Gravity and orbits

What gravity is

Gravity is the attraction between anything that has mass. You pull on Earth and Earth pulls on you; Earth just has far more mass, so you are the one that moves. The same force shapes stars, holds galaxies together and steers spacecraft.

In 1687 Isaac Newton described it with a simple rule. The pull between two objects grows with their masses and drops with the square of the distance between them. Double the distance and the pull falls to a quarter. Einstein's general relativity later refined the picture, describing gravity as the bending of space and time, but Newton's rule is still what engineers use to plan most missions.

Kepler's three laws in plain words

Decades before Newton, Johannes Kepler studied careful observations of Mars and found three patterns in how planets move.

First, orbits are ellipses, slightly stretched circles, with the Sun at one focus rather than at the center. Most planetary orbits are only a little stretched, so they look almost round.

Second, a planet moves faster when it is closer to the Sun and slower when it is farther away. A line from the Sun to the planet sweeps out equal areas in equal times. Earth moves fastest in early January and slowest in early July.

Third, farther planets take much longer to go around. The square of the orbital period is proportional to the cube of the average distance. Jupiter, about five times farther from the Sun than Earth, needs nearly 12 years per lap. Newton later showed that all three laws follow from his law of gravity.

Why the Moon doesn't fall

In a sense it does. The Moon is falling toward Earth all the time. But it also moves sideways at about 2,290 mph. As it falls, Earth's curved surface falls away beneath it, so it never gets closer. That endless falling-and-missing is what an orbit is.

Newton imagined a cannon on a very tall mountain. A slow cannonball drops nearby. A faster one lands farther away. Fire it fast enough and the ground curves away as fast as the ball drops, so it circles the planet. The International Space Station does exactly this at about 17,500 mph, and astronauts float because they are falling together with the station, not because gravity is missing.

Escape velocity

If you throw something faster, its orbit grows larger and more stretched. At a certain speed, called escape velocity, it never comes back. For Earth's surface, that is about 25,000 mph, or roughly 7 miles per second, ignoring air resistance.

Escape velocity depends on the mass and size of the body. On the Moon it is only about 5,300 mph; on Jupiter it is about 133,000 mph. Rockets do not need to reach escape velocity in one burst; they can climb gradually, but the total energy they need is the same.

Lagrange points and JWST

When two big bodies orbit each other, like the Sun and Earth, there are five special spots where a small object can keep its position relative to both. These are the Lagrange points, named after the 18th-century mathematician Joseph-Louis Lagrange.

L2 sits about 1 million miles from Earth, on the side facing away from the Sun. Something placed there would normally orbit the Sun more slowly than Earth, but the combined pull of Earth and Sun speeds it up just enough to keep pace with us. The James Webb Space Telescope circles around L2 in a large loop. From there the Sun, Earth and Moon are all in roughly the same direction, so one sunshield can block their heat and keep the telescope extremely cold.

L1, between Earth and the Sun, hosts solar observatories such as SOHO. L1, L2 and L3 are unstable, so spacecraft there make small thrusts to stay in place. L4 and L5 are stable, and asteroids gather there around Jupiter.

See orbits in action

You can watch orbital motion with your own eyes. Track the Moon's position against the stars over a few evenings; it shifts east by about its own width every hour. You can also look for the ISS crossing the sky as a bright, steady dot in the hours after sunset or before dawn; NASA's Spot the Station service lists passes for your location.

Over weeks, you can follow a bright planet such as Jupiter or Mars drifting against the background stars, and occasionally reversing direction for a while. That apparent loop happens because Earth, on its faster inner orbit, overtakes the planet.