v a and r O,p v p, the magnitude of the angular momentums satisfies L L O,p = L O,a. (5.6.1) Because ms << mp, the reduced mass µ ms and so the angular momentum condition becomes We can solve for v p in terms of the constants G, L = m s r p v p = m s r a v a (5.6.) m p, r a and r p as follows. Choose zero for the gravitational potential energy, U (r = ) =. When the satellite is at the maximum distance from the planet, the mechanical energy is When the satellite is at closest approach the energy is E a = K a +U a = 1 m v Gm m s p. (5.6.3) s a r a E p = 1 m v Gm m s p. (5.6.4) s p r p Mechanical energy is constant, therefore E E a = E p, (5.6.5) E = 1 m v Gm m s p s p r p = 1 m v Gm m s p. (5.6.6) s a r a From Eq. (5.6.) we know that v a = (r p / r a )v p. (5.6.7) Substitute Eq. (5.6.7) into Eq. (5.6.6) and divide through by m s / yields v p Gm p r p = r p r v Gm p. (5.6.8) p a r a We can solve this Eq. (5.6.8) for v p : 5-1

23 v p v p 1 r p 1 = Gm r p 1 a r p r a r a r p r = Gm a r p r p a r p r a (r v a r p )(r a + r p ) p r a = Gm r a r p p r p r a (5.6.9) v p = Gm p r a (r a + r p )r p. We now use Eq. (5.6.7) to determine that v a = (r p / r a )v p = Gm p r p (r a + r p )r a. (5.6.1) Example 5. The Motion of the Star SO- around the Black Hole at the Galactic Center The UCLA Galactic Center Group, headed by Dr. Andrea Ghez, measured the orbits of many stars within of the galactic center. The orbits of six of those stars are shown in Figure 5.1. Figure 5.1 Obits of six stars near black hole at center of Milky Way galaxy. 5-

24 We shall focus on the orbit of the star S- with the following orbit properties given in Table Distances are given in astronomical units, 1au = m, which is the mean distance between the earth and the sun. Table 5.1 Orbital Properties of S- Star Period (yrs) S- 15. (.68/.76) Eccentricity.8763 (.63) Semi-major axis ( 1 3 arcsec ) Periapse (au) Apoapse (au) 1.7 (4.5) (3.9) 181 (73) The period of S- satisfies Kepler s Third Law, given by T = 4π a 3 G(m 1 + m ), (5.6.11) where m 1 is the mass of S-, m is the mass of the black hole, and a is the semi-major axis of the elliptic orbit of S-. (a) Determine the mass of the black hole that the star S- is orbiting. What is the ratio of the mass of the black hole to the solar mass? (b) What is the speed of S- at periapse (distance of closest approach to the center of the galaxy) and apoapse (distance of furthest approach to the center of the galaxy)? Solution: (a) The semi-major axis is given by a = r p + r a = 119.5au +181au = 965.8au. (5.6.1) In SI units (meters), this is a = 965.8au m 1au = m. (5.6.13) The mass m 1 of the star S- is much less than the mass m of the black hole, and Equation (5.6.11) can be simplified to T = 4π a 3 G m. (5.6.14) Solving for the mass m and inserting the numerical values, yields 3 A.M.Ghez, et al., Stellar Orbits Around Galactic Center Black Hole, preprint arxiv:astro-ph/3613v1, 5 June,

25 m = 4π a 3 GT (4π )( m) 3 = ( N m kg )((15. yr)( s yr -1 )) = kg. (5.6.15) The ratio of the mass of the black hole to the solar mass is m m sun = kg kg = (5.6.16) The mass of black hole corresponds to nearly four million solar masses. (b) We can use our results from Example 5.1 that v p = Gm r a (r a + r p )r p = Gm r a ar p (5.6.17) v a = r p r a v p = Gm r p (r a + r p )r a = Gm r p ar a, (5.6.18) where a = (r a + r b ) / is the semi-major axis. Inserting numerical values, v p = Gm a r a r p = ( N m kg )( kg) ( m) = m s (5.6.19) The speed v a at apoapse is then v a = r p v r p = 181 a ( m s 1 ) = m s 1. (5.6.) Example 5.3 Central Force Proportional to Distance Cubed A particle of mass m moves in plane about a central point under an attractive central force of magnitude F = br 3. The magnitude of the angular momentum about the central 5-4

26 point is equal to L. (a) Find the effective potential energy and make sketch of effective potential energy as a function of r. (b) Indicate on a sketch of the effective potential the total energy for circular motion. (c) The radius of the particle s orbit varies between r and r. Find r. Solution: a) The potential energy, taking the zero of potential energy to be at r =, is The effective potential energy is r U (r) = ( b r 3 )d r = b 4 r 4 U eff (r) = L +U (r) = L mr mr + b 4 r 4. A plot is shown in Figure 5.13a, including the potential (yellow, right-most curve), the term L / m (green, left-most curve) and the effective potential (blue, center curve). The horizontal scale is in units of r (corresponding to radius of the lowest energy circular orbit) and the vertical scale is in units of the minimum effective potential. b) The minimum effective potential energy is the horizontal line (red) in Figure 5.13a. (a) (b) Figure 5.13 (a) Effective potential energy with lowest energy state (red line), (b) higher energy state (magenta line) c) We are trying to determine the value of r such that U eff (r ) = U eff (r ). Thus L mr + b 4 r 4 = L m(r ) + b 4 (r )4. 5-5

27 Rearranging and combining terms, we can then solve for r, 3 L 1 15 = br 8 m r 4 r 6 1 L =. 1 mb In the plot in Figure 5.13b, if we could move the red line up until it intersects the blue curve at two point whose value of the radius differ by a factor of, those would be the respective values for r and r. A graph, showing the corresponding energy as the horizontal magenta line, is shown in Figure 5.13b. Example 5.4 Transfer Orbit A space vehicle is in a circular orbit about the earth. The mass of the vehicle is m s = kg and the radius of the orbit is R e = km. It is desired to transfer the vehicle to a circular orbit of radius 4R e (Figure 4.14). The mass of the earth is M e = kg. (a) What is the minimum energy expenditure required for the transfer? (b) An efficient way to accomplish the transfer is to use an elliptical orbit from point A on the inner circular orbit to a point B on the outer circular orbit (known as a Hohmann transfer orbit). What changes in speed are required at the points of intersection, A and B? 4 Figure 4.1 Example 5.5 Solution: (a) The mechanical energy is the sum of the kinetic and potential energies, E = K +U = 1 m s v G m s M e R e. (5.6.1) 5-6

28 For a circular orbit, the orbital speed and orbital radius must be related by Newton s Second Law, F r = ma r G m s M e R e = m s v R e 1 m s v = 1 G m s M e R e. Substituting the last result in (5.6.) into Equation (5.6.1) yields (5.6.) E = 1 G m s M e R e G m s M e R e = 1 G m s M e R e = 1 U (R e ). (5.6.3) Equation (5.6.3) is one example of what is known as the Virial Theorem, in which the energy is equal to (1/) the potential energy for the circular orbit. In moving from a circular orbit of radius R e to a circular orbit of radius 4R e, the total energy increases, (as the energy becomes less negative). The change in energy is ΔE = E(r = 4R e ) E(r = R e ) = 1 G m s M e 4R e 1 G m M s e R e (5.6.4) = Gm s M e 8R e. Inserting the numerical values, ΔE = 1 8 G m s M e R e = 1 4 G m s M e R e = 1 4 ( m 3 kg 1 s ) (3. 13 kg)( kg) ( km) = J. (5.6.5) b) The satellite must increase its speed at point A in order to move to the larger orbit radius and increase its speed again at point B to stay in the new circular orbit. Denote the satellite speed at point A while in the circular orbit as v A, i and after the speed increase (a rocket burn ) as v A, f. Similarly, denote the satellite s speed when it first reaches point B as v B, i. Once the satellite reaches point B, it then needs to increase its speed in order to continue in a circular orbit. Denote the speed of the satellite in the circular orbit at point B by v B, f. The speeds v A, i and v B, f are given by Equation 5-7

29 (5.6.). While the satellite is moving from point A to point B in the elliptic orbit (that is, during the transfer, after the first burn and before the second), both mechanical energy and angular momentum are conserved. Conservation of energy relates the speeds and radii by 1 m (v s A, f ) G m m s e = 1 R e m (v s B,i ) G m m s e. (5.6.6) 4R e Conservation of angular momentum relates the speeds and radii by m s v A, f (R e ) = m s v B,i (4R e ) v A, f = v B,i. (5.6.7) Substitution of Equation (5.6.7) into Equation (5.6.6) yields, after minor algebra, We can now use Equation (5.6.) to determine that v A, f = G M e, v 3 R B,i = 1 GM e. (5.6.8) e 6 R e v A,i = 1 G M e, v R B, f = 1 G M e. (5.6.9) e 4 R e Thus the change in speeds at the respective points is given by Δv A = v A, f v A,i = Δv B = v B, f v B,i = G M e R e G M e R e. (5.6.3) Substitution of numerical values gives Δv A = m s, Δv B = 7. 1 m s. (5.6.31) 5-8

30 Appendix 5A Derivation of the Orbit Equation 5A.1 Derivation of the Orbit Equation: Method 1 Start from Equation (5.3.11) in the form dθ = L µ E L (1/ r ) µ r + G m m 1 r 1/ dr. (5.A.1) What follows involves a good deal of hindsight, allowing selection of convenient substitutions in the math in order to get a clean result. First, note the many factors of the reciprocal of r. So, we ll try the substitution u = 1/ r, du = (1/ r ) dr, with the result dθ = L du µ L µ E u + Gm1mu 1/. (5.A.) Experience in evaluating integrals suggests that we make the absolute value of the factor multiplying u inside the square root equal to unity. That is, multiplying numerator and denominator by µ /L, du dθ =. (5.A.3) ( µe / L u + (µg m 1 m / L 1/ )u) As both a check and a motivation for the next steps, note that the left side dθ of Equation (5.A.3) is dimensionless, and so the right side must be. This means that the factor of µ Gm m L in the square root must have the same dimensions as u, or 1 length ; so, 1 / define r L / µ Gm1m. This is of course the semilatus rectum as defined in Equation (5.3.1), and it s no coincidence; this is part of the hindsight mentioned above. The differential equation then becomes du dθ = (µe / L u + u / r ). 1/ (5.A.4) We now rewrite the denominator in order to express it terms of the eccentricity. 5-9

31 du dθ = µe / L +1/ r u ( + u / r 1/ r ) 1/ du = ( µe / L +1/ r (u 1/ r ) ) 1/ r = du ( µer / L +1 (r u 1) ). 1/ (5.A.5) We note that the combination of terms µ Er / L + 1 is dimensionless, and is in fact equal to the square of the eccentricity ε as defined in Equation (5.3.13); more hindsight. The last expression in (5.A.5) is then dθ = rdu ( ε ( ru 1) ) 1/. (5.A.6) From here, we ll combine a few calculus steps, going immediately to the substitution ru 1= ε cosα, rdu = ε sinα dα, with the final result that ε sinα dα dθ = = dα, (5.A.7) ( ε ε cos α) 1/ We now integrate Eq. (5.A.7) with the very simple result that θ = α + constant. (5.A.8) We have a choice in selecting the constant, and if we pick θ = α π, α = θ + π, cosα = cosθ, the result is r = 1 u = r 1 ε cosθ, (5.A.9) which is our desired result, Equation (5.3.11). Note that if we chose the constant of integration to be zero, the result would be r 1 r u 1 εcosθ = = (5.A.1) + which is the same trajectory reflected about the vertical axis in Figure 5.3, indeed the same as rotating by π. 5A. Derivation of the Orbit Equation: Method The derivation of Equation (5.A.9) in the form 5-3

32 1 u = ( 1 ε cos θ) (5.A.11) r suggests that the equation of motion for the one-body problem might be manipulated to obtain a simple differential equation. That is, start from F = µ a G m m 1 ˆr = µ d r r dt r dθ dt ˆr. (5.A.1) Setting the components equal, using the constant of motion L = µr (dθ / dt) and rearranging, Eq. (5.A.1) becomes dr L Gmm 1 3 µ =. (5.A.13) dt µ r r We now use the same substitution u = 1/ r and change the independent variable from t to r, using the chain rule twice, since Equation (5.A.13) is a second-order equation. That is, the first time derivative is dr dr du dr du dθ = =. (5.A.14) dt du dt du dθ dt From r = 1/ u we have dr / du = 1/ u. Combining with dθ / dt in terms of L and u, dθ / dt = Lu / µ, Equation (5.A.14) becomes dr 1 du Lu du L dt u d d = =, (5.A.15) θ µ θ µ a very tidy result, with the variable u appearing linearly. Taking the second derivative with respect to t, d r dt = d dt dr dt = d dθ dr dt dθ dt. Now substitute Eq. (5.A.15) into Eq. (5.A.16) with the result that (5.A.16) d r dt = d u L u dθ µ. (5.A.17) Substituting into Equation (5.A.13), with r = 1/ u yields 5-31

33 du L L 3 u = u Gmm 1 u. (5.A.18) dθ µ µ Canceling the common factor of u and rearranging, we arrive at du µ Gmm 1 u =. (5.A.19) dθ L Equation (5.A.19) is mathematically equivalent to the simple harmonic oscillator equation with an additional constant term. The solution consists of two parts: the angleindependent solution µ Gm1m u = (5.A.) L and a sinusoidally varying term of the form u H = Acos(θ θ ), (5.A.1) where A and θ are constants determined by the form of the orbit. The expression in Equation (5.A.) is the inhomogeneous solution and represents a circular orbit. The expression in Equation (5.A.1) is the homogeneous solution (as hinted by the subscript) and must have two independent constants. We can readily identify 1/u as the semilatus rectum r, with the result that u = u + u H = 1 ( 1+ r r A(θ θ )) r = 1 u = r 1+ r A(θ θ ). (5.A.) Choosing the product ra to be the eccentricity ε and θ = π (much as was done leading to Equation (5.A.9) above), Equation (5.A.9) is reproduced. 5-3

34 Appendix 5B Properties of an Elliptical Orbit 5B.1 Coordinate System for the Elliptic Orbit We now consider the special case of an elliptical orbit. Choose coordinates with the central point located at one focal point and coordinates (r,θ) for the position of the single body (Figure 5B.1a). In Figure 5B.1b, let a denote the semi-major axis, b denote the semi-minor axis and x denote the distance from the center of the ellipse to the origin of our coordinate system. (a) (b) Figure 5B.1 (a) Coordinate system for elliptic orbit, (b) semi-major axis 5B. The Semi-major Axis Recall the orbit equation, Eq, (5.A.9), describes r(θ), The major axis A= a is given by r(θ) = r 1 ε cosθ. A = a = r a + r p. (5.B.1) (5.B.) where the distance of furthest approach r a occurs when θ =, hence r a = r(θ = ) = r 1 ε, (5.B.3) and the distance of nearest approach r p occurs when θ = π, hence r p = r(θ = π ) = r 1+ ε. (5.B.4) 5-33

35 Figure 5B. shows the distances of nearest and furthest approach. We can now determine the semi-major axis Figure 5B. Furthest and nearest approach 1 r r r a = + =. (5.B.5) 1 ε 1+ ε 1 ε The semilatus rectum r can be re-expressed in terms of the semi-major axis and the eccentricity, r = a(1 ε ). (5.B.6) We can now express the distance of nearest approach, Equation (5.B.4), in terms of the semi-major axis and the eccentricity, In a similar fashion the distance of furthest approach is 5B..3 The Location x of the Center of the Ellipse r p = r 1+ ε = a(1 ε ) = a(1 ε). (5.B.7) 1+ ε r a = r 1 ε = a(1 ε ) = a(1+ ε). (5.B.8) 1 ε From Figure 5B.3a, the distance from a focus point to the center of the ellipse is x = a r p. (5.B.9) 5-34

36 (a) (b) Figure 5B.3 Location of the center of the ellipse and semi-minor axis. Using Equation (5.B.7) for r p, we have that x = a a(1 ε) = ε a. (5.B.1) 5B..4 The Semi-minor Axis From Figure 5B.3b, the semi-minor axis can be expressed as where b = (r b x ), (5.B.11) We can rewrite Eq. (5.B.1) as r b = r 1 ε cosθ b. (5.B.1) r r εcosθ = r. (5.B.13) b b b The horizontal projection of r b is given by (Figure 5B.b), x = rbcosθ, (5.B.14) b which upon substitution into Eq. (5.B.13) yields r = r + ε x. (5.B.15) b Substituting Equation (5.B.1) for x and Equation (5.B.6) for r into Equation (5.B.15) yields r b = a(1 ε ) + aε = a. (5.B.16) 5-35

37 The fact that r a b = is a well-known property of an ellipse reflected in the geometric construction, that the sum of the distances from the two foci to any point on the ellipse is a constant. We can now determine the semi-minor axis b by substituting Eq. (5.B.16) into Eq. (5.B.11) yielding ( b ) b= r x = a a = a. (5.B.17) ε 1 ε 5B..5 Constants of the Motion for Elliptic Motion We shall now express the parameters a, b and x in terms of the constants of the motion L, E, µ, m and 1 m. Using our results for r and ε from Equations (5.3.13) and (5.3.14) we have for the semi-major axis a = L 1 µgm 1 m (1 (1+ E L / µ(g m 1 m ) )) = G m 1 m E The energy is then determined by the semi-major axis, E Gm m a. (5.B.18) 1 =. (5.B.19) The angular momentum is related to the semilatus rectum r by Equation (5.3.13). Using Equation (5.B.6) for r, we can express the angular momentum (5.B.4) in terms of the semi-major axis and the eccentricity, Note that L = µgm 1 m r = µgm 1 m a(1 ε ). (5.B.) (1 ε ) = L µgm 1 m a, (5.B.1) Thus, from Equations (5.3.14), (5.B.1), and (5.B.18), the distance from the center of the ellipse to the focal point is ( 1 / ( ) ) Gm1m x = ε a= + EL µ Gm1m, (5.B.) E a result we will return to later. We can substitute Eq. (5.B.1) for 1 ε into Eq. (5.B.17), and determine that the semi-minor axis is 5-36

38 b al / µ Gm1m =. (5.B.3) We can now substitute Eq. (5.B.18) for a into Eq. (5.B.3), yielding Gm1m 1 b = al / µ Gm1m = L / µ Gm1m = L. (5.B.4) E µ E 5B..6 Speeds at Nearest and Furthest Approaches At nearest approach, the velocity vector is tangent to the orbit (Figure 5B.4), so the magnitude of the angular momentum is L = µr p v p, (5.B.5) and the speed at nearest approach is v p = L / µr p. (5.B.6) Figure 5B.4 Speeds at nearest and furthest approach Using Equation (5.B.) for the angular momentum and Equation (5.B.7) for r p, Equation (5.B.6) becomes v p = L = µgm m (1 ε ) 1 = Gm m (1 ε ) 1 = Gm m (1+ ε) 1 µr p µa(1 ε) µa(1 ε) µa(1 ε) A similar calculation show that the speed v a at furthest approach, ( ) v a = L = µgm m (1 ε ) 1 = Gm m 1 ε 1 = Gm m 1 ε 1 µr a µa(1+ ε) µa(1+ ε) µa(1+ ε). (5.B.7). (5.B.8) 5-37

39 Appendix 5C Analytic Geometric Properties of Ellipses Consider Equation (5.3.), and for now take ε < 1, so that the equation is that of an ellipse. We shall now show that we can write it as (x x ) a + y b = 1, (5.C.1) where the ellipse has axes parallel to the x - and y -coordinate axes, center at (x, ), semi-major axis a and semi-minor axis b. We begin by rewriting Equation (5.3.) as We next complete the square, x ε r y r. (5.C.) 1 ε 1 ε 1 ε x+ = x εr 1 ε x + ε r (1 ε ) + y 1 ε = r 1 ε + ε r (1 ε ) x εr 1 ε + y 1 ε = r (1 ε ) x εr 1 ε ( r / (1 ε )) + y = 1. (r / 1 ε ) (5.C.3) The last expression in (5.C.3) is the equation of an ellipse with semi-major axis semi-minor axis a r r 1 ε =, (5.C.4) b= = a 1 ε 1 ε, (5.C.5) and center at x = εr = εa, (5.C.6) (1 ε ) as found in Equation (5.B.1). 5-38