The Solar System

North

Pole

Ellipse

Ellipse

Circle

Hyberbola

Parabola

Center of

mass

NASANASA

Closed orbits return the orbiting object to its
starting point. The moon and artificial satellites
orbit Earth in closed orbits. Below, the cannonball
could follow an elliptical or a circular closed
orbit. If the cannonball travels as fast as escape velocity,
the velocity needed to leave a body, it will enter an
open orbit. An open orbit does not return
the cannonball to Earth.
It will escape.

A cannonball with a

velocity greater than

escape velocity will

follow a hyperbola and

escape from Earth.

A cannonball

with escape

velocity will follow

a parabola and

escape.

To be precise you should not

say that an object orbits Earth.

Rather the two objects orbit each other.

Gravitation is mutual, and if Earth pulls

on the moon, the moon pulls on Earth.

The two bodies revolve around their

commoncenter of mass, the balance

point of the system.

2

2a

3

3a

1c Astronauts in orbit around Earth feel weightless, but they are not “beyond Earth’s gravity,” to use a term from old science fiction movies. Like the moon,

the astronauts are accelerated toward Earth by Earth’s gravity, but they travel fast

enough along their orbits that they continually “miss the Earth.” They are literally

falling around Earth. Inside or outside a spacecraft, astronauts feel weightless

because they and their spacecraft are falling at the same rate. Rather than saying

they are weightless, you should more accurately say they are in free fall.

Two bodies of different mass balance at the

center of mass, which is located closer to the

more massive object. As the two objects orbit

each other, they revolve around their common

center of mass as shown at right. The center of

mass of the Earth–moon system lies only 4708

km (2926 miles) from the center of Earth —

inside the Earth. As the moon orbits the center of

mass on one side, the Earth swings around the

center of mass on the opposite side.

As described by Kepler’s second law, an

object in an elliptical orbit has its lowest

velocity when it is farthest from Earth (apogee),

and its highest velocity when it is closest to

Earth (perigee). Perigee must be above Earth’s

atmosphere, or friction will rob the satellite of

energy and it will eventually fall back to Earth.