🪐Intro to Astronomy Unit 3 – Orbits and Gravity

Key Concepts and Definitions

• Orbit refers to the path an object takes around another object due to the force of gravity
• Gravity is the force of attraction between any two objects with mass
• Kepler's laws of planetary motion describe the motion of planets around the sun and moons around planets
  • First law states planets orbit the sun in an elliptical path with the sun at one focus
  • Second law states a line connecting a planet to the sun sweeps out equal areas in equal times
  • Third law relates a planet's orbital period to its average distance from the sun
• Aphelion is the point in an orbit where an object is farthest from the sun
• Perihelion is the point in an orbit where an object is closest to the sun
• Escape velocity is the minimum speed needed for an object to break free of a planet or moon's gravitational force and leave without further propulsion

Historical Background

• Ancient Greeks believed Earth was the center of the universe with planets and stars orbiting in perfect circles (geocentric model)
• Nicolaus Copernicus proposed the sun-centered model of the solar system in the 16th century (heliocentric model)
• Galileo Galilei's observations of Jupiter's moons and Venus' phases provided evidence supporting Copernicus' heliocentric model
• Tycho Brahe made detailed observations of planetary positions without a telescope, which helped Kepler derive his laws
• Isaac Newton built upon Kepler's laws to develop the universal law of gravitation and laws of motion
  • Explained the force that keeps planets in orbit around the sun and moons around planets
• Albert Einstein's theory of general relativity further refined our understanding of gravity as a warping of spacetime

Laws of Planetary Motion

• Kepler's first law of planetary motion states that the orbit of a planet around the sun is an ellipse with the sun at one of the two foci
  • An ellipse is an elongated circle with two foci; the shape is determined by its eccentricity
  • Eccentricity ranges from 0 (perfect circle) to 1 (parabola); most planets have low eccentricity
• Kepler's second law states that a line segment joining a planet and the sun sweeps out equal areas during equal intervals of time
  • Planets move faster when they are closer to the sun and slower when farther away
  • This law is a consequence of the conservation of angular momentum
• Kepler's third law states that the square of a planet's orbital period is proportional to the cube of its average distance from the sun
  • Expressed mathematically as $P^2 = a^3$, where $P$ is the orbital period in years and $a$ is the semi-major axis in astronomical units (AU)
  • This law allows us to calculate the distance of a planet from the sun based on its orbital period

Gravitational Forces

• Gravity is a fundamental force of attraction between any two objects with mass
• Newton's law of universal gravitation states that the force of gravity between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them
  • Expressed mathematically as $F = G \frac{m_1 m_2}{r^2}$, where $F$ is the force of gravity, $G$ is the gravitational constant, $m_1$ and $m_2$ are the masses of the objects, and $r$ is the distance between their centers
• The strength of gravity decreases rapidly with distance; doubling the distance between objects reduces the force of gravity by a factor of four
• Tidal forces are the result of the uneven gravitational pull on an extended object (such as a moon or planet) by another object (like a planet or star)
  • Tidal forces cause the tides on Earth due to the moon's gravitational pull
  • Tidal forces can also cause tidal heating, which is responsible for the volcanic activity on Jupiter's moon Io

Types of Orbits

• Circular orbits have an eccentricity of 0 and a constant distance between the orbiting object and the center of mass
  • Most planets in our solar system have nearly circular orbits
• Elliptical orbits have an eccentricity between 0 and 1, with the center of mass at one focus of the ellipse
  • Comets often have highly elliptical orbits that take them close to the sun at perihelion and far out into the solar system at aphelion
• Parabolic orbits have an eccentricity of 1 and extend to infinity, with the object escaping the gravitational pull of the central mass
  • Objects in parabolic orbits have exactly the escape velocity at periapsis (closest approach)
• Hyperbolic orbits have an eccentricity greater than 1 and also extend to infinity, with the object having more than the escape velocity at periapsis
  • Interstellar objects like 'Oumuamua are thought to follow hyperbolic orbits through our solar system
• Geosynchronous orbits have a period equal to Earth's rotational period (23 hours, 56 minutes) and remain fixed over a point on Earth's equator
  • Many communication satellites are placed in geosynchronous orbits to provide continuous coverage

Orbital Mechanics

• Orbital mechanics is the study of the motion of artificial satellites and space vehicles in orbit around Earth or other celestial bodies
• The velocity required for an object to maintain a stable orbit is determined by the object's distance from the center of mass and the mass of the central object
  • Orbital velocity decreases with increasing distance from the central object
• Orbital maneuvers, such as changing the altitude or inclination of an orbit, require changes in velocity ($\Delta v$)
  • These maneuvers are accomplished through the use of thrusters or propulsion systems
• Orbital perturbations are deviations from the ideal orbit caused by factors such as atmospheric drag, gravitational influences of other bodies, and solar radiation pressure
  • These perturbations must be accounted for to maintain the desired orbit over time
• Orbital decay occurs when an object in orbit loses altitude over time due to atmospheric drag or other factors
  • Low Earth orbit (LEO) satellites experience more orbital decay than those in higher orbits due to the denser atmosphere

Applications in Space Exploration

• Orbits are essential for satellite communication, navigation, and Earth observation
  • GPS satellites are placed in medium Earth orbits (MEO) to provide global coverage
  • Weather satellites are often placed in polar orbits to observe the entire Earth's surface
• Interplanetary missions use orbits to navigate between planets and study celestial bodies
  • The Hohmann transfer orbit is an energy-efficient elliptical orbit used to transfer between two circular orbits (like Earth and Mars)
  • Gravity assist maneuvers use the gravitational pull of a planet to change a spacecraft's velocity and trajectory without expending fuel
• Space stations, like the International Space Station (ISS), are placed in low Earth orbit to allow for resupply missions and crew rotations
• Lagrange points, where the gravitational forces of two large bodies (like Earth and the sun) balance out, are used for space observatories and potential future space colonies
  • The James Webb Space Telescope (JWST) is located at the Earth-Sun L2 Lagrange point

Challenges and Future Developments

• Space debris, consisting of defunct satellites and other artificial objects in orbit, poses a collision risk to operational spacecraft
  • Mitigation strategies include deorbiting defunct satellites and designing spacecraft to minimize the creation of debris
• Orbital crowding, particularly in popular orbits like LEO and GEO, requires careful coordination and regulation to avoid interference between spacecraft
• Developing technologies, such as electric propulsion and autonomous navigation, could improve the efficiency and sustainability of orbital operations
• Mega-constellations, consisting of thousands of small satellites in LEO, are being developed to provide global internet coverage but raise concerns about their impact on astronomy and the space environment
• Future space exploration missions may require new types of orbits and trajectories, such as cycler orbits between Earth and Mars or orbits in the Martian moon system
  • These orbits could enable more efficient and sustainable exploration of the solar system