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Home , 2685 Masursky, Mean motion, Perturbation (astronomy)

Today in 111: , perturbations and orbital resonances  Leftovers: proofs of Kepler’s second and third laws  Elliptical and center of  More than two bodies: perturbations by on each other’s orbits  resonances Simulation of by the inner  Asteroids planets of the orbits of asteroids initially  Stability and instability in spread evenly along the ’s , by the belt Paul Wiegert (U. W. Ontario) and colleagues.

4 October 2011 Astronomy 111, Fall 2011 1 News flash: 2011 Nobel Prize in Physics

… goes for the detection of in the Universe’s expansion, by measurement of distances with supernovae of type Ia, to  Saul Perlmutter (UC Berkeley)  Adam Riess (STScI and JHU)  Brian Schmidt (ANU) Occasionally, physics prizes are given in astronomy.

4 October 2011 Astronomy 111, Fall 2011 2 Kepler’s Laws (continued)

 Kepler’s first law: we just showed, above, that elliptical orbits are consistent with and the laws of motion, and the relationship between the parameters of the and the conserved mechanical quantities.  Kepler’s second law (equal ): dη dr’ dA′= dr ′′( r dη ) dA r r2 dA′ r’ dA= dηη r′′ dr = d ∫0 2 r’dη dA r2 dη 1 = = rvη dt22 dt h 1 ==GMa 1 −=ε 2 constant. 22 ( ) 29 September2011 Leftovers Astronomy 111, Fall 2011 3 Kepler’s Laws (continued)

 Kepler’s third law (period and semimajor axis): dA h = Integrate over one : dt 2 APh dA′ = dt ∫∫002 h A= P = π ab 2 πε24− 2 44ππ222ab 41a ( ) 2 Pa23= = = . 2 h GMa(1 −ε 2 ) GM

29 September2011 Leftovers Astronomy 111, Fall 2011 4 Elliptical orbits and

As in the case of circular orbits, allowing the two to be finite in ratio changes the equations for elliptical orbits very little, if one just introduces the center of mass and reduced mass. The results for positions, velocities and conserved quantities: mm12 µµ µ =,r12 =−= rr ,, r mm12+ m1 m 2 µµ v12=−= vv,, v mm12 1 Gmµµ( ++ m) Gm( m) Ev=−=µ 2 12− 12, 22ra 2 L=µε Gm( 12 +− m) a(1.)

4 October 2011 Astronomy 111, Fall 2011 5 Elliptical orbits and center of mass (continued)

And for a few other useful relations: dA h 1 ==+−Gm( m) a1 ε 2 dt 22 12( ) 4π 2 Pa23= Gm( 12+ m)

2 21 v=+− Gm( 21 m) ra 22ππr Recall: P = = for circular orbits. v ω

4 October 2011 Astronomy 111, Fall 2011 6 A two-body-system simulator

Courtesy of Terry Herter (Cornell U.):

http://www.pas.rochester.edu/~dmw/ast111/Java/binary.htm

Chandra X-ray Observatory/TRW/NASA

4 October 2011 Astronomy 111, Fall 2011 7 Perturbation of orbits by other planets

In the two-body problem considered hitherto, each body is subjected to a purely 1 r 2 force centered at the other body.  When there is any deviation from the perfect inverse square law, orbits are no longer perfect .  Such departures would be produced, for instance, by additional bodies in the system, or by nonspherical symmetry of the mass density in one or other of the bodies.  In our these effects are small enough that orbits are usually very close to elliptical. We call the small additional effects perturbations.  The main effect of perturbations is that the orbits don’t exactly close: each successive orbit is very slightly different from the last.

4 October 2011 Astronomy 111, Fall 2011 8 Perturbation of orbits by other planets (cont’d)

 Because of perturbation and slight departure from closed elliptical orbits, the orientations of planetary orbits slowly change. We call this effect perihelion precession.  The best example is , with a perihelion precession of 5600.73 arcsec/century. 5025 arcsec/century of this is an artifact of our coordinate system (the precession of equinoxes, due to Earth’s rotational-axis precession, but the remaining 576 arcsec/century is due to perturbations: 280 arcsec/century from 151 arcsec/century from 102 arcsec/century from other planets (mostly Earth) 43 arcsec/century from general relativity (warping of space by the )

4 October 2011 Astronomy 111, Fall 2011 9 The many-body problem  The two-body problem is of a class mathematical physicists call completely integrable. That means that there is one conserved quantity per degree of freedom. • For point masses: t ↔ E;,, x y z ↔= px mv xyz,,. p p  Unique solutions can be obtained for completely integrable systems, so orbits can be predicted exactly for the two-body system.  For more bodies, even 3, there is no general analytical solution.  Even the restricted three-body problem (two massive, one massless particle) is complicated, as we shall see.  Among the unpredictable problems: • Cumulative effects of perturbations (resonances) • Non-deterministic solutions (“chaos”) 4 October 2011 Astronomy 111, Fall 2011 10 Orbital resonances

 Periodic perturbations on an object, such as the nearby passage of a , will add up if they are in phase with the orbital motion of the object.  If there are out of phase, then the perturbations will cancel on average and there will be no net change in the orbit of the object.

4 October 2011 Astronomy 111, Fall 2011 11 Orbital resonances (continued)

 Even small perturbations can be important: • The solar system is about 4.6 billion years old. • This means that everything in the inner Solar system has had more than a billion orbits. A billion small perturbations, if they add constructively (mostly in phase), can have large effects on the orbit of a small planet or asteroid.  The leading cause in our solar system of small perturbations in phase with orbital motion are the mean- motion resonances of three-body systems.

4 October 2011 Astronomy 111, Fall 2011 12 Mean-motion resonances

Consider the three-body system consisting of the Sun, a particle (large or small), and a perturbing planet. When do perturbations from the planet on the particle add constructively? That is, how could they be in phase with the orbital motion?  One way is for an integer times the orbital period of the planet is equal to an integer times the orbital period of the particle. • Define the mean motion to be the average value of angular velocity during a period: 2π GM Here, MM= , since it's our ω ≡=  P a3 Solar system we're discussing.

4 October 2011 Astronomy 111, Fall 2011 13 Mean-motion resonances (continued)

• If the mean motion of the particle, ω p , and that of the perturbing planet, ω q , are related by ijωω≅ , Note that we don’t need pqexact equality for there to be or equivalently jPpq≅ iP , a big effect over many orbits. where i and j are integers, then any possible orientation of the three bodies will recur every j orbits of the particle and every i orbits of the perturber – and if any of those orientations comprise a perturbation, that is manifestly in phase with the orbit. We would call this one the “i:j resonance.”

4 October 2011 Astronomy 111, Fall 2011 14 Mean-motion resonances (continued)

 Recall that 2π a3/2 P = GM (Kepler’s third law), so the semimajor axis ratio for a resonance is 23 2/3 aPpp   i  =  ≈  .    aPqq    j

 The largest convenient collection of small particles in orbit around the Sun, on which to see the effect of planetary perturbations, is the asteroids, a collection of small rocky bodies lying mostly between the orbits of and Jupiter.

4 October 2011 Astronomy 111, Fall 2011 15 Vital statistics of main-belt asteroids 24 -4 Total mass ~2.3×× 10 gm ( 3.9 10 M⊕ ) ~200,000 with diameter Number ≥ 1 km Mass range 1014− 10 24 gm Diameter range 1-1000 km Density range 0.5− 7 gm cm-3 Orbits: Major axis range 1.7− 3.7 AU Eccentricity range 0.05− 0.9 Inclination range 0−° 30 Typical separation 107 km (0.07 AU) Eight asteroids seen at close range, shown on a common scale (Dawn/JPL/NASA)

4 October 2011 Astronomy 111, Fall 2011 16 Visits to the asteroids Including NASA’s and Cassini, which came close to some asteroids on their way to the outer planets, eight have encountered ten different asteroids: Mission Launch Visited (flyby unless otherwise indicated)

Galileo (NASA) 1989 , 243 Ida (+ Dactyl, the first asteroidal )

NEAR (NASA) 1996 – orbiter, lander

Cassini (NASA) 1997

Deep Space 1 (NASA) 1998 9969 Braille

Stardust (NASA) 1999

Hayabusa (JAXA) 2003 – orbiter, lander, sample return

Rosetta (ESA) 2004 2867 Steins, 21 Lutetia

Dawn (NASA) 2007 , 1 (2015) – will orbit both

4 October 2011 Astronomy 111, Fall 2011 17 The main Number of asteroids of Number Distance from Sun (AU) Sun from Distance

Distance from Sun (AU) Distance from Sun (AU)

Snapshot of distribution of main-belt asteroids, as would be viewed from the pole (Bidstrup et al. 2005). The Sun’s is labelled with a blue , and the five inner planets by red blocks.

4 October 2011 Astronomy 111, Fall 2011 18 The main asteroid belt (continued) Distance above above Distance ecliptic (AU)

Distance from Sun (AU)

Snapshot of distribution of main-belt asteroids above and below the ecliptic (left), and perspective sketch of volume occupied by the main belt (right) (Bidstrup et al. 2005).

4 October 2011 Astronomy 111, Fall 2011 19 Resonances resulting in instability

Within the asteroid belt, particles often change orbits due to gravitation of other particles they encounter.  Most particles that are knocked thereby into strong mean motion resonances with Jupiter have their orbits slowly increase in eccentricity until they cross the orbit of a planet.  An encounter with that planet will eventually eject the particle from the inner Solar system.  This process is evident in the distribution of semimajor axes among the asteroids: the number of particles in the Jovian mean-motion resonances is very small. These sharp dips in the distribution are called the Kirkwood gaps.

4 October 2011 Astronomy 111, Fall 2011 20 Resonances resulting in instability (continued)

Mean-motion 3:1 resonances with Jupiter 5:2 7:3 The Kirk- wood gaps (Bidstrup et al. 2005)

Number of asteroids of Number 4:1 2:1 Earth Mars Jupiter 5:3

Semimajor axis length (AU)

4 October 2011 Astronomy 111, Fall 2011 21 Resonances resulting in stability

But some locations in some of the mean-motion resonances offer protection against being scattered away by a planet.  Suppose, for instance, that an encounter resulted in an asteroid in an orbit just like Jupiter’s, but placed opposite the Sun from Jupiter. Then it stays away from Jupiter, and out of the greatest scattering danger.  The Hildas, Thule, and Trojan groups are examples of this effect: they lie in Jovian orbital resonances, but keep well away from Jupiter all the time. • We will study this in more detail, under the rubric of the restricted three-body problem.

4 October 2011 Astronomy 111, Fall 2011 22 Resonances resulting in stability (continued)

Mean-motion resonances with Jupiter Jupiter (Bidstrup et al. 2005)

Number of asteroids of Number Hildas Trojans

Earth Mars 3:2 1:1

Semimajor axis length (AU)

4 October 2011 Astronomy 111, Fall 2011 23 Resonances resulting in stability (continued)

Simulation of asteroids locked in a 1:1 mean-motion resonance with Earth, by Paul Wiegert (U. W. Ontario) and colleagues.

4 October 2011 Astronomy 111, Fall 2011 24