One thousand million, or 1,000,000,000.

black hole

A region of space where the pull of gravity is so strong that nothing, not even light, can escape. One way that black holes are thought to form is when the cores of very massive stars fall in upon themselves.

combustion chamber

The part of a rocket engine in which the fuel is burned to produce exhaust gases that drive the spacecraft forward.


A tiny, negatively-charge particle, even smaller than a proton or neutron.


The waste material produced when a fuel is burned. The reactive force arising from the exhaust as it escapes from the combustion chamber causes a spacecraft to gain speed.

exhaust speed

The speed at which the exhaust material from a rocket engine leaves the spacecraft. A high exhaust speed is important if the spacecraft's final speed is also to be high.


A process in which the most basic particles, or nuclei, of a lighter substance combine to make nuclei of a heavier substance. During nuclear fusion, a small amount of mass is transformed into a very large amount of energy. The most common and energy-rich fusion reaction is that in which hydrogen nuclei collide, at high temperatures, and create helium nuclei.


A very large collection of stars. Galaxies come in a variety of shapes – round, oval, spiral, and odd-shaped – and a variety of sizes. Spiral galaxies, such as the one we live in, contain huge amounts of loose gas and dust from which future stars may be made.


A force exerted by any object that has mass. The Earth's gravity is the force that prevents the atmosphere,and people, from floating away into space.


The second lightest and second most common substance in the universe; normally found as a gas. At high temperatures, as in the center of stars, hydrogen will fuse to form helium, producing huge amounts of energy in the process.


The lightest and most common substance in the universe; normally found as a gas. At high temperatures, as in the center of stars, hydrogen will fuse to form helium, producing huge amounts of energy in the process.


A word meaning "between the stars."

interstellar ramjet

A type of spaceship in which the fuel (mainly hydrogen) is gathered and compressed from the interstellar space ahead of the ship, using a powerful magnetic scoop.


A device used for producing concentrated, well-directed beams of very pure light. The waves of light in a laser beam are exactly in step with one another and all of the same length. The first laser, built in 1960, used a rod-shaped crystal of ruby.


The distance traveled by light, moving at 186,282 miles per second, in one year. It equals 5.85 trillion miles.

magnetic field

A region in which a force of magnetism acts.


Anything that contains matter – for example, a rocket, a star, or a person – has mass. Mass gives a measure of how difficult it is to start moving an object that is still, or to change the speed of an object that is already moving.


The part of a spacecraft that does not include the fuel or propulsion system. In the case of a robot probe, the payload consists largely of scientific and communications equipment. In a spaceship with astronauts, though, a large part of the payload is made up of the support systems and living quarters for the human crew.


A round object, usually at least several thousands miles across, that is in orbit around a star. Planets give off no light of their own and are much cooler than stars. The Sun has eight planets and a number of dwarf planets (including Pluto). Many other stars also have systems of planets.

propulsion system

The means by which a spacecraft (or any other vehicle) is made to move.

solar system

The name given to the Sun and everything that goes around it, including the planets and their moons.


A round, gassy object in space that normally makes its own light and heat by nuclear fusion. The only exceptions are "dead" stars, such as white dwarfs, neutron stars, and black holes, that have used up their supplies of fusion energy.

theory of relativity

Developed by the great German-American scientist Albert Einstein in the early part of the twentieth century. The so-called special theory of relativity shows how quantities such as mass, length, and time change when objects move at different relative speeds. In his general theory of relativity, Einstein describes a new theory of gravity.


In rocketry, the forward force produced by a spacecraft's engine.


One million million, or 1,000,000,000,000.


The downward force acting on an object due to gravity. A person's weight, for instance, would be different on the Moon than on Earth because the pull of gravity is different. That person's mass, though, would stay the same wherever he or she was in the universe.