We are now ready to discuss the last three items on our list from the beginning of this section of six key things that gravity does:

Gravity keeps satellites, the International Space Station, and the Moon in orbit of Earth

Gravity keeps Earth and other planets in orbit around the Sun.

Gravity holds galaxies together.

All three of these relate to orbits, which means you already know how to explain them. Recall that once Newton came up with the universal law of gravitation, he was able to use mathematics to show that it predicted what Kepler had already discovered: that it leads less massive objects (like planets) to orbit more massive ones (like the Sun), and that the force holding the planets in orbit gets weaker with distance, thereby explaining Kepler’s discovery that more distant planets orbit the Sun at slower speeds.

In fact, Newton found that the universal law of gravitation applies in the same way to all orbiting objects. Applying this idea to the three things from the list above:

• Because Earth is much more massive than satellites, the International Space Station, or the Moon, Earth’s gravity can hold these objects in orbit.
• The Sun holds Earth and other planets in orbit because the Sun is so much more massive than any of the planets.
• Galaxies are made up of stars, so all of a galaxy’s stars together contribute to the overall gravity of the galaxy. This gravity is what keeps the stars all orbiting around the center of the galaxy. (Note: In addition to the gravity of stars, galaxies also have gravity from gas that they contain and from the mysterious “dark matter” that seems to make up the majority of the mass in most galaxies.)

Masses of Distant Objects

Have you ever wondered how scientists can know the masses of distant objects, such as of the Moon, or of the Sun, or of an entire galaxy? Or how we know the mass of our own planet? The answer is that the universal law of gravitation makes it possible to calculate masses in any case in which we can observe one object orbiting another.

To understand how this is possible, recall that Kepler discovered that planets farther from the Sun orbit more slowly than those closer to the Sun, and that this fact implies that it is a force from the Sun that holds the planets in orbit. Newton discovered this force to be gravity and his universal law of gravitation (in equation form) allows us to calculate the gravitational force that explains these orbits. Because the gravitational force depends on the Sun’s mass, it is possible to “work backward” from the orbital properties of any planet — more specifically, the planet’s average orbital distance and orbital period — to determine the mass of the Sun.

Note that because this idea works for any planet (you’ll get the same answer for the Sun’s mass no matter which planet you use), we can even use Earth’s orbital properties. In other words: Simply by knowing Earth’s orbital distance (about 150 million kilometers) and orbital period (one year), it is possible to calculate the mass of the Sun.

The same idea applies to all other cases in which one object orbits another. For example:

• Scientists can calculate Earth’s mass from the orbital period and distance of our Moon or of any artificial satellite.
• Scientists can calculate the mass of the Moon from the orbital period and distance of a spacecraft sent to orbit around it.
• Scientists can calculate the mass of Saturn from the orbital period and distance of any of its moons.
• Scientists can calculate the mass of a distant galaxy by measuring the orbital properties of stars or gas clouds orbiting its center.
• If a distant star is a member of a binary star system, scientists can calculate its mass by measuring the orbital properties of its companion star.

In fact, everything we know about masses of objects throughout the universe ultimately traces back to calculations based on the universal law of gravitation and observations of orbital properties.

The following box shows how you can calculate masses for yourself with the equation known as “Newton’s version of Kepler’s Third Law.” It gets this name because Kepler’s third law describes how orbital periods and distances are related, and Newton then used his universal law of gravitation to generalize this law into a form that also allows the determination of masses.

No Need for Fuel

Earth has been orbiting the Sun since the solar system was born some 4 ½ billion years ago. Have you ever thought about how it keeps going year after year?

The answer is that because there is no friction or air resistance in space, there is nothing to slow Earth down. As a result, it follows a path determined only by gravity, and the Sun’s gravity dominates the solar system. Therefore, Earth orbits the Sun on essentially the same path year after year. (The only deviations from being exactly the same path arise from the very small gravitational tugs on Earth caused by the Moon and by other planets.)

The same idea applies to other objects in space. As long as there is no friction, they will remain on orbital paths determined only by gravity. That is why, after a spacecraft is launched, it no longer needs any fuel to keep going. The only exception is for spacecraft in low Earth orbit, where there is still a tiny bit of friction from Earth’s upper atmosphere. These spacecraft can be slowed by this friction, causing their orbits to decay until they ultimately plummet back into the atmosphere (where they generally burn up, because they are coming in at such high speed).

Using Gravity to Steer Spacecraft

When you hear the term “orbit,” you probably think of one object going around another over and over again, just as Earth orbits the Sun year after year. However, gravity also allows objects to follow other paths. For example, some comets that come speeding into the inner solar system from afar simply curve around the Sun and then head back outward, never to return (Figure 3.34).

Scientists and engineers can use this same idea to “steer” spacecraft with gravity. Consider the New Horizons spacecraft, which flew past Pluto in 2015. Like any interplanetary spacecraft, New Horizons had to be launched from Earth with enough speed (called “escape velocity”) to allow it to escape the grip of Earth’s gravity and journey out into the solar system.

After that, however, New Horizons needed only very small amounts of fuel to make small course corrections, while all the rest of the “steering” was done by gravity. Engineers calculated exactly how the gravity of the Sun and planets would affect the spacecraft, and then chose a path that would make sure it reached Pluto. Figure 3.35 shows the path along with one of the spectacular photos of Pluto that the spacecraft took as it flew past.

Orbiting Disks

As we’ve discussed, gravity’s central pull explains why virtually every large individual object in the universe — including stars, planets, dwarf planets, and large moons — is round.

There is also a second very common shape in the universe, though it applies only to collections of orbiting objects: this shape is a flattened disk. Examples include (Slide Show Figure 3.36):

• The rings of Saturn, which are a collection of individual particles orbiting Saturn.
• Disks of material orbiting young stars.
• The disks of spiral galaxies.
• The orbits of the planets in our solar system, which all trace a fairly flat disk around the Sun.

Let’s consider how a star and its planets are born. Observations tell us that stars are born from giant clouds of gas and dust in space.  Two things happen at the same time:

1. Gravity pulls the material inward, making the cloud shrink in size.
2. Like an ice skater pulling in his arms as he spins, the cloud rotates faster and faster as it shrinks (Figure 3.37). (You may recall that this is a result of what we call the “law of conservation of angular momentum.”)

A round star forms at the center of this spinning cloud, where so much material is pulled in by gravity that it essentially become a single object (the star). Meanwhile, the surrounding material flattens into a disk as it orbits around the central star, for much the same reason that spinning a ball of dough will flatten it into a pizza crust (Figure 3.38). Planets (along with moons, asteroids, and comets) can later form from within this disk, which explains why the orbits of planets end up tracing the shape of a disk.

All other disk-like structures in the universe arose for similar reasons: In all cases, gravity drew material together, and the material spun faster and formed a disk as it was drawn inward.