Orbital Mechanics I

Discussion

circular orbits

Newton's laws only. Nothing about energy or momentum. Centripetal force and gravitational force.

F_c = F_g

Kepler's third law. Derive Kepler's third law of planetary motion (the harmonic law) from first principles.

The "constant" depends on the object at the focus. Although formulated from the data for objects orbiting the Sun, Newton showed that Kepler's third law can be applied to any family of objects orbiting a common body.

orbit families

• LEO: low earth orbit, typical altitude < 2000 km
  • space shuttle
  • space station
  • satellite telephony/messaging/internet/data: Iridium, Starlink, Globalstar, Orbcomm
  • remote sensing: EROS, Landsat
  • Hubble Space Telescope
• MEO: medium earth orbit, typical altitude 10,000 to 20,000 km
  • Satellite navigation systems like GPS occupy a special MEO with a 12 hour, half day period (semisynchronous)
  • GPS 32 semisynchronous MEOs (6 orbits × 4 satellites per orbit, plus 5 spares and 3 offline)
• GEO: geosynchronous earth orbit, seven Earth radii, one-ninth of the distance to the Moon, altitude = 36,000 km
  • types
    • GSO geosynchronous earth orbit, geosynchronous in general
  • GEO geostationary earth orbit, geosynchronous equatorial orbit
  • IGSO inclined geosynchronous earth orbit, synchronous but not stationary
  • QZO quasi-zenith orbit, highly inclined, slightly elliptical, geosynchronous orbit
  • quasi-geostationary?
  • Arthur C. Clarke: In 1945, while still in his late 20s, he was the first to propose the concept of using a network of satellites in the geosynchronous orbit for television and telecommunications
  • meteorology: GOES - Geosynchronous (Geostationary) Operational Environmental Satellites
  • communication:
    • signal relays for terrestrial broadcast and cable systems
    • direct broadcast satellite TV and radio
  • TDRS: Tracking and Data Relay Satellite
  • GEOs tend to drift toward a large gravity anomaly over the Indian Ocean — an area of above average gravity thought to be above a mantle subduction zone

Magnify

A snapshot of the Earth with 7,560 of its artificial satellites in 2022 (Esri, UCS, NORAD). Satellites on the ring are in geosynchronous earth orbit (GEO). Those in the dense web near the Earth are in low earth orbit (LEO). Scattered in between are satellites in medium earth orbits (MEO). The most prominant of these are the constellation of GPS satellites. The Moon, Earth's only natural satellite, is approximately nine times farther from the Earth than the ring of geosynchronous satellites.

binary systems

Circular motion about the center of mass

Still just a balance between centripetal and gravitational force, but slightly more complicated

Lagrange points

The three body problem. Lagrange libration points are the simplest solutions (sometimes called Lagrangian points or just Lagrange points).

Still just a balance between centripetal and gravitational forces, but now more complicated

The five Lagrange points of the Earth-Sun system. Satellites in orbit at these locations remain fixed with respect to the Earth and Sun. This figure is not drawn to scale.

L1 and L2 are approximately four times farther from the Earth than the Moon. L3 is a near the "anti-Earth" point.

L4 and L5 are at the vertex of an equilateral triangle formed with the Earth and Sun. L4 leads the Earth and L5 follows.

Paraphrase needed: Objects can settle in an orbit around a Lagrange point. Orbits around the three collinear points, L1, L2, and L3, are unstable. They last but days before the object will break away. L1 and L2 last about 23 days. Objects orbiting around L4 and L5 are stable because of the Coriolis force.

• L1
  • Artificial satellites between the Sun and Earth
    • International Cometary Explorer (ICE), 1978. Originally named International Sun-Earth Explorer 3 (ISEE 3). Moved to solar orbit in 1983.
    • Wind, 1994.
    • Solar and Heliospheric Observatory (SOHO), 1995.
    • Advanced Composition Explorer (ACE), 1997.
    • Genesis, 2001. Moved to solar orbit in 2005.
    • Deep Space Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC), 2015. Provides advanced warning of potentially adverse space weather events and monitors the environment of the Earth on a hemispheric scale. DSCOVR spent ten years locked in storage for dumb-assed political reasons. Originally known as Triana (in honor of Rodrigo de Triana, the first member of Columbus's crew to spot America). Derisively known as the "GoreSat" and a "multimillion-dollar screen saver" by political opponents of US Vice President Al Gore. (Al Gore was a proponent of DSCOVR during the time he was a presidential candidate.)
    • LISA Pathfinder, 2015? LISA stands for Laser Interferometer Space Antenna. Originally named Small Missions for Advanced Research in Technology 2 (SMART 2). The LISA Pathfinder mission is one component of the larger LISA project to look for low-frequency gravitational waves.
    • Kuafu Project (夸父計劃), 2017? A Chinese space weather monitoring system composed of three satellites. Kuafu A will be placed at L1. Kuafu B1 and B2 will be placed in polar orbits. The project is named after Kua Fu (夸父), a giant in Chinese mythology who died trying to catch the Sun. Autotranslators translate Kua Fu (夸父) as "braggadocio" or "boast father".
    • Global Sunshade. A proposed geoengineering project to place large opaque satellites at L1 to reduce solar radiation received by Earth and counteract global warming.
• L2
  • Artificial satellites in Earth's shadow
    • Wilkinson Microwave Anisotropy Probe (WMAP), 2001. Moved to solar orbit in 2010.
    • Eddington. Proposed for 2008. Canceled in 2004.
    • Herschel Space Observatory and Planck Surveyor, 2009.
    • Chang'e 2 (嫦娥二号), 2011–2012. The spacecraft left lunar orbit on 9 June 2011 to fly to L2. After spending 235 days at L2, Chang'e 2 departed on 15 April 2012 to explore deep space, including a flyby of the small near Earth asteroid 4179 Toutatis.
    • Global Astrometric Interferometer for Astrophysics (GAIA), 2013.
    • James Webb Space Telescope. Originally known as the Next Generation Space Telescope. Launch date uncertain. Last reported date was 2019.
    • Darwin. Proposed for 2015, but canceled in 2009. (More precisely, it was never approved.)
    • Terrestrial Planet Finder (TPF). Would have launched sometime after 2015. Deferred indefinitely in 2007. Canceled in 2011.
    • New Worlds Mission. Starshade and telescope. Built, but not funded.
    • Advanced Technology Large-Aperture Space Telescope (ATLAST or ATLAS Telescope), 2025-2035?
    • Spectrum-Roentgen-Gamma (Спектр Рентген-Гамма, Spektr-RG), 2018.
    • Euclid, 2020. Euclid is an ESA mission to map the geometry of the dark Universe.
      • Euclid Consortium
      • ESA page
    • Planetary Transits and Oscillations of Stars (PLATO), 2018. Detection and characterization of terrestrial exoplanets around bright solar-type stars, with emphasis on planets orbiting in the habitable zone.
    • Wide Field Infrared Survey Telescope (WFIRST), 2025. WFIRST is a NASA observatory designed to settle essential questions in the areas of dark energy, exoplanets, and infrared astrophysics.
    • Advanced Telescope for High Energy Astrophysics (ATHENA), 2028. ATHENA will be an x-ray telescope designed to address the Cosmic Vision science theme 'The Hot and Energetic Universe'.
      • Athena X-ray Observatory: Community Support Portal
      • ESA page
• L3
  • Fictional "anti-Earths"
    • Pythagorean counter Earth behind the "central fire", which is different from the Sun. Part of their numerological belief in a 10 body universe. 10 since 1 + 2 + 3 + 4 (the sum of the harmonic series) equals 10
    • Journey to the Far Side of the Sun. (a.k.a. Doppelgangers) 1969
    • Bizarro World, a.k.a. htraE (Earth spelled backwards). Home of Bizarro Superman
• L4 & L5
  • Jupiter trojans
    • 4269 in the L4 Greek camp ahead of Jupiter
    • 2432 in the L5 Trojan camp behind Jupiter
  • Venus trojans
    • 1 temporary trojan at the L4 point, 2013 ND15
  • Mars trojans
    • 1 L4 trojan, 1999 UJ7 (121514)
    • 8 L5 trojans. The first confirmed was 5261 Eureka.
    • 2 co-orbital asteroids may become Trojans.
  • Saturnian satellite groups
    • Telesto-Tethys-Calypso
    • Helene-Dione-Polydeuces
  • Uranus trojans
    • 1 L4 trojan, 2011 QF99
  • Neptune trojans
    • 13 L4 trojans. The first confirmed was 385571 Otrera
    • 4 L5 trojans
  • Earth trojans
    • 2010 TK7 Earth's first confirmed trojan asteroid
    • The twin STEREO spacecraft will spend some time around L4 and L5 but won't enter into true Lagrangian orbits. STEREO A (for ahead) is approaching L4 and STEREO B (for behind) is approaching L5. STEREO is an acronym for Solar Terrestrial Relations Observatory. We want to encourage good relations between the Sun and the Earth.
    • Spitzer Space Telescope, 2013–2015. Was in an approximate L5 orbit (an Earth trailing orbit).
  • Earth-Moon (as opposed to Sun-Earth)
    • The L4 and L5 points in the Earth-Moon system have been proposed as prime locations for large "cities in space". The most famous proponent of such proposals was the American physicist Gerard K. O'Neill (1927–1992) who wrote a popular book on the subject in 1977 filled with fantastic illustrations called The High Frontier: Human Colonies in Space (paid link).
• Co-orbital states
  • Prograde, orbiting in the same general direction as the planet
    • The body goes through a cycle of catching up and falling behind the planet. This makes it appear to change direction in the planet's frame of reference
      • Horseshoe orbit
        • 3753 Cruithne oscillates back and forth between the Earth's L4 and L5 points (passing through L3 in the middle) once every 770 years. The orbit appears circular in the Sun's frame of reference but horseshoe shaped (or c-shaped) in the Earth's. Every 385 years it comes to within 15 million kilometers of Earth. The last time was 1900. The next will be 2285.
        • 2002 AA29, 2006 JY26, 2010 SO16, 2013 BS45
      • Tadpole orbit
        • An apparent orbit around L4 or L5. Elliptical when viewed from the Sun, but tadpole shaped (or comma shaped) when viewed from the Earth.
    • When the orbits of two bodies have different eccentricities but similar periods (1:1 orbital resonance), the one on the more elliptical orbit appears to loop around the one with the more circular orbit
      • Quasi-satellites
        • Venus: 2002 VE68
        • Earth: 2001 GO2, 2003 YN107, 2004 GU9, 2006 FV35, 2013 LX28, 2014 OL339, 2016 HO3
        • Neptune: 2007 RW10
        • Pluto: 15810 Arawn
  • Retrograde, orbiting in a direction opposite that of the planet
    • Trisectrix (limaçon). A type of roulette curve that crosses itself once resulting in a smaller, more elliptical loop inside a larger, more circular loop.
      • 2015 BZ509

noncircular orbits

qualitative description of noncircular orbits

centripetal-gravitational forces don't balance

uses

• Molniya orbit
  Developed for coverage of areas above 60° (?) latitude. Typically uses three satellites in an unusually elliptical orbit. Each satellite rotates into the farthest point from Earth, where it spends about 8 (?) hours. The satellite obeys Kepler's second law of planetary motion, so the speed at this point is low. If the period of the satellite is set just right, the satellite will appear relatively motionless during this period.
• Hohmann transfer orbit, opportunities
  The point in an orbit where the engines are fired becomes a point in a new orbit. The burn point is an intersection between the old and new orbits, a point of common tangency in most cases. The burn must occur where the current and desired orbits intersect.