Luke Skywalker leads a group of Rebel X-wing fighters in an attack on the Imperial Death Star. As the fighters bank and roll towards the gargantuan spacecraft, you see laser weapons firing from both sides. Luke does some fancy flying, fires his weapons, lands his torpedo in the vent, and, with a loud explosion, the Death Star is no more. This climactic scene from "Star Wars: Episode IV" is typical of many action science fiction movies. It makes for a great movie going experience, but is the science real? Could spacecraft really move like this? Could you see laser blasts? Will we hear the deafening explosions? And should we care about any of these things?

Image courtesy © Lucasfilm Ltd. & TM. All Rights Reserved. Luke Skywalker and the Rebels' attack on the Death Star. See more science fiction pictures.

We'll answer the last question first: "Yes, definitely!" Science is essential to any work of science fiction; in fact, it separates science fiction from fantasy or other works of fiction. Furthermore, sci-fi fans are very discriminating. Sometimes, minor errors in the science do not detract from the story and may not be noticeable, except by the discriminating viewer. In other cases, the mistakes in science are so blatant that the story becomes totally unbelievable and the movie falls apart.

In this article, we'll explore some major mistakes and misconceptions in sci-fi movies and TV shows. A few notes before we begin:

• Our list is not comprehensive – we have chosen several topics, but there are many more.
• You may disagree with our choices. Discussion of sci-fi is always a good thing.
• We love sci-fi movies, TV shows, novels and short stories. Our goal is to inform, not to "pick on" a particular work.
• We realize that the primary goal of moviemakers is to entertain, not necessarily to educate. Sometimes emphasizing the science may not make the scene work.
• We realize that sci-fi movies are constrained by budgets, technical capabilities and matters that are critical to entertainment.

With this in mind, let's look at how science fiction doesn't work.

The Laws of Science Fiction In their book "Space Travel," sci-fi author Ben Bova and Anthony R. Lewis state two laws of science fiction: Science fiction stories are those in which some aspect of future science or technology is so integral to the story that, if you take away the science or technology, the story collapses. Image courtesy Amazon Science fiction writers are free to extrapolate from today's knowledge or to invent anything they can imagine –so long as no one can prove that what they have "imagined" is wrong. This first law separates sci-fi from fantasy, space opera, or just plain fiction. For example, in "Contact," a radio astronomer receives a radio message from another civilization in the galaxy. The sciences of radio astronomy and interstellar travel are critical to the story and the late author/astronomer Carl Sagan took great efforts to get the science right. Bova and Lewis's second law gives science fiction authors great license. Let's examine how this law works with a common science fiction film problem. The distances between stars in the galaxy are vast (light years and parsecs). So, for characters to travel among the stars in a reasonable time, they must travel faster than light. But Einstein's theory of special relativity places the speed of light (300,000 km/s) as the cosmic speed limit. Nothing that has mass can travel as fast as light because it would take an infinite amount of energy to accelerate it to the speed of light, and the mass would increase infinitely. Science-fiction authors have invented ways around this problem, such as "warp drives" that distort space on "Star Trek." These methods enable the characters to traverse the great distances without incurring the penalties of relativity (increases in mass, shortened lengths, time dilation). In fact, these inventions have stimulated theoretical physicists to explore them. So far, none of these ideas can be disproved and have become staples of science fiction.

Willing Suspension of Disbelief

Image courtesy Amazon "The Core" bombed at movie theaters in part because it broke the laws of science fiction.

There is a principle in movie making called "the willing suspension of disbelief," in which moviegoers can accept a certain level of implausibility in favor of the story. For example, fantasy stories rely on magic and readers and viewers accept this. This also happens with some science fiction stories. For example, the work may be dated. Jules Verne's "Journey to the Center of the Earth" was written before geologists knew anything about the internal structure of the Earth or plate tectonics, so you can suspend belief and enjoy the story. Finding the line at which viewers are unwilling to suspend their belief can be tricky.

So, science is important to make a work of science fiction and authors and film makers should strive to make the science in their works as real as possible. If the science is not real, the responses can vary. Some viewers may be willing to suspend their disbelief. "Star Wars" fans are certainly willing to suspend disbelief. However, if the science is too "out there," Viewers can be turned off. "The Core" was so unbelievable that it bombed at the box office. How filmmakers choose to tackle the believability factor could mean the difference between a success and a bomb.

How Spaceships Move
We opened this article with a description of banking X-wing fighters and other spacecraft, from "Star Wars." You can see similar movements in the Viper fighters of the original "Battlestar Galactica" TV series. Designers modeled these spacecraft after modern jet aircraft fighters (like the F-14 and the MiG) and they engage in dogfights like those in "Top Gun." The banking of an aircraft is a consequence of air moving over the surfaces of the wing, ailerons and rudder. When a plane turns, the ailerons on one wing move up on one side and down on the other, which causes the aircraft to roll in the direction of the turn. Simultaneously, the tail rudder moves in the opposite direction of the turn and deflects air to make the turn. These combined air movements cause the plane to bank in the direction of the turn as the plane continually thrusts forward. They could not happen without air.

While an aircraft moves through the medium of air, a spacecraft moves in a vacuum. Newton's Third Law of Motion ("for every action, there is an equal, but opposite reaction") governs the movement of a spacecraft. For a spacecraft to turn, it must fire a rocket thruster (eject mass as hot gasses) in the opposite direction from where it must go. There are three axes of rotation: pitch, roll and yaw. If the pilot wants to turn right, then the rocket thrusters fire left and usually the roll and yaw thrusters fire simultaneously. Such maneuvering thrusters are located in various places along the body of the spacecraft and allow it to move in all three axes of rotation. So, the turn of a spacecraft looks like an abrupt flip in one or more directions simultaneously rather than a smooth bank. You can see such movements of the Apollo spacecraft in the HBO miniseries "From the Earth to the Moon" and in the Viper fighters of the new "Battlestar Galactica" series on the SciFi Channel.

Next, we'll learn about sci-fi mistakes with planets and asteroids.