Have you ever thrown a rock straight up into the sky to see how far it would go or accidentally let go of a helium balloon? Let’s not kid ourselves. Either as a kid or an adult, we probably all had. During this time, you probably noticed that the balloon was as good as gone, while the rock only flew to a certain point and then dropped back to the ground. Now, we know this ‘phenomenon’ to be the result of gravity, which is also the force that keeps our feet grounded on the Earth. However, you might not have known gravity to be the force field that dictates the structure of our universe. Intrigued? Let me show you how it works.
Gravity is a force that acts on every object in the universe, and its strength of attraction largely depends on the mass and distance between the center of these bodies. In other words, the universe can be imagined as a gigantic envelope created by gravitational force fields that act on every object within it. Gravity literally holds our solar system and entire galaxies together.
This article will illustrate how gravity functions in space relative to the positioning of celestial bodies in the universe. It will also go further to clarify the controversies that exist between the density of objects and gravity.
Where does Gravity Act in Space?
If you thought gravity only existed on Earth, then you are unfortunately more than wrong. Gravity is everywhere.
Even though space shuttles in orbit seemingly portray their inhabitants to experience zero gravity, the term ‘zero gravity’ does not accurately describe this spectacle.
The feeling of weightlessness results from the space shuttle escaping far enough from the gravitational force field of the Earth to be in orbit. The further the shuttle moves from the center of the Earth, the less gravitational pull the Earth has on the space shuttle. But this does not mean there is no longer any attraction between the two bodies.
To really understand how all this works, let’s first figure out how the mass, weight, and density of an object relate to gravity.
Mass
The mass of an object is the fundamental measure of the amount of matter within it. Although definitions may vary, it’s important to understand that this quantity is what makes one object unique from another. Generally speaking, the gravitational field strength is directly proportional to an object’s mass. The greater the mass, the greater the field strength.
Weight
The weight of an object will vary if its distance from the center of the Earth is changed at any given instant, which can be attributed to the gravitational pull between the two bodies. Simply put, the weight of an object can be defined as the gravitational force exerted by one object on the mass of another and vice versa. For example, at 125 miles above the Earth’s surface, the gravity an astronaut experience is only 94% of what they would experience on Earth (1).
The formula for Weight (w) equates to Mass (m) · Acceleration of Gravity (g), and the only changeable variable here is the acceleration of gravity as its value is dictated by the distance between one object and another. As such, the weight of an object will be ever-changing while floating through the darkness of space.
Density
Contrary to popular belief, or just what most of us are taught in school, density can actually be split into two types: ‘weight density’ and ‘mass density.’
When we use the term ‘density’, it usually refers to mass density. This is because mass density forms the basis of scientific evaluations, as it quantifies the mass of an object per unit volume (e.g., cubic centimetres), and this will never change. For example, under the metric system, the density of water is approximately 1g per cubic centimetre at 4°C.
Weight density, on the other hand, is referred to as ‘specific weight’ or ‘unit weight’, with its units being weight (newton) per unit volume of a material. Similar to weight, specific weight is subject to change depending on the acceleration of gravity.
The density of an object compared to another will influence its behaviour in a given gravitational force field. Although the gravity of the Earth will, without a doubt, exert its force on a helium-filled balloon, this balloon will fly towards the sky without stopping, as helium molecules are less dense (lighter) than the oxygen and nitrogen molecules of our atmosphere (2).
Although not to be mistaken, the capability of astronauts to float around in their space shuttles does not mean they are less dense than what surrounds them (space is a vacuum). So then why do they float anyways?
You will get your answer soon enough.
So, does gravity exist in space? The answer is, without a doubt, yes. But where and what gravity will act upon fully depends on which objects are within its force field. Since all objects with mass exert a gravitational attraction, no matter how small, every object will influence another’s position. There are exceptions, however, but we will get into that later.
How Does Gravity Influence Objects in Space?
Before I get any messages telling me I have got gravity all wrong, it must be made clear that the gravity I have been describing acts as a force under Newtonian Physics, which is generally true if we remain within our galaxy group.
However, the most accurate theory of gravity is Einstein’s General Theory of Relativity. With the cosmological constant included, the Theory of Relativity describes the effect of gravity as the movement of an object within a warped/curved spacetime due to the effect of a massive body like the Earth or our Sun.
This “behaviour of spacetime depends on how much mass and energy is present, how it is distributed, and how it is moving.” So, General Relativity does explain more than the traditional gravitational attraction between objects, but within a localized galaxy group/groups, we can be safe to assume spacetime will continue to affect objects under the guise of Newtonian Gravity. (6)
With that out of the way, we understand the mass of an object to be the unique measure of the amount of matter it contains. And this mass is considered when determining the weight of an object under the gravitational influence of another. As such, we should be able to identify some examples that will showcase such an attraction between objects:
- Although mass as a quantity cannot be changed, but it can experience a force such as gravity. And this downward force in turn, expresses a quantity we know to be the weight of an object. For example, an object of mass, 200 kg, will occupy the same volume at any point in space when immersed in water, but its weight will vary across these different locations in space.
- The weight of an object is relative to the gravitational force exerted by another. And the strength of this influence depends on the strength of the gravitational field and their separation distance. It means that the weight of a space shuttle on Earth will significantly change when the shuttle lands on the moon or Mars.
- The gravitational force field is flexible to an extent throughout space. Imagine spreading a sheet over four wooden shafts nailed to the ground. When you place two balls of different masses on the sheet’s surface, you will notice that the ball with the lesser mass will roll towards the ball with the greater mass. The distance separating the balls will primarily depend on the tension present on the sheet. And such is a simple illustration of a gravitational force field (if we could see it, that is).
- The International Space station orbits the Earth at an altitude of about 250 miles. The gravity at this altitude is about 90% of the gravity experienced on the Earth’s surface. So, It is safe to say that a person who weighs 100kg on the Earth’s surface would weigh approximately 90kg at that altitude (3).
What causes Objects to Float in Space?
First off, our understanding of gravity was achieved through hypotheses and plausible assumptions using mathematical proofs. It is impossible to see or touch a “force”, much less have an accurate measure of it. Therefore, to provide accurate assumptions is to give theories backed by proofs solved by people far more academically inclined than us. As such, the comparisons we will make between objects and celestial bodies are of their behaviour in a vacuum.
With that out of the way, let’s talk orbit.
As previously mentioned, ~90% of the Earth’s gravity still reaches our astronauts up in their shuttles and space station, so technically, everything we have sent up to space is falling, except not towards but around the Earth. When an orbiting space shuttle can move at a speed that matches the curvature of its fall to the Earth’s curvature (17,500 miles per hour), the spacecraft and everything within it will be in a state of constant freefall (6). And the same holds true for our moon as well.
With no external force pushing or pulling on these bodies except gravity, freefalling in the vacuum of space allows all objects to fall at the exact same rate disregarding their mass. So the astronauts will appear to float as they fall at the exact same speed as the shuttles that contain them, and gravity will seem to have disappeared.
Due to the free fall effect, astronauts can seemingly move objects weighing hundreds of pounds in mass with the touch of their little fingers.
To be more precise, the mass will not affect the speed of a freefalling object in a vacuum. A rock weighing 100kg will fall at the same speed as a feather. Why this occurs can be understood through the equation for the acceleration of gravity:
a = GM / r^2
Where gravity is the only force acting on the object, the acceleration of gravity is independent of the object’s mass. The ‘M’ in the equation only refers to the Mass of the Earth.
Strictly speaking, at least within a galaxy, most bodies are in orbit around another. The moon is in orbit around the Earth, the Earth is in orbit around the Sun, and the Sun is in orbit around a supermassive black hole named Sagittarius A*, which is at the center of the Milky Way Galaxy. This is important because technically, anything that we assume to be floating is actually in a state of freefall around a body with a larger mass and thus stronger gravitational pull.
If I ever decide to skydive out of the International Space Station, with nothing to sustain my speed – I would, unfortunately, begin to fall out of orbit and begin to burn up during my fall back into the Earth’s atmosphere.
What happens to gravity in between celestial bodies?
Even though we can’t see the effects, the gaps between celestial bodies and galaxies are not entirely void and also experience the force of gravity. This space is a hard vacuum filled with particles of lower densities, such as a plasma of Hydrogen and Helium, magnetic fields, neutrinos, dust, and cosmic rays. It also features a baseline temperature set by the background radiation from the Big Bang of approximately -270°C.
Since the local concentrations of matter in space have condensed to form stars and galaxies, recent studies suggest that 90% of the mass of most galaxies exists in an uncharacterized form known as Dark Matter.
Although we aren’t sure what it’s made of, we do know dark matter interacts with other matter through gravitational forces. The reason we can’t feel the pull of dark matter is that everything, including our Sun and satellites, is being pulled by it at the same time.
Further observations suggest that a more significant part of the mass-energy complex in the observable universe is composed of Dark Energy, which behaves quite opposite to gravity as it exerts negative and repulsive pressure.
Does Gravity Begin or End Somewhere in Space?
Many might think the Earth to be the origin of gravity, and it continuously and infinitely stretches out into the universe. However, if we were to truly believe that the effect of gravity is the Earth’s doing, then logically speaking, it would mean that the Earth is the most massive object in the entirety of the universe. But, we know this to be simply not true.
To counter such claims, based on the assumption that the strength of gravity largely depends on the mass of an object, the following table shows the respective masses of the celestial bodies (planets, stars, and moons) in the solar system.
| Celestial Body | Mass (kg) |
| Sun | 1.9891×1030 |
| Jupiter | 1.8986×1027 |
| Saturn | 5.6846×1026 |
| Neptune | 10.243×1025 |
| Uranus | 8.6810×1025 |
| Earth | 5.9736×1024 |
| Venus | 4.8685×1024 |
| Mars | 6.4185×1023 |
| Mercury | 3.3022×1023 |
| Moon | 7.349×1022 |
| Pluto (5) | 1.25×1022 |
| Other Celestial Bodies Include: | |
| Phobos | 1.08×1016 |
| Deimos | 1.80×1015 |
| Metis | 9.56×1016 |
| Adrastea | 1.9×1016 |
| Amalthea | 1.3×1018 |
| Thebe | 7.77×1017 |
Evaluating the table above will show that the order of the celestial bodies in our solar system is by no means a function of their mass. Earth is not the largest body in the solar system and certainly not the largest body in the universe.
So, the answer to the question of where gravity begins is very simple. Everything in the universe will exert a gravitational pull on each other. But to know where gravity ends might prove to be a little more complicated.
From a mathematical and Newtonian physics perspective, the amount of gravitational force (F) acting on a single body depends solely on how far apart they are, which is denoted by the variable “r” in the equation:
F = (G)(m1)(m2) / r^2
This means as an object gets further away from a gravitational body such as the Sun or Earth, the force of gravity will continually weaken but never actually disappear.
Even though this might be true in a traditional sense, but as mentioned previously, Newton’s law is not the most accurate theory of gravity.
Dr. Christopher S. Baird (6) expresses that within a localized galaxy, such as our Milky Way, “there is enough localized mass present to make spacetime act like traditional gravity.” Yet, on scales larger than galaxy groups, the mass of stars and planets are too sparse across the universe, resulting in a spacetime that is mostly uniform and flat. According to General Relativity, “two distant galaxies in such a spacetime no longer move towards each other. They move away from each other.”
Yes, the galaxies are moving away from each other, but beware, this does not mean the two galaxies are repelling. The nature of spacetime, when it’s mostly empty and flat, is that it continually expands, causing the galaxy groups to get farther and farther away from each other. Although whether this phenomenon can be attributed to the effects of Dark Energy is up for further debate and research in the Astrophysics community.
In the end, to answer the question of where gravity ends: in the sense of traditional Newtonian physics, the gravitational force between two celestial bodies will extend infinitely. But Einstein’s General Theory of Relativity theorizes the end of gravitational attraction to be where spacetime is no longer warped due to the mass of a gravitational body.
FAQ On How Gravity Works in Space
1. What is the difference between free-fall and orbiting?
An object in orbit is in the act of free-fall at all points around the Earth’s or any other celestial body’s curvature.
2. Can a force act without a cause?
No. Energy is required to effect a push or pull. The universe is an ocean of energy, whether it be through the warping of spacetime, dark matter, or dark energy.
Sources:
- Union University: https://www.uu.edu/dept/physics/scienceguys/2004Oct.cfm
- Mass, Weight and, Density: http://www.physics.ucla.edu/k-6connection/Mass,w,d.htm
- NASA; What is Microgravity? https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-microgravity-58.html
- Wikipedia; Outer Space: https://en.wikipedia.org/wiki/Outer_space#:~:text=outer%20space%20is%20not%20completely,%20C%20dust%20C%20and%20cosmic%20rays.
- Smart Conversion; Mass of Planets and the Sun: https://www.smartconversion.com/otherinfo/Mass_of_planets_and_the_Sun.aspx
- WTAMU: https://www.wtamu.edu/~cbaird/sq/2015/06/09/does-the-influence-of-gravity-extend-out-forever/#:~:text=No.,the%20dependence%201%2Fr2.
