Gravity: Celestial Bodies & Space Travel
- Sylvia Rose
- Apr 2
- 6 min read
Gravity is the invisible force defining the paths of celestial bodies and spacecraft. It affects the way planets orbit and space navigation. Einstein's relativity theory says gravity is a curvature of spacetime caused by mass.
Gravity pulls objects toward one another. The larger an object, the stronger its gravitational pull. Jupiter, the largest planet in the solar system, has 24.79 m/s² of gravitational acceleration compared to Earth's 9.81 m/s².
In theory, the reason mass is proportional to gravity is because everything with mass emits tiny particles, gravitons. The gravitons create attraction. The more mass, the more gravitons. This has yet to be proven.
Earth's gravity is integral to weather patterns. It influences air pressure, wind, and movement of water in the atmosphere, affecting precipitation and cloud formation.
Valleys generally slightly higher gravity than hilltops or mountain peaks they're are closer to the Earth's center. Pollution and airborne particles tend to accumulate in low-lying regions.
The gravitational pull between Earth and Moon causes Moon's orbit. The balance between pull and inertia keeps the Moon traveling around the planet at an average distance of about 384,400 km.
Gravity of the Moon and to a lesser degree, the Sun, creates tides on Earth. The Moon's gravitational pull causes oceans to swell on the side nearest to the Moon and the side opposite it, creating high tides.
Without Earth's gravity the Moon would drift away into space. Similarly Earth is held in orbit by the Sun’s gravity.
According to NASA, the only known way to create artificial gravity it to apply a force on an astronaut, generating the same acceleration as on the surface of earth: 9.8 meters/sec2 or 32 feet/sec2.
It can be done with bungee chords or body restraints. Another methods is to spin the spacecraft fast enough to create enough centrifugal acceleration. Centrifugal force is force generated by rotation.
A centrifuge uses the force to separate mixture components according to density. It's often used in labs to divide liquids from solids, or liquids of varying densities.
The Earth functions on centrifugal force due to its rotation. Earth spins on its axis at about 1,600 km/hr. Force is strongest at the equator and non-existent at the poles.
As a result the Earth is slightly oblate. Earth has an equatorial radius of 6378.1 km, or 0.34% greater than the polar radius of 6356.8.
Centrifugal force pushes objects outward, while gravity pulls them inward.
Centrifugal force is often combined with gravity to determine apparent weight of objects.
Gravity & Orbit
An orbit is a dynamic equilibrium. The Sun's immense gravity constantly pulls Earth towards it. However, Earth is also moving forward with high velocity. Earth orbits the Sun at a speed of 107,000 km/h.
Forward motion prevents the Earth from falling into the Sun. Instead, the Earth continuously falls around the Sun.
Objects in orbit are in a constant state of free fall. They’re falling towards the body they orbit, but also have enough forward velocity to avoid crashing into it.
Communication satellites are placed in geostationary orbit, where they maintain a fixed position relative to Earth. This type of orbit is 35,785 km above Earth’s equator, for telecommunications and weather monitoring.
Different types of orbits are heavily influenced by gravity. For instance, low Earth orbit (LEO) is 160 to 1,930 km above Earth. Satellites in this orbit, like the International Space Station (ISS), make frequent trips around the Earth.
One cycle is about 90 minutes. Satellites in LEO travel about 28,000 km/h. Those in geostationary orbit (GEO) 35,406 km above the equator move more slowly.
Earth's magnetic field has no effect on gravity, and vice versa. They're different forces, overall independent of each other.
Speed and Distance: Speed of an orbiting object is directly related to its distance from the body it orbits. An object closer to the central body gets a stronger gravitational pull and needs to move faster to maintain orbit.
Perturbations: Gravitational influences from other celestial bodies, like the Moon or planets, can subtly alter an object's orbit.
Orbital Decay: Objects in low Earth orbit experience slight atmospheric drag, even in the thin upper reaches of the atmosphere. This gradually slows the object down, causing it to lose altitude and spiral towards Earth.
Gravity & Space Flight
Launch & Escape Velocity: Overcoming Earth's gravity is the first hurdle in any space mission. Rockets must achieve minimum speed, or escape velocity (4,0234 km/h), to break free of Earth's gravitational pull.
Once in space, spacecraft enter microgravity. Although gravity is still present, astronauts can float. Aboard the ISS, gravity is one-sixth less than on Earth, a weightless environment conducive to scientific experiments.
Orbital Mechanics Navigating in space relies heavily on orbital mechanics. Mission planners determine the precise trajectories needed to reach specific destinations.
Gravity affects a spacecraft's velocity and trajectory. The craft can perform maneuvers like orbital transfers, moving from one orbit to another; and rendezvous with other spacecraft.
Gravity Assists: In a gravity assist or slingshot maneuver, a spacecraft uses the gravitational field of a planet to change its speed and direction. By carefully approaching a planet, a spacecraft can use some of the planet's orbital momentum.
This increases its velocity without expending fuel. This technique has been instrumental in missions to the outer solar system, like the Voyager probes.
Artificial Gravity: One problem of long-duration space travel is lack of gravity. Prolonged exposure to microgravity can cause bone loss, muscle atrophy, and other health problems.
Gravity in Space Missions
For re-entry into Earth's atmosphere, spacecraft are subject to the pull of gravity. Operators try to decrease their speed to avoid overheating. The angle of approach, known as the angle of attack, helps adjust velocity.
Long-duration spaceflight introduces physiological challenges. Muscle atrophy and bone density loss are common afflictions of astronauts on the ISS. People lose 1-2% of bone mass per month in space.
Missions can last several months. Astronauts use daily resistance exercises to maintain physical health.
Gravity governs the movement of galaxies and the universe. Gravitational waves or oscillating states of gravity are created by massive cosmic events like black hole collisions.
Galaxies form complex gravitational interactions, shaping the larger cosmic structure. When galaxies come into proximity, their gravitational forces begin to influence each other.
Attraction causes galactic merging, formation of tidal tails and transition of gas and stars from galaxy to another. A tidal tail is a thin stream of material created when gravitational force strips cosmic matter to form long "tails" extending into space.
Galaxy clusters consist of hundreds to thousands of galaxies bound together by gravity. Formation is a gradual process beginning with small density fluctuations in the early universe.
As the universe expands and cools, regions with higher densities attract more matter, forming protoclusters. Over billions of years, protocluster gravity pulls in more galaxies and dark matter.
Dark matter is a major component of gravitational interaction in galaxy formation. It creates a large part of total mass in the universe. In galaxies it uses gravity to trap galaxies and gas, facilitating cluster formation.
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