Spacecraft need an extra boost to travel between stars 

accelerate: To experience a change in velocity (speed). 

atmosphere: The envelope of gases surrounding Earth, another planet or a moon. 

atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons. 

atomic: Having to do with atoms, the smallest possible unit that makes up a chemical element. 

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H₂O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds. 

constant: Continuous or uninterrupted. (in mathematics) A number that is known and unchanging, usually based on some mathematical definition. For example, π (pi) is a constant equal to 3.14. . . and defined as the circumference of a circle divided by its diameter. 

cosmos: (adj. cosmic) A term that refers to the universe and everything within it. 

dimension: Descriptive features of something that can be measured, such as length, width or time. 

engine: A machine designed to convert energy into useful mechanical motion. Sometimes an engine is called a motor. (in computer science) A computer program that performs a particular, narrow range of functions. 

engineer: A person who uses science and math to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need. 

exoplanet: Short for extrasolar planet, it’s a planet that orbits a star outside our solar system. 

fiction: (adj. fictional) An idea or a story that is made-up, not a depiction of real events. 

force: Some outside influence that can change the motion of an object, hold objects close to one another, or produce motion or stress in a stationary object. 

friction: The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage a surface of some material as it rubs against another. 

fuel: Any material that will release energy during a controlled chemical or nuclear reaction. Fossil fuels (coal, natural gas and petroleum) are a common type that liberate their energy through chemical reactions that take place when heated (usually to the point of burning). 

fusion: (v. to fuse) The merging of two things to form a new combined entity. (in physics) The process of forcing together the nuclei of atoms. This nuclear fusion is the phenomenon that powers the sun and most other stars, producing heat and forging the creation of new, heavier elements. 

gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity. 

interstellar: Between stars. 

ion: (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms. 

lever: A rigid bar resting on a pivot. It is used to help move a heavy or firmly fixed load with one end when pressure is applied to the other. Or, as part of some devices, it can act like a switch to turn some system on or off. 

light-year: The distance light travels in one year, about 9.46 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year. 

Mars: The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s. 

mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from. 

momentum: A measure of the motion of something, made by multiplying its mass and velocity. Changing the speed or direction of an object will also alter its momentum. 

nuclear power: Energy derived from processes that produce heat by splitting apart the nuclei of atoms (fission) or forcing atomic nuclei to merge (fusion). A nuclear power plant uses that heat to drive turbines that create electricity. 

oxidizer: A substance that yields oxygen, resulting in the combustion of another material, such as a fuel. 

particle: A minute amount of something. 

photon: A particle representing the smallest possible amount of light or other type of electromagnetic radiation. 

physics: The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist. 

planet: A large celestial object that orbits a star but unlike a star does not generate any visible light. 

plastic: Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation. (adj.) A material that is able to adapt by changing shape or possibly even changing its function. 

polymer: A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example). 

pressure: Force applied uniformly over a surface, measured as force per unit of area. 

propulsion: The act or process of driving something forward, using a force. For instance, jet engines are one source of propulsion used for keeping airplanes aloft. 

Red Planet: A nickname for Mars. 

reflective: Adjective that refers to the ability of something to reflect light strongly. Such objects can produce a strong bright glare when sunlight bounces off of them. Examples of reflective objects include a mirror, a smooth metal can, a car window, a glass bottle, ice, snow or the watery surface of a lake. 

rocket: Something propelled into the air or through space, sometimes as a weapon of war. A rocket usually is lofted by the release of exhaust gases as some fuel burns. (v.) Something that flings into space at high speed as if fueled by combustion. 

science fiction: A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds. 

sea: An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty. 

society: An integrated group of people or animals that generally cooperate and support one another for the greater good of them all. 

solar: Having to do with the sun or the radiation it emits. It comes from sol, Latin for sun. 

solar system: The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets. 

solar wind: A flow of charged particles (including atomic nuclei) that have been ejected from the surface of the star, such as our sun. It can permeate the solar system. When emitted by a star other than the sun, this radiation is known as a stellar wind. 

speed of light: A constant often used in physics, corresponding to 1.08 billion kilometers (671 million miles) per hour. 

star: The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become hot enough, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star. 

system: A network of parts that together work to achieve some function. For instance, the blood, vessels and heart are primary components of the human body’s circulatory system. Similarly, trains, platforms, tracks, roadway signals and overpasses are among the potential components of a nation’s railway system. System can even be applied to the processes or ideas that are part of some method or ordered set of procedures for getting a task done. 

technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts. 

thrust: A force that makes an object move forward. 

thruster: An engine that pushes or drives with force by expelling a jet of fluid, gas or stream of particles. 

unique: Something that is unlike anything else; the only one of its kind. 

universe: The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years). 

Venus: The second planet out from the sun, it has a rocky core, just as Earth does. Venus lost most of its water long ago. The sun’s ultraviolet radiation broke apart those water molecules, allowing their hydrogen atoms to escape into space. Volcanoes on the planet’s surface spewed high levels of carbon dioxide, which built up in the planet’s atmosphere. Today the air pressure at the planet’s surface is 100 times greater than on Earth, and the atmosphere now keeps the surface of Venus a brutal 460° Celsius (860° Fahrenheit).